Rebreather apparatus and controller

Abstract

A method of operating a rebreather apparatus is disclosed The method comprises receiving a first output signal from a pressure sensor in the rebreather apparatus, wherein the first output signal is indicative of variations in a gas pressure within the rebreather apparatus. A second output signal is received from a carbon dioxide sensor in the rebreather apparatus, wherein the second output signal is indicative of carbon dioxide exhaled by a user via a mouthpiece of the rebreather apparatus. The first output signal is used to detect a breathing cycle of the user. The method further comprises determining, based on the breathing cycle and the second output signal, a value indicative of end-tidal carbon dioxide for the user.

Description

[0001] REBREATHER APPARATUS AND CONTROLLER

[0002] Field of the Invention

[0003] The present invention relates to a rebreather apparatus, and method for controlling a rebreather apparatus. A controller for a rebreather apparatus is also provided.

[0004] Background

[0005] A rebreather is a type of breathing apparatus that is used for underwater diving. In a rebreather, used gas exhaled by a user of the rebreather is recirculated and reused, to permit the rebreathing (i.e. recycling) of the unused oxygen content of the used gas. Used gas has a lower oxygen content than unused gas, because some of the oxygen is used (metabolised) by the user of the rebreather. Oxygen is therefore normally added to the used gas to replace the oxygen used by the user of the rebreather.

[0006] It is important to control the oxygen content of the gas in the rebreather. In particular, it is important to control the concentration (or the proportion) of oxygen in the gas in the rebreather, which is normally expressed by the partial pressure of oxygen in the gas in the rebreather. The partial pressure of oxygen (PPO2) in a gas mixture is given by:

[0007] PPO2= P X FO2(1) where P is the total pressure of the gas mixture and FO2is the volume fraction of oxygen in the gas mixture.

[0008] If the partial pressure of oxygen in the gas in the rebreather falls below a minimum safe partial pressure (which may depend on e.g. the user’s physiology and level of exertion and on the exposure time), the user of the rebreather may be at risk of unconsciousness or death due to hypoxia. On the other hand, if the partial pressure of oxygen in the gas in the rebreather rises above a maximum safe partial pressure (which may also depend on e.g. the user’s physiology and level of exertion and on the exposure time), the user of the breathing apparatus may be at risk of oxygen toxicity known as hyperoxia, which may result in seizures and even death in severe cases.

[0009] Therefore, it is important to control the partial pressure of oxygen in the gas in the rebreather, so that the partial pressure of oxygen remains within safe limits. In known rebreathers, this is achieved by providing the rebreather with one or more oxygen sensors for sensing the partial pressure of oxygen in the gas in the rebreather, and a control system for controlling the partial pressure of oxygen in the gas in the rebreather (e.g. to be equal to a target partial pressure of oxygen, which may be referred to as the target set-point of the rebreather) based on the output of the one or more oxygen sensors.

[0010] A rebreather may also typically include a carbon dioxide absorbent unit (often referred to as a “scrubber”), which absorbs carbon dioxide exhaled by the user, in order to avoid build-up of carbon dioxide in the rebreather.

[0011] The present invention has been devised in light of the above considerations.

[0012] Summary of the Invention

[0013] At its most general, the invention provides a method of operating a rebreather apparatus, which enables the user’s end-tidal carbon dioxide (ETCO2) to be monitored. ETCO2 corresponds to a level (e.g. concentration) of carbon dioxide released by a person at an end of an exhalation in their breathing cycle. Thus, ETCO2 typically corresponds to a maximum expired level of carbon dioxide during the breathing cycle. ETCO2 provides an indication of an effectiveness with which carbon dioxide is carried in the bloodstream back to the lungs and exhaled, and correlates with arterial carbon dioxide, i.e. the concentration of carbon dioxide in arterial. ETCO2 can also provide an indication of cardiac output and pulmonary blood flow.

[0014] For a rebreather user, ETCO2 levels provide a useful indicator for safety of the user. In particular, an increase in ETCO2 may be indicative of increased carbon dioxide retention by the user, such that the user may be at risk of hypercapnia. Increases in ETCO2 could result, for example, from a high breathing rate, a high work (exertion) level, a high work of breathing with the rebreather, and / or increased gas density in the rebreather. In some cases, an increase in ETCO2 may result from a malfunction of the CO2 absorbent unit (e.g. the carbon dioxide absorbent unit is not removing enough carbon dioxide from the rebreather). Accordingly, monitoring the user’s ETCO2 levels contributes to improving a safety of the user, e.g. by enabling the user to be warned when their ETCO2 increases and / or reaches an unsafe level. The user can then take appropriate action, to bring their ETCO2 down to a safe level and avoid hypercapnia, for instance by reducing their breathing rate and / or work level, or by changing their diving depth to control the gas density in the rebreather and therefore the efficiency of the CO2 absorbent unit.

[0015] According to a first aspect of the invention, there is provided a method of operating a rebreather apparatus, the method comprising: receiving a first output signal from a pressure sensor in the rebreather apparatus, wherein the first output signal is indicative of variations in a gas pressure within the rebreather apparatus; receiving a second output signal from a carbon dioxide sensor in the rebreather apparatus, wherein the second output signal is indicative of carbon dioxide exhaled by a user via a mouthpiece of the rebreather apparatus; detecting, using the first output signal, a breathing cycle of the user; and determining, based on the breathing cycle and the second output signal, a value indicative of end-tidal carbon dioxide for the user.

[0016] The method may be implemented by a controller of the rebreather apparatus, e.g. as discussed below in relation to the second and third aspects of the invention. Thus, the method may be a computer-implemented method.

[0017] The pressure sensor may be any suitable sensor for detecting variations (i.e. changes) in gas pressure within the rebreather apparatus.

[0018] The pressure sensor may detect variations in a total pressure of a gas mixture present in the breathing apparatus. Thus, the pressure sensor may be sensitive to the pressure of the gas mixture as a whole and need not be sensitive to partial pressures of individual components of the gas mixture in the rebreather. In particular, the pressure sensor may not be an oxygen sensor that detects a partial pressure of oxygen. Accordingly, the first output signal may be indicative of variations in the total gas pressure in the rebreather apparatus.

[0019] Herein, a total gas pressure in the breathing apparatus may refer to a pressure of a gas mixture that is present in the rebreather apparatus. In contrast, a partial pressure may correspond to an individual component (such as oxygen or carbon dioxide) of the gas mixture in the rebreather apparatus.

[0020] The pressure sensor may be arranged to detect pressure variations in a breathing volume of the rebreather apparatus. For example, the pressure sensor may detect variations in a breathing loop of the rebreather apparatus, e.g. the pressure sensor may be located in the breathing loop.

[0021] The rebreather apparatus may comprise a breathing loop and a mouthpiece for breathing gas in the breathing loop. A pressurised oxygen supply tank may be connected to the breathing loop via a supply valve.

[0022] As an example, the pressure sensor may be a differential pressure sensor. Such a differential pressure sensor may detect variations in the total gas pressure within the breathing apparatus relative to a reference pressure. For instance, the reference pressure may correspond to a pressure outside the breathing apparatus. The differential pressure sensor may comprise a first pressure sensor which is located inside the breathing apparatus, and which is linked to an ambient pressure sensor located outside the breathing apparatus, so that a pressure variation resulting from breathing of the user can be calculated.

[0023] The first output signal varies in accordance with the pressure variations in the rebreather apparatus. Thus, a period and amplitude of the first output signal may be related to a period and amplitude of the pressure variations in the rebreather apparatus. For example, the pressure sensor may comprise a transducer for converting variations in the total gas pressure in the rebreather apparatus into an electrical signal, e.g. to measure variations in pressure in the breathing loop which are reflective of a breathing rate (pattern) and volume.

[0024] The carbon dioxide sensor is arranged to detect carbon dioxide exhaled by a user of the rebreather apparatus via the mouthpiece. Thus, the carbon dioxide sensor may be arranged to receive gas exhaled by the user via the mouthpiece. For example, the carbon dioxide sensor may be located in the mouthpiece, e.g. on an exhale side of the mouthpiece. The mouthpiece may comprise an exhale valve via which gas exhaled by the user enters the breathing volume (e.g. breathing loop). The carbon dioxide sensor may then be located at (e.g. next to) the exhale valve to receive gas exhaled by the user. In some cases, the carbon dioxide sensor may be located in the breathing loop, between the exhale side of the mouthpiece and a carbon dioxide absorbent unit of the apparatus. In this manner, the carbon dioxide sensor is exposed to the exhaled gas from the user before it passes through the carbon dioxide absorbent unit.

[0025] The second output signal from the carbon dioxide sensor may be indicative of a level of carbon dioxide in the gas exhaled by the user. For example, the output signal may be indicative of an amount or proportion of carbon dioxide in the gas exhaled by the user.

[0026] The second output signal may be indicative of a partial pressure of carbon dioxide (PPCO2) in the gas exhaled by the user. The PPCO2 detected by the carbon dioxide sensor may be given by:

[0027] PPCO2 = P x FCO2(2) where P is the total pressure of the gas mixture in the rebreather apparatus at the carbon dioxide sensor, and FCO2 is the volume fraction of carbon dioxide in the gas mixture. The partial pressure may be used to determine a concentration of the carbon dioxide in the gas exhaled by the user.

[0028] The first output signal from the pressure sensor is used to detect a breathing cycle of the user. The breathing cycle may comprise an inhalation phase (where the user breathes in) and an exhalation phase (where the user breathes out). The breathing cycle is repeated by the user, such that a breathing pattern of the user is made up of a sequence of breathing cycles.

[0029] During use of the rebreather apparatus, the gas pressure in the rebreather apparatus will vary over time as the user inhales and exhales. These variations in the pressure are reflected in the first output signal, which may vary over time in accordance with the user’s breathing cycle and breathing pattern. For example, the first output signal may comprise a series of peaks and troughs, corresponding to peaks and dips in the total gas pressure in the rebreather apparatus, which may be caused by the user exhaling and inhaling, respectively. Accordingly, the first output signal may be representative of the user’s breathing cycles, and of their breathing pattern more generally.

[0030] Thus, the first output signal can be monitored (and / or recorded) over time to detect a breathing cycle of the user. For example, the first output signal may be substantially periodic in accordance with a period of the user’s breathing cycle. Accordingly, a period of the first output signal may be determined to determine a period of the user’s breathing cycle. Likewise, a start time and an end time of the user’s breathing cycle (i.e. from a start of the inhalation phase to an end of the exhalation phase) can be determined from the first output signal. Moreover, different phases of the user’s breathing cycle can be detected from the first output signal. For example, a time interval over which the first output signal indicates a decrease in pressure in the rebreather apparatus may correspond to an inhalation phase, and a time interval of which the first output signal indicates an increase in pressure in the rebreather apparatus may correspond to an exhalation phase.

[0031] The method then involves determining, using the detected breathing cycle and the second output signal from carbon dioxide sensor, a value indicative of ETCO2 for the user. Thus, both information relating to the breathing cycle and the second output signal are used as inputs for determining the value indicative of ETCO2. Taking into account the detected breathing cycle can facilitate determination of the value indicative of ETCO2 for the user. In particular, the inventors have found that, using knowledge of the user’s breathing cycle obtained from the first output signal, a level of the ETCO2 and / or a trend of the ETCO2 can be accurately determined. This also enables ETCO2 to be monitored with a variety of different carbon dioxide sensors, including slower-reacting carbon dioxide sensors which may be used in a rebreather apparatus.

[0032] In more detail, an amount of carbon dioxide exhaled by the user will vary throughout their breathing cycle, with end-tidal carbon dioxide corresponding to a level of carbon dioxide exhaled at an end of the breathing cycle. Thus, the second output signal may vary throughout the breathing cycle, in response to changes in the amount of carbon dioxide exhaled by the user. By detecting the breathing cycle, it is possible determine a portion (e.g. timestamp) of the second output signal which is indicative of ETCO2.

[0033] The value indicative of ETCO2 may be indicative of a level of carbon dioxide at an end of the user’s breathing cycle, i.e. at the end of the exhalation phase. The value indicative of ETCO2 may, for example, be indicative of an amount, proportion, partial pressure, and / or concentration of carbon dioxide at the end of the user’s breathing cycle. The value indicative of ETCO2 may be determined from (or correspond to) a magnitude of the second output signal at the end of the user’s breathing cycle. In some cases, the value indicative of ETCO2 may be indicative of a trend of ETCO2, e.g. as discussed in more detail below. The method may further comprise generating an alert if the value indicative of the end-tidal carbon dioxide exceeds a predetermined threshold. In this manner, the user may be automatically notified if their ETCO2 exceeds the predetermined threshold. This may enable the user to take preventive action, to avoid hypercapnia. The alert (or notification) may take any suitable form for notifying (informing) the user. For example, generating an alert may comprise one or more of: displaying an alert or notification on a display unit, generating an audible alert (e.g. via a speaker), and providing haptic feedback to the user (e.g. via a wearable device).

[0034] The method may comprise detecting an end of an exhalation phase in the breathing cycle from the first output signal, wherein the value indicative of end-tidal carbon dioxide is determined from a value of the second output signal at the end of the exhalation phase. In this manner, the end of the exhalation phase can be accurately determined, to facilitate extraction of the value indicative of the ETCO2. In particular, this technique ensures that a reading is taken from the end of the exhalation phase, to provide an accurate indication of the ETCO2 and / or a trend of the user’s ETCO2.

[0035] As an example, a timestamp corresponding to the end of the exhalation phase may be determined from the first output signal. For instance, the end of the exhalation phase may correspond to a maximum in the pressure in the rebreather apparatus, which may be reflected as a maximum in the first output signal. A value of the second output signal corresponding to the determined timestamp may then be used to determine (e.g. provide) the indication of the ETCO2.

[0036] In some cases, the value indicative of ETCO2 may be determined based on an average over one or more breaths (breathing cycles). For example, a value of the second output signal may be obtained at the end of the exhalation phase of two or more successive breathing cycles. The obtained values for the two or more successive breathing cycles can then be averaged together, to provide a value indicative of ETCO2.

[0037] In some cases, the method may comprise detecting an end of an exhalation phase in the breathing cycle from the first output signal and, in response to detecting the end of the exhalation phase, triggering reading of a value of the second output signal. The value read from the second output signal may then be used to determine the (e.g. provide) the indication of the ETCO2.

[0038] The method may comprise detecting a sequence of breathing cycles of the user using the first output signal, wherein, for each breathing cycle in the sequence, a respective value indicative of end-tidal carbon dioxide for the user is determined using the second output signal. In this manner, the user’s ETCO2 may be monitored and tracked over time. Thus, for each breathing cycle of the user, a respective value indicative of ETCO2 can be determined, so that an evolution (or trend) of ETCO2 can be observed. Each respective value indicative of ETCO2 can be associated with a respective timestamp (i.e. a time at which the corresponding value of the second output signal was obtained), so that the values indicative of ETCO2 can be recorded as a function of time.

[0039] The respective value indicative of ETCO2 for each breathing cycle in the sequence can be obtained as discussed above. Thus, for each breathing cycle, the second output signal is read at a time corresponding to an end of the exhalation phase. In more detail, for each breathing cycle, an end of an exhalation phase in the breathing cycle can be determined from the first output signal. A value of the second output signal corresponding to the end of the exhalation phase is then used to determine the value indicative of ETCO2.

[0040] The method may further comprise determining a trend of the end-tidal carbon dioxide over time, based on the respective value indicative of end-tidal carbon dioxide for each breathing cycle in the sequence. For example, by monitoring an evolution of the ETCO2 over time, it can be determined if the ETCO2 is stable, if it is increasing, or if it is decreasing. This can provide valuable information regarding safety of the user. For example, if the ETCO2 is stable or decreasing over time, this may be indicative of a relatively low risk of hypercapnia. On the other hand, if the determined trend shows an increase in ETCO2, then the user may be at increased risk of hypercapnia. Accordingly, the determined trend can be used to provide the user with an indication of risk level, which may enable them to take preventive action in order to reduce risk of hypercapnia.

[0041] By way of example, the ETCO2 trend may be determined to be increasing if the determined value indicative of ETCO2 increases continuously over a predetermined number of breathing cycles or over a predetermined amount of time. The increase over the predetermined number of breathing cycles or over the predetermined amount of time may be more than a standard deviation of the value indicative of ETCO2. Similarly, the ETCO2 trend may be determined to be decreasing if the determined value indicative of ETCO2 decreases continuously over a predetermined number of breathing cycles or over a predetermined amount of time. The decrease over the predetermined number of breathing cycles or over the predetermined amount of time may be more than a standard deviation of the value indicative of ETCO2. The predetermined amount of time used for determining the ETCO2 trend may, for example, be between 1 to 5 minutes. The ETCO2 trend may be determined to be stable if the value indicative of ETCO2 does not vary by more than a predetermined amount (e.g. the standard variation) over successive breathing cycles or over a predetermined amount of time.

[0042] If the determined trend fulfils a predetermined condition, the method may further comprise generating an alert. In this manner, the user may be automatically notified, e.g. if the trend shows that they are at increased risk of hypercapnia. In particular, the predetermined condition may be defined to generate an alert of the determined trend is indicative of an increase in ETCO2 and / or a risk of hypercapnia. For example, the predetermined condition may be fulfilled if the ETCO2 trend is determined to be increasing (e.g. as described above). The generated alert may correspond to any of the alert types mentioned above.

[0043] Advantageously, the method of the invention enables accurate determination of the trend of ETCO2 over time, without necessarily having to determine the value of ETCO2 to high accuracy. This enables the invention to be implemented with slower-reacting carbon dioxide sensors which may be used in a rebreather apparatus. In particular, slow-reacting carbon dioxide sensors may be used in a rebreather apparatus, as such sensors can be generally less sensitive to moisture (in contrast to other faster-reacting sensors), providing higher reliability in the operating conditions of a rebreather. Such slow-reacting sensors may, for example, have a T90 response time of 10 seconds or more, meaning it takes the sensor 10 seconds or more to settle to a final output value in response to a sudden change in partial pressure of carbon dioxide. In contrast, a person’s breathing cycle typically has a duration of 4 to 6 seconds. As a result, a slow-reacting carbon dioxide sensor may not have time to settle within the duration of a breathing cycle. As a result, the slow-reacting carbon dioxide sensor may, on its own, not provide an accurate reading of ETCO2 as it will not have fully settled at the end of the exhalation phase before the next inhalation phase is started. However, by measuring the second output signal across a sequence of breathing cycles (e.g. at the end of the exhalation phase of each breathing cycle), it is possible to reliably establish a trend (time evolution) in ETCO2. For example, as discussed above, it is possible to compare the respective value indicative of ETCO2 across successive breathing cycles to determine if the ETCO2 is increasing, decreasing, or stable. In this manner, the trend of the ETCO2 can be reliably determined using either a slow- reacting sensor (e.g. T90 reaction time of 10 seconds or more) or a fast-reacting sensor, which can be used to assess a risk of carbon dioxide retention and / or hypercapnia of the user.

[0044] The method may further comprise receiving a third output signal from a from a second carbon dioxide sensor in the rebreather apparatus, wherein the third output signal is indicative of a partial pressure of carbon dioxide at an outlet of a carbon dioxide absorbent unit in the breathing apparatus. In this manner, the two carbon dioxide sensors can be used to obtain both an indication of carbon dioxide exhaled by the user, and carbon dioxide levels in gas exiting the carbon dioxide absorbent unit.

[0045] The carbon dioxide absorbent unit (also referred to as a “scrubber”), is arranged to absorb carbon dioxide exhaled by the user, in order to avoid build-up of carbon dioxide in the rebreather apparatus. As the carbon dioxide absorbent unit absorbs carbon dioxide, it gradually becomes saturated until it no longer absorbs any more carbon dioxide. Accordingly, the third output signal may provide an indication of an effectiveness of the carbon dioxide absorbent unit at absorbing carbon dioxide. For example, an increase in the third output signal may be indicative of saturation of the carbon dioxide absorbent unit, and / or malfunction of the carbon dioxide absorbent unit. The value indicative of end-tidal carbon dioxide may further based on a comparison between the second output signal and the third output signal. In this manner, an accuracy with which the value indicative of ETCO2 is determined can be improved. In particular, the third output signal may serve to provide an indication of a background carbon dioxide level in the rebreather apparatus, against which carbon dioxide exhaled from the user can be estimated. For example, determining the value indicative of ETCO2 may comprise subtracting a value of the third output signal from a value of the second output signal. Thus, at the end of an exhalation phase of the breathing cycle values from the second and third output signals may be obtained and compared (e.g. subtracted) to determine the value indicative of ETCO2.

[0046] The third output signal from the carbon dioxide sensor may be used for determining the trend of ETCO2 over time. For example, the trend of ETCO2 may be determined based on a comparison between the second output signal and the third output signal. For instance, if a difference between the PPCO2 at the outlet of the carbon dioxide absorbent unit (as determined from the third output signal) and the amount of CO2 exhaled by the user (as determined from the second output signal) increases, this may be indicative of increasing ETCO2.

[0047] The second carbon dioxide sensor may be a same type of carbon dioxide sensor as the first carbon dioxide sensor described above. This may facilitate comparing the second and third output signals.

[0048] The method may further comprise determining, using the first output signal, a breathing rate of the user. If the breathing rate of the user exceeds a predetermined value for a predetermined amount of time, an alert may be generated. A high breathing (ventilation) rate may be indicative of increasing ETCO2, which may be a precursor to a hypercapnic event. This provides an additional mechanism for warning the user of a risk of carbon dioxide retention. The predetermined amount of time may, for instance, be in the range of 1 to 5 minutes.

[0049] The method may further comprise determining, using the first output signal, a breathing rate of the user; and determining, based on the breathing rate, a risk of carbon dioxide retention for the user. This provides an additional means for assessing risk of carbon dioxide retention and hypercapnia of the user, which may further contribute to safety of the user. The user may be notified of a determined risk level carbon dioxide retention level (e.g. low, medium, or high risk). An alert may be generated if the determined risk level exceeds a predetermined threshold.

[0050] The risk of carbon dioxide retention determined from the breathing rate can be used to crosscheck the value indicative of ETCO2 and / or the ETCO2 trend, e.g. to improve reliability of the method. For example, if the value indicative of ETCO2 and / or the ETCO2 trend indicate that there is an increased risk of carbon dioxide retention, the risk level can be confirmed by also checking the breathing rate of the user. In general, carbon dioxide retention varies as a function of breathing rate, with an increase in breathing rate typically being indicative of increased carbon dioxide retention. Accordingly, an increase in breathing rate may be associated with an increase in carbon dioxide retention. If the breathing rate exceeds a predetermined threshold, the method may determine that there is an increased (or high) risk of carbon dioxide retention, and an alert may optionally be generated.

[0051] In order to determine the risk (risk level) of carbon dioxide retention, the method may use a predetermined relationship between breathing rate and risk of carbon dioxide retention. Such a relationship may be determined experimentally, and / or using a theoretical model.

[0052] The breathing rate may be determined from the first output signal as discussed above, e.g. by determining a period of the breathing cycle.

[0053] The method may further comprise: receiving a fourth output signal from a flow sensor in the rebreather apparatus, wherein the fourth output signal is indicative of a gas flow rate in the rebreather apparatus; and determining a tidal volume of the user based on the fourth output signal. The tidal volume provides an indication of a volume of gas displaced by the user during the inhalation phase (or the exhalation phase) of their breathing cycle. The tidal volume therefore provides another parameter of the user’s breathing cycle which can be used to monitor their safety. For example, shallow breathing of the user (e.g. where the tidal volume is low) may be indicative of carbon dioxide retention.

[0054] The flow sensor may be arranged to detect a gas flow rate (e.g. in litres per minute) in the rebreather apparatus. This can then be used to determine the tidal volume of the user, e.g. by integrating the gas flow rate over the inhalation phase (or exhalation phase) of the breathing cycle. For example, the flow sensor may be arranged to detect a gas flow rate of gas inhaled via the mouthpiece by the user, or a gas flow rate of gas exhaled via the mouthpiece by the user.

[0055] The method may further comprise determining a risk of carbon dioxide retention based on the determined tidal volume. This provides an additional means for assessing risk of carbon dioxide retention and hypercapnia of the user, which may further contribute to safety of the user. The user may be notified of a determined risk level carbon dioxide retention level (e.g. low, medium, or high risk). An alert may be generated if the determined risk level exceeds a predetermined threshold.

[0056] The risk of carbon dioxide retention determined from the tidal volume can be used to crosscheck the value indicative of ETCO2 and / or the ETCO2 trend, e.g. to improve reliability of the method. For example, if the value indicative of ETCO2 and / or the ETCO2 trend indicate that there is an increased risk of carbon dioxide retention, the risk level can be confirmed by also checking the tidal volume of the user. In order to determine the risk (risk level) of carbon dioxide retention, the method may use a predetermined relationship between tidal volume and risk of carbon dioxide retention. Such a relationship may be determined experimentally, and / or using a theoretical model.

[0057] In some cases, both the breathing rate and the tidal volume of the user may be used to determine the risk of carbon dioxide retention.

[0058] The method may further comprise generating an output based on the determined value indicative of ETCO2 and / or the determined trend (where applicable). Such an output may serve to notify (or alert) the user of their ETCO2 level, enabling them to take preventive measures to reduce their ETCO2 if needed. Such an output may take any suitable form, such as a visual output (display), audible output, and / or haptic feedback.

[0059] The method may further comprise generating a display output based on the value indicative of the end-tidal carbon dioxide. For example, the method may display an output on a display unit, based on (as a function of) the value indicative of ETCO2.

[0060] The display output may comprise a visualisation (graphical representation) having a colour determined based on the value indicative of the end-tidal carbon dioxide. In this manner, information regarding the user’s ETCO2 may be efficiently conveyed to them, which may be highly beneficial in diving situations where the user’s visibility and ability to read information may be limited. In particular, colour coding of the visualisation may avoid the user having to read small digits on a display (which may be difficult in underwater conditions), to provide them with a quick indication of their ETCO2.

[0061] By way of example, the visualisation may be a first colour (e.g. red) if the value indicative of ETCO2 is above a predetermined value, and a second colour (e.g. green) if the value indicative of ETCO2 is below the predetermined value.

[0062] The visualisation may comprise a graphical representation of the user’s breathing cycle. The graphical representation may be coloured as described above. In this manner, information regarding multiple parameters of the user’s breathing may be efficiently conveyed to them in one place, without having to read or look at multiple displays or display areas. The graphical representation may comprise a shape having a size that changes (e.g. oscillates) in time with the user’s breathing cycle (as determined from the first output signal). The shape may be colour coded as described above.

[0063] For example, the graphical representation may comprise a bar whose length changes (e.g. oscillates) in time with the user’s breathing cycle. This may provide the user with a clear representation of their breathing cycle and breathing rate, suitable for interpretation by the user in underwater situations. In some cases, an amplitude of the oscillations may be determined based on the determined tidal volume. The display output may further include a graphical representation (or visualisation) of a target (or ‘ideal’) breathing cycle. In this manner, the user can compare their breathing cycle to a target breathing cycle, which may assist them in controlling their breathing cycle to bring it into line with the target breathing cycle. The target breathing cycle may be determined so as to reduce the user’s ETCO2. For example, a breathing rate and / or tidal volume of the target breathing cycle may be determined to reduce excessive ETCO2, e.g. through proper ventilation techniques. So, for instance, where it is determined that the user’s breathing rate is excessive (e.g. it is indicative of a risk of CO2 retention) the breathing rate of the target breathing cycle may be lower than the user’s breathing rate, e.g. to prompt the user to reduce their breathing rate. Similarly, where it is determined that the user’s tidal volume is low (e.g. it is indicative of a risk of CO2 retention), the tidal volume of the target breathing cycle may be greater than the user’s tidal volume, e.g. to prompt them to increase their tidal volume.

[0064] The graphical representation of the target breathing cycle may be analogous to the graphical representation of the user’s breathing cycle described above. This facilitates direct comparison between the user’s breathing cycle and the target breathing cycle.

[0065] According to a second aspect of the invention, there is provided a rebreather apparatus, comprising: a mouthpiece; a breathing volume connected to the mouthpiece; a pressure sensor configured to detect variations in a gas pressure in the breathing volume, and to produce an output signal indicative of the variations in gas pressure; a carbon dioxide sensor configured to detect carbon dioxide exhaled from a user via the mouthpiece; and a controller configured to perform a method according to the first aspect of the invention.

[0066] The rebreather apparatus of the second aspect designed to implement the method of the first aspect. Therefore, any of the features discussed above in relation to the first aspect of the invention may be shared with the rebreather apparatus of the second aspect of the invention (and vice versa). In particular, any features of the method described above may be implemented by the controller of the rebreather apparatus. Likewise, any features relating to the rebreather apparatus discussed in relation to the method of the first aspect can be included in the rebreather apparatus of the second aspect.

[0067] The carbon dioxide sensor may be located in the breathing volume on an exhale side of the mouthpiece. For example, the carbon dioxide sensor may be located between an exhale side (e.g. exhale valve) of the mouthpiece and an inlet of a carbon dioxide absorbent unit of the apparatus.

[0068] The carbon dioxide sensor may have a T90 response time of 1 second or less.

[0069] The carbon dioxide sensor may comprise an optical carbon dioxide sensor, such as an infrared carbon dioxide sensor. Such an infrared-based sensor may have a relatively rapid response time (e.g. T90 response time less than 1 second). This may enable the ETCO2 level to be determined to high accuracy for each breathing cycle.

[0070] The carbon dioxide sensor may have a T90 response time of 10 seconds or more. Such a sensor may be ideally suited to use in a rebreather apparatus, as it may be less sensitive to moisture build-up on the sensor which can commonly occur in a rebreather apparatus. For example, a Polestar Technologies CO2 sensor may have such a response time.

[0071] The rebreather apparatus may further comprise a carbon dioxide absorbent unit configured to absorb carbon dioxide present inside the breathing volume; and a second carbon dioxide sensor configured to detect a partial pressure of carbon dioxide at an outlet of the carbon dioxide absorbent unit, and to produce a third output signal indicative of the partial pressure of carbon dioxide.

[0072] The rebreather apparatus may further comprise a flow sensor configured to detect a gas flow rate in the breathing volume, and to produce a fourth output signal indicative of the gas flow rate.

[0073] The rebreather apparatus may further comprise a display unit configured to display an output based on the value indicative of the end-tidal carbon dioxide. The display unit may comprise, for example, a screen or a touchscreen for displaying the output.

[0074] The rebreather apparatus may also include one or more additional sensors. For example, the rebreather apparatus may comprise an oxygen sensor configured to detect a partial pressure of oxygen in the rebreather apparatus (e.g. in the breathing volume). The controller may then configured to: receive an output signal from the oxygen sensor, wherein the output signal is indicative of a partial pressure of oxygen in the rebreather apparatus; and determine a current value of the partial pressure of oxygen in the breathing apparatus, using the output signal. This may facilitate monitoring and control of PPO2 in the rebreather apparatus.

[0075] The rebreather apparatus may comprise a high pressure sensor configured to detect a pressure in an oxygen supply tank of the rebreather apparatus, and to produce an output signal indicative of the pressure in the oxygen supply tank.

[0076] According to a third aspect of the invention, there is provided a controller for a rebreather apparatus, the controller having a processing unit and a memory storing instructions which, when executed by the processing unit, cause the processing unit to perform a method according to the first aspect of the invention. The controller may, for example, correspond to the controller of the rebreather apparatus in the second aspect of the invention. Features described above in relation to preceding aspects of the invention may be shared with the third aspect of the invention (and vice versa).

[0077] The controller may be implemented using any suitable computing or processing device. The invention includes combinations of the aspects and features described except where such a combination is clearly impermissible or expressly avoided.

[0078] Summary of the Figures

[0079] Embodiments and experiments illustrating the principles of the invention will now be discussed with reference to the accompanying figures in which:

[0080] Fig. 1 shows a schematic diagram of a rebreather apparatus according to an embodiment of the invention;

[0081] Fig. 2 shows a flow diagram of a method according to an embodiment of the invention;

[0082] Fig. 3 shows an example graph of an output signal of a pressure sensor in a rebreather according to an embodiment of the invention;

[0083] Fig. 4 shows an example graph of a value indicative of end-tidal carbon dioxide as a function of time; and

[0084] Fig. 5 shows a schematic diagram of a display unit that may form part of a rebreather apparatus according to an embodiment of the invention.

[0085] Detailed Description of the Invention

[0086] 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.

[0087] Fig. 1 is a schematic diagram of a breathing apparatus having a controller 102 according to an embodiment of the invention. The breathing apparatus is a rebreather 100, which may typically be used for underwater diving. Of course, the rebreather 100 may have many more parts or components than those illustrated in Fig. 1 , and the arrangement, shape, etc. of the rebreather 100 and its parts or components may be different to that illustrated in Fig. 1.

[0088] The rebreather 100 has a mouthpiece 104 through which a user of the rebreather can breathe a gas 5 in the rebreather. The mouthpiece 104 may, for example, comprise a dive surface valve (DSV), or a bailout valve (BOV). The rebreather 100 further comprises a breathing volume in the form of a breathing loop 106, having an upstream side 106a which is connected to the mouthpiece 104 via an upstream (inhale) valve 108, and a downstream side 106b which is connected to the mouthpiece 104 via a downstream (exhale) valve 110. Gas 5 is supplied to the mouthpiece 104 via the upstream side 106a of the breathing loop and the upstream valve 108. When the user exhales through the mouthpiece 104, the exhaled gas 5 is removed from the mouthpiece 104 via the downstream valve 110 and the downstream side 106b of the breathing loop. The upstream valve 108 and the downstream valve 110 are one-way valves, which are arranged to ensure that the gas 5 flows into the mouthpiece 104 from the upstream side 106a of the breathing loop when the user inhales, and that the gas 5 flows from the mouthpiece 104 into the downstream side 106b of the breathing loop 106 when the user exhales. In other words, the valves 108, 110 serve to ensure that the gas 5 flows around the breathing loop 106 in a single direction, as indicated by the arrows 112 in Fig. 1. Any suitable type of one-way valve may be used for the upstream and downstream valves 108, 110, such as mushroom valves.

[0089] A pressurised oxygen supply tank 114 is connected to the breathing loop 106 via a supply valve 116. When the user breathes the gas 5, oxygen in the gas 5 is used by the user and therefore removed from the gas 5. Accordingly, oxygen is injected (i.e. added) into the breathing loop 106 from the oxygen supply tank 114 via the supply valve 116, in order to replenish the oxygen used by the user. The supply valve 116 can be controlled by the controller 102, in order to control amount (volume) or rate of oxygen into injected into the rebreather 100. The supply valve 116 may be any suitable type of valve whose opening and closing can be electronically controlled, such as a solenoid valve. The controller 102 may be connected to the supply valve 116 via a wired connection (not shown), so that the controller 102 can transmit an electrical signal to the supply valve 116 in order to control a position of the supply valve 116. For example, the controller 102 may control whether the supply valve 116 is opened or closed, and in some cases an extent to which the supply valve is opened 116, in order to control the rate of injection of oxygen into the breathing loop 106. A pressurised air tank (not shown) may also be connected to the breathing loop, so that air (or diluent) can be injected into the breathing loop 106. The controller 102 may be configured to control injection of air into breathing loop 106 in a similar manner to how injection of oxygen is controlled. In this manner, a desired gas mixture may be maintained within the breathing loop 106.

[0090] The rebreather 100 further comprises a carbon dioxide absorbent unit 118 (or “scrubber”), which is a sealed canister containing a carbon dioxide absorbent material. A typical material used in such applications is soda lime, however other materials may also be used. The carbon dioxide absorbent unit 118 is connected to the breathing loop 106 such that gas exhaled by the user passes through the carbon dioxide absorbent unit 118 before it is recirculated to the mouthpiece 104. In this manner, the carbon dioxide absorbent unit 118 captures carbon dioxide exhaled by the user, to prevent a build-up of carbon dioxide inside the breathing loop 106.

[0091] The rebreather 100 may also comprise one or more counterlungs in the breathing loop 106. In the example shown, the rebreather 100 comprises an upstream counterlung 120 through which gas 5 on the upstream side 106a of the breathing loop passes before reaching the mouthpiece 104, and a downstream counterlung 122 on the downstream side 106b of the breathing loop through which exhaled gas passes. The volume of the counterlungs 120, 122 may vary in accordance with the user’s tidal volume, to facilitate breathing with the rebreather 100. The controller 102 may be in the form of a portable computing device, which has software installed thereon for performing the functions described herein. The controller 102 may comprise a memory in which data from sensors can be recorded. The controller 102 may also include a user interface (not shown), which enables a user to interact with the controller 102, e.g. in order to control various aspects of the rebreather 100. For example, the controller 102 may comprise one or more buttons and / or a display unit (e.g. a screen or touchscreen). The controller 102 may also be connected to a display (not shown), such as a screen or heads-up display, so that the controller 102 can notify the user of relevant information, such as status information for the rebreather 100.

[0092] The rebreather 100 further includes a series of sensors, which are used by the controller 102, to perform various process and monitor performance of the rebreather 100. The various sensors and their uses are discussed below.

[0093] The rebreather 100 comprises a pressure sensor 124 in the breathing loop 106, and arranged to detect a pressure of the gas 5 in the breathing loop 106. In particular, the pressure sensor 124 is sensitive to variations (i.e. changes) of pressure of the gas 5 in the breathing loop 106. Any suitable type of pressure sensor may be used. For example, the pressure sensor 124 may be a differential pressure sensor that detects changes in pressure of the gas 5 relative to a reference pressure. In the example shown, the pressure sensor 124 is located in the upstream side 106a of the breathing loop. However, in other examples, the pressure sensor 124 may be arranged at a different location in the breathing loop 124. In some cases, there may be multiple pressure sensors arranged around the breathing loop 106, in order to detect the gas pressure at different locations in the breathing loop 106.

[0094] As the user of the rebreather 100 breathes, the pressure of the gas 5 in the breathing loop 106 will vary. Thus, the pressure in the breathing loop 106 will decrease when the user inhales, and will increase when the user exhales. Accordingly, the pressure sensor 124 can detect the variations in pressure inside the breathing loop 106 caused by the user’s breathing. The pressure sensor 124 produces a first output signal that is indicative of the detected pressure variations, such that the first output signal can be used to monitor the user’s breathing pattern.

[0095] The pressure sensor 124 is connected to the controller 102, e.g. via a wired or wireless connection (not shown), such that the controller 102 can receive the first output signal from the pressure sensor 124. The controller 102 is configured to detect a breathing cycle of the user using the first output signal, as described in more detail below.

[0096] The rebreather 100 may further comprise an ambient pressure sensor 125, which is located outside the breathing loop 106, such that it is exposed to an ambient (e.g. water) pressure located outside the breathing loop 106. The ambient pressure sensor 125 is sensitive to the ambient pressure outside the breathing loop 106, and is configured to produce an output signal that is indicative of the ambient pressure. The ambient pressure sensor 125 is connected to the controller 102, e.g. via a wired or wireless connection (not shown), such that the controller 102 can receive the output signal from the pressure sensor 125. The controller 102 may then be configured to use the ambient pressure determined from the output signal from the ambient pressure sensor 125 as a reference for pressure variations in the breathing loop 106 detected with the pressure sensor 124. In other words, the controller 102 may measure pressure variations in the breathing loop 106 relative to the ambient pressure outside the breathing loop 106.

[0097] The rebreather 100 further comprises a first carbon dioxide sensor 132 for detecting carbon dioxide exhaled by the user. In the example shown, the carbon dioxide sensor 132 is located in the downstream side 106b of the breathing loop 106, between the downstream (exhale) valve 110 and the carbon dioxide absorbent unit 118. In this manner, the carbon dioxide sensor 132 is exposed to carbon dioxide in the gas exhaled by the user. In another example, the carbon dioxide sensor 132 may be located in the mouthpiece 104.

[0098] The first carbon dioxide sensor 132 may be configured to detect the PPCO2 in gas exhaled by the user through the mouthpiece 104, and to produce an output signal indicative of the exhaled PPCO2. The first carbon dioxide sensor 132 be slow-reacting, e.g. with a T90 reaction time of 10 seconds or more. In other words, it may take the sensor 10 seconds or more to settle to a final output value in response to a sudden change in partial pressure of carbon dioxide. For example, a Polestar Technologies CO2 sensor may be used. Alternatively, a fast-reacting sensor, e.g. having a T90 of 1 second or less, may be used. This may enable breath-by-breath analysis of the exhaled carbon dioxide. As an example, an infrared carbon dioxide sensor such as the SprintlR®-R CO2 Sensor, which is a nondispersive infrared (NDIR) carbon dioxide sensor, may be used.

[0099] The first carbon dioxide sensor 132 is connected to the controller 102, e.g. via a wired or wireless connection (not shown), such that the controller 102 can receive the output signal from the carbon dioxide sensor 132. The controller 102 can then determine the PPCO2 in the exhaled gas from the output signal from the first carbon dioxide sensor 132. For example, the controller 102 may use one or more calibration curves associated with the first carbon dioxide sensor 132 for determining the PPCO2 from the output signal.

[0100] The rebreather 100 may further comprise a second carbon dioxide sensor 128, located in the breathing loop 106 on an outlet side of the carbon dioxide absorbent unit 118. In this manner, the second carbon dioxide sensor 128 is arranged to detect carbon dioxide in gas exiting the carbon dioxide absorbent unit 118. Thus, the second carbon dioxide sensor 128 can be used to monitor performance of the carbon dioxide absorbent unit 118, i.e. its effectiveness at removing carbon dioxide from the gas in the breathing loop 106. The second carbon dioxide sensor 128 may be a same type of carbon dioxide sensor 128 as the first carbon dioxide sensor described above, e.g. both may be slow or fast-reacting sensors. Alternatively, different types of carbon dioxide sensors may be used, e.g. one sensor may be slow-reacting and the other may be fastreacting.

[0101] The rebreather 100 may further comprise one or more oxygen sensors 126 in the breathing loop 106. In the example shown, there are three oxygen sensors 126 located adjacent to one another, next to the carbon dioxide absorbent unit. However, in other examples, there may be more or fewer oxygen sensors, and the oxygen sensors may be arranged at different locations in the breathing loop 106. Each of the oxygen sensors is configured to detect the PPO2 in the breathing loop 106, and to produce an output signal that is indicative of the PPO2. Any suitable type of oxygen sensor may be used, such as an electro-galvanic sensor, a paramagnetic sensor, or a luminescent oxygen sensor. The oxygen sensors 126 are connected to the controller 102, e.g. via a wired or wireless connection (not shown), such that the controller 102 can receive the output signals from the oxygen sensors 126. The controller 102 is then configured to determine the PPO2 in the breathing loop 106 from the output signals from the oxygen sensors 126. For example, the controller 102 may use one or more calibration curves for determining the PPO2 in the breathing loop 106 from the output signals. The purpose of providing multiple oxygen sensors 126 is to provide a level of redundancy, in case one or more of the oxygen sensors 126 fails. The controller 102 may determine the PPO2 in the breathing loop 106 from the different output signals in any suitable manner. For instance, in some cases, the controller 102 may average the results obtained from the different output signals. The controller 102 may also apply the “voting logic” described in GB2525973B, for determining which oxygen sensor output signal(s) should be used for determining the PPO2.

[0102] The controller 102 may be configured to use both the user’s oxygen consumption rate, and the determined value of PPO2, in order to control the PPO2 in the breathing loop 106. For example the controller 102 may control injection of oxygen into the breathing loop 106 (by controlling the supply valve 116), to maintain a target value of PPO2 in the breathing loop 106.

[0103] The rebreather 100 may further comprise a flow sensor 130 in the breathing loop, for detecting a gas flow rate of the gas 5 in the breathing loop 106. An output signal produced by the flow sensor 130 may be indicative of the detected gas flow rate (e.g. in litres per minute or other suitable unit). The flow sensor 130 is connected to the controller 102, e.g. via a wired or wireless connection (not shown), such that the controller 102 can receive the output signal from the flow sensor 130. The controller 102 can then determine the gas flow rate in the breathing loop 106 from the output signal from the flow sensor 130. For example, the controller 102 may use one or more calibration curves associated with the flow sensor 130 for determining the gas flow rate from the output signal. In the example shown, the flow sensor 130 is located in the downstream side 106b of the breathing loop 106. However, in other examples, the flow sensor 130 may be located in the upstream side 106a of the breathing loop 106. Fig. 2 shows a flow diagram of a method 200 according to an embodiment of the invention. The method 200 may, for example, be implemented by the controller 102. For convenience, the method 200 is described in the context of the rebreather 100; however, the method 200 may be applied to other rebreathers.

[0104] In a step 202, the method comprises receiving, by the controller 102, a first output signal from the pressure sensor 124. As discussed above, the first output signal from the pressure sensor 124 is indicative of variations in the gas pressure in the breathing loop 106. When the user breathes, this causes changes in the gas pressure in the breathing loop 106. In particular, the gas pressure may decrease when the user inhales, and increase when the user exhales. This is illustrated in Fig. 3, which shows an example graph 300 of the first output signal from the pressure sensor 124 over time. The first output signal is substantially periodic shape in accordance with a breathing rate of the user. A portion of the first output signal where the pressure decreases from a maximum to a minimum may be determined as an inhalation phase 302, and a second portion of the first output signal where the pressure increases from the minimum to a maximum may be determined as the exhalation phase 304. Together, the inhalation phase 302 and the exhalation phase 304 constitute a breathing cycle 306 of the user. Thus, at step 204, the method 200 comprises detecting the user’s breathing cycle 306 using the first output signal from the pressure sensor 124. This may comprise monitoring and / or recording the first output signal over time to detect the breathing cycle 306, e.g. by detecting local maxima and local minima in the first input signal.

[0105] At step 206, the method comprises receiving, by the controller 102, a second output signal from the carbon dioxide sensor 132. As discussed above, the second output signal from the carbon dioxide sensor is indicative of carbon dioxide exhaled by the user. The step 206 may be performed in parallel with (i.e. at the same time as) step 202. In other words, the controller 102 may continuously receive the first and second output signals from the pressure sensor 124 and the carbon dioxide sensor 132, respectively. Alternatively, reading of the second output signal by the controller 102 may be triggered based on the detection of the breathing cycle in step 204, as discussed below.

[0106] At step 208, the method 200 comprises determining, by the controller 102, a value indicative of end-tidal carbon dioxide (ETCO2) for the user. The value of ETCO2 is determined using both the detection of the user’s breathing cycle (from step 204) and the second output signal from the carbon dioxide sensor 132 (from step 206). In more detail, an end of the exhalation phase 302 of the breathing cycle 306 can be determined from the first output signal. This may be achieved, for example, by detecting a maximum in the first output signal, which may be indicative of a transition between the exhalation phase 304 and the inhalation phase of a next breathing cycle. The end of the exhalation phase 304 is indicated as h in Fig. 3. A value of the second output signal at time h may then be used to provide an indication of ETCO2 at the end of the breathing cycle 306. In some cases, a raw value of the second output signal at time ti can be used to provide the indication of ETCO2. Alternatively, the value of the second output signal at time h can be converted into a value indicative of ETCO2, e.g. using calibration data associated with the carbon dioxide sensor 132. In some cases, detecting of the end of the exhalation phase from the first output signal may be used to trigger reading (or recording) of a value from the second output signal, to provide the indication of ETCO2. Thus, the carbon dioxide sensor 132 may only need to be read once per breathing cycle.

[0107] In some cases, the output signal from the second carbon dioxide sensor 128 can be used to improve an accuracy of the value indicative of ETCO2. In particular, the output signal from the second carbon dioxide sensor 128 can be used to account for carbon dioxide present in the upstream side of the breathing loop 106a, so as to more accurately determine an amount of carbon dioxide exhaled by the user. For example, the output signal from the second carbon dioxide sensor 128 may be treated as a background signal which is subtracted from the output signal from the first carbon dioxide sensor 132. Thus, to determine the value indicative of ETCO2, a value obtained from the output signal of the second carbon dioxide sensor 128 may be subtracted from a value obtained from the output signal of the first carbon dioxide sensor 132 (e.g. at time ti), to provide a difference value. The difference value can then be used to determine the value indicative of ETCO2. Depending on the types of carbon dioxide sensors used, various signal normalisation, calibration, and / or conversion processes may be performed to enable effective comparison between the two signals.

[0108] The value indicative of ETCO2 obtained from the second output signal for time ti can be recorded. In a similar manner, for each successive breathing cycle of the user, a value indicative ETCO2 may be obtained from the second output signal at a time corresponding to the end of the exhalation phase for that breathing cycle. Accordingly, ETCO2 can be monitored and / or recorded over a sequence (plurality) of breathing cycles, to monitor evolution of ETCO2 over time. This is illustrated in Fig. 4, which shows an example graph of determined values of indicative of ETCO2 as a function of time, for a sequence of breathing cycles. The graph 400 has six data points, corresponding to determined values indicative of ETCO2 in each of a sequence of six breathing cycles at times ti to te, respectively. Each time ti to te indicates the end of the exhalation phase in the corresponding breathing cycle, as determined from the first output signal. The same process as for time ti discussed above may be performed for each of times t2 to te, to obtain the data points in the graph 400. Accordingly, a value indicative of ETCO2 can be determined for each breathing cycle, to monitor an evolution of the ETCO2 over time.

[0109] Where a fast-reacting carbon dioxide sensor is used, the value indicative of ETCO2 determined at step 208 may provide an accurate indication of ETCO2 level at the end of the corresponding breathing cycle. On the other hand, where a slow-reacting carbon dioxide sensor is used, the output signal may not have time to settle within the breathing cycle. As such, the value indicative of ETCO2 determined at step 208 using a slow-reacting carbon dioxide sensor may be less accurate. However, by monitoring evolution of the value indicative of ETCO2 over time, it is possible to reliably determine a trend for evolution of the ETCO2. In particular, using the techniques described, it can be reliably determined if the ETCO2 is increasing over time, even where a slow-reacting sensor is used.

[0110] Following step 208, the method 200 may further comprise generating an output based on the determined value indicative of ETCO2. For example, the controller 102 may be connected to an output device (e.g. display unit, speaker, haptic device) for providing notifying the user of the determined value. In some cases, an alert (or alarm) may be generated, depending on the determined value. For example, if the determined value exceeds a predetermined threshold, the controller 102 may generate an alert via a suitable output device to warn the user. As another example, an alert can be generated if it is determined that a trend of the determined value fulfils one or more predetermined conditions. For instance, if it is determined that the value indicative of ETCO2 is increasing over time, this may be indicative of an increasing risk of hypercapnia, and an alert can be generated to notify the user. This can be achieved by monitoring evolution of the determined value over time, e.g. as shown in Fig. 4. If the determined value indicative of ETCO2 increases continuously over a predetermined number of breathing cycles or over a predetermined amount of time, then the ETCO2 trend may be determined to be increasing, and an alert can be generated. Additionally or alternatively, if the determined value indicative of ETCO2 increases by more than a threshold amount over a predetermined number of breathing cycles or over a predetermined amount of time, then the ETCO2 trend may be determined to be increasing, and an alert can be generated. The determined value indicative of ETCO2, provides an indication of carbon dioxide retention by the user, and hence of a risk of hypercapnia.

[0111] The method 200 may further include determining, by the controller 102, a breathing rate of the user from the first output signal from the pressure sensor 124. In particular, as discussed in relation to Fig. 3, a breathing cycle can be detected from the first output signal, thus enabling the period of the breathing cycle and hence the breathing rate to be determined. ETCO2 and carbon dioxide retention typically vary as a function of the user’s breathing rate, such that the user’s breathing rate can be used as an indicator of risk of carbon dioxide retention. Thus, the determined breathing rate can be used to assess a risk of carbon dioxide retention by the user. For example, the controller may store a predetermined relationship between breathing rate and a risk level of carbon dioxide retention. Then, using the predetermined relationship and the determined breathing rate, the controller 102 can look up an associated risk level. If it is determined that there is a risk (e.g. a high or increased) risk of carbon dioxide retention based on the breathing rate, then an alert can be generated to notify the user. The method 200 may also involve using the output signal from the flow sensor 130 to determine a tidal volume of the user. For example, the output signal from the flow sensor can be integrated over the inhalation phase (or exhalation phase) of a breathing cycle of the user, to determine their tidal volume. Similarly to breathing rate, tidal volume also provides an indicator for risk of carbon dioxide retention, with lower tidal volumes typically being indicative of increased risk of carbon dioxide retention. Accordingly, the determined tidal volume can be used to asses risk of carbon dioxide retention. For example, the controller may store a predetermined relationship between tidal volume and a risk level of carbon dioxide retention. Then, using the predetermined relationship and the determined tidal volume, the controller 102 can look up an associated risk level. If it is determined that there is a risk (e.g. a high or increased) risk of carbon dioxide retention based on the tidal volume, then an alert can be generated to notify the user.

[0112] In some cases, both the breathing rate and the tidal volume can be factored into the determination of the risk level for carbon dioxide retention, e.g. based on a set of predetermined rules for determining risk level as a function of breathing rate and tidal volume. The value indicative of ETCO2 and / or a determined ETCO2 trend may be used to confirm or cross-check the risk level determined from the breathing rate and / or the tidal volume.

[0113] Fig. 5 shows a schematic diagram of a display unit 500 that may be used with a rebreather in an embodiment. For example, the display unit 500 may be used with the rebreather 100 described above, the display unit 500 may be communicatively coupled to the controller 102, e.g. via a wired or wireless connection. The display unit 500 is arranged to display an output generated by the controller 102 based on the determined value indicative of ETCO2. The display unit 500 has a display (or screen) 502, which may for example be a LED display or LCD display. The display unit is arranged to provide a graphical representation of parameters determined by the controller 102. In this manner, information can efficiently be conveyed to the user, without them having to read characters or symbols on the display 502.

[0114] The graphical representation includes a shape 504 (in the example the shape is in the form of a bar), where a size of the shape 504 varies with the user’s breathing cycle (as shown by the arrows 506). For instance, the size of the shape 504 (e.g. length of the bar) may vary in accordance with a magnitude with the first output signal from the pressure sensor 124. Accordingly, as the user breathes, the shape 504 may extend and contract in time with the user’s breathing. In some cases, a maximum size (amplitude) of the shape 504 may be determined based on the tidal volume (as determined from the flow sensor 130). Thus, when the user has a higher tidal volume, the shape may have a larger maximum size than when the user has a lower tidal volume. Additionally, a display colour of the shape 504 may be set based on the determined value indicative of ETCO2. So, if the value indicative of ETCO2 changes over time, the colour of the shape 504 will change accordingly. For example, where the value indicative of ETCO2 is below a predetermined threshold, the shape 504 may be a first colour (e.g. green), and where the value indicative of ETCO2 is above the predetermined threshold, the shape 504 may be a second colour (e.g. red). In some cases, the display colour of the shape 504 may be based on a determined trend of the ETCO2. For example, if the trend is determined to be stable (or decreasing), then the shape 504 may be a first colour (e.g. green), and if the trend is determined to be increasing, then the shape 504 may be a second colour (e.g. red). It should be noted that, whilst a bar is used as an illustration of the graphical representation provided by the display unit 500, other shapes instead of the bar can be used.

[0115] In some embodiments, the display unit 500 may further be configured to display a graphical representation of a target (or ‘ideal’) breathing cycle, to guide the user’s breathing and assist them in regulating their breathing cycle. The graphical representation of the target breathing cycle includes a shape 508. In an analogous manner to the shape 504 discussed above, the size of the shape 508 (e.g. the length of the bar) varies over time in accordance with a breathing rate of the target breathing cycle. The maximum size of the shape 508 may be determined based on the tidal volume of the target breathing cycle. Thus, by watching the shapes 504 and 508, the user can attempt to bring their breathing cycle (represented by shape 504) into line with the target breathing cycle (represented by shape 508). The controller 102 may store a breathing rate and / or tidal volume for the target breathing cycle. The breathing rate and / or tidal volume of the target breathing cycle may, for example, be based on an ideal breathing cycle, or on a regular breathing cycle of the user. In some cases, the controller 102 may determine the breathing rate and / or tidal volume of the target breathing cycle so as to reduce the user’s ETCO2. For example, if the controller 102 determines that the user has an excessive breathing rate, the controller 102 may set the breathing rate of the target breathing cycle lower than the user’s breathing rate, to prompt the user to lower their breathing rate. Similarly, if the controller 102 determines that the user’s tidal volume is too low, the controller 102 may set the tidal volume of the target breathing cycle higher than the user’s tidal volume, to prompt the user to increase their tidal volume.

[0116] 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.

[0117] 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.

[0118] 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.

[0119] Any section headings used herein are for organizational purposes only and are not to be construed as limiting the subject matter described.

[0120] 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.

[0121] 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

Claims:1 . A method of operating a rebreather apparatus, the method comprising: receiving a first output signal from a pressure sensor in the rebreather apparatus, wherein the first output signal is indicative of variations in a gas pressure within the rebreather apparatus; receiving a second output signal from a carbon dioxide sensor in the rebreather apparatus, wherein the second output signal is indicative of carbon dioxide exhaled by a user via a mouthpiece of the rebreather apparatus; detecting, using the first output signal, a breathing cycle of the user; and determining, based on the breathing cycle and the second output signal, a value indicative of end-tidal carbon dioxide for the user.

2. A method according to claim 1 , further comprising generating an alert if the value indicative of the end-tidal carbon dioxide exceeds a predetermined threshold.

3. A method according to claim 1 or 2, further comprising detecting an end of an exhalation phase in the breathing cycle from the first output signal, wherein the value indicative of end-tidal carbon dioxide is determined from a value of the second output signal at the end of the exhalation phase.

4. A method according to any preceding claim, comprising detecting a sequence of breathing cycles of the user using the first output signal, wherein, for each breathing cycle in the sequence, a respective value indicative of end-tidal carbon dioxide for the user is determined using the second output signal.

5. A method according to claim 4, further comprising determining a trend of the end-tidal carbon dioxide over time, based on the respective value indicative of end-tidal carbon dioxide for each breathing cycle in the sequence; and optionally generating an alert if the trend fulfils a predetermined condition.

6. A method according to any preceding claim, further comprising receiving a third output signal from a from a second carbon dioxide sensor in the rebreather apparatus, wherein the third output signal is indicative of a partial pressure of carbon dioxide at an outlet of a carbon dioxide absorbent unit in the breathing apparatus.

7. A method according to claim 6, wherein the value indicative of end-tidal carbon dioxide is further based on a comparison between the second output signal and the third output signal.

8. A method according to any preceding claim, further comprising: determining, using the first output signal, a breathing rate of the user; and determining, based on the breathing rate, a risk of carbon dioxide retention for the user.

9. A method according to any preceding claim, further comprising: receiving a fourth output signal from a flow sensor in the rebreather apparatus, wherein the fourth output signal is indicative of a gas flow rate in the rebreather apparatus; and determining a tidal volume of the user based on the fourth output signal.

10. A method according to claim 9, further comprising determining a risk of carbon dioxide retention based on the determined tidal volume.

11. A method according to any preceding claim, further comprising generating a display output based on the value indicative of the end-tidal carbon dioxide.

12. A method according to claim 11 , wherein the display output comprises a visualisation having a colour determined based on the value indicative of the end-tidal carbon dioxide.

13. A rebreather apparatus, comprising: a mouthpiece; a breathing volume connected to the mouthpiece; a pressure sensor configured to detect variations in a gas pressure in the breathing volume, and to produce an output signal indicative of the variations in gas pressure; a carbon dioxide sensor configured to detect carbon dioxide exhaled from a user via the mouthpiece; and a controller configured to perform a method according to any preceding claim.

14. A rebreather apparatus according to claim 13, wherein the carbon dioxide sensor is located in the breathing volume on an exhale side of the mouthpiece.

15. A rebreather apparatus according to claim 13 or 14, wherein the carbon dioxide sensor has a T90 response time of 1 second or less.

16. A rebreather apparatus according to any of claims 13 to 15, wherein the carbon dioxide sensor has a T90 response time of 10 seconds or more.

17. A rebreather apparatus according to any of claims 13 to 16, further comprising: a carbon dioxide absorbent unit configured to absorb carbon dioxide present inside the breathing volume; and a second carbon dioxide sensor configured to detect a partial pressure of carbon dioxide at an outlet of the carbon dioxide absorbent unit, and to produce a third output signal indicative of the partial pressure of carbon dioxide.

18. A rebreather apparatus according to any of claims 13 to 17, further comprising a flow sensor configured to detect a gas flow rate in the breathing volume, and to produce a fourth output signal indicative of the gas flow rate.

19. A rebreather apparatus according to any of claims 13 to 18, further comprising a display unit configured to display an output based on the value indicative of the end-tidal carbon dioxide.

20. A controller for a rebreather apparatus, the controller having a processing unit and a memory storing instructions which, when executed by the processing unit, cause the processing unit to perform a method according to any of claims 1 to 12.

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