WO2018172203A1

WO2018172203A1 - Respiration gas monitor with automated resistance calibration

Info

Publication number: WO2018172203A1
Application number: PCT/EP2018/056647
Authority: WO
Inventors: Eugene Peter GERETY
Original assignee: Koninklijke Philips N.V.
Priority date: 2017-03-20
Filing date: 2018-03-16
Publication date: 2018-09-27

Abstract

A respiration gas monitor (RGM) device (10) includes a respired air flow path (36) for carrying respired air. An infrared light source (30) is arranged to launch infrared light (34) through the respired air flow path. An optical detector (40) is arranged to detect the infrared light after passing through the respired air flow path. A resistance measurement circuit (38) is connected to measure a resistance value across the infrared light source. A control device (46) is configured to transform the measured resistance value to a reference infrared light signal value using a calibration (50).

Description

RESPIRATION GAS MONITOR WITH AUTOMATED RESISTANCE

CALIBRATION

FIELD

The following relates generally to the respiration gas monitor (RGM) device arts, gas detection cell calibration arts, and related arts.

BACKGROUND

Respiration Gas Monitor (RGM) devices are used for measuring partial pressure or concentration of carbon dioxide (C0₂) in respired air, or some other respired gas such as oxygen (0₂), nitrous oxide (N₂0), or an administered anesthetic gas. An RGM device for measuring C0₂ is commonly referred to as a capnometer. Various gas component detection technologies may be employed. In certain RGM devices employing an optical detector, an infrared light source launches broadband infrared light that passes through a sampling cell through which respired air flows. The opposing optical detector module includes a narrowband filter and an infrared detector. The filter is tuned to pass a wavelength that is strongly absorbed by the target gas, e.g. 4.3 micron for C0₂.

However, variations in the infrared light output of an optical emitter can occur. This issue can be solved by compensating the capnometer for variations in the infrared light output of the emitter without actually measuring the infrared light output of the emitter. Normally, this compensation can be performed by using a second reference channel. There are several ways of measuring an emitter's light output, but they require additional components (e.g., detectors, optical elements, amplifiers, etc.). This technique infers the instantaneous light output of the emitter by measuring the instantaneous resistance of the emitter. The emitter's resistance is a monotonic function of the emitter temperature, and the infrared light output of the emitter at any wavelength is a monotonic function of the emitter temperature. Consequently, it can be deduced that the instantaneous infrared light output of the emitter is a monotonic function of the instantaneous emitter resistance.

The following discloses new and improved systems and methods.

SUMMARY

In one disclosed aspect, a respiration gas monitor device includes a respired air flow path for carrying respired air. An infrared light source is arranged to launch infrared light through the respired air flow path. An optical detector is arranged to detect the infrared light after passing through the respired air flow path. A resistance measurement circuit is connected to measure a resistance value across the infrared light source. A control device is configured to transform the measured resistance value to a reference infrared light signal value using a calibration.

In another disclosed aspect, a respiration gas monitoring method includes: with an infrared light source, launching infrared light through a respired air flow path; with an optical detector, detecting the infrared light after passing through the respired air flow path; with a resistance measurement circuit, measuring a resistance value across the infrared light source; and with a control device, transforming the measured resistance value to a reference infrared light signal value using a calibration.

In another disclosed aspect, a respiration gas monitor (RGM) device includes a respired air flow path for carrying respired air. An infrared light source is arranged to launch infrared light through the respired air flow path. An optical detector is arranged to detect the infrared light after passing through the respired air flow path. A resistance measurement circuit is connected to measure a resistance value across the infrared light source. A control device is configured to: transform the measured resistance value to a reference infrared light signal value using a calibration; and measure an absorption value of a target gas flowing through the respired air flow path based upon a value of the detected infrared light signal and the reference infrared light signal value.

One advantage resides in providing more accurate monitoring of carbon dioxide or another target gas in respired air.

Another advantage resides in providing more frequent calibration of a respiration gas monitor (RGM) device.

Another advantage resides in providing an RGM device that does not require connection to nitrogen or another calibration gas.

Another advantage resides in providing an RGM device in which the respired gas flow through the RGM device is not interrupted to perform calibration.

A given embodiment may provide none, one, two, more, or all of the foregoing advantages, and/or may provide other advantages as will become apparent to one of ordinary skill in the art upon reading and understanding the present disclosure. BRIEF DESCRIPTION OF THE DRAWINGS

The disclosure may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating the preferred embodiments and are not to be construed as limiting the invention.

FIGURE 1 diagrammatically illustrates a respiration gas monitor (RGM) device.

FIGURE 2 diagrammatically shows an operational flow chart for operation of the RGM device of FIGURE 1.

DETAILED DESCRIPTION

In a respiration gas monitor (RGM) device employing optical detection of the target gas, the infrared light source typically includes an infrared emitting element (e.g. a ceramic element) that is resistively heated by conducting an electrical current pulse train through the infrared element. The infrared element is resistively heated is to a temperature effective to produce broadband blackbody radiation with strong emission in the infrared. This infrared emission is strongly dependent on the precise temperature of the heated infrared element. The temperature of the heated infrared emitting element can drift over time during operation of the RGM device. The infrared light detector may be a lead selenide (PbSe) detector, a microbolometer, thermocouple, pyroelectric detector, or the like.

Depending upon the thermal stability of the IR source, the calibration of the optical detection cell may need to be repeated at intervals as frequent as every few minutes to tens of minutes to ensure sufficient accuracy for robust medical monitoring of a critically ill patient.

The disclosed improvement is premised on an observation that the resistance across the infrared source used to launch the infrared light correlates with the detected signal with almost no time lag. The infrared light source is essentially analogous to an incandescent lamp filament, except that the temperature is lower (around 500 - 600°C) so as to emit blackbody radiation with a strong infrared component.

On this basis, a calibration can be performed in which, in one example, peak and valley resistance of the infrared light source is mapped to maximum and minimum received signal. This calibration is done under reference conditions, i.e. with no C02 flowing through the sampling chamber, and can be done once at the factory and programmed into the RGM device controller. Thereafter, the controller monitors resistance over the infrared light source and uses the calibration to transform the resistance to reference signal level.

One difficulty is that the reference signal level inferred from the IR source resistance does not take into account absorption due to moisture in the sampling chamber. However, this problem is not significant in the side stream configuration in which a water trap is installed in the diverted flow line. In this case liquid water in the flow is removed by the trap, and heating produced by the infrared light source ensures that any water vapor remaining in the flow does not condense into fluid droplets in the sampling chamber.

With reference to FIGURE 1, an illustrative respiration gas monitor (RGM) device 10 is connected with a patient 12 by a suitable patient accessory, such as a nasal cannula 14 in the illustrative example, or by an airway adaptor connecting with an endotracheal tube used for mechanical ventilation, or so forth. The patient accessory 14 may optionally include one or more ancillary components, such as an air filter, water trap, or the like (not shown). In the illustrative RGM device 10, respired air is drawn from the patient accessory 14 into an air inlet 16 and through target gas measurement cell 20 by a pump 22. The respired air is then discharged via an air outlet 24 of the RGM device 10 to atmosphere or, as in the illustrative embodiment, is discharged through the air outlet 24 into a scavenging system 26 to remove an inhaled anesthetic or other inhaled medicinal agent before discharge into the atmosphere. It should be noted that the respired air generally has a composition that is different from the ambient air, for example the respired air may contain different concentrations of C0₂, oxygen, and/or may contain added gas such as an administered anesthetic gas.

The illustrative RGM device setup has a sidestream configuration in which respired air is drawn into the RGM device using the pump 22, and the target gas measurement cell 20 is located inside the RGM device 10. The sidestream configuration is suitably used for a spontaneously breathing patient, i.e. a patient who is breathing on his or her own without assistance of a mechanical ventilator. In an alternative configuration, known as a mainstream configuration (not illustrated), the target gas measurement cell is located externally from the RGM device housing, typically as a target gas measurement cell patient accessory that is inserted into the "mainstream" airway flow of the patient. Such a mainstream configuration may, for example, be employed in conjunction with a mechanically ventilated patient, with the target gas measurement cell patient accessory being designed to mate into an accessory receptacle of the ventilator unit, or is installed on an airway hose feeding into the ventilator. The target gas measurement cell 20 comprises an infrared optical absorption cell in which the target gas in the respired air drawn from the patient accessory 14 produces absorption in the infrared that is detected optically. By way of non-limiting illustration, C0₂ has an absorption peak at about 4.3 micron. By way of further non-limiting illustration, other target gases may include oxygen (0₂), nitrous oxide (N₂0), or an administered anesthetic gas, each of which have specific characteristic absorption lines in the infrared.

An infrared light source 30 includes an infrared emitting element 32 that is resistively heated by a drive current Ip which in some embodiments comprises an electric current pulse train. The electric current I_P heats the infrared emitting element (e.g. a ceramic element or other thermally radiating element) to emit blackbody radiation with strong emission in the infrared. Thusly launched broadband infrared light 34 (diagrammatically indicated by a block arrow in FIGURE 1) transmits through a respired air flow path 36 (diagrammatically indicated by a curve arrow in FIGURE 1) along which the respired air flows. The flow path 36 may be defined by a tube or other conduit defining a cuvette with walls made of a plastic, glass, sapphire, or other material that is substantially transparent for the infrared light 34. The pump 22 actively drives the flow of respired air through the flow path 36; in a mainstream configuration the flow may be driven by mechanical ventilation of the patient, and/or by active breathing of the patient. A resistance measurement circuit 38 is operatively connected to the infrared light source 30 to measure a resistance value thereacross. The resistance measurement circuit 38 may employ an analog or digital resistance measurement configuration, and as illustrative examples may be a series-type ohmmeter, a shunt-type ohmmeter, or so forth.

An optical detector 40 is configured to detect the infrared light 34. In some illustrative embodiments, the optical detector 40 may be a lead selenide (PbSe) detector, a microbolometer, thermocouple, pyroelectric detector, or the like. To provide specificity to the target gas, a bandpass filter 42 having a passband tuned to an absorption line of the target gas is interposed between the infrared emitting element 32 and the optical detector 40. For example, in the case of the target gas being C0₂, the bandpass filter 42 suitably has a passband that encompasses, and is preferably centered at, 4.3 micron which is a wavelength at which carbon dioxide is strongly absorbing. For other target gases, the bandpass filter is designed to have a passband that encompasses, and preferably is centered on, a strong absorption line of the other target gas. The absorption line bandpass filter may, for example, comprise a stack of layers on an infrared light-transmissive substrate such as sapphire, in which the layers of the stack of layers have thicknesses, refractive indices, and arrangement designed to form an interference filter with a narrow passband having the requisite center frequency (e.g. 4.3 micron for C0₂ detection).

The RGM device 10 further includes RGM device electronics 46 that provide electrical biasing of, and readout for, the optical detector 40. The electronics 46 optionally provide the drive current I_P for the infrared light source 30 (connection not shown in FIGURE 1; moreover other driving configurations are contemplated such as a separate current or voltage drive power supply). The RGM device electronics 46 also include analog signal processing circuitry and/or digital signal processing (DSP) suitable for converting the detected signal into a gas signal 48, e.g. a concentration or partial pressure of C0₂ in the respired air flow, and optionally for performing further processing such as detecting a breath interval and/or an end-tidal C0₂ level (etC0₂ level, relevant for capnometry embodiments). The conversion to C0₂ level or other target gas signal can employ suitable empirical calibration - for example, in general, concentration or partial pressure of the target gas produces greater absorption and hence a reduced signal from the optical detector 40. The empirical calibration may take into account other factors such as flow rate or pressure, and/or the effects of gases other than the target gas (for example, oxygen and nitrous oxide are known to affect the infrared absorption characteristics due to C0₂), and can be suitably programmed as a look-up table, mathematical equation, non-linear op-amp circuit, or so forth.

Another aspect of the conversion is compensating for drift in the signal output by the optical detector 40 due to drift in the intensity of the infrared radiation 34 launched by the infrared light source 30. Conventionally, such drift is accounted for by measuring a reference infrared (IR) signal while a calibration gas flows through the flow path 36. The calibration gas does not contain the target gas, or contains a negligible concentration or partial pressure of the target gas. For the case of the target gas being C0₂, the calibration gas is commonly air or nitrogen. Since the calibration gas does not contain the target gas (or, in the case of air, contains negligibly low concentration of the target gas), it is substantially transparent to the infrared radiation 34, so that the IR signal serves as the reference signal level. During monitoring, the target gas in the respired air reduces the IR signal compared with this baseline or reference IR signal, and the concentration or partial pressure of the target gas can be related to the ratio of the measured IR signal during monitoring versus the reference IR signal measured while the calibration gas is flowing.

The foregoing conventional approach has certain disadvantages as recognized herein. For example, the change out of the flow in the path 36 from respired air to calibration gas, and then back to respired air, requires appropriate valves and valve actuators. In the case of a calibration gas other than air, a connection to a calibration gas source is also needed. The flow change-out introduces delays during which respired air monitoring is interrupted, thus interrupting the monitoring of the target gas. Still further, measurement error can be introduced if monitoring is resumed before the flow change-out is complete and the respired air flow has again attained steady state.

To account for drift without requiring change-out to flow of a calibration gas, in embodiments disclosed herein a calibration 50 is performed using the resistance measurement circuit 38 as described elsewhere herein in order to generate a reference infrared (IR) light signal value. In brief, it is recognized herein that the resistance over the IR light source 32 is closely related to the intensity of the infrared light 34 launched by the IR light source 32. Thus, from the measured resistance the reference IR light signal value can be estimated using the resistance-to-intensity calibration 50 with sufficient accuracy. By employing a ratio of the signal from the optical detector 40 versus the reference IR signal estimated from the resistance measurement, the drift is compensated.

In the case of the RGM device electronics 46 being implemented at least in part by DSP, such DSP may be implemented by a microcontroller or microprocessor or the like programmed by instructions stored on a read only memory (ROM), electronically programmable read-only memory (EPROM), CMOS memory, flash memory, or other electronic, magnetic, optical or other non-transitory storage medium that is readable and executable by the microcontroller or microprocessor or the like to perform the digital signal processing. For DSP processing, a front-end analog-to-digital (A/D) conversion circuit is typically provided to digitize the detector signal from the optical detector 40. To provide useful target gas monitoring, an output component 52 is provided. In the illustrative embodiment, the output component 52 is a display 52, e.g. an LCD display or the like. The illustrative display 52 plots target gas concentration or partial pressure versus time as a trend line. Additionally or alternatively, the display may show a numerical value, e.g. of the target gas concentration at a particular time in the respiratory cycle, e.g. etC0₂ in the case of a capnometer. The output component may additionally or alternatively take other forms, such as being or including (possibly in addition to the display 52) a USB port or other data port via which the target gas data may be read out.

It will be further appreciated that the RGM device 10 may include numerous other components not illustrated in simplified diagrammatic FIGURE 1, such as a pressure gauge and/or flow meter for monitoring the respired air flow, a keypad or other user input components, and/or so forth.

With continuing reference to FIGURE 1, in order to determine the reference IR signal from the resistance measured by the resistance measurement circuit 38, a calibration is first determined, which is then used in transforming the measured resistance value output by the resistance measurement circuit 38 to the reference IR signal. In one approach for generating the calibration 50, the resistance measurement circuit 38 is configured to measure resistance values of the infrared light source 30 for different operating power levels of the infrared light source. The optical detector 40 is configured to measure infrared light signals corresponding to the different operating power levels of the infrared light source. The control device 46 is configured to receive the measured resistance values from the infrared light source 30 (communication path not shown in FIGURE 1, may for example be a conductive wired connection although wireless communication, e.g. Bluetooth, is also contemplated) and the infrared light signals from the optical detector 40. The control device 46 is programmed to map the measured resistance values for the different operating power levels to the corresponding measured infrared light signals to construct the calibration 50. In some examples, the control device 46 can include a memory 54 to store the constructed calibration 50.

No respired air or other gas containing the target gas should be flowing through the respired air flow path 36 during the generation of the calibration 50. This ensures that the intensity data of the (resistance, intensity) data pairs represent the reference IR light intensity level without attenuation due to the target gas. Advantageously, however, the construction of the calibration 50 can be done once prior to deploying the RGM device 10 in the field. For example, the calibration 50 can be generated at the factory during manufacture of the RGM device 10. The resulting calibration 50 is stored in the memory 54 (which is preferably a non- volatile memory so that its contents are retained when the RGM device 10 is not powered) and thereafter the calibration 50 is used during monitoring to estimate the reference IR signal level at any instant from the resistance currently measured by the resistance measurement circuit 38. Thus, in a straightforward implementation, the calibration 50 is generated without any flow through the path 36, i.e. with only ambient air filling this path 36.

In one illustrative approach for generating the calibration 50, the IR light source 32 is driven using electrical pulses, e.g. the current I_P indicated in FIGURE 1 comprises a pulse train. It has been found that the measured resistance and the intensity of the launched infrared light 34 tracks closely with these pulses, so that the peaks and valleys of the pulse train provide two (resistance, intensity) data point pairs for constructing the calibration relating resistance to intensity. In other embodiments, additional data point pairs may be obtained by adjusting the peak and/or valley current (or voltage) values of the electrical energizing.

The control device 46 is also programmed to measure an absorption value of a target gas flowing through the respired air flow path 36 using the IR signal measured by the optical detector 40. To do so, the optical detector 40 is configured to detect an infrared light signal transmitted from the infrared light source 30. The control device 46 receives the infrared light signal from the optical detector 40 and also receives the resistance value for the IR light source 30 measured concurrently by the resistance measurement circuit 38, and is then programmed to compute the absorption value of the target gas from the detected infrared light signal and the reference infrared light signal value estimated from the measured resistance using the stored calibration 50.

In one example, the control device 46 is programmed to input a determined temperature value of the infrared light source 32 (determined by the resistance measurement circuit 38) into a lookup table to determine a corresponding launch power value; and input the determined launch power value into a lookup table to determine a corresponding absorption value. This approach is based on the first principles concept that the resistance should map closely to the temperature of the infrared light source 32, which in turn should map closely to the blackbody radiation output (including the infrared component). However, it is not generally necessary to make the intermediate temperature estimation.

Thus, in another example, the control device 46 is programmed to employ the calibration 50 which maps the resistance value of the infrared light source 30 (measured by the resistance measurement circuit 38) directly to the reference optical power value, which is then used to determine the corresponding absorption value. The control device 46 is preferably further programmed to compute a concentration or partial pressure of the target gas flowing through the respired air flow path 36 from the computed absorption value of the target gas. To do so, the control device 46 can also include an empirical calibration table (not shown) mapping the measured absorption to concentration or partial pressure. Other corrections can be made as is known in the art - for example, it is known to correct for oxygen and/or nitrous oxide when estimating [C0₂].

With reference to FIGURE 2, operation of the RGM device 10 of FIGURE 1 is diagrammatically flowcharted as a method 100. At 102, infrared light is launched with the infrared light source 30 through a respired air flow path 36. At 104, the infrared light signal is filtered with a bandpass filter 42 so that the infrared light signal includes a passband encompassing an absorption line of the target gas. At 106, the launched infrared light is detected with the optical detector 40 after the light passes through the respired air flow path 36. At 108, a resistance value across the infrared light source 30 with the resistance measurement circuit 38. At 110, a calibration is constructed with a control device 46 as a mapping of the measured resistance values to the corresponding measured infrared light signals. At 112, the measured resistance value is transformed with the control device 46 to a reference infrared light signal value using the constructed calibration. At 114, an absorption value of the target gas is computed with the control device 46 from the detected infrared light signal and the reference infrared light signal value. At 116, a concentration or partial pressure of the target gas flowing through the respired air flow path 36 is computed with the control device 46 from the computed absorption value of the target gas.

A possible disadvantage of the disclosed approach for estimating the reference IR signal from the measured IR source resistance is that this approach does not take into account other sources of drift, in particular sensitivity drift of the optical detector 40 or drift due to build-up of condensates and/or air bubbles on the walls of the respired air flow path 36. These additional sources of drift can be addressed as follows. As to sensitivity drift of the optical detector 40, this is generally associated with temperature drift of the optical detector 40, that is, the sensitivity is a function of the operating temperature of the optical detector 40. If the optical detector 40 is kept at a constant temperature, e.g. using a Peltier device (not shown), then this temperature related drift is reduced or eliminated. As to the potential for build-up of condensates and/or air bubbles on the walls of the respired air flow path 36, these are believed to be effectively suppressed by heating of the path 36 by transmission of the launched IR light 34 during monitoring. Accordingly, it is believed that the principle source of drift in the reference IR signal during target gas monitoring is due to temperature drift of the IR light source 30, which is corrected using the approach disclosed herein.

The disclosure has been described with reference to the preferred embodiments. Modifications and alterations may occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.

Claims

CLAIMS:

1. A respiration gas monitor (RGM) device (10), comprising:

a respired air flow path (36) for carrying respired air;

an infrared light source (30) arranged to launch infrared light (34) through the respired air flow path;

an optical detector (40) arranged to detect the infrared light after passing through the respired air flow path;

a resistance measurement circuit (38) connected to measure a resistance value across the infrared light source; and

a control device (46) configured to transform the measured resistance value to a reference infrared light signal value using a calibration (50).

2. The RGM device (10) of claim 1, wherein the control device (46) comprises an electronic processor programmed to determine the calibration (50) by operations including:

using the resistance measurement circuit (38), measuring resistance values of the infrared light source (30) for different operating power levels of the infrared light source;

using the optical detector (40), measuring infrared light signals corresponding to the different operating power levels of the infrared light source; and

constructing the calibration as a mapping of the measured resistance values for the different operating power levels to the corresponding measured infrared light signals.

3. The RGM device (10) of either one of claims 1 and 2, further comprising:

a memory (54) storing the calibration (50) as a mapping of resistance to infrared light signal.

4. The RGM device (10) of any one of claims 1-3, wherein the control device (46) comprises an electronic processor programmed to measure an absorption value of a target gas flowing through the respired air flow path (36) by operations including:

detecting an infrared light signal using the optical detector (40); and computing the absorption value of the target gas from the detected infrared light signal and the reference infrared light signal value.

5. The RGM device (10) of claim 4, wherein the electronic processor (46) is further programmed to compute a concentration or partial pressure of the target gas flowing through the respired air flow path (36) from the computed absorption value of the target gas.

6. The RGM device (10) of either one of claims 4 and 5, further comprising a bandpass filter (42) having a passband encompassing an absorption line of the target gas interposed between the infrared light source (30) and the optical detector (40).

7. The RGM device (10) of claim 6, wherein the target gas is carbon dioxide and the passband of the bandpass filter (42) encompasses the 4.3 micron absorption line of carbon dioxide.

8. The RGM device (10) of any one of claims 4-7, wherein the control device (46) comprises an electronic processor programmed to:

input the determined temperature value into a lookup table to determine a corresponding launch power value; and

input the determined launch power value into a lookup table to determine a corresponding absorption value.

9. A respiration gas monitoring method (100), comprising:

with an infrared light source (30), launching infrared light (34) through a respired air flow path (36);

with an optical detector (40), detecting the infrared light after passing through the respired air flow path;

with a resistance measurement circuit (38), measuring a resistance value across the infrared light source; and

with a control device (46), transforming the measured resistance value to a reference infrared light signal value using a calibration (50).

10. The method (100) of claim 9, wherein the calibration (50) is determined by operations including: with the resistance measurement circuit (38), measuring resistance values of the infrared light source (30) for different operating power levels of the infrared light source;

with the optical detector (40), measuring infrared light signals corresponding to the different operating power levels of the infrared light source; and

with the control device (46), constructing the calibration as a mapping of the measured resistance values for the different operating power levels to the corresponding measured infrared light signals.

11. The method (100) of claim 10, wherein the operations performed to determine the calibration (50) are performed with no respired air flowing through the respired air flow path (36).

12. The method (100) of any one of claims 9-11 , further including measuring an absorption value of a target gas flowing through the respired air flow path (36) by operations including:

with the optical detector (40), detecting an infrared light signal emitted from the infrared light source (30); and

with the control device (46), computing the absorption value of the target gas from the detected infrared light signal and the reference infrared light signal value.

13. The method (100) of claim 12, further including:

with the control device (46), computing a concentration or partial pressure of the target gas flowing through the respired air flow path (36) from the computed absorption value of the target gas.

14. The method (100) of either one of claims 12 and 13, further including:

with a bandpass filter (42), filtering the infrared light signal so that the infrared light signal includes a passband encompassing an absorption line of the target gas.

15. The method (100) of claim 14, wherein the target gas is carbon dioxide and the passband of the bandpass filter (42) encompasses the 4.3 micron absorption line of carbon dioxide.

16. A respiration gas monitor (RGM) device (10), comprising:

a respired air flow path (36) for carrying respired air;

a control device (46) configured to:

transform the measured resistance value to a reference infrared light signal value using a calibration (50); and

measure an absorption value of a target gas flowing through the respired air flow path based upon a value of the detected infrared light signal and the reference infrared light signal value.

17. The RGM device (10) of claim 16, wherein the control device (46) comprises an electronic processor programmed to determine the calibration (50) by operations including:

18. The RGM device (10) of claim 16, wherein the electronic processor (46) is further programmed to compute a concentration or partial pressure of the target gas flowing through the respired air flow path (36) from the computed absorption value of the target gas.

19. The RGM device (10) of claim 18, further comprising a bandpass filter (42) having a passband encompassing an absorption line of the target gas interposed between the infrared light source (30) and the optical detector (40).

20. The RGM device (10) of claim 19, wherein the target gas is carbon dioxide and the passband of the bandpass filter (42) encompasses the 4.3 micron absorption line of carbon dioxide.