Adapter and computer-implemented method

Abstract

A computer implemented method of calculating an indication dose of a substance delivered to a patient from an inhaler device, the method comprising detecting an inhalation signal 702 from an airflow r

Description

Field of the Invention The present invention relates to an adapter for fitting to an inhaler device and a computer-implemented method for calculating a dose of a substance delivered to a patient from an inhaler device. Background The effective deposition of inhaled respiratory medication is known to be strongly dependent on medication particle size, and the flow rate of inhalation. Aerosols with larger particle sizes (e.g., >5 microns) tend to deposit in the upper airway of a patient, which can result in the medication being swallowed and / or delivered to the wrong tissue type. When an aerosolised medication has a smaller particle size (e.g., <5 microns), the particles are small enough to be carried by the airflow into the lung. A preferred inhalation airflow rate to ensure satisfactory delivery of medication to the patient may be in the range of approximately 10-40 litres per minute (LPM). A slower air flow rate risks that the aerosol particles expelled by the inhaler device will settle before being delivered to the required anatomy of the patient (typically to the bronchial tubes, and / or deeper into the lungs). A higher air flow rate risks the aerosol particles simply hitting and adhering to the back of the patient's throat, without penetrating deeper into the patient's pulmonary system. Adapters, such as spacers or holding chambers, can be used with drug delivery inhaler devices to assist patients with their inhaler use. Valved holding chambers (which may also be referred to as spacers) are typically provided with a one-way valve allowing inhalation from, but not exhalation into, the chamber. The medication from the inhaler device can be trapped within the holding chamber allowing the medication to be inhaled slowly and if necessary, for patents with low tidal volume, in more than one inhalation cycle. The use of these valved holding chambers eliminates the requirement that slow deep inhalation coincides with the actuation of the drug delivery inhaler device. Adapters, such as spacers or valved holding chambers, are therefore generally recommended to improve the effectiveness and consistency of inhaled aerosol medication. Some adapters comprise a whistle located at an upstream end of the adapter, to provide an indication of poor technique (e.g., when a inhalation flow rate is too high). However, the inhalation flow rate at which the whistle in these adapters is actuated is highly dependent on the air flow resistance of the drug delivery inhaler device used with the adapter. This means that the whistle may actuate at a different inhalation flow rate depending on the type of inhaler device used with the actuator, as shown in Figure 24, which shows the airflow rate at which the whistles in various existing spacers, actuate for different inhaler devices. An example spacer has a reed whistle located at an upstream end of the adapter. When the patient inhales from such a valved holding chamber having a whistle, air will be drawn in through both the whistle and the bypass channel (e.g., a gap formed between a drug cannisterand an inner surface of a housing of the inhaler device). The balance between these flow paths will dictate the flow rate at which the whistle sounds. For example, if the drug delivery inhaler device has a smaller bypass channel, and therefore a higher airflow resistance, a patient is able to generate a lower differential pressure compared to other inhaler devices such that a greater proportion of the airflow is drawn through the whistle and therefore the whistle will sound at a lower inhalation airflow rate. Conversely, if the drug delivery inhaler device has a lower resistance (e.g., a larger bypass channel), it will be necessary to reach a higher inhalation air flow rate before there is sufficient negative pressure to cause air to pass through and sound the whistle. Therefore, using the actuation of a whistle in a valved holding chamber as an indication of an inhalation rate may be misleading as it may not accurately indicate the inhalation air flow rate. The present invention has been devised in light of the above considerations. Summary of the Invention According to a first aspect, there is provided an adapter for fitting to an inhaler device, the adapter comprising: a body defining an inlet for connection to a mouthpiece of the inhaler device, an outlet for communication with the mouth of a patient, and an airflow path between the inlet and the outlet; a valve positioned between the inlet and the outlet; and, a whistle positioned between the valve and the outlet, the whistle being operable to generate a sound signal indicating when an inhalation airflow rate through the outlet is greater than or equal to an inhalation airflow rate threshold, and wherein the whistle is operable to generate the sound signal when the airflow rate through the whistle is greater than or equal to a whistle actuation flow rate, the whistle actuation flow rate being less than or equal to 5 litres per minute (LPM). By positioning the whistle between the valve and the outlet (i.e., downstream of the valve), the inhalation airflow rate at which the whistle is actuated (i.e., the airflow rate at which the whistle generates a sound signal) is less dependent on the air flow resistance of the particular inhaler device connected to the inlet of the adapter. This is due to the significant drop in pressure which occurs as the airflow passes through the valve. However, even with such an arrangement, there is still a difference in the inhalation airflow rates at which the whistle is actuated for different types of inhaler device. The effect of this difference may be further reduced by using a whistle that is actuated at a lower flow rate therethrough (than e.g., a reed whistle which generally have an actuation flow rate of approximately 8 LPM), and in particular at a whistle actuation flow rate of less than or equal to 5 LPM. Using such a whistle means that, for any given inhaler device, the inhalation airflow rate along the airflow path required to actuate the whistle (i.e., the threshold airflow rate) is lower. This may allow the whistle to more consistently and accurately indicate whether or not the user is inhaling at the correct rate, regardless of the inhaler device used with the adapter. Furthermore, by positioning the whistle between the valve and the outlet and by using the whistle having an activation flow rate of less than or equal to 5 LPM, dilution of the aerosol from the inhaler device, due to air being drawn into the adapter (e.g., through the inlet and / or the whistle), can be reduced. Optional features of the adapter will now be set out. Litres per minute (LPM) may be understood to mean standard litres per minute (SLPM). The whistle may comprise a kettle whistle. Kettle whistles have been found to consistently / reliably have an activation airflow rate through the whistle of less than or equal to 5 LPM. Alternatively, the whistle may comprise a reed whistle or a tube whistle (stepped / corrugated or otherwise) having an activation flow rate of less than or equal to 5 LPM. The frequency (e.g., of the fundamental harmonic) of the sound signal generated by the whistle may vary linearly or monotonically (which may be approximated as linearly) with inhalation airflow rate through the outlet. In this way, the frequency of the sound signal can be used to determine the inhalation airflow rate through the outlet. It will be appreciated that, during user inhalation, the airflow path through the adapter may have a direction pointing from the inlet (i.e., an upstream end of the airflow path) to the outlet (i.e., a downstream end of the airflow path). The valve may be configured to enable airflow from the inlet to the outlet, and to inhibit / prevent airflow from the outlet to the inlet (e.g., the valve may be a one-way valve). Positioning the whistle between the valve and the outlet may be understood as meaning that the whistle is positioned downstream of the valve (e.g., with reference to the airflow path). The inhaler device may store a substance. For example, the inhaler device may correspond to a drug delivery inhaler device and may store a drug. The inhaler device may be actuatable to eject a metered dose of the substance (e.g., into the adapter when the inhaler device is fitted to the adapter), for example in the form of an aerosol. The adapter may be for fitting to different types of inhaler devices (e.g., drug delivery inhaler devices made by different manufacturers). As will be described in more detail below, the sound signal generated by the whistle may provide positive feedback indicating that the user is inhaling at a rate above a predetermined minimum level, and / or below a predetermined maximum level. In these examples, the inhalation airflow rate threshold may correspond to a minimum airflow rate threshold (e.g., a minimum airflow rate threshold suitable / effective for delivery of the substance stored in the inhaler device to the patient). When the whistle is generating a sound signal, the user is therefore provided positive feedback that they are inhaling at a preferred inhalation rate (and in particular, above a predetermined minimum level and / or below a predetermined maximum level). The sound signal generated by the whistle may provide negative feedback indicating that the user is inhaling too quickly. In these examples, the inhalation airflow rate threshold may correspond to a maximum airflow rate threshold (e.g., a maximum airflow rate threshold above which suitable / effective for dose delivery, of the substance stored in the inhaler device, to the patient is reduced). When the whistle is generating a sound signal, the user is therefore provided negative feedback that they are inhaling too fast (and in particular, above a predetermined maximum level). It will be appreciated that the inhalation airflow rate through the outlet may refer to the inhalation airflow rate when a mouthpiece of an inhaler device is connected to the adapter inlet. For completeness, even with the adapters disclosed herein, the whistle may sound at a slightly different inhalation airflow rate through the outlet depending on the inhaler device used with the adapter. However, the whistle more accurately and consistently indicates when the inhalation airflow rate through the outlet is greater than or equal to an inhalation airflow rate threshold, regardless of the inhalation device used with the adapter. It may be understood that the whistle is operable to generate a sound signal when the differential pressure across the (kettle) whistle (e.g., between an environment external to the adapter and the environment internal to the adapter) reaches and / or is above a threshold differential pressure. In some examples, the whistle actuation flow rate may be less than or equal to 4 LPM, for example less than or equal to 3 LPM, or less than or equal to 2.5 LPM. In some examples, a whistle actuation flow rate of 2.5 LPM may be 10% or less of the inhalation airflow rate (i.e., the total airflow rate through the outlet). The whistle actuation flow rate may be greater than or equal to 1 LPM, for example greater than or equal to 2 LPM. In some examples, the kettle whistle may have a whistle axis. The kettle whistle may comprise a first plate and a second plate spaced apart from the first plate along the whistle axis. The first plate may include a first surface, and the second plate may include a second surface facing and / or opposite the first surface. The first surface and / or the second surface may be transverse / perpendicular to the whistle axis. The first plate may define a first aperture, and the second plate may define a second aperture aligned with the first aperture along the whistle axis. In this way, a cavity may be defined between the first plate and the second plate. In some examples, the diameter of the first aperture and / or the diameter of the second aperture may be greater than or equal to 0.5 mm, and / or less than or equal to 10 mm. For example, the diameter of the first aperture and / or the diameter of the second aperture may be greater than or equal to 1 mm and / or less than or equal to 4 mm. The diameter of the first aperture and / or the second aperture may be greater than or equal to 2 mm and / or less than or equal to 3 mm. For example, the diameter of the first aperture and / or the second aperture may be (approximately) 2.3 mm. In some examples, the distance between the first plate and the second plate along the whistle axis may be greater than or equal to 0.5 mm, and / or less than or equal to 20 mm. For example, the distance between the first plate and the second plate along the whistle axis may be greater than or equal to 2 mm, and / or less than or equal to 5 mm. The distance between the first plate and the second plate along the whistle axis may be greater than or equal to 3 mm, and / or less than or equal to 4 mm. For example, the distance between the first plate and the second plate along the whistle axis may be (approximately) 3.4 mm. The thickness of the first plate in a direction parallel to the whistle axis and / or the thickness of the second plate in a direction parallel to the whistle axis may be at least 0.1 mm and / or less than or equal to 1 mm. For example, the thickness of the first plate along the whistle axis and / or the thickness of the second plate along the whistle axis may be at least 0.3 mm and / or less than or equal to 0.7 mm. The thickness of the first plate along the whistle axis and / or the thickness of the second plate along the whistle axis may be at least 0.4 mm and / or less than or equal to 0.6 mm. For example, the thickness of the first plate along the whistle axis and / or the thickness of the second plate along the whistle axis may be (approximately) 0.5 mm. A Reynolds number operating range of the kettle whistle may be at least 2000. In some examples, the whistle may comprise a positive whistle that provides positive feedback that the user is inhaling at a preferred inhalation rate. The positive whistle may be operable to generate a sound signal indicating when the inhalation airflow rate through the outlet reaches and / or is above a minimum airflow rate threshold suitable / effective for delivery of a substance (e.g., a drug) stored in the inhaler device to the patient. In this way, positive feedback in the form of a whistle sound / sound signal may be provided by the whistle when the inhalation airflow rate through the adapter is high enough to effectively deliver the substance to the patient. By using a whistle having a whistle actuation flow rate of less than or equal to 5LPM (such as a kettle whistle) and positioning the whistle downstream of the valve, the positive whistle may be actuated at a lower flow rate along the airflow path for a given inhaler device. Such a whistle is therefore well suited to being a positive whistle operable to generate a sound signal at a minimum airflow rate threshold, indicating that the user is inhaling at a preferred inhalation rate. The minimum airflow rate threshold may be greater than or equal to approximately 5 LPM, greater than or equal to approximately 10 LPM, or greater than or equal to approximately 15 LPM, or greater than or equal to approximately 20 LPM. The minimum airflow rate threshold may be less than or equal to approximately 40 LPM, for example, less than or equal to approximately 30 LPM, or less than or equal to approximately 25 LPM. In particular, the minimum airflow rate threshold may be approximately 20 LPM. In these examples, the whistle generating a sound signal indicates that the inhalation airflow rate through the outlet (and thus into a mouth of a user inhaling from the outlet) is approximately 20 LPM. The whistle can therefore provide positive feedback that the user is inhaling at a rate equal to or exceeding a predetermined minimum level suitable for effective drug delivery (e.g., 20LPM). Approximately may mean equal to within a margin of 25%, or a margin of 10%. Thus, approximately 20 LPM may mean between (and including) 15 LPM and 25 LPM. For the positive whistle, the threshold differential pressure may be less than or equal to 50 Pa, for example less than or equal to 40 Pa. The threshold differential pressure may be greater than or equal to 15 Pa, greater than or equal to 20 Pa, or greater than or equal to 30 Pa. The threshold differential pressure may be between approximately 30 and 50 Pa, more preferably between approximately 35 and 45 Pa, for example approximately 40 Pa. In some examples, a differential pressure required to open the valve (i.e., the valve located between the inlet and the outlet) may be lower than the threshold differential pressure required to actuate the whistle. In this way, the whistle may not be actuated until there is an airflow along the airflow path from the inlet to the outlet. In some examples, the inhalation airflow rate threshold may correspond to the minimum airflow rate along the airflow path required to open the valve. In this way, the whistle may generate a sound signal indicating when the valve opens, or when there is an airflow rate along the airflow path between the inlet and the outlet. In some examples, the positive whistle may be operable to generate a sound signal indicating when the inhalation airflow rate through the outlet is below a maximum airflow rate threshold suitable / effective for delivery of a substance (e.g., a drug) stored in the inhaler device to the patient. For example, the maximum airflow rate threshold may be greater than or equal to 30 LPM, for example greater than or equal to 40 LPM, or greater than or equal to 50 LPM. In some examples, the maximum airflow rate threshold may be 50 LPM or less, for example 40 LPM or less. In some examples, the adapter may further comprise a shut off valve fluidly connected to the whistle (e.g., fluidly connected to the cavity of the whistle). The shut off valve may be operable to inhibit or prevent the whistle from generating a sound signal when the inhalation airflow rate through the outlet is greater than a shut off airflow rate threshold. In particular, the shut off valve may be operable to inhibit / prevent the whistle from generating a sound signal when the airflow rate through the whistle is greater than or equal to a predefined whistle shut off threshold, the predefined whistle shut off threshold being greater than the whistle actuation flow rate. As such, the whistle may be operable to generate a sound signal when the airflow rate therethrough is between the whistle actuation flow rate and the predefined whistle shut off threshold. It may be understood that the shut off valve may be operable to inhibit or prevent the whistle from generating a sound signal when the differential pressure across the whistle reaches a shut off threshold differential pressure. When the differential pressure across the whistle reaches and / or is above the shut off threshold differential pressure, the inhalation airflow rate through the outlet may be considered to be above the shut off airflow rate threshold (e.g., for a range of different inhaler devices). In this way, the whistle may be inhibited or prevented from generating a sound signal / positive feedback when the inhalation air flow rate through the outlet reaches and / or is above a maximum airflow rate threshold suitable / effective for delivery of the substance stored in the inhaler device to the patient. As such, the user receives positive feedback only when they are inhaling within the preferred range for effective drug delivery. In some examples, the shut off valve may comprise a flapper valve. The flapper valve may be operable to open (and thus generate a bypass channel inhibiting the whistle from generating a sound signal), when the airflow rate through the whistle is greater than or equal to the predefined whistle shut off threshold. The flapper valve may be operable to open when the internal pressure within the whistle (e.g., within the cavity of a kettle whistle) is greater than or equal to a threshold internal pressure, and to remain closed when the pressure within the whistle is less than the threshold internal pressure. For example, the flapper valve may be operable to open when the differential pressure between the kettle whistle cavity and the environment exterior to the adapter (which may be at atmospheric pressure) is greater than or equal to a threshold internal differential pressure, and to remain closed when the differential pressure between the kettle whistle cavity and the environment exterior to the adapter is less than the threshold internal differential pressure. In this way, the shut off valve may open to form a bypass channel inhibiting air from passing through (via the apertures) both plates of the kettle whistle, and thus inhibiting the kettle whistle from generating a sound signal when the inhalation airflow rate is too high. In some examples, the shut off airflow rate threshold may be greater than or equal to 30 LPM, for example greater than or equal to 40 LPM, or greater than or equal to 50 LPM. In some examples, the shut off airflow rate threshold may be 50 LPM or less, for example 40 LPM or less. For example, the shut off airflow rate threshold may be approximately 40 LPM, meaning that the positive whistle may be inhibited / prevented from generating a whistle sound / sound signal when the inhalation airflow rate through the outlet is approximately 40 LPM. The shut off threshold differential pressure may be greater than or equal to 90 Pa, greater than or equal to 100 Pa, greater than or equal to 250 Pa, or greater than or equal to 290 Pa, and less than or equal to 350 Pa, or less than or equal to 300 Pa. The shut off threshold differential pressure may be approximately 295 Pa. In some examples, the whistle may comprise a negative whistle that provides negative feedback that the user is inhaling too quickly. The negative whistle may be operable to generate a sound signal indicating when the inhalation airflow rate through the outlet reaches and / or is above a maximum airflow rate threshold suitable / effective for delivery of a substance (e.g., a drug) stored in the inhaler device to the patient. In this way, negative feedback in the form of a whistle sound / sound signal may be provided by the whistle indicating when the flow rate through the adapter is too high to effectively deliver the substance to the patient. It will be appreciated that the maximum threshold airflow rate may be greater than the minimum threshold airflow rate. In some examples, the maximum airflow rate threshold may be greater than or equal to 30 LPM, for example greater than or equal to 40 LPM, or greater than or equal to 50 LPM. In some examples, the maximum airflow rate threshold may be less than or equal to 50 LPM, for example less than or equal to 40 LPM. For example, the maximum airflow rate threshold may be approximately 40 LPM, meaning that the negative kettle whistle may generate a whistle sound / sound signal indicating when the inhalation air flow rate through the outlet reaches and / or is above approximately 40 LPM. For a negative kettle whistle, the threshold differential pressure may be greater than or equal to 90 Pa, greater than or equal to 100 Pa, greater than or equal to 250 Pa, or greater than or equal to 290 Pa, and less than or equal to 350 Pa, or less than or equal to 300 Pa. The shut off threshold differential pressure may be approximately 295 Pa. In some examples, in addition to the positive kettle whistle described above, the adapter may further comprise the negative kettle whistle described above (i.e., a negative whistle operable to generate a sound signal indicating when the inhalation airflow rate through the outlet reaches and / or is above a maximum airflow rate threshold suitable / effective for delivery of a substance (e.g., a drug) stored in the inhaler device to the patient). In other words, the adapter may comprise both a positive and negative whistle. In this way, positive feedback in the form of a whistle sound / sound signal may be provided by the positive whistle indicating when the inhalation airflow rate through the adapter is high enough to effectively deliver the substance to the patient (e.g., when the inhalation airflow rate is greater than or equal to 20 LPM), and negative feedback in the form of another whistle sound / sound signal may be provided by the negative whistle indicating when the inhalation airflow rate through the adapter is too high to effectively deliver the substance to the patient (e.g., when the flow rate is greater than or equal to 40 LPM). This provides improved feedback to the user. Rather than providing negative feedback only (e.g., that the airflow rate is above the maximum threshold airflow rate), as in some known devices, positive feedback, or positive and negative feedback, may therefore be provided. In some examples, when the adapter comprises the shut off valve fluidly coupled to the positive whistle, the shut off airflow rate threshold may be equal to the maximum airflow rate threshold, and / or the shut off threshold differential pressure may be equal to the threshold differential pressure of the negative whistle. In this way, the positive whistle may be inhibited / prevented from generating a whistle sound / sound signal at the same time as the negative whistle generates a whistle sound / sound signal. The negative kettle whistle may be positioned between the valve and the outlet, (e.g., may be positioned downstream of the valve). As such, in examples where the adapter comprises both a positive and a negative whistle, both whistles may be positioned between the valve and the outlet. In some examples where the whistle is a kettle whistle, the distance between the first plate and the second plate, the diameter of the first aperture and the second aperture, the thickness of the first plate and the second plate and / or the operating range of the kettle whistle may be different for the positive kettle whistle and the negative kettle whistle. For example, a diameter of the first and second aperture of the negative kettle whistle, and / or the distance between the first plate and the second plate along the whistle axis may be greater than that of the positive kettle whistle. A larger distance between the first plate and the second plate may provide an increased amplification of the sound signal. For example, for the positive kettle whistle, the diameter of the first aperture and the second aperture may be (approximately) 2.3 mm. The distance between the first plate and the second plate along the whistle axis may be (approximately) 3.4 mm. The thickness of the first plate and the second plate along the whistle axis may be (approximately) 0.5 mm. For the negative kettle whistle, the diameter of the first aperture and the second aperture may be (approximately) 2.8 mm. The distance between the first plate and the second plate along the whistle axis may be (approximately) 3.5 mm. The thickness of the first plate and the second plate along the whistle axis may be (approximately) 0.5 mm. In this way, the positive kettle whistle and the negative whistle may be operable at different flow rates along the airflow path, and / or at different differential pressures. In some examples, the sound signal generated by the positive whistle may be different from the sound signal generated by the negative whistle. For example, an acoustic characteristic (e.g., a frequency) of the positive whistle sound signal may be different from an acoustic characteristic of the negative whistle sound signal. In this way, the patient and / or a sound signal detection algorithm may be able to differentiate between the positive whistle sound signal and the negative whistle sound signal. In some examples, the adapter may further comprise an exhalation whistle operable to generate a sound signal when air flows into the body via the outlet. The exhalation whistle may be positioned between the valve and the outlet. In this way, feedback in the form of a whistle sound / sound signal may be provided when a patient exhales into the adapter. In some examples, the exhalation whistle may be operable to generate a sound signal only when air flows into the body via the outlet. That is, the exhalation whistle may be inhibited / prevented from generating a sound signal when air flows along the air flow path from the inlet to the outlet. In some examples, the exhalation whistle may be fluidly coupled to an exhalation valve. The exhalation valve may comprise a one-way valve operable to: allow airflow through the exhalation whistle in a direction out of the body; and, inhibit airflow through the exhalation whistle in a direction into the body. In some examples, the exhalation whistle may be a kettle whistle. In some examples, the inhalation whistle (e.g., the positive whistle and / or the negative whistle described above) may be operable as an exhalation whistle. The inhalation whistle may be operable to generate a sound signal when air flows into the body via the outlet. The sound signal(s) generated by the whistle(s) (e.g., the positive whistle and / or the negative whistle) and / or the exhalation whistle may be audible and / or may be detectable by a microphone (e.g., of a mobile device such as a smartphone) and / or a sound signal detection algorithm. In some examples where the whistle comprises a kettle whistle, the distance between the first plate and the second plate, the diameter of the first aperture and the second aperture, the thickness of the first plate and the second plate and / or the operating range may be different for the exhalation whistle and the kettle whistle (e.g., the positive kettle whistle and / or the negative kettle whistle). In some examples, the sound signal generated by the exhalation whistle may be different from the sound signal generated by the inhalation whistle (e.g., the positive whistle and / or the negative whistle described above). For example, an acoustic characteristic of the exhalation whistle sound signal may be different from the acoustic characteristic of the inhalation whistle sound signal. The acoustic spectrum of the exhalation whistle may have more harmonics and / or may be broader than the acoustic spectrum of the whistle (e.g., the positive whistle and / or the negative whistle). In this way, the patient and / or the sound signal detection algorithm may be able to differentiate between the exhalation whistle sound signal and the inhalation whistle sound signal. In some examples, the whistle (e.g., the positive whistle and / or the negative whistle) may be operable to generate a sound signal comprising two or more harmonics when the inhalation airflow rate through the outlet is greater than or equal to the inhalation airflow rate threshold. In this way, the sound signal may be more accurately detected (e.g., may be more accurately / easily differentiated from background noise) by the sound signal detection algorithm. In some examples, the whistle (e.g., the positive kettle whistle and / or the negative kettle whistle) and / or the exhalation whistle may be operable to generate a sound signal with a frequency of at least 1 kHz (e.g., when the inhalation airflow rate reaches and / or is above the inhalation airflow rate threshold). In this way, the sound signal may be more accurately detected (e.g., may be more accurately / easily differentiated from background noise) by the sound signal detection algorithm. It will be appreciated that the adapter may correspond to (e.g., be) a spacer. The body may comprise a holding chamber including the inlet, and / or a mouthpiece including the outlet. The valve may be positioned between the holding chamber and the mouthpiece. The holding chamber may be fluidly separated from the mouthpiece when the valve is closed, and / or may be fluidly coupled to the mouthpiece when the valve is open. In some examples, the valve may be a duckbill valve, a diaphragm valve, or a butterfly valve. In this way, a wide range of airflow rates may be enabled to flow along the airflow path. In some examples, the whistle (e.g., the positive and / or the negative kettle whistle) may be positioned on a sidewall of the body (e.g., a sidewall of the mouthpiece). The whistle may be at least partially formed by the sidewall of the body. For example, a sidewall of the body may form the first plate of the kettle whistle. A second plate of the kettle whistle may be coupled to the sidewall, for example by an interference fit. In this way, the kettle whistle may be more robust and / or less prone to damage. The kettle whistle axis may extend transversely to a longitudinal axis of the body (e.g., the mouthpiece). In some examples, the mouthpiece may be integrally formed. In some examples, the mouthpiece may comprise the exhalation valve. The exhalation valve may be a one-way valve operable to allow airflow in a direction from the mouthpiece outlet out of the mouthpiece through the exhalation valve, and to prevent airflow in a direction through the exhalation valve into the mouthpiece. In some examples, the exhalation whistle may be fluidly coupled to the exhalation valve. In some examples, the adapter may comprise a removable cap for covering the outlet of the body and / or the whistle (e.g., the positive kettle whistle and / or the negative kettle whistle). In this way, dirt and / or debris may be prevented from entering the adapter and / or the kettle whistle. According to a second aspect, there is provided a computer-implemented method of calculating an indication of a dose of a substance delivered to a patient from an inhaler device, the computer-implemented method comprising: detecting an inhalation signal from an airflow rate indicator, the airflow rate indicator operable to generate the inhalation signal indicating when an airflow rate along an airflow path defined by the inhaler device and / or by an adapter fitted to the inhaler device is greater than or equal to a minimum airflow rate threshold; determining, from the inhalation signal, an airflow rate along the airflow path; and, calculating, based on the determined airflow rate, an indication of a dose of the substance delivered to the patient. An airflow rate at which a patient inhales a substance may affect the proportion of the substance which is effectively delivered to the required anatomy of the patient (e.g., the bronchial tubes, and / or the lungs). For example, a slower air flow rate may risk that the aerosol particles expelled by the inhaler device will settle before being delivered to the required anatomy of the patient. A higher air flow rate may risk the aerosol particles simply hitting and adhering to the back of the patient's throat, without penetrating deeper into the patient's pulmonary system. By using the inhalation signal from the airflow rate indicator, which indicates that the inhalation airflow rate is greater than or equal to a minimum airflow rate threshold (e.g., suitable for delivery of a substance to the patient), the computer-implemented method may enable an indication of a dose of the substance delivered to the patient (e.g., to the required anatomy of the patient) to be accurately calculated. In this way, the patient may be provided with an indication of whether a sufficient amount of the dose has been delivered. It will be appreciated that the inhaler device may correspond to a drug delivery inhaler device and the substance may be a drug (e.g., medication). The inhaler device may store the substance. The inhaler device may be actuatable to eject a metered dose of the substance (e.g., into the adapter when the inhaler device is fitted to an adapter), for example in the form of an aerosol. The indication of the dose may correspond to a proportion of a metred dose of a substance delivered to the patient (and in particular to the required anatomy of the patient). The indication of the dose may correspond to an amount of a substance delivered to the patient (and in particular, to the required anatomy of the patient). In some examples, the airflow rate indicator may be coupled to a body of an adapter. The body may define an inlet for connection to a mouthpiece of the inhaler device, and an outlet for communication with the mouth of the patient. The airflow path may extend from the inlet to the outlet. In some examples, the adapter may comprise a valve positioned between the inlet and the outlet. In such examples, the airflow rate indicator may be positioned between the valve and the outlet. In some examples, the minimum airflow rate threshold may be 0 LPM, and the inhalation signal may indicate when the airflow rate along the airflow path is greater than 0 LPM. It may be understood that the airflow rate indicator may be operable to generate the inhalation signal when the differential pressure across the airflow rate indicator reaches and / or is above a threshold differential pressure. For example, when the airflow rate indicator is coupled to the adapter body, the differential pressure across the airflow rate indicator may be a differential pressure between an environment external to the adapter body and the environment internal to the adapter body In some examples, the airflow rate indicator may be or may comprise a whistle. In such examples, the inhalation signal may comprise a sound signal generated by the whistle. The whistle may be operable to generate the sound signal to indicate when the airflow rate along the airflow path reaches and / or is above the minimum airflow rate threshold. For example, the airflow rate indicator may comprise a kettle whistle, such as the kettle whistle detailed above with respect to the first aspect. In this way, the inhalation signal may be detected using a microphone, and the computer-implemented method may be carried out by a device such as a smartphone, for example. In some examples, a frequency (e.g., a fundamental frequency) of the inhalation signal (e.g., the sound signal) may be dependent upon and / or may correlate (e.g., linearly or monotonically which may be approximated as linearly) with the airflow rate along the airflow path and / or the differential pressure across the airflow rate indicator. In this way, the determination of the airflow rate may be facilitated based on the frequency of the inhalation signal. In some examples, determining the airflow rate may comprise determining (e.g., measuring) the frequency of the inhalation signal. Determining the airflow rate may comprise selecting and / or extracting a fundamental harmonic from the inhalation signal and measuring the frequency of the fundamental harmonic (the “fundamental frequency”). Selecting the fundamental harmonic may be based on an amplitude of the fundamental harmonic, and / or an amplitude of one or more other harmonics. For example, selecting the fundamental harmonic may comprise selecting the harmonic with the largest amplitude. Selecting the fundamental harmonic may be based on a spectral peak (e.g., a position of the spectral peak) of the fundamental harmonic and / or a spectral peak of one or more other harmonics (e.g., the positions of the spectral peaks of the one or more other harmonics). For example, selecting the fundamental harmonic may comprise selecting, from a plurality of (e.g., acoustic) spectral peaks, the spectral peak which has the lowest (e.g., central) frequency. Selecting the fundamental harmonic may comprise determining, from a plurality of (e.g., acoustic) spectral peaks the central frequency of the spectral peak with the lowest frequency. Determining the airflow rate may comprise calculating, from the measured frequency and / or from a predetermined (e.g., linear or monotonic) correlation between frequency (e.g., fundamental frequency) and the airflow rate, the airflow rate along the airflow path. The computer-implemented method may comprise receiving the predetermined correlation (e.g., from a memory, or from a user input). It will be appreciated that the predetermined correlation may correspond to a formula for calculating airflow rate from frequency of an inhalation signal. The predetermined correlation may depend on the inhaler device (e.g., on the inhaler device fitted to the adapter). For example, different types of inhaler devices may be associated with different predetermined correlations. In this way, the computer-implemented method may enable an airflow rate (through e.g., the inhaler device and / or the adapter), and a proportion of a dose of the substance delivered to the patient to be accurately determined. In some examples, the computer-implemented method may comprise measuring a duration of the inhalation signal (e.g., a duration over which the inhalation signal is detected). In such examples, the indication of the dose (e.g., the proportion ofthe metred dose) may be calculated based on the determined airflow rate and the measured duration. Measuring the duration ofthe inhalation signal may comprise detecting a start point and an endpoint ofthe inhalation signal. The start point may be detected by detecting an actuation ofthe airflow rate indicator, for example by detecting the inhalation signal. The endpoint may be detected by detecting an actuation of an exhalation indicator (e.g., an exhalation whistle) operable to generate an exhalation signal (e.g., a sound signal) when airflows (e.g., in response to air flowing) into an outlet ofthe inhaler device or the adapter. Detecting the actuation ofthe exhalation indicator may comprise detecting an exhalation signal from an exhalation indicator. It may be the case that an entire metered dose is not inhaled by the patient (e.g., does not enter the patient’s mouth), for example if the patient does not inhale for a long enough duration. Therefore, in some examples, calculating the proportion ofthe dose (e.g., the proportion ofthe metered dose) may comprise calculating an inhaled proportion ofthe dose (e.g., a proportion ofthe dose which enters the patient’s mouth). In some examples, the inhaled proportion ofthe dose may be calculated by: y. , , . Inhaled proportion =-------if Vinhaled <V; Inhaled proportion = lif Vinhaled >V; where Vinhaied is an inhaled volume of air (e.g., a volume of air which enters the patient’s mouth), and V is an internal volume ofthe adapter or the inhaler device (e.g., a volume defined by the adapter body or the inhaler device). In this way the proportion ofthe dose may be calculated more accurately. Calculating the indication ofthe dose may comprise calculating the inhaled volume of air. For example, the inhaled proportion ofthe dose may comprise calculating the inhaled volume of air. The inhaled volume may be calculated based on the determined airflow rate and the measured duration over which the inhalation signal is detected. For example, the inhaled volume may be calculated by: Vinhaled = Airflow rate x duration It may be the case that the patient inhales multiple times (e.g., from the adapter) after actuating the inhaler device to release the substance. Therefore, calculating the inhaled volume (i.e., a total inhaled volume) may comprise combining (e.g., adding) a plurality of individual inhaled volumes, each individual inhaled volume representing a respective volume inhaled by the patient after actuating the inhaler device. Each individual inhaled volume may be calculated based on a respective airflow rate and corresponding measured duration, for example using the above equation for Vinhaied. In this way, the inhaled volume and / or the proportion ofthe dose may be calculated more accurately. In some examples, it may be assumed that the inhaled volume is equal to the internal volume of the adapter and / or the volume of the metred dose. For example, it may be assumed that adults can inhale the full metred dose. In some examples, the computer-implemented method may comprise receiving a value of the internal volume of the adapter or the inhaler device (e.g., from a memory or a user input). In some examples, the proportion of the dose may be calculated based on the determined airflow rate and a predetermined (e.g., linear) correlation between airflow rate and a delivered proportion of a dose (e.g., a delivered proportion of an inhaled dose, or metered dose), for example by inserting the determined airflow rate into the predetermined correlation. A delivered proportion of a dose may be understood as a proportion of the dose delivered to the required anatomy (e.g., the lung). In particular, although all of a metred dose may be inhaled, it may not all be delivered to the required anatomy of the patient (due to e.g., inhaling too fast or too slow). The computer-implemented method may comprise receiving the predetermined correlation (e.g., from a memory, or from a user input). It will be appreciated that the predetermined correlation may correspond to a formula for calculating a delivered proportion of a dose from an airflow rate. In an example in which the volume of the adapter is less than 500 mL, or less than an average tidal volume for adults (e.g., when the volume of the adapter is approximately 140mL), the predetermined correlation may be1: % Delivered proportion for Adult patients = 44.9% — 0.11 x Airflow rate', wherein Delivered proportion is a proportion (specifically a percentage) of a dose (e.g., the metered dose) which is (e.g., effectively) delivered to the required anatomy of the patient and Airflow rate is the determined airflow rate in units of LPM. In some examples, the proportion of the dose may be calculated from the inhaled proportion and the delivered proportion, for example from a product of the inhaled proportion and the delivered proportion. Calculating the proportion of the dose may comprise multiplying the inhaled proportion by the delivered proportion. In some examples, the proportion of the dose may be calculated based on a tidal volume of the patient and a predetermined (e.g., linear) correlation between the tidal volume and an inhaled proportion of the dose (e.g., a delivered proportion of a metered dose), or a delivered proportion of the dose (e.g., a delivered proportion of a metered dose), for example by inserting the tidal volume into the predetermined correlation. The computer-implemented method may comprise receiving the predetermined correlation (e.g., from a memory, or from a user input). For example, for paediatric patients or certain adult patients, with a lung volume, or tidal volume of the order of the adapter volume (e.g.140mL) or less, the predetermined correlation may between tidal volume and an inhaled proportion of the dose may be: 1 Use of functional respiratory imaging to characterize the effect of inhalation profile and particle size on lung deposition of inhaled corticosteroid / long-acting (32-agonists delivered via a pressurized metered-dose inhaler; Van Holsbeke et. al.; Therapeutic Advances in Respiratory Disease; 2018; Vol. 12: 1-15 % Inhaled proportion for Pediatric patients = 0.085 x Tidal Volume + 6.7%; wherein Inhaled proportion is an inhaled proportion (specifically a percentage) of a dose (e.g., of a metered dose) and Tidal Volume is in units of mL. In some examples, calculating the proportion of the dose (e.g., the delivered proportion of the dose) may comprise multiplying the inhaled proportion of the dose by a correction factor. For example, an example of a predetermined correlation between the tidal volume and a delivered proportion of the dose (e.g., a delivered proportion of the metered dose) may comprise the above example predetermined correlation between the inhaled proportion and the tidal volume, multiplied by a correction factor. The correction factor may be different for different patients. In this way, the proportion of the dose may be calculated more accurately. In some examples, the tidal volume of the patient may be measured based on the determined airflow rate along the airflow path. The tidal volume may be calculated based on the determined airflow rate and the measured duration over which the inhalation signal is detected. For example, the tidal volume may be calculated by: Vtidai = Airflow rate x duration The equations for inhaled proportion and delivered proportion provided above may assume that paediatric patients may not inhale the full volume of the metered dose, butthat for adult patients that they will. These equations may be valid for airflow rates greater than or equal to 5 LPM. The equations for inhaled proportion and delivered proportion provided above may be adjusted for different inhaler devices, types of substance and / or particle size of the substance ejected from the inhaler device. In some examples, the computer-implemented method may comprise choosing between calculating the proportion of the dose based on: the tidal volume of the patient and the predetermined correlation between the tidal volume and an inhaled proportion of the dose or a delivered proportion of the dose; or based on the determined airflow rate and the predetermined correlation between airflow rate and a delivered proportion of the dose. The computer-implemented method may choose based on the tidal volume of the patient, and / or a pattern of inhalation of the patient (e.g., a number and / or depth of inhalations of the patient). For example, the tidal volume of the patient may be calculated as discussed above. The computer-implemented method may comprise determining whether the patient has a tidal volume greater than the volume of the adapter, or less than or equal to the volume of the adapter. If it is determined that the patient has a tidal volume greater than the volume of the adapter, the proportion of the dose may be calculated based on the determined airflow rate and the predetermined correlation between airflow rate and a delivered proportion of the dose (e.g., using the “delivered proportion for adult patients” equation provided above). If it is determined that the patient has a tidal volume less than or equal to the volume of the adapter, the proportion of the dose may be calculated based on the tidal volume of the patient and the predetermined correlation between the tidal volume and an inhaled proportion of the dose or a delivered proportion of the dose (e.g., using the “inhaled proportion for paediatric patients” equation provided above).. In some examples, the computer-implemented method may comprise detecting an actuation signal generated by actuating the inhaler device. For example, the actuation signal may comprise a sound signal generated by actuating the inhaler device. The sound signal may comprise a characteristic acoustic signature, for example with a predetermined frequency, bandwidth (e.g., full-width half maximum) and / or amplitude (of a fundamental harmonic and / or one or more other harmonics). Detecting the actuation signal may comprise detecting the characteristic acoustic signature, for example by detecting one or more of the predetermined frequency, bandwidth and / or amplitude. In some examples, the indication of the dose of the substance may be calculated only when the actuation signal is detected (e.g., in response to detection of the actuation signal). In some examples, when the actuation signal is not detected, the proportion of the dose may be calculated to be zero. In this way, inaccurate calculations of the indication of the dose and / or inaccurate indications of whether a sufficient amount of a substance has been delivered to the patient may be prevented. In some examples, the computer-implemented method may comprise measuring a time period between the actuation signal and the inhalation signal. The indication of the dose of the substance may be calculated based on the measured time period (e.g., in addition to the determined airflow rate). For example, a longer period between the actuation signal and the inhalation signal may result in a lower calculated indication of the dose. Calculating the indication of the dose may comprise reducing a value of a calculated indication of the dose (calculated based e.g., the airflow rate) when the measured period is above a threshold period. In some examples, the computer-implemented method may comprise detecting an exhalation signal from an exhalation indicator (e.g., an exhalation whistle) operable to generate an exhalation signal (e.g., a sound signal) when air flows (e.g., in response to air flowing) into an outlet of the inhaler device or the adapter. In such examples, the computer-implemented method may further comprise: determining, from the exhalation signal, an exhalation airflow rate along the airflow path (e.g., in an opposite direction along the airflow path to the direction or airflow of an inhalation signal); measuring a duration of the exhalation signal; and, calculating, based on the determined exhalation airflow rate and the measured duration, a volume of air exhaled by the patient into the outlet of the inhaler device or adapter. For example, calculating the volume of air exhaled may comprise multiplying the determined exhalation airflow rate by the measured duration. In this way, the computer-implemented method may enable a determination of a lung volume available for inhalation and / or of lung preparation for subsequent receiving of the substance. In some examples, a frequency (e.g., a fundamental frequency) of the exhalation signal (e.g., the sound signal) may be dependent upon and / or may correlate (e.g., linearly or monotonically) with the exhalation airflow rate along the airflow path (e.g., in the backwards direction along the airflow path) and / or the differential pressure across the exhalation indicator. For example, the exhalation indicator may comprise a whistle such as a kettle whistle. The exhalation indicator may comprise the airflow rate indicator. In this way, the determination of the exhalation airflow rate may be facilitated in a similar manner to the inhalation airflow rate described above. In some examples, determining the airflow rate may comprise measuring the frequency of the exhalation signal. Determining the exhalation airflow rate may comprise selecting and / or extracting a fundamental harmonic from the exhalation signal and measuring the frequency of the fundamental harmonic (the “fundamental frequency”). Selecting the fundamental harmonic may be based on an amplitude of the fundamental harmonic, and / or an amplitude of one or more other harmonics. For example, selecting the fundamental harmonic may comprise selecting the harmonic with the largest amplitude. Selecting the fundamental harmonic may be based on a spectral peak of the fundamental harmonic and / or a spectral peak of one or more other harmonics. For example, selecting the fundamental harmonic may comprise selecting, from a plurality of (e.g., acoustic) spectral peaks, the spectral peak which has the lowest (e.g., central) frequency. Selecting the fundamental harmonic may comprise determining, from a plurality of (e.g., acoustic) spectral peaks the central frequency of the spectral peak with the lowest frequency. Determining the exhalation airflow rate may comprise calculating, from the measured frequency and / or from a predetermined (e.g., linear or monotonic) correlation between frequency (e.g., fundamental frequency) and the exhalation airflow rate, the exhalation airflow rate along the airflow path. The computer-implemented method may comprise receiving the predetermined correlation (e.g., from a memory, or from a user input). It will be appreciated that the predetermined correlation may correspond to a formula for calculating exhalation airflow rate from frequency of an exhalation signal. In this way, the computer-implemented method may enable the volume of air exhaled to be accurately determined. A volume of air exhaled prior to inhalation of the dose may affect the delivery of the dose to the patient (e.g., to the required anatomy)2. In some examples, the indication of the dose of the substance may be calculated based on the calculated exhaled volume (e.g., in addition to the airflow rate of the inhalation). For example, if the exhaled volume is greater than a patient’s tidal volume (e.g., the amount of air that moves into and out of the patient’s lungs with each breath), the calculated indication of the dose (e.g., proportion of a metred dose) may be greater. In some examples, the computer-implemented method may comprise determining whether the calculated exhaled volume is greater than a tidal volume of the patient. The computer-implemented method may comprise, if it is determined that the calculated exhaled volume is greater than the tidal volume of the patient, modifying the calculated proportion of the dose. Modifying the calculated proportion of the dose may comprise increasing the calculated proportion of the dose, for example according to: Modified proportion = 0.65 x Proportion x Vexhalef vtidal 2 Does It Really Matter What Volume to Exhale Before Using Asthma Inhalation Devices? H. Self et al; Journal of Asthma; Volume 46; 2009; Issue 3 where Proportion is the calculated proportion of the dose, Modified proportion is the modified proportion of the dose, Vexhaied is the calculated volume of air exhaled, and Vtidai is the patient’s tidal volume. In some examples, the computer-implemented method may comprise receiving the patient’s tidal volume (e.g., from a memory or user input). In some examples, the calculated proportion of the dose may be modified only if it is determined that the calculated exhaled volume is greater than the tidal volume of the patient. In this way, a more accurate indication of the proportion of substance effectively delivered to the patient (e.g., to the required anatomy) may be provided. In some examples, if the exhaled volume is such that a residual volume of the patient’s lungs is reached, the calculated indication of the dose (e.g., the calculated proportion of the metered dose) may be modified to a modified indication of the dose (e.g., a modified proportion of the metered dose), which may be 30% higher than the calculated indication of the dose. In some examples, the computer-implemented method may comprise receiving the patient’s residual volume (e.g., from a memory or user input). In some examples, the exhalation signal generated by the exhalation indicator may be different from the inhalation signal generated by the airflow rate indicator. For example, an acoustic characteristic of the exhalation sound signal may be different (e.g., may have more harmonics and / or may be broader) from the acoustic characteristic of the inhalation sound signal. In this way, it may be possible to differentiate between the exhalation signal and the inhalation signal. In some examples, detecting the signal (e.g., detecting the inhalation signal or the exhalation signal) may comprise distinguishing the signal from background noise. In some examples, distinguishing the signal from background noise may comprise determining whether the signal has a signal to noise ratio over a predetermined signal to noise ratio threshold. The computer-implemented method may comprise only using the signal to calculate the proportion of the dose if the signal to noise ratio is exceeds a predetermined signal to noise ratio threshold. For example, the computer-implemented method may comprise only determining the airflow rate from the signal if the signal to noise ratio is over the predetermined signal to noise ratio threshold. In this way, false detections may be avoided and / or reduced. In some examples, detecting the signal (e.g., detecting the inhalation signal or the exhalation signal) may comprise detecting a signal comprising two or more predefined harmonics, for example three or four predefined harmonics. For example, distinguishing the signal from background noise may comprise determining whether the signal comprises two or more predefined harmonics, for example three or four predefined harmonics (e.g., a fundamental harmonic and one or more other harmonics), for example based on one or more amplitudes or spectral peak positions (e.g., frequencies of spectral peaks) of the signal. Determining whether the signal comprises two or more harmonics may include determining whether there is a spectral peak position which is an integer factor of another spectral peak position (e.g., the spectral peak positions may be 1 f, 2f, 3f, 4f, where f is the position of the fundamental harmonic spectral peak). The computer-implemented method may comprise only using the signal to calculate the proportion of the dose if the signal comprises two predefined harmonics. For example, the computer-implemented method may comprise only determining the airflow rate from the signal if the signal comprises two predefined harmonics. In this way, it may be possible to more accurately distinguish the signal from background noise. In some examples, distinguishing the signal from background noise may comprise analysing a relative amplitude of two harmonics or more in the signal, and / or analysing a relative position of one or more spectral peaks in the signal (e.g., determining whether there is a spectral peak position which is an integer factor of another spectral peak position). In some examples, detecting the signal (e.g., detecting the inhalation signal or the exhalation signal) may comprise detecting a signal in a predetermined frequency band. For example, distinguishing the signal from background noise may comprise determining whether the signal is within the predetermined frequency band. The computer-implemented method may comprise only using the signal to calculate the proportion of the dose if the signal is within the predetermined frequency band. For example, the computer-implemented method may comprise only determining the airflow rate from the signal if the signal is in the predetermined frequency band. For example, the airflow indicator may generate an inhalation signal with a frequency in the predetermined frequency band. The frequency band may extend from (and include) a lower frequency, and / or may extend to (and include) an upper frequency. The lower frequency may be 0.5 kHz or more, or 1 kHz or more, or 3kHz or more. The upper frequency may be 10 kHz or less, 9 kHz or less, or 8 kHz or less. In this way, it may be possible to more accurately distinguish the signal from background noise. In some examples, the computer-implemented method may comprise outputting (e.g., transmitting and / or displaying) and / or storing the calculated indication (e.g., proportion) of the dose. In this way, the patient may be provided with an indication of whether a sufficient amount of a dose has been delivered, and / or may keep track overtime of amounts of the substance (e.g., drug) which have been effectively delivered. In some examples, the computer-implemented method may comprise determining, using the calculated indication of the dose, the determined airflow rate and / or the measured duration over which the inhalation signal is detected, an adherence to a recommended dose and / or technique of inhalation (e.g., a recommended airflow rate, or airflow rate range such as 20-40LPM). The computer-implemented method may further comprise storing and / or outputting (e.g., transmitting or displaying) the determined adherence. For example, if the calculated indication of the dose (e.g., calculated proportion or modified proportion of the dose) is within a predetermined deviation from the from the recommended dose, it may be determined that the patient adheres to the recommended dose. If the calculated indication of the dose (e.g., calculated proportion or modified proportion of the dose) is outside of a predetermined deviation from the from the recommended dose, it may be determined that the patient does not adhere to the recommended dose. An adult or patient with a tidal volume of 200 ml or more, a recommended technique of inhalation may correspond to a single inhalation of 300 ml or more, and / or an airflow rate of 40 LPM or less. For a child, or a patient with a tidal volume of less than 200 ml, a recommended technique of inhalation may correspond to repeated inhalations of the patient’s tidal volume, where a combined volume of the repeated inhalations is 300 ml or more. In this way, the patient and / or their care-giver may be provided with an indication of their adherence to a recommended dose and / or technique of inhalation. In some examples, a patient, or their care-giver may enter physiological data associated with the patient. Physiological data may include, for example, spirometry data, residual lung volume, total lung volume, tidal volume. In some examples, the computer-implemented method may comprise receiving physiological data, for example from a user input, or from a memory. The computer-implemented method may comprise storing the physiological data. The physiological data may be used in the calculation of the indication (e.g., proportion) of the dose. In some examples, a patient may enter dose information, for example a medication prescription including dose information. The dose information may specify a recommended dose. The computer-implemented method may comprise receiving dose information, for example from a user input, or from a memory. The computer-implemented method may comprise storing the dose information. In some examples, determining an adherence to the recommended dose may be based on the dose information. It will be appreciated that the features set out with reference to the first aspect may be equally included in the second aspect, unless clearly impermissible, and vice versa. For example, the adapter of the second aspect may correspond to the adapter of the first aspect. The airflow rate indicator of the second aspect may correspond to the whistle (e.g., the kettle whistle) of the first aspect. The inhalation signal of the second aspect may correspond to the sound signal from the whistle (e.g., the kettle whistle) of the first aspect. The exhalation indicator of the second aspect may correspond to the exhalation whistle of the first aspect. The airflow rate along the airflow path referred to in the second aspect may be understood to mean the airflow rate through the outlet of the adapter. As such, the method of the second aspect may comprise detecting the inhalation signal from a whistle of an adapter fitted to an inhaler device, the adapter comprising: a body defining: an inlet for connection to a mouthpiece of the inhaler device; an outlet for communication with the mouth of the patient, wherein the airflow path is between the inlet and the outlet; and a valve positioned between the inlet and the outlet; wherein the whistle is positioned between the valve and the outlet, the whistle being operable to generate a sound signal indicating when an inhalation airflow rate through the outlet is greater than or equal to the minimum airflow rate threshold, and wherein the whistle is operable to generate a sound signal when the airflow rate through the whistle is greater than or equal to a whistle actuation flow rate, the whistle actuation flow rate being less than or equal to 5 LPM. According to a third aspect, the disclosure provides a software application comprising instructions which, when executed by a processor of a device, cause the processor to execute the steps of the computer-implemented method according to the second aspect. The computer-implemented method of the second aspect may be carried out by a software application. The software application may be a smartphone application, for example. The software application may be downloadable, e.g., from a server. According to a fourth aspect, the disclosure provides a device comprising a memory and a processor. The memory stores instructions which, when executed by the processor of the device, cause the processor to execute the steps of the computer-implemented method according to the second aspect. According to a fifth aspect, the disclosure provides a kit of parts comprising the adapter of the first aspect and the software application of the third aspect. The disclosure includes the combination of the aspects and features described except where such a combination is clearly impermissible or expressly avoided. Optional features are applicable singly or in any combination with any aspect. Summary of the Figures Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which: Figure 1 shows a perspective cross-sectional view of an adapter according to the present disclosure. Figure 2 shows a side view of an adapter according to the present disclosure. Figure 3 shows a cross-sectional view of a kettle whistle according to the present disclosure. Figure 4 shows a cross-sectional view of a portion of an adapter according to the present disclosure. Figure 5 shows a data table of experimental results measured using the adapter of Figure 1. Figure 6 is a plot showing the relationship between the square root of differential pressure (between an external environment and the inhaler device bypass channel) and airflow rate through the inhaler device for different inhaler devices. Figure 7 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 8 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 9 is a plot showing an acoustic spectrum of a sound signal generated by a kettle whistle. Figure 10 is a plot showing an acoustic spectrum of a sound signal generated by a kettle whistle. Figure 11 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 12A is a plot showing the frequency of a sound signal fundamental harmonic generated by a kettle whistle coupled to an adapter against airflow rate through the adapter for various inhalers fitted to the adapter. Figure 12B is a data table showing, for different inhaler devices, parameters for correlations between frequency of an inhalation signal and airflow rate through an inhaler device. Figure 13 is a plot showing the frequency of a sound signal fundamental harmonic generated by a kettle whistle coupled to an adapter against airflow rate through the adapter. Figure 14 is a plot showing the frequency of a sound signal second harmonic generated by a kettle whistle coupled to an adapter against airflow rate through the adapter for various inhalers fitted to the adapter. Figure 15 is a plot showing differential pressure within an adapter against airflow rate through the adapter for various inhaler devices fitted to the adapter. Figure 16 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 17 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 18 is a plot showing a delivered portion of a dose from an inhaler device against airflow rate through an adapter coupled to the inhaler device. Figure 19 is a plot showing a delivered portion of a dose from an inhaler device against tidal volume of patient lungs. Figure 20 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 21A is a plot showing an acoustic spectrum of an actuation sound signal generated by actuating an inhaler device which is coupled to an adapter. Figure 21B is a plot showing an acoustic spectrum of an actuation sound signal generated by actuating an inhaler device. Figure 22 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 23 is a flow diagram representing a computer-implemented method according to the present disclosure. Figure 24 is a plot showing the airflow rate at which the whistle in various existing spacers actuate for different inhaler devices. Detailed Description of the Invention Aspects and embodiments of the present invention will now be discussed with reference to the accompanying figures. Further aspects and embodiments will be apparent to those skilled in the art. All documents mentioned in this text are incorporated herein by reference. Figure 1 shows a perspective cross-sectional view of an adapter 100 for fitting to various types of drug delivery inhaler devices. The adapter 100 comprises an elongate, roughly cylindrical body 102 which defines an inlet 104 for connection to a mouthpiece of an inhaler device, an outlet 106 for communication with the mouth of a patient, and an airflow path from the inlet 104 to the outlet 106. The body 102 includes a holding chamber 108 which defines the inlet 104, and a mouthpiece 110 which defines the outlet 106. The adapter 100 further comprises a one-way duckbill valve 112 positioned between the holding chamber 108 and the mouthpiece 110, the one-way valve 112 operable to enable airflow from the holding chamber 108 to the mouthpiece 110, but to prevent airflow from the mouthpiece 110 to the holding chamber 108. Thus, the one-way valve 112 inhibits airflow from the outlet 106 to the inlet 104 but enables airflow from the inlet 104 to the outlet 106. The adapter 100 further comprises a kettle whistle 114 positioned on a sidewall of the mouthpiece 110 such that the kettle whistle 114 is downstream of the one-way valve 112. The kettle whistle 114 is a positive kettle whistle 114 which is operable to generate a sound signal indicating when the inhalation airflow rate along the airflow path (and in particular through the outlet 106) reaches a minimum inhalation airflow rate threshold suitable for delivery of the drug stored in the inhaler device to the patient. Therefore, positive feedback in the form of a whistle sound / sound signal is provided by the positive kettle whistle 114 indicating when the inhalation airflow rate through the adapter 100 (and in particular through the outlet 106) is high enough to effectively deliver the drug to the patient. The kettle whistle 114 positioned between the one-way valve 112 and the outlet 106 is well suited to being a positive kettle whistle 114, because the positive kettle whistle 114 may be actuatable by lower flow rates through the adapter 100. For example, the kettle whistle 114 may provide positive feedback indicating when the airflow rate along the airflow path (and in particular through the outlet) reaches approximately 20 LPM or more for a range of different inhaler devices (i.e., the minimum inhalation airflow rate threshold may be approximately 20 LPM, e.g., between 15 LPM and 25 LPM, for a range of different inhaler devices). Figure 2 shows a side view of the adapter 100 shown in Figure 1, in which the mouthpiece 110, the kettle whistle 114 and the holding chamber 108 can be seen. As shown in Figure 2, the kettle whistle 114 comprises a circular plate 116b. The positive kettle whistle 114 is shown in more detail in Figure 3. As shown in Figure 3, the positive kettle whistle 114 is partially formed by the sidewall of the mouthpiece 110. That is, the sidewall of the mouthpiece 110 forms a first plate 116a of the kettle whistle 114 and a second plate 116b of the kettle whistle 114 (or the circular plate 116b) is coupled to the sidewall via an interference fit such as to oppose the first plate 116a. Each plate 116a / b of the kettle whistle 114 defines an aperture 118a / b, and the apertures 118a / b are aligned with one another. In a particular example, the diameter of each of the apertures 118a / b is 2.3 mm, and the distance between the plates is 3.4 mm. A Reynolds number operating range of the positive kettle whistle 114 is at least 2000. As further shown in Figure 3, the adapter 100 additionally comprises a shut off valve 120 fluidly connected to the positive kettle whistle 114. The shut off valve 120 is operable to inhibit or prevent the kettle whistle 114 from generating a sound signal when the inhalation airflow rate along the airflow path (and in particular, through the outlet) is greater than a shut off airflow rate threshold. In this way, the positive kettle whistle 114, may be prevented from generating positive feedback (a sound signal) when the air flow rate along the airflow path reaches the shut off airflow rate threshold. The shut off valve 120 is in the form of a flapper valve operable to open when the differential pressure between the kettle whistle cavity 122 between the plates 118a / b and the environment exterior to the adapter 100 (which may be at atmospheric pressure) is greater than or equal to a threshold internal differential pressure, and to remain closed when the differential pressure between the kettle whistle cavity and the environment exterior to the adapter 100 is less than the threshold internal differential pressure. The shut off airflow rate threshold is approximately 40 LPM. Figure 4 shows a cross-sectional view of a portion of another adapter 200 according to the present disclosure. The adapter 200 shares several features in common with the adapter 200 shown in Figures 1 to 3. For conciseness, these features will not be repeated. As shown in Figure 4, the mouthpiece 210 of the adapter 200 comprises three whistles: a positive kettle whistle 214, a negative kettle whistle 224, and an exhalation whistle 226. The positive kettle whistle 214 shares several features in common with the positive kettle whistle 114 described above with reference to Figures 1 to 3. For conciseness, these features will not be repeated. The negative kettle whistle 224 is operable to generate a sound signal indicating when the inhalation airflow rate along the airflow path (in particular, through the outlet 206) reaches a maximum inhalation airflow rate threshold suitable for delivery of the drug stored in the inhaler device to the patient. Therefore, negative feedback in the form of a whistle sound / sound signal is provided by the negative kettle whistle 224 indicating when the airflow rate through the adapter 200 (and in particular, through the outlet 206) is high enough to effectively deliver the drug to the patient. The negative kettle whistle 224 may provide negative feedback indicating when the inhalation airflow rate reaches approximately 40 LPM or more for a range of different inhaler devices (i.e., the maximum inhalation airflow rate threshold may be approximately 40 LPM for a range of different inhaler devices). Therefore, both positive feedback in the form of a whistle sound / sound signal may be provided by the positive kettle whistle 214 indicating when the flow rate through the adapter 200 is high enough to effectively deliver the substance to the patient (e.g., when the flow rate is greater than or equal to 20 LPM), and negative feedback in the form of a whistle sound / sound signal may be provided by the negative kettle whistle 224 indicating when the flow rate through the adapter 200 is too high to effectively deliver the substance to the patient (e.g., when the flow rate is greater than or equal to 40 LPM). The dimensions of the negative kettle whistle 224 (e.g., the diameter of the apertures, and / or the spacing between the plates) can be different from those of the positive kettle whistle 214. Therefore, the sound signal generated by the positive kettle whistle 214 is different from that of the negative kettle whistle 224, and the two sound signals may be differentiated from one another (e.g., by a patient, or by software). The exhalation whistle 226 is positioned between the one-way duckbill valve 212 and the outlet and is operable to generate a sound signal when air flows into the body 202 via the outlet 206. In this way, feedback in the form of a whistle sound / sound signal may be provided when a patient exhales into the adapter 200. The exhalation whistle 226 is fluidly coupled to an exhalation valve 228, which is a one-way valve which allows airflow through the exhalation whistle 226 in a direction out of the mouthpiece 210 and inhibits airflow through the exhalation whistle 226 in a direction into the mouthpiece 210. The sound signal generated by the exhalation whistle 226 may be different to those generated by the positive kettle 214 and the negative kettle whistle 224. Thus, the sound signals may be differentiated from one another. The valve in the adapter 200 shown in Figure 4 is a duckbill valve. Further, the adapter 200 shown in Figure 4 comprises a removable cap 230, which is shown in Figure 4 covering the outlet 206 of the body 202. Figure 5 is a data table 500 showing experimental results measured using the adapter 100 of Figures 1 to 3. The data table 500 shows the minimum inhalation airflow rates at which the positive kettle whistle 114 is first triggered, for different types of inhaler devices (pMDIs) coupled to the inlet 104. As shown in Figure 5, the minimum inhalation airflow rates are all within a margin of 25% of 20 LPM. Three of the minimum inhalation airflow rates are equal to 22.5 LPM. Therefore, for each of the inhaler devices, the positive kettle whistle is triggered at an inhalation airflow rate which is high enough for suitable delivery to the patient of the drug stored in the inhaler device. Thus, the positive kettle whistle provides accurate feedback of proper use of the adapter when used with different types of inhaler devices. Figure 6 is a plot showing the relationship between the square root of differential pressure (between an external environment and the inhaler device bypass channel) and airflow rate through the inhaler device for different inhaler devices. Figure 6 shows that different inhaler devices have different resistances to airflow. Turning to an example of the second aspect, the present disclosure provides a computer-implemented method of calculating a proportion of a metered dose of a substance (e.g., a drug) delivered to a patient from an inhaler device. Figure 7 shows a flowchart representing such a computer-implemented method. As shown in Figure 7, the method includes, at step 702, detecting an inhalation signal. The inhalation signal is from an airflow rate indicator. In the specific example now described, the airflow rate indicator is the kettle whistle described with reference to Figures 1 to 6, and the inhalation signal is a sound signal generated by the kettle whistle, although it will be appreciated that this method could be applied to sound signals generated by other whistles. The kettle whistle generates the inhalation signal indicating when the airflow rate along the airflow path defined by the adapter (i.e., the airflow path extending from the inlet of the adapter to the outlet of the adapter) is above a minimum airflow rate threshold. The method then includes, at step 704, determining, from the detected inhalation signal, an airflow rate along the airflow path and, at step 706, calculating, based on the determined airflow rate, an indication of a dose of the substance delivered to the patient. Each of the steps 702, 704 and 706 shown in Figure 7 will now be described in more detail. The step 702 of detecting the inhalation signal is shown in more detail in Figure 8. As shown in Figure 8, detecting the inhalation signal comprises, at step 802, receiving a sound signal. At step 804, it is determined whether the sound signal comprises two (predefined) harmonics. If the signal only comprises a single harmonic, or does not comprise the predefined harmonic, the method returns to the start. If the sound signal comprises two harmonics, the method proceeds to step 806 in which it is determined whether the fundamental harmonic is in a predetermined frequency band of 0.5 kHz or more and 8 kHz or less. If the fundamental frequency is not in the predetermined frequency band, the method returns to the start (e.g. step 802). If the fundamental frequency is in the predetermined frequency band, at step 808, it is determined that the inhalation signal is detected. This ensures that the signal can be distinguished from the background noise. Figure 9 shows is a plot showing an acoustic spectrum of a sound signal generated by the kettle whistle in the adapter shown in Figure 1, for a flow rate of 50 LPM along the airflow path, when an Atrovent® inhaler is fitted to the adapter. As shown in Figure 9, the acoustic spectrum has both a fundamental frequency (with a frequency of 3.522 kHz) and a first higher harmonic (with a frequency of 7 kHz). It is to be understood that the fundamental frequency can be considered a harmonic. Thus, using the method set out in Figure 8, the sound signal generated by the kettle whistle may be detected as the inhalation signal. Figure 10 shows is a plot showing an acoustic spectrum of a sound signal generated by the kettle whistle in the adapter shown in Figure 1, for a flow rate of 10 LPM along the airflow path, when a Ventolin inhaler is fitted to the adapter. As shown in Figure 10, the acoustic spectrum has a fundamental frequency (“first harmonic” in Figure 10), a first higher harmonic (“second harmonic” in Figure 10), and a second higher harmonic (“third harmonic” in Figure 10). The fundamental frequency is between 1 kHz and 2 kHz. Thus, again, using the method set out in Figure 8, the sound signal generated by the kettle whistle may be detected as the inhalation signal. Turning back to Figure 7, the step 704 of determining, from the inhalation signal, the airflow rate along the airflow path is shown in more detail in Figure 11. As shown in Figure 11, determining the airflow rate comprises, at step 1102, measuring a frequency of the fundamental harmonic inhalation signal, and at step 1104, calculating, from the measured frequency, the airflow rate along the airflow path. The fundamental frequency of the inhalation signal from the kettle whistle correlates approximately linearly with the airflow rate along the airflow path. Therefore, it is possible to determine the airflow rate by measuring the frequency of the inhalation signal. The correlation of the frequency with airflow rate is shown in Figure 12A for various different inhaler devices being coupled to the adapter. As shown in Figure 12A, for various different inhaler devices, the fundamental frequency correlates approximately linearly with the airflow rate along the airflow path, and the exact correlation depends on the inhaler device fitted to the adapter. The correlation for each inhaler device can be expressed as y = mx + c, where y is the frequency (in units of e.g., per minute) and x is the airflow rate (in units of e.g., LPM). The values of m and c for each of the inhalers are provided in the table shown in Figure 12B. The correlations for each of the inhaler devices may be stored in a memory. Therefore, turning back to Figure 11, in step 1104 the airflow rate along the airflow path is calculated by inserting the measured frequency into a predetermined correlation equation between frequency and airflow rate for the specific inhaler device coupled to the adapter. The predetermined correlation may be received from a memory. Figure 13 shows the correlation of the frequency with airflow rate for the Boeringer Atrovent® inhaler in more detail. Figure 14 shows that the frequency of the second harmonic (e.g., first higher harmonic) of the inhalation signal from the kettle whistle also correlates approximately linearly with the airflow rate along the airflow path. The correlation of the frequency with airflow rate is shown in Figure 14 for various different inhaler devices being coupled to an adapter such as that shown in Figure 1. Figure 15 shows the variation of differential pressure across the kettle whistle with airflow rate along the airflow path, for various different inhaler devices being coupled to the adapter. Turning back to Figure 7, the step 706 of calculating, based on the determined airflow rate, an indication of a dose, and in particular a proportion of the dose, of the substance delivered to the patient, is shown in more detail in Figure 16. As shown in Figure 16, calculating the proportion of the dose comprises, in step 1602 calculating an inhaled proportion of the dose (i.e., a proportion of the dose which enters the patient’s mouth), in step 1604, calculating a delivered proportion of the inhaled dose, and in step 1606 multiplying the inhaled proportion and the delivered proportion to end up with the proportion of the metered dose which is delivered to the required anatomy of the patient. The step 1602 of calculating the inhaled proportion of the dose is shown in more detail in Figure 17. As shown in Figure 17, calculating the inhaled proportion of the dose comprises, in step 1702, calculating an inhaled volume of air. The inhaled volume of air is calculated by multiplying the airflow rate along the airflow path by the duration over which the inhalation signal is detected. In step 1704, it is determined whether the inhaled volume is less than a volume of the adapter. The volume of the adapter used in step 1704 may have been received from a memory. If the inhaled volume is less than the volume of the adapter, in step 1706, the inhaled proportion is calculated by dividing the inhaled volume by the volume of the adapter. If the inhaled volume is greater than or equal to the volume of the adapter, then in step 1708 the value of the inhaled proportion is determined to be 1. The proportion of the inhaled volume which is delivered to the required anatomy of the patient correlates approximately linearly with the airflow rate along the airflow path. Turning back to Figure 16, the delivered proportion is calculated by substituting the determined airflow rate into the following equation: % Delivered proportion for Adult patients = 44.9% — 0.11 x Airflow rate', Figure 18 is a plot showing the correlation between the delivered proportion and the airflow rate along the airflow path for adult patients. In children, the lung tidal volume can affect the delivered proportion of the dose3. Figure 19 is a plot showing the correlation between the delivered proportion and the tidal volume. Although the inhaled proportion is calculated in the specific examples described in Figures 16 and 17, in other examples, this step may be removed, and it may be assumed that the entire metered dose is inhaled by the patient (i.e., enters the patient’s mouth). For example, this may be a reasonable assumption for patients who have a tidal volume greater than the volume of the adapter (e.g., adults). Therefore, the delivered proportion may represent the delivered proportion of the metered dose. Figure 20 shows another method for calculating a proportion of a metered dose. As shown in Figure 20, this method includes the steps 702, 704, 706 described with reference to Figure 7. For conciseness, these steps will not be described again. The method shown in Figure 20 includes an additional step 2002, carried out before detecting the inhalation signal at step 702, of determining whether an actuation signal is detected. The actuation signal is a sound signal generated by actuating the inhaler device. As shown in Figure 20, if the actuation signal is not detected, the method returns to the start. If the actuation signal is detected, the method proceeds to the next step 702 of detecting the inhalation signal. Therefore, a calculation of a proportion of a dose is only carried out if the inhaler is actuated to release a dose. Figure 21A shows an acoustic spectrum of the sound signal generated by actuating an inhaler device which is coupled to an adapter. Figure 21B is a plot showing an acoustic spectrum of a sound signal generated by actuating an inhaler device (which is not coupled to the adapter). As shown in Figures 21A and 21B, the sound signals have characteristic acoustic signatures, which can be detected according to the present method. In other examples, the method may additionally, or alternatively comprise measuring a period between detecting the actuation signal and detecting the inhalation signal. The proportion of the metered dose may be calculated based on the measured period. 3 Valved holding chamber drug delivery is dependent on breathing pattern and device design; Csonka et al; ERJ Open Research 2019, 5: 00158-2018 Figure 22 shows additional steps which may be carried out in the methods for calculating a proportion of the metered dose described with reference to Figures 7 and 20. These steps may be carried out before the inhalation signal is detected. As shown in Figure 22, in step 2202, an exhalation signal is detected. The exhalation signal is from an exhalation indicator. In the specific example now described, the exhalation indicator is the exhalation whistle described with reference to Figure 4, and the exhalation signal is a sound signal generated by the exhalation whistle. The exhalation whistle generates an exhalation signal when air flows into the outlet of the adapter (e.g., from a user inhaling). In step 2204, the duration of the exhalation signal is detected. In step 2206, the airflow rate of the exhalation along the airflow path is determined. The airflow rate is determined using a similar method to that described with reference to Figure 11. The fundamental frequency of the exhalation signal correlates approximately linearly with the airflow rate of the exhalation along the airflow path. The airflow rate is calculated using a predetermined correlation between airflow rate of exhalation along the airflow path and frequency of an exhalation signal, and a measured frequency of the exhalation signal. In step 2208, a volume of air exhaled by the patient is calculated by multiplying the measured duration and the determined airflow rate. The volume of air exhaled can be used to modify the proportion of the dose calculated in step 706. For example, if the volume of air exhaled is greater than the tidal volume of the patient (which may be received from a memory), the calculated proportion of the dose may be modified, after step 706, according to: Modified proportion = 0.65 x Proportion x *tidal where Proportion is the calculated proportion of the dose, Modified proportion is the modified proportion of the dose, Vexhaled is the calculated volume of air exhaled, and Vtidal is the patient’s tidal volume. Figure 23 shows additional steps which may be carried out in the methods for calculating a proportion of the metered dose described with reference to Figures 7, 20 and 22. These steps are carried out after the proportion of the metered dose (or the modified proportion of the metered dose) is calculated. As shown in Figure 23, in step 2302 dose information is received (e.g., from a storage, or from a user input). The dose information specifies a recommended dose of the substance for the patient. In step 2304, the calculated proportion of the dose (or the modified proportion of the dose) is compared to the recommended dose, and in step 2306, this comparison is used to determine an adherence to a recommended dose. For example, if the calculated proportion of the dose (or modified proportion of the dose) is within a predetermined deviation from the from the recommended dose, it may be determined that the patient adheres to the recommended dose. If the calculated proportion of the dose (or modified proportion of the dose) is outside of a predetermined deviation from the from the recommended dose, it may be determined that the patient does not adhere to the recommended dose. The determined adherence and / or the calculated proportion of the dose (or modified proportion of the dose) may then be stored (e.g., in a memory). In other examples, the method may alternatively, or additionally include using the determined airflow rate and / or measured duration over which the inhalation signal is detected to determine an adherence to a recommended technique of inhalation. The features disclosed in the foregoing description, or in the following claims, or in the accompanying drawings, expressed in their specific forms or in terms of a means for performing the disclosed function, or a method or process for obtaining the disclosed results, as appropriate, may, separately, or in any combination of such features, be utilised for realising the invention in diverse forms thereof. While the invention has been described in conjunction with the exemplary embodiments described above, many equivalent modifications and variations will be apparent to those skilled in the art when given this disclosure. Accordingly, the exemplary embodiments of the invention set forth above are considered to be illustrative and not limiting. Various changes to the described embodiments may be made without departing from the spirit and scope of the invention. For the avoidance of any doubt, any theoretical explanations provided herein are provided for the purposes of improving the understanding of a reader. The inventors do not wish to be bound by any of these theoretical explanations. Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described. Throughout this specification, including the claims which follow, unless the context requires otherwise, the word “comprise” and “include”, and variations such as “comprises”, “comprising”, and “including” will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps. It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the context clearly dictates otherwise. Ranges may be expressed herein as from “about” one particular value, and / or to “about” another particular value. When such a range is expressed, another embodiment includes from the one particular value and / or to the other particular value. Similarly, when values are expressed as approximations, by the use of the antecedent “about,” it will be understood that the particular value forms another embodiment. The term “about” in relation to a numerical value is optional and means for example + / -10%.

Claims

1. A computer-implemented method of calculating an indication of a dose of a substance delivered to a patient from an inhaler device, the computer-implemented method comprising:detecting an inhalation signal from an airflow rate indicator, the airflow rate indicator operable to generate the inhalation signal indicating when an airflow rate along an airflow path defined by the inhaler device and / or by an adapter fitted to the inhaler device is greater than or equal to a minimum airflow rate threshold;determining, from the inhalation signal, an airflow rate along the airflow path; and,calculating, based on the determined airflow rate, an indication of a dose of the substance delivered to the patient.

2. A computer-implemented method according to any claim 1, wherein:the airflow rate indicator comprises a whistle;the inhalation signal comprises a sound signal generated by the whistle; and,the whistle is operable to generate the sound signal indicating when the airflow rate along the airflow path is greater than or equal to a minimum airflow rate threshold.

3. A computer-implemented method according to any of the preceding claims, wherein determining the airflow rate comprises:measuring a frequency of the inhalation signal; and,calculating, from the measured frequency, the airflow rate along the airflow path.

4. A computer-implemented method according to any of the preceding claims, further comprising measuring a duration of the inhalation signal, wherein the indication of the dose is calculated based on the determined airflow rate and the measured duration5. A computer-implemented method according to any of the preceding claims, further comprising: detecting an actuation signal generated by actuating the inhaler device; and, calculating the indication of the dose of the substance delivered to the patient in response to the actuation signal.

6. A computer-implemented method according to claim 5, wherein the actuation signal is a sound signal generated by actuating the inhaler device.

7. A computer-implemented method according to claim 5 or claim 6, further comprising:measuring a time period between the actuation signal and the inhalation signal; and,wherein the indication of the dose of the substance is calculated based on the measured time period.

8. A computer-implemented method according to any of the preceding claims, further comprising:detecting an exhalation signal from an exhalation indicator operable to generate an exhalation signal when air flows into an outlet of the inhaler device or the adapter;determining, from the exhalation signal, an exhalation airflow rate along the airflow path;measuring a duration over which the exhalation signal is detected; and, calculating, based on the determined exhalation airflow rate and the measured duration, a volume of air exhaled by the patient into the outlet of the inhaler device or adapter.

9. A computer-implemented method according to claim 8, wherein the indication of the dose of the substance is calculated based on the calculated exhaled volume.

10. A computer-implemented method according to any of the preceding claims, wherein detecting the inhalation signal comprises detecting a signal in a predetermined frequency band.

11. A computer-implemented method according to any of the preceding claims, wherein detecting the inhalation signal comprises detecting a sound signal comprising two harmonics.

12. A computer-implemented method according to any of the preceding claims, further comprising outputting and / or storing the calculated indication of the dose.

13. A computer-implemented method according to any of the preceding claims, wherein the airflow path is defined by an adapter connected to a mouthpiece of the inhaler device, and the airflow path extends from an inlet of the adapter to an outlet of the adapter.

14. A computer-implemented method according to any of the preceding claims, wherein the inhalation signal is a sound signal, and the method further comprises:detecting the inhalation sound signal from a whistle of an adapter fitted to an inhaler device, the adapter comprising:a body defining:an inlet for connection to a mouthpiece of the inhaler device; andan outlet for communication with the mouth of a patient; wherein the airflow pathis between the inlet and the outlet; and,a valve positioned between the inlet and the outlet;wherein the whistle is positioned between the valve and the outlet, the whistle being operable to generate a sound signal when the airflow rate along the airflow path is greater than or equal to the minimum airflow rate threshold, and wherein the whistle is operable to generate a sound signal when the airflow rate through the whistle is greater than or equal to a whistle actuation flow rate, the whistle actuation flow rate being less than or equal to 5 LPM..

15. A software application comprising instructions which, when executed by a processor of a device, cause the processor to execute the steps of the computer-implemented method according to any of the preceding claims.

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