Method and system for delivering gas to a patient

Abstract

To provide a system for delivering a gas to a patient during a medical treatment.SOLUTION: The system comprises a heater arranged to heat at least one of a gas and a humidification liquid. The system comprises a controller 21 arranged to control the system according to a first mode during delivery of a first flow rate of the gas and a second mode during delivery of a second flow rate of the gas. The controller monitors electrical characteristics of the heater to select a mode of operation and / or to determine an operating state of the system.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present disclosure relates to methods, systems, and devices for delivering humidified gas to a patient. [Background technology]

[0002] A variety of medical procedures require the provision of a gas, typically carbon dioxide, to a patient during the procedure. For example, two broad categories of medical procedures that often require the provision of a gas to a patient include closed medical procedures and open medical procedures.

[0003] In closed medical procedures, an insufflator is positioned to deliver gas to a patient's body cavity during a medical procedure to inflate the cavity or prevent the cavity from collapsing. Examples of such medical procedures include laparoscopy and endoscopy, although the insufflator may be used with any other type of medical procedure as needed. Endoscopic procedures allow a physician to visualize a body cavity by inserting an endoscope or the like through a natural orifice or small puncture to generate an image of the cavity. In a laparoscopic procedure, a physician generally inserts surgical instruments through a natural orifice or small puncture to perform a surgical procedure within the body cavity. In some cases, an initial endoscopic procedure is performed to evaluate the body cavity, and then a subsequent laparoscopy may be performed to operate on the body cavity. Such procedures are widely used, for example, in the peritoneal cavity or during thoracoscopy, colonoscopy, gastroscopy, or bronchoscopy.

[0004] In open medical procedures, such as open surgeries, gas is used to fill the surgical cavity, with excess gas leaking out through the opening. Gas may also be used to provide a layer of gas over exposed internal body areas where there is no discernible cavity. In these procedures, gas may not serve to inflate the cavity, but may be used to prevent or reduce desiccation and infection by covering the exposed internal body area with a layer of heated, humidified, sterile gas.

[0005] Systems for delivering gases during these medical procedures may include an adjustable throttling pressure regulator and a gas flow controller. The systems generally include or are connected to a gas source, which may be a remote pressurized gas source, a gas cylinder, or the like, and operate to control the pressure and / or flow of gas from the gas source to a suitable level for delivery to a body cavity, usually through a cannula or needle inserted into the body cavity, or through a diffuser positioned to diffuse the gas over and into a wound or surgical cavity.

[0006] Body temperature is generally about 37°C, and it may be desirable for the temperature of the gas delivered from the system to closely match normal body temperature. Similarly, it may be desirable to increase the humidity level of the gas provided by the gas source, as it may be relatively dry and may cause damage to the body cavity, including cell death and / or adhesion. Therefore, a humidifier may be placed in the gas flow path to heat and humidify the gas before it enters the body cavity.

[0007] U.S. Patent No. 8,206,337 to Fisher & Paykel Healthcare Limited (incorporated herein by reference in its entirety) discloses a system including an infuser arranged to connect to a remote source of pressurized gas, such as may be provided via a hospital's gas supply system. The infuser delivers gas via tubing to a humidifier including a liquid container and a heater, which heats the liquid to produce vapor. The humidified gas is delivered to the patient via another tube, which may also be heated. In one example, the infuser and humidifier are located in separate housings connected together by suitable tubing and / or electrical connections. In another example, the infuser and humidifier are located in a common housing arranged to connect to a remote gas supply via suitable tubing.

[0008] Additionally, the same system may be used to provide respiratory assistance by delivering heated and / or humidified respiratory gas to a patient, typically via a gas delivery conduit connected to a patient interface, which may include, for example, a face mask or nasal cannula. Such systems generally include, or are arranged to be connected to, a humidifier that humidifies the gas before it is delivered to the patient. Summary of the Invention [Means for solving the problem]

[0009] For example, the same equipment, including an infuser and humidifier, may be used for closed medical procedures, open medical procedures, and respiratory assistance, but the operation of this equipment for each type of use may be very different. The present disclosure provides a system and method for controlling a gas supply and humidification system that automatically determines which type of use the system is being used for and adjusts its operation to suit the different conditions of each type of use. This operates to prevent user error or unintended harm to the patient.

[0010] For example, when a gas supply and humidification system is used in an open surgical procedure, a relatively high flow rate of gas is required and a correspondingly relatively large amount of liquid is consumed to maintain the desired water content to sufficiently humidify the gas.

[0011] When used in closed surgical procedures, a lower flow rate and / or more intermittent gas delivery, and a correspondingly smaller volume of liquid, is required.

[0012] External sensing probes, typically temperature and flow probes, may be used to enable the gas delivery and humidification system to control gas flow and humidity as appropriate for each type of treatment or use. The outputs of these probes may be used by the gas delivery system to control gas flow and humidity to suit the type of medical treatment or use. However, providing such probes can be expensive and it can be difficult for medical personnel to correctly connect the probes for proper operation. External probes also reduce the overall ease of use of the system and add yet another external part that can be lost or broken and must be sterilized after each use.

[0013] Additionally, other conditions may arise that can significantly affect the operation of the gas supply and humidification system. For example, if the flow of gas is restricted or stopped, this can create a number of problems for the rest of the system. For example, these problems may arise in systems that are supplied by a separate gas source, such as a gas bottle or cylinder, that runs out of gas. Also, the system may be significantly affected if a humidifier associated with the system runs out of liquid, for example, if all of the humidification liquid evaporates.

[0014] The present disclosure provides a control method and system for delivering gas to a patient that overcomes, or at least ameliorates, one or more of these drawbacks.

[0015] In one embodiment, a system is described for delivering gas to a patient during a medical procedure. The system includes or is arranged to include a heater configured to heat at least one of a gas and a humidification liquid contained in a humidification chamber. The system is operable according to a first mode, where the medical procedure is a first medical procedure in which the system delivers gas at a first flow rate, and according to a second mode, where the medical procedure is a second medical procedure in which the system delivers gas at a second, different flow rate. The system is configured to monitor an electrical characteristic of the heater and subsequently select either the first mode or the second mode in response to the monitored electrical characteristic.

[0016] The first medical procedure may be an open medical procedure during which, in a first mode, the system delivers a relatively high flow rate of gas.

[0017] The second medical procedure may be a closed medical procedure during which, in the second mode, the system delivers a relatively low flow rate and / or an intermittent flow of gas.

[0018] The flow rate of gas in either or each mode may be a constant flow of gas, a variable flow of gas, or an intermittent flow of gas. The desired flow rate of gas may be achieved by controlling a blower or pump and / or by controlling a control valve and / or regulator.

[0019] The monitored electrical characteristic may be the power drawn by the heater. For example, the monitored electrical characteristic may be the power duty cycle of the heater. The power duty cycle may be calculated, for example, as the percentage of time that the heater draws power. In one embodiment, the monitored characteristic may be the current drawn by the heater.

[0020] The heater's electrical characteristic may be monitored for a predetermined period of time and a calculated average value of the heater's electrical characteristic for that period. In one embodiment, the system may determine the operating mode by comparing the calculated average value of the heater's electrical characteristic to a predetermined threshold.

[0021] In one embodiment, in an initial step, the heater is turned on and allowed to warm up during a warm-up period, for example, the heater may be turned on and allowed to warm up before the electrical characteristics of the heater are monitored.

[0022] In one embodiment, a method of controlling components of a gas supply and humidification system is described, the method including monitoring a power duty cycle of a heater and subsequently determining a deficiency of humidification liquid in a humidification chamber by analyzing the monitored power duty cycle of the heater.

[0023] The power duty cycle and / or current draw of the heater may be monitored for a predetermined period of time and one or more average values calculated over the predetermined period of time, and the one or more average values are used to determine a deficiency of humidification liquid in the humidification chamber.

[0024] The short-term average power duty cycle and / or current draw of the heater may be calculated over a first predetermined period of time, and the long-term average power duty cycle and / or current draw of the heater may be calculated over a longer predetermined period of time. For example, in one embodiment, the long-term average is calculated over a window that is 2 to 10 times longer than the time window of the short-term average. Of course, other relative differences in window size may be used. The difference between the long-term average and the short-term average may be used to determine a lack of humidification liquid in the humidification chamber. In one embodiment, if the long-term average exceeds the short-term average by at least a first positive threshold amount, the level of humidification liquid in the humidification chamber is determined to be low.

[0025] Alternatively or additionally, the difference between the long-term and short-term average values of the heater power duty cycle and / or current draw may be used to determine whether humidification liquid has been added to the humidification chamber. In one embodiment, if the short-term average value exceeds the long-term average value by at least a second positive threshold amount, it is determined that humidification liquid has been added to the humidification chamber.

[0026] If it is determined that humidification liquid has been added to the humidification chamber, further monitoring of the heater power duty cycle and / or current may be delayed until a stabilization period has elapsed, thereby allowing the system to stabilize.

[0027] The system can activate an indicator if it determines that the humidification liquid is low in the humidification chamber.

[0028] In one embodiment, the system can determine whether the level of humidification liquid in the humidification chamber is low or empty at the start of treatment before monitoring the power duty cycle of the heater. This can be done by comparing at least one characteristic of the humidification system to at least one predetermined characteristic of the humidification system when a predetermined sufficient amount of humidification liquid is present in the humidification chamber. The characteristic compared can be, for example, the output of a temperature sensor associated with the heater. In another embodiment, the characteristic compared is the power duty cycle of the heater.

[0029] In one embodiment, before entering normal operating mode, the system detects whether gas is flowing through the system.

[0030] In one embodiment, a humidifier for providing humidified gas to a patient is described. The humidifier may include a humidification chamber and a heater that heats humidification liquid in the humidification chamber to generate vapor and humidify the gas delivered to the patient. The humidifier includes a controller that is arranged to monitor a power duty cycle of the heater, i.e., the percentage of time that the heater draws power, and subsequently, in response to the monitored power duty cycle of the heater, determine that the humidification liquid in the humidification chamber is depleted or insufficient.

[0031] In one embodiment, an electrical characteristic of the heater is monitored. A determination is made as to whether gas is flowing through the system based on the monitored electrical characteristic. In one embodiment, the electrical characteristic may be a current drawn by the heater. In one embodiment, the electrical characteristic is a power duty cycle of the heater. The electrical characteristic may be monitored for a predetermined period of time and an average value of the electrical characteristic calculated over the predetermined period of time. The average value of the electrical characteristic may be used to determine whether gas is flowing through the system.

[0032] In one embodiment, a short-term average value of the electrical property is calculated over a first predetermined period of time, and a long-term average value of the electrical property is calculated over a second, longer predetermined period of time. The difference between the long-term and short-term average values of the electrical property may be used to determine whether gas is flowing.

[0033] If the long-term average value of the electrical characteristic exceeds the short-term average value by at least a first positive threshold amount, it is determined that the gas flow rate has fallen below the normal operating range. The system can activate an indicator when it is determined that the gas flow rate has fallen.

[0034] If the short-term average value exceeds the long-term average value by at least a second positive threshold amount, the flow rate of the gas is determined to have increased.

[0035] In one embodiment, where the electrical properties of the heater are monitored, a calibration can be performed and a calibrated average value of the electrical properties calculated over a predetermined period of time, which can be used as a reference value to allow the electrical resistance of the heater to be varied.

[0036] In one embodiment, the heater is allowed to warm up before being turned on and the electrical characteristics of the heater are monitored. The warm-up process may occur at a calculated rate for a predetermined period of time to avoid the buildup of condensation within the system.

[0037] In one embodiment, before monitoring the electrical characteristics of the heater, the system determines whether a supply of gas has been brought on. If a supply of gas is not detected, the system can activate an indicator.

[0038] Further objects of the disclosed method, system, and apparatus will become apparent from the description that follows.

[0039] Several embodiments of the disclosed methods, systems and apparatus will now be described, by way of example only, with reference to the following drawings, in which: [Brief explanation of the drawings]

[0040] [Figure 1] 1 is a schematic diagram of an exemplary embodiment of a medical gas delivery system. [Figure 2] 1 is a schematic diagram of an exemplary embodiment of a medical gas delivery system. [Figure 3] 1 is a schematic diagram of an exemplary embodiment of a medical gas delivery system. [Figure 4] FIG. 1 is a flow diagram of an exemplary embodiment of a method for controlling a medical gas delivery system. [Figure 5] 5 shows further details of the flow diagram of FIG. [Figure 6] 5 shows further details of the flow diagram of FIG. [Figure 7] 5 shows further details of the flow diagram of FIG. [Figure 8] 5 shows further details of the flow diagram of FIG. [Figure 9] 5 shows further details of the flow diagram of FIG. [Figure 10] 5 shows further details of the flow diagram of FIG. [Figure 11] FIG. 1 is a flow diagram of an exemplary embodiment of a method for determining whether a medical gas delivery system is being used in an open or closed medical procedure. [Figure 12-1] 1 is a flow diagram of an exemplary embodiment of a method for controlling a medical gas delivery system in an open mode. [Figure 12-2] 1 is a flow diagram of an exemplary embodiment of a method for controlling a medical gas delivery system in an open mode. [Figure 13] 1 is a flow diagram of an exemplary embodiment of a method for controlling a medical gas delivery system in an open mode. [Figure 14] 10 is a table showing exemplary values for the heater plate setpoint temperature, measured heater plate temperature, and heater plate duty cycle during an embodiment of a warm-up period. [Figure 15] 10 is a table showing a comparison of potential condensation during different embodiments of the warm-up period. [Figure 16] 10 is a table showing a comparison of potential condensation during different embodiments of the warm-up period. DETAILED DESCRIPTION OF THE INVENTION

[0041] 1 and 2 are schematic diagrams of an exemplary embodiment of a humidification and gas delivery device 1. Device 1 includes a base unit 3 and a humidification chamber 5 removably mounted to base unit 3. Humidification chamber 5 includes a gas inlet 7 arranged to be connected to a gas source 9 via an inlet conduit 10 to deliver a gas, e.g., carbon dioxide, to chamber 5. Chamber 5 further includes a gas outlet 11 arranged to be connected to a gas delivery conduit 13 for delivery of humidified gas to a patient. A filter 12 (shown in FIG. 2) may be provided between gas source 9 and chamber 5 to filter the inlet gas.

[0042] In the embodiment of Figure 1, the end of the gas delivery conduit 13 includes a trocar 15 arranged to be connected to a patient for use in closed medical procedures such as endoscopy and laparoscopy. In the embodiment of Figure 2, the end of the gas delivery conduit 13 includes a diffuser 17 arranged to diffuse humidified gas into a patient's wound during an open medical procedure such as open surgery.

[0043] The device 1 includes, for example, a heater. The heater may include, for example, a heating plate on the base unit 3. The heater is configured to heat humidifying liquid in the chamber 5 to generate vapor. The humidifying liquid is typically, but not necessarily, water. Gas from the gas source 9 flows into the chamber 5 and passes over the heated humidifying liquid, thereby absorbing vapor and increasing the humidity level, before being delivered to the patient via the gas delivery conduit 13. Alternatively or additionally, the chamber 5 may include an integral heater or a heater disposed within the chamber 5.

[0044] The gas delivery conduit 13 may also include or be equipped with a heater. A heater for the gas delivery conduit 13 may ensure that the gas temperature is maintained at a desired level along the conduit 13 and minimize or eliminate the formation of condensation. The heater for the gas delivery conduit 13 may have resistance wire provided or attached to the conduit 13, or a wire or other heating element provided within the conduit 13. The heater for the gas delivery conduit 13 may be electrically connected to the base unit 3 or chamber 5, for example, by an electrical cable 19, to provide power to the heater. Additionally or alternatively, the conduit 13 may be thermally insulated.

[0045] The device 1 includes a controller 21, which is arranged to control the device 1, particularly the flow rate, temperature, and humidity of the gas delivered to the patient, as appropriate for the type of medical procedure for which the device is being used. Thus, the controller 21 controls, among other things, the heater for the humidification chamber 5 and / or the heater for the gas delivery conduit 13, if provided. The controller 21 may also control a regulator that adjusts the flow rate of gas through the device 1. The regulator may include a flow inducer and / or restrictor, such as an electric fan. Additionally or alternatively, valves and / or vents may be used to control the flow rate. The controller 21 may include an electronic controller, which may be, for example, microprocessor-based. The system may include memory and any electronic components capable of performing calculations, as will be understood by those skilled in the art.

[0046] 3 is a schematic diagram of an embodiment of apparatus 1. Apparatus 1 may include, for example, a user interface 301, a controller 21, a conduit heater 313, a chamber heater 315, and a gas flow regulator 317. User interface 301 may be used to operate controller 21. Controller 21 may provide electrical control signals to conduit heater 313, chamber heater 315, and, in some embodiments, gas flow regulator 317 to control the operation of those devices as described elsewhere herein. Controller 21 may also control gas source 309 and / or injector 319. In some embodiments, the injector controls the flow, and apparatus 1 responds to that flow. In some embodiments, a gas container may provide a flow that may be controlled by a gas flow regulator or valve system.

[0047] The device 1 is controlled so that it can operate in at least two modes.

[0048] In a first mode, for example for use in an open medical procedure such as open surgery, the controller 21 is configured to control a relatively high gas flow rate through the device 1. Accordingly, the controller 21 controls the device 1 to generate a relatively large amount of vapor to ensure that the relatively high volumetric flow rate of gas is adequately humidified.

[0049] In a second mode, for use in a closed medical procedure, such as an endoscopic or laparoscopic procedure, the controller 21 is arranged to control a relatively low gas flow rate through the device 1. Accordingly, the controller 21 controls the device 1 to produce a relatively small amount of steam to ensure that the relatively low volumetric flow rate of gas is not overly humidified.

[0050] The controller 21 is configured to at least (1) monitor one or more electrical characteristics of the one or more heaters in use, (2) generate an output used to control the operating mode of the device 1, (3) detect the flow of gas through the device 1, and / or (4) detect a water-out condition in the humidification chamber 5. A water-out condition occurs when the humidification liquid in the humidification chamber 5 is depleted or insufficient.

[0051] In one embodiment, the controller 21 is arranged to monitor electrical characteristics of one or more heaters, such as power drawn, current drawn, or power duty cycle, to determine the type of medical procedure the device 1 is being used in, and subsequently control the device 1 according to a first or second mode.

[0052] In one embodiment, the controller 21 is configured to monitor one or more heater electrical characteristics, such as power drawn, current drawn, resistance, or power duty cycle, and use one or more of these electrical characteristics to determine the presence or absence of gas flow through the device 1 and subsequently activate a low or zero gas flow indicator.

[0053] In one embodiment, the controller 21 is configured to monitor an electrical characteristic of one or more heaters, such as power drawn, current drawn, or power duty cycle, to determine if the humidification liquid in the chamber 5 is depleted or insufficient. If low or depleted humidification liquid is detected, the controller 21 activates a low or depleted humidification liquid indicator. In one embodiment, under such conditions, the controller 21 may deactivate at least a portion of the device 1. For example, the controller 21 may deactivate the chamber heater 315, the conduit heater 313, and / or the flow director or regulator 317.

[0054] As described above, the device 1 can be used in a first mode suitable for open medical procedures, such as open surgery. In the first mode, the flow rate of gas delivered to the patient can be relatively high and substantially constant, for example, about 10 L / min. The flow rate can be adjusted either automatically or manually. The constant flow rate can be determined automatically or manually. The constant flow rate can be set according to the specific patient's requirements. The flow rate can be controlled to rise from a lower initial level and then reach a desired constant level.

[0055] The device 1 may also be used in a second mode suitable for closed medical procedures such as endoscopy and laparoscopy. In the second mode, a lower flow rate of gas is typically delivered to the patient, and in one example, may be delivered as a relatively low, pulsed flow rate rather than a constant low flow rate. Additionally, there may be periods of time when the delivered flow is very low or no flow is delivered.

[0056] In each mode, the humidity of the delivered gas is controlled and can be controlled to reach a minimum threshold, which in one example is about 30 mg of humidifying liquid per liter of gas.

[0057] Before entering any mode, the controller controls the device 1 according to a warm-up procedure, which may be arranged to preheat one or more heaters of the device 1 for a given period of time.

[0058] For purposes of the following description, reference to the heater refers to the chamber heater 315 (as shown in FIG. 3). Note, however, that the heater may alternatively or additionally refer to the conduit heater 313, if provided.

[0059] 4 is a flow diagram of an embodiment of a method used by controller 21 to determine the operating mode. The system is powered up and allowed to warm up in warm-up process 401. Warm-up process 401 may include, for example, providing power to a heater until a predetermined threshold temperature is reached. After warm-up process 401, the system moves to mode detection process 403. Mode detection process 403 determines the type of medical procedure in which device 1 is being used. Mode detection process 403 may be used as part of or to initiate subsequent control processes, including, for example, closed mode process 405 and open mode process 407.

[0060] 5 shows the warm-up process 401 in detail. The system is powered on at 501. At 503, the heater receives power during the warm-up process. Then, at 505, the system determines whether a sufficient warm-up period has elapsed. If not, the system continues the warm-up process at 503. If a predetermined warm-up period n1 has elapsed, the process moves to the mode detection process 403.

[0061] The mode detection process 403 determines the type of medical procedure in which the device 1 is being used. The gas flow requirements for open medical procedures, such as open surgery, are relatively high; therefore, the temperature of the humidification chamber heater and / or the temperature of the gas delivery conduit heater must also be high to maintain a relatively high gas flow at the desired humidity level. One or more heaters may be controlled to a setpoint temperature sufficient to provide gas at the required humidity level. One or more temperature sensors may be provided to measure the temperature of the one or more heaters. This measurement is used by the controller 21 to maintain the desired setpoint temperature.

[0062] 6, the mode detection process 403 determines whether an open medical procedure, such as open surgery, is being performed by monitoring the electrical characteristics of the heater, such as the power drawn by the heater. Relatively high gas flow rates, such as those used during open medical procedures, require more power to the heater than lower gas flow rates, such as those used during closed medical procedures.

[0063] In one embodiment, the mode detection process 403 monitors 601 one or more electrical characteristics of the heater, such as, for example, the heater's power duty cycle, i.e., the percentage of time the heater is drawing power. The one or more electrical characteristics are monitored for a predetermined period n2 for use in the determination in 603. In one embodiment, the one or more electrical characteristics are averaged over that period. The one or more electrical characteristics, or the one or more electrical characteristics averaged over the period n2, are then compared to a specified threshold in 605. If the one or more electrical characteristics, or the averaged one or more electrical characteristics, exceed the threshold, the controller 21 determines that the device 1 is being used in an open medical procedure, such as open surgery, and controls the device 1 in accordance with the open mode process 407.

[0064] If the one or more electrical characteristics, or the average one or more electrical characteristics, are below the threshold, the mode detection process 403 continues to monitor the one or more electrical characteristics for an extended period n3 to be used in the determination at 607. After the extended period n3, the one or more electrical characteristics, or the average one or more electrical characteristics, are again compared to the threshold, and if still below the threshold, the controller 21 controls the apparatus 1 according to the closed mode process 405. In the closed mode process 405, the gas flow can be controlled at a relatively low constant flow rate or an intermittent or pulsed flow rate.

[0065] Figure 7 illustrates an embodiment of the open mode process 407. The open mode process 407 may include an additional warm-up process 701. As shown in Figure 8, the additional warm-up process 701 includes applying additional power to the heater for a predetermined period of time n4 at 801 until the additional warm-up period expires.

[0066] Once the additional warm-up process 701 is complete, the system continues to monitor 703 one or more electrical characteristics for a predetermined period of time. In one embodiment, the predetermined period is 40 minutes. In 703, the one or more electrical characteristics may include, for example, duty cycle and / or current drawn by the heater. The system then performs a flow detection process 705 and / or a liquid level detection process 707, as described in further detail herein.

[0067] The voltage supplied to the heater is made constant by the power supply of the device 1, and therefore the current drawn by the heater varies depending on the amount of heat the heater needs to generate to maintain the heater temperature at the set point temperature determined by the controller 21. The heater temperature is measured by a temperature sensor in the heater.

[0068] In one embodiment, the flow detection process 705 and the liquid level detection process 707 are implemented only as part of the open mode process 407. However, the flow detection process 705 and / or the liquid level detection process 707 may also or instead be implemented as part of the closed mode process 405.

[0069] In one embodiment, flow and / or temperature sensors may be incorporated into any component of device 1 to provide additional data. For example, a temperature sensor may be used to determine the temperature of the humidification liquid in humidification chamber 5.

[0070] In an embodiment, the power or current drawn by the heater may be used to determine and / or monitor ambient conditions and to calibrate the device 1.

[0071] 9 shows additional details of the flow detection process 705. In one embodiment, the heater's electrical characteristics, such as the power duty cycle and the current drawn, are continuously averaged over two different time periods. These time periods include a short-term and a long-term. The long-term average value L AVE is determined in 901, and the short-term average value S of the same electrical characteristic AVE is determined at 903. Long-term and short-term average values may be determined for all of one or more electrical characteristics, including, for example, the heater's power duty cycle and current drawn. Thus, for each electrical characteristic of the heater, the controller 21 calculates a short-term and a long-term average value. In one embodiment, these characteristics may be monitored during the warm-up procedure 701 and / or after the warm-up procedure 701 is completed.

[0072] In one embodiment, the flow detection process 705 is initiated after the warm-up procedure 701 is completed (as shown in FIG. 7), i.e., after the heater power duty cycle and drawn current have been averaged for a predetermined period of time. Alternatively, as described above, the heater power duty cycle and drawn current may be averaged for a predetermined period of time after the warm-up process is completed as part of the flow detection process 705.

[0073] The flow detection process 705 determines whether gas is flowing by comparing the difference between the long-term average and short-term average values of the heater current drawn to a threshold value. While the described embodiment includes one particular type of difference comparison, i.e., subtracting the short-term average value from the long-term average value and comparing the result to a threshold value, other types of difference comparisons can also be used; for example, the long-term average value can be subtracted from the short-term average value and compared to a negative threshold value. When using different types of measurements to compare similar data, those skilled in the art will understand that the determination steps in the described embodiment will be modified accordingly. Therefore, this embodiment, and all embodiments described herein, are provided by way of example and not by way of limitation.

[0074] If the difference is less than the threshold value τ1 at 905, the flow detection process 705 moves to the liquid level detection process 707. If the difference is greater than or equal to the threshold value τ1 at 905, the flow detection process 705 may determine low or no gas flow at 907 and activate a low gas flow indicator. This is because too low or no gas flow is undesirable as it can adversely affect the medical procedure being performed and cause excessive humidity, which can impair visual clarity in the body cavity for the physician.

[0075] If the difference between the long-term and short-term average values calculated at 905 is near zero, this indicates that gas flow is constant. However, if the difference is not near zero, this may indicate that gas flow has decreased and / or been stopped, or that it has increased, depending on whether the difference is positive or negative. The flow detection process 705 is further operable to determine whether gas flow is increasing, for example, when the device 1 is initially connected to a gas source or when connected to a new or replacement gas source. The determination again begins at 909, determining the difference between the calculated long-term and short-term average values of drawn heater current. If the difference (calculated by subtracting the short-term average value from the long-term average value) is negative, i.e., less than zero, the flow detection process 705 determines that gas flow has been added and adjusts the settings of the device 1 accordingly. The system then waits a predetermined period for the settings of the device 1 to take effect before returning to monitoring the long-term and short-term average values at 905. If the difference is not negative, the process returns to monitoring flow out at 907.

[0076] In one embodiment, the power drawn by the heater may be used to detect gas flow using the heater power duty cycle instead of the heater current drawn.

[0077] In one embodiment, an alternative process for detecting gas flow may be used that requires only a single average value of the heater current drawn. This alternative method involves first calibrating the device 1 to determine a threshold above which gas flow will not be provided, measuring the heater resistance, and subsequently using a look-up table to determine the voltage corresponding to the measured resistance of the heater wire.

[0078] In one embodiment, the device 1 can be used to detect low or no gas flow, for example, starting by calibrating the heater. Thus, once the device 1 is warmed up, a threshold or cutoff value can be used to detect whether there is no flow.

[0079] Liquid level detection process 707 helps ensure that dry gas is not delivered to the patient. An embodiment of the operation of process 707 is shown in FIG.

[0080] The liquid level detection process 707 may be initiated once the common initial warm-up procedure 701 is complete, or after the flow detection process 705 determines that there is gas flow, or independently of either condition.

[0081] The liquid level detection process 707 detects the long-term average value L of an electrical characteristic of the heater, such as the power duty cycle of the heater. AVE and the short-term average value S AVE It determines whether there is humidification liquid in the humidifier chamber by comparing the difference between the long-term average value and the short-term average value. If, in step 1010, the difference between the long-term average value and the short-term average value is greater than or equal to threshold value τ2, then liquid level detection process 707 may determine that there is no humidification liquid or only a low level of humidification liquid in chamber 5 and activate a water outflow indicator in step 1012. The above discussion regarding using multiple types of difference comparisons also applies here.

[0082] The liquid level detection process 707 may also be configured to determine, in step 1014, whether the difference, again calculated by subtracting the short-term average value from the long-term average value of the heater current, is less than a predetermined threshold value τ3. If the difference is below the threshold, this indicates that humidification liquid is present in chamber 5. If humidification liquid is determined to be present in the humidification chamber, further monitoring of the heater power duty cycle and / or current drawn may be delayed in step 1016 until a stabilization period has elapsed, allowing the system to stabilize. Thus, the liquid level detection process 707 may also use this method to detect whether humidification liquid has been added. As noted above, other types of calculations may be performed using similar data, and the embodiments described herein are not intended to be limiting.

[0083] The use of long-term and short-term average values of the heater power duty cycle and / or heater current drawn helps minimize any variations that may occur under different environmental conditions. This may include, for example, the use of different types of humidification chambers or heating plates, power fluctuations, and / or changes in air temperature. The use of long-term and short-term average values also allows for calibration of the system.

[0084] The controller 21 may be configured to detect whether the chamber 5 is empty at the start of a medical procedure by having a predetermined profile of the system's electrical characteristics relative to the level of humidification liquid in the chamber 5. The profile may be determined, for example, by experimentation or by using pre-stored data. Thus, the controller 21 may perform an initial monitoring step of monitoring the system's electrical characteristics during use. The controller 21 may, for example, determine whether the chamber 5 is empty by monitoring the heater current drawn during use and comparing it to a predetermined profile of the heater current drawn relative to the level of humidification liquid in the chamber 5. If the chamber 5 is empty, the heater temperature may overshoot or become unstable.

[0085] The controller 21 may further be configured to detect the flow of gas entering the device, since, as mentioned above, excessive humidity in the system may impair optical clarity to the physician, and the controller may therefore be arranged to initially control the generation of humidified steam depending on the detected gas flow.

[0086] Alternatively or additionally, liquid level detection may be performed using optical sensing, capacitance, or other methods.

[0087] Alternate embodiments may include any additional components as desired.

[0088] The above-described processes have been described with respect to a medical gas delivery system for use in a medical or surgical procedure, such as during open or closed surgery, or during closed medical procedures such as endoscopy or laparoscopy. However, it will be appreciated that the described flow detection process 705 and liquid level detection process 707 may equally be used in any other type of gas delivery system designed to deliver heated and / or humidified gas to a patient. For example, the system may instead include a respiratory assistance device arranged to deliver respiratory gas to a patient to assist with breathing.

[0089] FIG. 11 illustrates another embodiment of a process for determining whether an open procedure (high flow rate) or a closed procedure (low flow rate) is being performed by a gas supply system. Process 1100 begins at 1101, where the heating plate (HP) is powered on at time zero (0). At 1103, the heating plate is warmed up to a variable temperature setpoint n1 based on the elapsed time since the warm-up process began. This has the effect of warming the heating plate at a predetermined rate so that the system does not heat up too quickly. As discussed below with reference to FIGS. 14-16, heating the system too quickly increases condensation within the system. Therefore, it is advantageous to provide a warm-up process that achieves the desired temperature setpoint without undue delay while reducing condensation. In one embodiment, the warm-up process lasts for a predetermined period T1, as confirmed at 1105. In one embodiment, period T1 is 15 minutes. The warm-up period may also be passed through an initial temperature and air temperature and / or humidity level determined for the system.

[0090] Once the initial warm-up period is complete, the process moves to 1107 and 1109, where the process checks whether the temperature of the heater plate is equal to a predetermined laparoscope setpoint temperature for a predetermined period of time, T2. In one embodiment, the predetermined period of time, T2, is 21 minutes. The process then moves to 1111 and 1113, where the process monitors an average value of the heater plate duty cycle. The average value of the heater plate is determined over a third period of time, T3. In one embodiment, T3 is 25 minutes. The process then checks whether the previously determined average value of the heater plate duty cycle exceeds a predetermined duty cycle, n2, at 115. In one embodiment, n2 is equal to 5. If the average value duty cycle does not exceed n2, the system checks at 1119 whether it has already monitored the duty cycle for period T4. If not, the process continues to monitor the average value duty cycle. In one embodiment, T4 is 37 minutes. If the time exceeds T4 at 1119, the process moves to 1121 where the process determines that the respiratory assist system is in laparoscopic mode. If the average value duty cycle exceeds n2 at 1115, the process moves to 1117 where the process determines that the respiratory assist system is in open (or high flow) mode. At this point, the system moves to the open mode process described in FIG. 12.

[0091] Figure 12 shows an embodiment of an open (or high flow) process. This process recognizes that the history of several electrical properties of the system is an important factor in determining the process and current state of the respiratory assistance device. Therefore, the open mode process of Figure 12 involves using "Integrals," which are running summations of several electrical properties over time that are held under several conditions.

[0092] For example, in FIG. 9 (and related figures), in step 905, the difference between the long-term average and short-term average values of the system's electrical characteristics, such as the heater current drawn, is used to determine the flow and / or water out condition or the flow and / or water in condition.

[0093] The open mode process of FIG. 12 involves an alternative, more robust comparison of the difference over time between long-term and short-term average values using an integral or running sum of the differences. Integral = Integral + (L AVE -S AVE ) Equation 1

[0094] This more robust comparison can help reduce random fluctuations that may occur in the system. It can be used for both water runoff detection and stream runoff detection. Equation 1 is used, for example, in step 1231 shown in FIG. 12C. (L AVE -S AVE ) falls within a specified range, Integral is set to zero. Once Integral falls outside the specified range, information about flow and / or water outflow conditions or flow and / or water inflow conditions can be provided. The system can also turn off the heater wires to prevent hot dry gas from being delivered if no water is detected in the chamber.

[0095] Another alternative formula for comparing the difference utilizes both an upper and lower bound. Condition:((L AVE -S AVE ) <a)&((L AVE -S AVE )>b) If true: Integral=0 If not true: Integral = Integral + (L AVE -S AVE ) Equation 2

[0096] The values of variables a and b are determined by experiment. Once the Integral value reaches its set upper or lower limit, flow or water inflow / outflow detection is determined.

[0097] Referring to the process illustrated in FIG. 12 (divided into FIG. 12-1 and FIG. 12-2), the process begins at 1201 once it is determined that the respiratory assistance system is in open mode. At 1203, the process begins a separate open mode warm-up process. The warm-up process continues at 1205 until it is determined that a predetermined warm-up period T1 has elapsed. In one embodiment, T1 is 5 minutes. The process then moves to 1207, where various electrical characteristics are monitored. These characteristics may include those described above with respect to FIGS. 7-9 and may also include heater wire resistance. In the embodiment of FIG. 12, short-term and long-term average values of the heater wire resistance are calculated, in addition to short-term and long-term average values of the heater plate duty cycle. These characteristics are monitored at 1209 for a period T2. In some embodiments, T2 is 10 minutes. The process then moves to 1211, where the difference between the long-term and short-term heater wire resistance values is analyzed to determine whether the difference falls within a specified range, such as, for example, -0.085 to 0.35. In an alternative embodiment, the specified range may be -0.05 to 2.0. If the difference between the average long-term and short-term heater wire resistance values falls within this range, the process moves to 1227, where the heater wire Integral is set to zero. If the difference between the average long-term and short-term heater wire resistance values falls outside the aforementioned range, the process moves to 1213, where the heater wire Integral is updated to be the previous (or initial) heater wire Integral plus the difference between the average long-term and short-term heater wire resistance values. This process allows for monitoring of the system over a period of time so that small blips or inconsistencies in the monitored data do not unnecessarily trigger alarm events. After 1213, the process moves to 1215 where the updated heater wire Integral is analyzed to determine if the Integral falls within a certain predetermined range, such as, for example, −500 to 200. If the updated heater wire Integral falls within this range, the process moves to 1229, described below.If the heater line Integral does not fall within this range, if the heater line Integral is below a threshold, e.g., −500, at 1217, a flow out alarm is activated at 1219 and the process returns to 1211. If the heater line Integral is above a threshold, e.g., 200, at 1221, the process turns off the flow back in alarm at 1223 and waits for a predetermined time T3, e.g., 30 minutes, before returning to 1211.

[0098] Returning to step 1227, once the heater wire Integral has been zeroed, the process moves to step 1229, where the difference between the long-term and short-term average values of the heater plate duty cycle is analyzed to determine whether the average value falls within a predetermined range. This range may be, for example, -3.5 to 2.5. If the difference between the short-term and long-term average values of the heater plate duty cycle falls within this range, the heater plate Integral is set equal to zero in 1230, and the process returns to step 1211. If the difference between the short-term and long-term average values of the heater plate duty cycle does not fall within the above range, the process moves to 1231, where the heater plate Integral is updated to be equal to the current heater plate Integral value plus the difference between the short-term and long-term heater plate duty cycle averages. The process then moves to 1233, where the updated Hotplate Integral is analyzed to determine whether it falls within a second range, e.g., -200 to 1000. If the Hotplate Integral falls within this range, the process returns to step 1211. If the Hotplate Integral does not fall within this range at 1233, the process moves to step 1235, where if the Hotplate Integral exceeds a predetermined threshold, e.g., 1000, the process determines that water is flowing back in and displays an alarm at 1237. The process then returns to step 1211. If the Hotplate Integral falls below a threshold, e.g., -200, at 1237, the process determines that water is flowing back in and turns off the water flow alarm at 1241. The system then waits for a predetermined period T4, e.g., 30 minutes, and then returns to step 1211.

[0099] FIG. 13 illustrates another embodiment of the open mode process, including a determination of faster water return. The process begins at 1301 once it is determined that the respiratory assistance system is in open mode. At 1303, the process begins a separate open mode warm-up process. The warm-up process continues at 1305 until it is determined that a predetermined warm-up period T1 has elapsed. In one embodiment, T1 is 5 minutes. The process then proceeds to 1307, where various electrical characteristics are monitored. These characteristics may include, for example, short-term and long-term average values of the heater plate duty cycle. Alternatively or additionally, any of the characteristics are described above with respect to FIGS. 7-9. These characteristics are monitored at 1309 for a period T2. In some embodiments, T2 is 10 minutes. The process then proceeds to 1311, where the difference between the long-term and short-term heater plate duty cycles is analyzed to determine whether the difference falls within a specified range or exceeds a threshold. As shown in FIG. 13, if the long-term average value of the hotplate duty cycle minus the short-term average value is less than a specified threshold, X2, the process moves to step 1313, where the hotplate Integral is set equal to zero in 1313, and the process returns to step 1311. The threshold, X2, may be a nominally positive value selected to avoid hysteresis. Steps 1311 and 1313 may be considered normal operation where power usage is relatively constant. If the difference is greater than or equal to the specified threshold, X2, in 1311, the process moves to step 1315, where the hotplate Integral is updated to equal the current hotplate Integral value plus the difference between the long-term and short-term hotplate duty cycle averages, as shown in FIG. 13. The Integral represents the total amount of power usage, and is analyzed in step 1317 to determine if the power usage has entered a certain total amount. The updated heater plate Integral is analyzed to determine if it falls within a second range at 1317. For example, if the Integral is less than or equal to the threshold X3 at 1317, the process returns to normal operation at 1311.If Integral is greater than or equal to X3 at 1317, the process returns to step 1311. At step 1319, the process determines whether the most recently calculated short-term average heater plate duty cycle is approximately equal to the most recent short-term average heater plate duty cycle. If so, the system activates a water and / or flow out status indicator, which may be a toggled or illuminated LED. The process then waits for a period of T3 at 1321 while activating the indicator at 1323. Once time T3 has elapsed, the process moves to 1325, where the process continues to activate the water and / or flow out indicator until the process determines that water and / or flow has returned to the system. At 1325, the process determines the most recent short-term average heater plate duty cycle and subtracts the most recent short-term average duty cycle. If the result is greater than or equal to threshold X4, the process determines that water and / or flow has not returned and continues to activate the indicator at 1327. If the result is greater than or equal to threshold X4 at 1325, the process moves to 1329 where the system turns off the indicator because it has determined that water and / or flow has been added back in. The system then waits at 1331 for a period T4 to allow the system to stabilize before returning to normal operating mode at 1311.

[0100] As will be appreciated by those skilled in the art, the periods and thresholds of the above described embodiments may be selected according to various criteria to provide systems exhibiting various advantages and disadvantages. For example, short wait periods and narrow thresholds may speed up various processes and systems, but may come at the expense of reliability. Thus, the above examples are provided by way of illustration, not limitation.

[0101] FIG. 14 is a graph illustrating an embodiment of controlling the temperature of a heating plate during a first operating period as a means of providing controlled humidity delivery. This process may be implemented for any and / or all of the warm-up processes described in this disclosure. FIG. 14 illustrates the heating plate set point 1403, the heating plate temperature 1405, and the heating plate duty cycle 1407. This controlled humidity delivery process may be used to improve optical clarity for the surgeon. By controlling the level of humidity delivered, cooler surgical equipment has an opportunity to warm up to the same temperature as the gas, reducing the formation of condensation on the equipment. In some embodiments, a linear equation can be used to adjust the heating plate set point temperature based on the initial set point temperature. The formula is: Setpoint temperature = slope * time + initial setpoint temperature Equation 3 is.

[0102] In the embodiment shown in Figure 14, the initial set point temperature is 21°C and the slope is 1 / 60, with the set point temperature increasing at a rate of 1°C per minute for a time value measured in seconds.

[0103] If the starting hotplate temperature, i.e., the temperature of the hotplate when the system is first turned on, is below the initial setpoint temperature, the system continuously supplies power to the hotplate to reach the initial setpoint temperature as quickly as possible, and then continues as defined by Equation 3. If the starting hotplate temperature is above the initial setpoint temperature, the system does not supply power to the hotplate until the setpoint temperature equals the hotplate temperature. This process continues for a fixed time, e.g., 15 minutes. For example, if the starting hotplate temperature is 30°C, the hotplate will not begin heating the liquid until time = 9 minutes, but will then continue according to Equation 3. Depending on the starting hotplate temperature, the system will only begin supplying power to the hotplate at a time and rate determined by Equation 3, so that the hotplate temperature reaches the desired temperature at the end of the warm-up period.

[0104] After a designated warm-up period, the temperature of the heating plate reaches a temperature that ensures optimal humidity output for the type of procedure being performed. In one embodiment, the desired temperature is 43°C for laparoscopy or 53°C for open surgery.

[0105] FIGS. 15 and 16 show comparative examples of condensation reduction using the processes described in this disclosure. FIG. 15 illustrates the effects of condensation in a system where the warm-up process is not controlled; rather, as is typical of prior art humidification systems, the heater plate is warmed up as quickly as possible. As shown in FIG. 15, the heater plate's temperature 1505 is quickly brought to its operating temperature of approximately 43° C. The water chamber temperature 1507 rises quickly with the heater plate temperature, but the internal cannula temperature 1511 lags significantly behind the heater plate and chamber. Therefore, the potential for condensation, shown as the temperature difference region 1503 between the chamber temperature and the cannula temperature, is large. FIG. 16, on the other hand, illustrates the potential condensation reduction using the warm-up procedures described in this disclosure. FIG. 16 also shows the heater plate temperature 1605; however, in this example, the heater plate's temperature rise during the warm-up process is specifically controlled to increase at a slower rate before reaching its operating temperature of approximately 43° C. This can result in a controlled, but equally elevated, chamber temperature. As a result of this controlled warm-up process, the temperature difference between the internal cannula temperature 1611 and the chamber temperature 1607 is much smaller during the warm-up period. This reduces the likelihood of condensation 1603.

[0106] From the foregoing, it can be seen that a medical gas delivery apparatus and method are provided that can, at least in part, control the operating mode of the apparatus or activate an indicator of zero or reduced gas flow and / or zero or low levels of humidification liquid. This is done by analyzing the electrical characteristics of the humidifier heater and / or delivery gas conduit heater without requiring the use of external temperature or flow sensor probes. That is, the response, or change in response, exhibited by one or more of the heaters can be used to determine the occurrence of a particular event and / or control the apparatus accordingly.

[0107] Unless the context clearly requires otherwise, throughout the description, the words "comprise", "comprising", etc. are to be construed inclusively, i.e., "including but not limited to", as opposed to exclusive or exhaustive.

[0108] While the disclosed apparatus and method have been described by way of example and with reference to contemplated embodiments thereof, it should be understood that modifications or improvements may be made thereto without departing from the scope of the present disclosure. The disclosed apparatus and method may also be broadly referred to in any or all of the parts, elements, and features referred to or shown in the specification of this application, individually or collectively, as consisting of said parts, elements, and features. Furthermore, when reference is made to particular components or entire parts of the disclosed apparatus and method known to be equivalent, such equivalents are incorporated herein as if individually set forth.

[0109] Any discussion of prior art throughout this specification is not to be taken as an acknowledgement that such prior art is widely known or forms part of the common general knowledge in the field.

Claims

1. 1. An apparatus for delivering gas to a patient during a medical procedure, comprising: the apparatus comprises a humidifier; The humidifier comprises: a heater for heating the humidifying liquid in the humidifying chamber; a controller configured to perform an initial heater warm-up process, control the heating of the heater at a predetermined or calculated rate before the heater reaches its operating temperature, and select an operating mode from a plurality of operating modes including at least a first mode associated with an open medical procedure and a second mode associated with a closed medical procedure by monitoring an electrical characteristic of the heater; The apparatus wherein the operating mode is selected in response to the monitored electrical characteristic without using input from external temperature or flow sensors.

2. The apparatus of claim 1 , wherein the controller is further configured to perform the initial heater warm-up process for a predetermined period of time.

3. The apparatus of claim 2 , wherein the predetermined period is 5 minutes, 10 minutes, or 15 minutes.

4. The apparatus of claim 1 , wherein the controller is further configured to perform the initial heater warm-up process at the calculated rate for a predetermined period of time.

5. 5. The apparatus of claim 1, wherein performing the initial heater warm-up process comprises providing power to the heater until the heater reaches its operating temperature and / or until a predetermined period of time expires.

6. The apparatus of any one of claims 1 to 4, wherein performing the initial heater warm-up process comprises heating the heater at 1°C per minute.

7. The controller adjusts the heater setpoint temperature based on an initial setpoint temperature using the following equation: Setpoint temperature = slope x time + initial setpoint temperature The apparatus of any one of claims 1 to 4, further configured to perform heater temperature control for the initial heater warm-up process according to:

8. 8. The apparatus of claim 7, wherein the initial setpoint temperature is 21°C and the slope is 1 / 60, such that the setpoint temperature increases at a predetermined rate of 1°C per minute for a time value measured in seconds.

9. 5. The apparatus of claim 1, wherein the apparatus determines whether a predetermined warm-up period has expired or whether a predetermined temperature has been reached, and if the determination is affirmative, initiates a mode detection process, otherwise continues the initial heater warm-up process.

10. The apparatus of claim 9 , wherein the mode detection process initiates a subsequent control process.

11. The apparatus of claim 10 , wherein the subsequent control process comprises a flow detection process and / or a liquid level detection process.

12. The apparatus of any one of claims 1 to 4, wherein the controller is further configured to monitor an electrical characteristic of the heater during the initial heater warm-up process.

13. 5. The apparatus of claim 1, wherein upon completion of the initial heater warm-up process, the controller is further configured to determine whether the temperature of the heater is equal to a predetermined temperature for a second predetermined period of time.

14. 14. The apparatus of claim 13, wherein the second predetermined period is 21 minutes.

15. 5. The apparatus of claim 1, wherein the controller is further configured to perform an additional warm-up process comprising applying additional power to the heater for a predetermined period of time.

16. The apparatus of any one of claims 1 to 4, wherein the controller is further configured to perform the initial heater warm-up process to achieve a desired temperature set point while reducing condensation.

17. 5. The apparatus of claim 1, wherein the controller is further configured to switch on the heater and allow it to warm up before an electrical characteristic of the heater is monitored.

18. 5. The device of any one of claims 1 to 4, wherein the device is configured to deliver humidified gas during an open or closed medical procedure, which is an endoscopic or laparoscopic medical procedure using surgical instruments.

19. 19. Apparatus according to any one of the preceding claims, wherein the humidifier comprises a humidification chamber.

20. 1. A system for controlling the delivery of gas to a patient during a medical procedure, comprising: A system comprising an apparatus according to any one of claims 1 to 19.

21. A method of controlling a system arranged to deliver humidified gas, comprising: a controller executing an initial heater warm-up process to warm up a heater for heating humidification liquid in a humidification chamber of the humidifier; the controller controlling the heating of the heater at a predetermined or calculated rate during the initial heater warm-up process before the heater reaches its operating temperature; the controller selecting an operating mode from a plurality of operating modes including at least a first mode associated with an open medical procedure and a second mode associated with a closed medical procedure by monitoring an electrical characteristic of the heater; A method wherein the operating mode is selected in response to the monitored electrical characteristic without using input from external temperature or flow sensors.

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