WO2004041084A1

WO2004041084A1 - Indirect calorimeter

Info

Publication number: WO2004041084A1
Application number: PCT/US2003/035428
Authority: WO
Inventors: Craig Thomas Flanagan
Original assignee: Craig Thomas Flanagan
Priority date: 2002-11-05
Filing date: 2003-11-04
Publication date: 2004-05-21
Also published as: AU2003291343A1

Abstract

An indirect calorimeter for measuring the metabolic rate and related respiratory parameters of a user includes a non-disposable section (10) and a disposable facemask (28). The calorimeter uses exhalation gas flow and exhalation gas oxygen content to determine the user's metabolic rate and other respiratory parameters. Other exhalation gas properties can also be sensed and used in the determination. Exhalation flow is directed though an exhalation gas pathway through the non-disposable section (10) in a one-way flow regime so as to expose exhalation gas to device sensors including an exhalation gas flow transducer (32) and an oxygen sensor (40). Electrical signals from the sensors are transmitted to a computational unit, such as a microprocessor, which performs the metabolic rate and other calculations and displays the results on a display (20).

Description

INDIRECT CALORIMETER

BACKGROUND OF THE INVENTION Field: This invention is in the field of Indirect Calorimeters for measuring Resting Metabolic Rate (RMR), oxygen consumption (VO2), and related respiratory parameters.

State of the Art: Indirect calorimetry is a method by which nutritional substrate utilization and heat production are measured in vivo starting from gas exchange measurements. Specifically^" indirect calorimetry is useful in calculating the resting metabolic rate (RMR) or the caloric intake of an individual at rest. Indirect Calorimetry first proved its value in a clinical setting, where it allowed physicians to obtain a direct assessment of the metabolic state of a patient undergoing mechanical ventilation therapy for prescription of the proper amount and makeup of parenteral nutritional support.

Proper nutrition is important to the critically ill patient population. Improper patient nutrition has been associated with poor clinical outcome. Specifically, overfeeding can produce hyperglycemia and fatty infiltration producing liver dysfunction as well as respiratory acidosis/hypercapnia which are associated with ventilator weaning difficulties. Underfeeding can depress the immune response and lead to loss of lean body mass.

The establishment of Indirect Calorimetry as a useful clinical tool in patient nutritional assessment and management has extended to common usage in the following patient populations:

Chronic pulmonary disease Cardiac failure Multiple organ failure Cancer

Hypovolemic shock Sepsis

Burn injury Major trauma or surgery

Inflammatory bowel disease Obesity

Thyroid disease Recently, Indirect Calorimetry systems have proven to be an effective tool in weight management and weight loss programs. Such use allows a user to estimate their daily caloric burn rate based on measured RMR values from Indirect Calorimetry systems in combination with an estimate of Activity Related Energy Expenditure (AEE). Such measurements of RMR coupled with estimates of AEE produce good estimates of Total Energy Expenditure (TEE) as RMR + AEE = TEE. The RMR component is generally about 60-70% of TEE.

Formula-based estimates have existed for many decades to estimate RMR. Perhaps the most widely used of such estimates is the so-called Harris-Benedict equation. This equation provides an estimate of metabolic rate which, when coupled with actual measured metabolic rate (as from an Indirect Calorimetry system) can be of great diagnostic use. in establishing., metabolic abnormalities in patients. However, the Harris-Benedict Equation itself is of little value in diagnosing metabolic abnormalities.

The need thus exists for the direct measurement of metabolic rate. Such measurements are made using Indirect Calorimetry systems. The prior art consists of a number of different strategies for obtaining resting metabolic rate (RMR). The first strategy, employed in Patent Nos. 4,917,108 and 5,038,792 to Mault, employ the now outdated use of a carbon dioxide scrubber. Such a scrubber was found to be bulky and required frequent replacement. A number of patents by Mault and assigned to Healthetech (Golden Co.) measure inhaled and exhaled flow, as well as either oxygen or carbon dioxide or a combination of both in order to obtain RMR. Patent Nos. 5,178,155, 5,179,958, and 6,572,561 employ the use of a bi-directional flow meter to measure inhale and exhale flow and, by extension through mathematical integration techniques, gas volume. Mault's Patent No. 6,506,608 is slightly different from the above patents in that a determination of gas consumption is made by measuring changes in exhaled gas mass using ultrasonic transducers. Mault's Patent No. 6,620,106 teaches an indirect calorimetry system which either requires the use of ultrasonic transducers or the use of an intravenous pump. Mault's Patent No. 6,599,253 teaches a breath monitoring apparatus which relies on collimated beams of radiation for sample assessment. Mault's Patent No. 6,616,615 teaches a respiratory calorimeter which uses a capnometer. Mault's Patent Nos. 6,402,698 and 6,468,222 involve indirect calorimeters in which both inhaled and exhaled flow are directed through a single flow tube. Bi-directional flow transducers measure the flow in each direction and gas composition is determined by sensors in commumcation with the bi-directional flow tube. All of the Mault patents and methods for determination of RMR are based on measurements made during both inhalation and exhalation and require a flow path used for both inhalation and exhalation. Systems relying on both inhalation and exhalation flow measurement and using a bidirectional flow pathway, such as Mault's systems, require the user to rebreathe end- expiration gas which remains in the flow tube of the device at the end of the exhalation phase. Such end-expiration gas contains the highest concentration of carbon dioxide present during the breathing cycle,- as this end-expiration gas represents deep lung gas where carbon dioxide is exchanged with oxygen, at the alveolar leveLof the lung. Rebreathing..of carbon dioxide is . known to produce agitation and discomfort in users as well as abnormal breathing patterns due to the user's desire to clear, or remove from the lungs, the extra CO₂ breathed in. Agitation and discomfort have been associated with reduced accuracy in such indirect calorimetry systems.

Patent Nos. 5,285,794 to Lynch and 6,475,158 to Orr and Kofoed use unidirectional flow of only exhaled gas through their devices. Mixing chambers are used in these devices to mix and equilibrate or average exhaled air from each exhalation. This averaging is necessary in their devices to obtain desired RMR measurements. However, the addition of these mixing chambers makes their devices bulky which is a disadvantageous in a small handheld device.

SUMMARY OF THE INVENTION

According to the present invention, a system for the determination of the metabolic rate of an individual uses separate inhalation and exhalation flow pathways with a unidirectional flow of exhalation gas in the exhalation pathway. A flow sensor disposed in the exhalation flow pathway measures instantaneous exhalation gas flow through the pathway and transduces the instantaneous exhalation gas flow measurement into electrical signals representative of the instantaneous exhalation gas flow. Further, the system incorporates an oxygen sensor disposed to measure the instantaneous oxygen content of the exhalation gas in the exhalation pathway. The oxygen sensor transduces the oxygen measurement into electrical signals representative of the oxygen content measured. Because instantaneous oxygen measurements are used, no mixing chamber is necessary and the exhalation flow pathway is configured to minimize any mixing or averaging of the exhalation gas until after the desired measurements are made.

The exhalation gas flow and the oxygen sensors produce an adequate amount of information for a mathematical algorithm to calculate the volumetric oxygen consumption of a user breathing through the device, and from this the RMR can be determined. An algorithm based on a modified Haldane Transform has been found satisfactory to calculate the RMR of a user from only exhalation flow rate and exhalation gas oxygen content measurements, and to do so with more tolerance of flow transducer zero and gain errors, and thus with more accuracy, than in currently available indirect calorimeter measurement systems. The algorithm is -implemented -in a microprocessor- system -in commumcation with each of .the -sensors. The results of calculations performed by the microprocessor using the algorithm are displayed for the user via a graphical display. Additional sensors to determine additional desired parameters, such as ambient and/or airway humidity, ambient and/or airway temperature, and ambient pressure, may be included in the device. Incorporation of such additional measurements into the calculations of some of the calculated parameters, such as RMR, will increase the accuracy of those calculated parameters.

The device of the invention is more comfortable to the user, less prone to algorithm- induced errors, and significantly less expensive to produce than current systems on the market, while not sacrificing the accuracy of the calculated respiratory parameters such as RMR. A preferred indirect calorimetry system of the invention includes a handheld device having an exhalation gas inlet and exhalation gas outlet with an exhalation gas pathway extending between the inlet and outlet. A facemask or mouthpiece is removably secured in the inlet to direct the exhalation gas from a user into the inlet of the device. The exhalation gas pathway has a gas flow sensor therein to measure the instantaneous gas flow through the pathway and has an oxygen sensor in flow communication with the pathway to measure the instantaneous oxygen content of the gas passing through the pathway. While some oxygen sensors can be placed directly in the exhalation gas pathway to measure instantaneous oxygen content of the gas passing the oxygen sensor as the gas passes through the pathway, for use with a preferred oxygen sensor which requires a small constant flow of gas through the sensor, a measurement pathway, which includes the oxygen sensor therein, is provided in flow communication with the exhalation gas pathway. A vacuum pump draws exhalation gas from the exhalation gas pathway through a drying tube, the oxygen sensor, and a flow restrictor and pneumatic accumulator which together ensure a constant flow rate through the measurement pathway. A three way valve is preferably included in the measurement pathway upstream of the oxygen sensor which selectively connects the measurement pathway to the exhalation gas pathway for measurement of the oxygen content of the exhalation gas passing through the exhalation gas pathway or to the atmosphere to pull atmospheric air through the oxygen sensor for calibration of the oxygen sensor. The face mask is designed for single use and means may be provided, such as a bar code placed on a portion of the face mask that is inserted into the inlet of the-device in combination with a bar code reader in the inlet of the device to read the bar code, to ensure that a particular face mask is used only once. The device preferably includes a display to display the measurements obtained and calculated by the device to the user.

BRIEF DESCRIPTION OF THE DRAWINGS The best mode presently contemplated for carrying out the invention in actual practice is illustrated in the accompanying drawings, in which:

FIG. 1 is a perspective view of an embodiment of an indirect calorimetry system of the invention, showing a facemask engaged with and extending from the body of the device;

FIG. 2, a perspective view of the device showing the top portion of the body separated from the bottom portion so that interior components are visible;

FIG. 3, a perspective view of the device similar to that of FIG. 2, but from the opposite side;

FIG. 4, a perspective view of the disposable facemask for use with the embodiment of FIG. 1; FIG. 5, a perspective view from a different angle of the disposable facemask of FIG. 4;

FIG. 6, a block diagram of the signal processing portion of the system; and FIG. 7, a flow diagram of the exhalation gas pathway and measurement pathway of the system.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENT It has been found by the inventor that determination of a person's resting metobolic rate, RMR, and various other respiratory parameters, can be calculated from only a measurement of a person's instantaneous exhalation breath flow rate through an exhalation gas pathway along with a measurement of the instantaneous oxygen content of that exhalation gas. This allows an indirect colorimeter device to be built with only these two measurements necessary, eliminating the need for a bi-directional flow pathway through the device, and, since only instantaneous measurements of these parameters are required, to eliminate mixing chambers that mix the exhalation gas to obtain averaged measurements.

The measurement technique used is based on a modified Haldane Transform. Haldane _ . was the first to described a mathematical derLvatio-x. currently known-as.the-.Haldane_Transform in which a person's oxygen consumption (N0 ), carbon dioxide production (NC0₂), and respiratory quotient (RQ = NC0₂/Vθ₂) were found to be mathematically interrelated. The Haldane Transform provides the foundation for the algorithmic approach used in this invention. Such a mathematical derivation is repeated below:

Let:

VT = Volume of inspired air in ml (STPD - standard temperature (0°C), standard pressure pressure (760 mmHg), dry (0% relative humidity). V_E = Volume of expired air in ml (STPD).

F = Fraction of gas in alveolar air by volumes (STPD). VO₂ = mis of O₂ removed from Vi in lung (STPD).

Then, we have:

VO₂ = Vι*FτO₂ - V_E*F_EO₂ (1.1)

By Nitrogen balance we have:

V_I*FιN₂ = V_E*F_EN₂ (1.2) Next, plugging in Nitrogen volume equivalents yields:

Vι*(l-FτO₂) - V_E*(l-F_EO₂ - F_ECO₂) (1.3)

This is equivalent to:

Vi = V_E*(1 -F_EO₂ - F_ECO₂)/(l -FιO₂) (1.4)

Finally, Equation 1.4 can be plugged into Equation 1.1. to yield the Haldane , _ __

Transform:

VO₂ = (FιO₂*(l-F_EO₂ - FECO2)/(1-FΓO2)-FEO2)*VE (1.5)

This equation yields oxygen consumption. A further formulation known as the Weir equation can be used to obtain the RMR from VO₂ and VCO₂ (VCO₂ is either measured or obtained from an estimate of RQ, RQ = VCO₂/VO₂) . The Weir Equation is as follows:

RMR = [3.9 * (VO₂) + 1.1* (VCO₂)]* 1.44 (1.6)

This technique is commonly used in many indirect calorimetry devices. The above derivation, for simplicity sake, does not take into consideration changes in inhalation and exhalation water vapor content during breathing. Such a correction can be mathematically appended to the above relation and is known in the art. Now, let's derive the Modified Haldane Transform, which is a derivative of the above approach, and is the approach used in this invention:

Once again let:

Vi = Volume of inspired air in ml (STPD - standard temperature (0°C), standard pressure (760 m Hg), dry (0% relative humidity). V_E = Volume of expired air in ml (STPD).

F = Fraction of gas in alveolar air by volumes (STPD).

VO₂ = mis of O₂ removed from Vi in lung (STPD).

Then, we have:

VO₂ = Vι*FιO₂ - V_E*F_EO₂ (1.7)

However, let's assume that the inhaled volume Vj is simply a proportionate function of exhaled volume.N_E—- This can be represented symbolically as follows:

Vι = f(Ω*V_E) (1.8)

Where "Ω" represents a constant of proportionality

Now, plugging the above relation into equation 1.7 yields the following:

VO₂ = Ω*V_E *FιO₂ - V_E*F_EO₂ (1.9)

Next, lets obtain a mathematical representation of inhaled volume in terms of other parameters. Once again by nitrogen mass we have (during exhalation):

VN₂ = V_E - V_ECO₂ - V_EO₂ (1.10)

During inhalation we have:

VN₂ = VE - V_ECO₂ - V_EO₂ (1.11)

Setting Equations 1.10 and 1.11 equal to each other and rearranging yields: VI =V_E-V_EO₂- V_ECO₂-V_IO₂ (1.12)

By clinical definition, the respiratory quotient RQ = VCO₂/VO₂, and also by clinical definition VO₂ = VιO₂ - V_EO_2, substituting these definitions into equation 1.12 produces:

VI =V_E - (1-RQΓVO₂ (1.13)

Now,-, substituting equation .1.8 into 1.13 yields the following useful expression of Vi as estimated from other breath parameters:

Vi = V_E - (l-RQ)*V_E*(Ω*F,O₂- F_EO₂) (1.14)

Given Vi, we can now use the following expression to determine VO₂:

VO₂ = Vι*FIO₂ - V_E*FEO₂ (1.15)

Finally, the Weir Equation shown in Equation 1.6 can be used to estimate RMR given a starting estimate of RQ to obtain the VCO₂ required for this equation.

Expression 1.14 is the expression we will use to eliminate the need for inspiratory flow/volume determination and, by extension, the need to monitor inhalation breath parameters on the device. Thus, the device does not need to sense gas volume or gas concentration during the inspiratory phase of breathing. The above equations are used as follows:

1. The device measures the instantaneous partial pressure of oxygen during exhalation and instantaneous value of flow and the algorithm flow averages these values to obtain PEO₂ (or the average partial pressure of oxygen during exhalation). 2. Depending on device configuration, the device either measures the current barometric pressure using an on-board barometric pressure transducer (use of such a transducer is currently preferred) or estimates the barometric pressure using an altitude value inputted by the user. This barometric pressure is used to convert partial pressures of oxygen to fractions of oxygen as follows:

FEO₂ = PEθ₂/PBarometric 3. The device measures the fraction of inspired oxygen, FτO_2; by obtaining the partial pressure of oxygen at the beginning of exhalation and assuming, since no gas consumption has occurred in this airway gas, that it is equal to the partial pressure of ambient oxygen (such an assumption is very accurate). The partial pressure is then divided by current barometric pressure as in step 2:

FιO₂ = PEθ₂Begir-ning of Exhalation/ PBarometric

4. Next, the value of RQ is assumed to be a typical clinical value of ~0.8 because, in this device as with other indirect calorimetry devices, carbon dioxide volume (VCO ) necessary for RQ calculation tends to require a long measurement duration before stability can be assumed, and errors in this parameter do not contribute significantly to overall algorithm error. Therefore the value of carbon dioxide volume is rarely measured in practice.

5. Plugging these values of RQ, FτO₂, F_EO₂ into equation 1.14, and assuming that Ω = 1, which is a reasonable assumption, the determination of Vi as a function of these other respiratory parameters can be estimated. An even more accurate estimate of the value of VT can be obtained by iterating. Namely by performing the following steps below: a. Obtain a value of Vi using step 5 above. b. Determine a new value of Ω by dividing the resulting Vj by V_E (since by definition Ω = V_E). c. Plug this value of Ω into equation 1.14 and repeat.

6. Once VT has been determined, it can be plugged into 1.15 to obtain VO₂. This VO₂ can in turn be plugged into the Weir Equation to obtain RMR.

As mentioned earlier, additional corrections to this algorithm can be made to correct for differences in inhaled and exhaled gas water vapor content during breathing if these parameters are measured or calculated. Such corrections are minor and are not mentioned here for ease of analysis. However, making these corrections add to the accuracy of the calculated parameters. If such corrections are to be made, an ambient humidity sensor can be used, and is used in preferred embodiments of the invention. The ambient humidity sensor is used to determine the water vapor pressure in the inhaled gas stream. A determination of inhaled gas stream humidity is required to make correction for changes in water vapor content between the inhaled and exhaled breathing gas. With only the ambient humidity sensor, exhaled gas humidity is assumed to be at a relative humidity of 100% (and at a temperature slightly below body temperature or approximately 36°C). This is a very good assumption and generally an exhalation gas humidity sensor will not be used. However, an exhalation gas humidity sensor which would be located in the exhalation gas pathway could also be used, but the inventor is not currently aware of an inexpensive and robust sensor that can be used as an exhalation gas humidity sensor. Advances in micro-machined electro-mechanical device technologies (MEMS) may make such a sensor available in the next few years. Such a sensor would improve estimates of water vapor content of the exhaled gas and thereby improve overall algorithm/device accuracy. An exhalation gas temperature sensor could also be used in the future in conjunction with the exhalation gas humidity sensor for determining real time, intra- breath values of relative humidity. The determination of the relative humidity of a gas requires both gas temperature and absolute humidity readings.

Further additional corrections to this algorithm can be made to more accurately convert gas volumes to standard temperature and pressure, dry (STPD), if the ambient temperature is measured. The gas laws require the use of temperature information for converting gas volumes to STPD conditions. A preferred embodiment of the invention includes an ambient temperature sensor.

For comparison with the above exhalation-only algorithm, an Integration-type algorithm, which is based on a comparison of inhalation and exhalation gas parameters, is commonly used in practice in prior art systems. Such an algorithm will be termed an "Integration Algorithm" for sake of discussion in this patent. Such Integration Algorithms are mentioned as being preferentially used by Mault in patents such as No. 6,402,698, and are based on the following mathematical expression:

.=« _ ... VO₂ = vinspi * Fins_Pi02 * dt - \ VEXP * Fa≠Ol * dt

I=i

This mathematical expression reads that the flow during inhalation at times i (vlnsp , multiplied by the fraction (partial pressure/barometric pressure) of oxygen during inhalation at that same time i (FInspjO2) and integrated from time i = 1 (beginning of inhalation) to time N (end of inhalation) all minus the flow during exhalation at times i (vExpi), multiplied by the fraction of oxygen during exhalation at that same time i (FInspιO2) and integrated from time i = N+l (hegin ing of exhalation) to time M (end of exhalation) is equal to the volume of oxygen consumed for that breath. This VO₂ value can then be plugged into further mathematical expressions such as the Weir, described herein, to obtain RMR. It is clear upon inspection of the terms of this Integration Algorithm that implementation of such an Integration Algorithm requires sensor-based information on inhalation flow.

The Modified Haldane Transform provides a significant computational benefit over the Integration Algorithm. Specifically, implementation of a device using this Modified Haldane Transform results in improved algorithm accuracy in the setting of flow transducer zero and gain errors. Such errors are an unavoidable fact in practical application of such sensor-based systems. The following Table shows the results of calculations made on a spreadsheet given increasing errors in flow transducer gain. The table shows that transducer gain significantly affects the accuracy of the VO₂ readings of the algorithm as determined by the Integration Algorithm. Errors in VO₂ readings propagate though the subsequent mathematical formulations (such as the Weir Equation) used by prior art devices thus affecting the ultimate calculation accuracy of resting metabolic rate. These large errors seen in the Integration Algorithm are due to the imbalance of inspiratory and expiratory flow serving to produce divergent errors in the values of VιO₂ and V_EO₂. Such divergent errors are not present in the Modified Haldane Transform as inspiratory volume is estimated from expiratory volume and the errors tend to fall out of the formulation.

Table 1.1. Flow Transducer Gain Error Effects on Modified Haldane and Integration Algorithms:

The system described herein based on exhalation flow only does not require a bidirectional flow meter. Implementation of systems such as that described by Mault in Patent No. 6,402,698 are based on two-way flow and require the use of bi-directional flow meters. Such a requirement narrows the selection of candidate flow meters. For example, such flow meters as low-cost flapper type differential pressure flow meters cannot be used as they generally measure flow in one direction only. In addition to limiting the selection of flow meter technologies which can be used, implementation of a two-way flow regime also effectively halves the analog to digital resolution of the sensor as seen by the microprocessor. For example, if a 10-bit analog to digital converter is used to convert sensor output to a digital value that the microprocessor can read, then 1028 bits or 1028 analog voltage levels can be represented to the processor. If a bi-directional flow meter is used, this number must be halved to represent 512 bits representing negative flow values and 512 bits representing positive flow values. Such a halving of the analog-to-digital converter output need not occur with a one- way- flow-, transducer, effectivel -doubling the-ATD. resolution, as seen, at he-microprocessor. Such a doubling means that a lower bit count A/D converter may be used reducing the cost of device manufacture.

A specific embodiment of an indirect calorimeter of the invention is illustrated in the drawings. As illustrated, the indirect calorimeter, indicated generally as 10, Figs. 1-3, includes an exhalation gas pathway therethrough with an inlet 14 and outlet 16. The calorimeter body includes an upper body portion 17 and a lower body portion 18. A flip up screen 19, with a graphic or alphanumeric display 20, can be closed when the device is not in use by pivoting the screen 20 down into a receiving recess 21 in the upper body portion 17, as shown in Fig. 3. A power switch 22 can be pressed to initiate power-up of the device, i.e., to turn "on" the device. LED 24 indicates when the device is connected to a source of AC power and LED 25 indicates when the device is turned "on." A rotary selector switch 26 with built in "press switch," such as a Grayhill 61 C series switch, allows the user to rotate the switch knob to select the desired operation of the device or to input information into the device and to confirm the desired selection by pressing the knob to activate the "press switch" which enters the information. While the rotary switch with press switch is currently preferred, other arrangements such as separate selection and confirmation switches can be used.

In use of the device, a user exhales into the inlet 14 of the exhalation air pathway through the device. It is important that the entire exhaled breath of the user pass through the exhalation gas pathway. For this purpose, the inlet 14 to the exhalation gas pathway is coupled to a user's mouth by a face mask 28, Fig. 1, or by other suitable mouthpiece so that as a user exhales, the entire exhaled breath is directed into the inlet 14 of the exhalation air pathway. The face mask or other suitable mouthpiece will also include an inlet for atmospheric air for inhalation by the user so that the user can easily inhale and exhale while using the device with the exhaled breath directed into inlet 14 of the device. Figs. 2 and 3 show the upper and lower portions of the calorimeter body separated to expose the inside of the calorimeter. The exhalation gas flow pathway through the device extends from exhalation gas inlet 14 to exhalation gas outlet 16, and includes inlet tube 30 positioned in exhalation gas inlet 14 to receive the exhalation gas outlet of face mask 28 thereover. An O-ring may be positioned in groove 31 of inlet tube 30 to seal the connection to the face mask. The inlet tube 30, after several bends, connects to a flow sensor 32_. which measures the real time or instantaneous flow of exhalation gas through the flow sensor 32, which is the same as the flow of exhalation gas through the exhalation gas flow pathway. A heated screen element differential pressure transducer, similar in construct to a Hans Rudolph Model 8300 works well as the flow sensor and is currently preferred, although other types of flow sensors can also be used. The flow sensor produces an electrical signal representative of the measured instantaneous flow of exhalation gas through the sensor, which is also a measure of the instantaneous gas flow through the exhalation gas pathway. Exit tube 33 extends from the outlet of flow sensor 32 and after several bends, discharges the exhalation gas to the atmosphere through exhalation gas outlet 16. For use with the presently preferred types of oxygen sensors which perform best when a small constant flow of gas is pulled through the sensor, a measurement flow pathway extends from an inlet tube 35 in gas flow communication with the exhalation gas flow pathway, here shown as communication with exit tube 33, to a measurement pathway outlet 36. Inlet tube 35 may be a length of Nafion tubing which connects the exhalation gas pathway to an inlet of an electrically operated three way valve 37. The Nafion tube dries the exhalation gas drawn into the measurement pathway before it reaches the oxygen sensor. A second inlet of the three way valve 37 communicates with the atmosphere through inlet 38. Tube 39 connects the outlet of the three way valve 37 to oxygen sensor 40. The presently preferred oxygen sensor is a cylindrical fast fuel cell (Galvanic) oxygen sensor, such as a Teledyne UFO-130. Other types of oxygen sensors may be used, such as paramagnetic oxygen sensors, for example fast acoustic paramagnetic sensors as manufactured by Datex of Helsinki, Finland. The oxygen sensor produces an electrical signal representative of the measured instantaneous partial pressure of oxygen of gas passing through the sensor. Tube 41 extends from oxygen sensor 40 to flow restrictor 42 connected by tube 43 to pneumatic accumulator 44. Tube 45 connects pneumatic accumulator 44 to vacuum pump 46 which discharges to the atmosphere through outlet 36. Vacuum pump 46 can be any of a number of small DC pumps available today as OEM units, such as a Gast miniature diaphragm 2D series. Three-way valve 37 can switch the inlet for the measurement pathway and oxygen sensor 40 between exhalation gas from the exhalation gas pathway and atmospheric air. Pneumatic accumulator 44, in combination with flow restrictor 42,,form-a..pneumatic damping. system in the measurement pathway. to ..damp out. ump pressure oscillations and provide a constant gas or air flow through the measurement pathway and oxygen sensor 40.

Figs. 4 and 5 show a preferred embodiment of disposable face mask 28 for use with the calorimeter of the invention and includes a face mask body 50 configured to fit over the nose and mouth of a user and a mask outlet tube 52. Mask outlet tube 52 is sized to slide over and closely fit over inlet tube 30, Fig. 3, in calorimeter inlet 14, with the connection sealed by an O- ring in inlet tube groove 31. A keyed protrusion 53 on mask outlet tube 52 fits into receiving groove 54, Fig. 3, in calorimeter inlet 14 to ensure that the mask is correctly oriented when mask outlet tube 52 is inserted into calorimeter inlet 14, and that it does not rotate when placed over inlet tube 30. The mask has two one-way check valves 56 and 58. These two check valves ensure that gas enters the mask only through valve 56 on inhalation by the user, and exits the mask only through outlet tube 52 after passing though valve 58 on exhalation by the user.

The mask is designed to minimize dead space volume between the inner surface of the mask and the surface of the face. Such a minimization of dead space volume is achieved by inward protrusions 60 which extend inward towards the face to ensure only the nose area and mouth area have a large opening. Such a minimization of dead space volume reduces the amount of CO re-breathed by the user and as a result minimizes the consequent negative effects thereof previously described for two way flow pathways.

For health reasons, it is preferred that the masks be disposable and be used only once. This prevents transmission of disease from one user to the next through use of the same face mask. Further, it is preferred that the masks include a filter which forms a biological barrier between the user and the calorimeter to prevent contamination of the calorimeter from the face mask and of the face mask from the calorimeter. This also protects the user from infection resulting from calorimeter reuse. To ensure the face masks are not reused, each face mask may be provided with a bar code indication 61, Fig. 5, placed on a flat area 62 on the top of outlet tube 52. A bar code reader is built into the top of calorimeter inlet 14 as at 63, Fig. 3, so that as the face mask outlet tube 52 is slid into inlet 14, the bar code is read by the bar code reader which produces electrical signals indicative of the bar code that has been read. The alignment of the mask resulting from the protrusion 53 and receiving groove 54 ensures that bar code indication 52 is correctly oriented to be read by the bar code reader. A "Single Use" label 64 is preferably located on the front of the mask.

The calorimeter includes a microprocessor, Fig. 6, and associated circuitry to process signals from the various sensors and produce desired information therefrom and display that information to a user. Thus, the flow sensor and oxygen sensor are connected to the microprocessor. The indicated "microprocessor" block is meant to include not only the microprocessor itself, but all associated circuitry, such as interface circuitry and memory circuitry. Any other sensors, such as the illustrated CO₂ sensor, Barometric Pressure sensor, Temperature sensor, and Humidity sensor are also connected to the microprocessor. The microprocessor processes the signals supplied to it from the sensors and using the "algorithm" as previously described and programmed into the microprocessor, calculates the desired information which is then sent to the display. The calculated information may also be stored by the microprocessor. This is all done in customary fashion so is not further described in detail. The circuitry is housed in the calorimeter body in any desired location, such as inside the upper portion of the calorimeter body and is not visible in the drawings. The calorimeter may be battery powered and/or may include a socket 70, Fig. 3, for attachment of a plug from a power jack which allows wall power to be plugged into the device alleviating the necessity for batteries or prolonging the life of batteries when provided.

Fig. 7 shows a pneumatic flow schematic of the device indicating the flow pathways of the preferred embodiment. The arrows represent gas flow in direction and width of the arrows gives some indication of degree of volumetric flow. Upon inhalation by the user, air is draw in through the check valve in the facemask through the ambient gas inlet. Upon exhalation by the user, gas is exhausted from the outlet of the face mask and through the exhalation gas pathway in the device. The second check valve shown located in the indirect calorimetry device can be either located in the indirect calorimetry device or located in the mask at or near the mask's connection to the indirect calorimetry device. Such location in the mask has been described for the mask of Figs. 4 and 5. A filter is preferably located in the facemask so as to form a biologic barrier between the user and the device to protect the user from infection resulting from device reuse (the facemasks are not reused). A small volumetric stream of exhalation gas is drawn through a pneumatic oxygen sampling system consisting of a Nafion Tube segment to dry the gas (optional), a three-way solenoid valve, a restrictor, an accumulator, and a vacuum gas pump. The three-way valve allows the device to switch (under microprocessor control) between supplying ambient air to the oxygen sensor (path 2 connected to path 3) and supplying exhalation gas to the oxygen sensor (path 1 connected to path 2). With the three-way valve switched to supply ambient air, ambient atmospheric air passes through the oxygen sensor and can be used to calibrate the sensor to 20.9% oxygen. The purpose of the restrictor and accumulator is to pneumatically damp out pressure oscillations resulting from pump action, thus allowing a smooth and uninterrupted flow of gas though the oxygen sensor. Finally, the vacuum pump produces a vacuum in the line proximal to the pump for drawing gas through this measurement pathway. It should be noted that the accumulator is preferentially located distal to the oxygen sensor so as to prevent gas mixing before sampling, thus ensuring a sample representative of exhalation oxygen partial pressure at a given point in time.

In an alternative embodiment the oxygen sensor can be placed in the exhalation flow pathway thus eliminating the need for the pump-based sampling system.

As can be seen, particularly from Fig. 7 showing the flow path and location of the sensors in a preferred embodiment of the invention, that depending upon the particular embodiment, the exhalation gas does not pass all sensors at the same time. As shown in Fig. 7, the exhalation gas will pass the flow sensor where the flow measurement is taken and then that gas will flow further through the exhalation gas pathway and a portion of it will flow through the measurement pathway to the oxygen sensor so the oxygen reading for particular gas passing through the flow sensor will be delayed from the flow reading. Therefore, the flow signal from the flow sensor or transducer should be delayed, such as in the algorithm, to appropriately synchronize it with the oxygen signal which is delayed due to gas transport through the pathways of the device.

In order to provide additional information that might be desired by a user, a carbon dioxide sensor may be included in the exhalation gas pathway to determine the carbon dioxide partial pressure of the exhalation gas stream. Such a determination would allow the device to calculate the proportionate amount of carbohydrate and fat being metabolized (given an estimate of protein metabolism). Such formulations of carbohydrate and fat metabolism, given a ratio of carbon dioxide volumetric production to oxygen volumetric consumption are known in the art. The invention has been described as using instantaneous or real time measurements of oxygen content and flow. Any exhalation breath will vary in oxygen content from start of exhalation to the end of exhalation, with less oxygen contained in the gas exhaled at the end of exhalation than at the beginning of exhalation. Further, the flow rate or speed of exhalation will also generally vary during exhalation. Instantaneous or real time measurement of oxygen content and flow rate means that the output of the sensor will be substantially representative of the actual value of oxygen content and flow rate of the gas passing the sensor at the time of the output and that the device is configured so that the character of the gas passing through the device maintains the character of the gas as it was exhaled, i.e., that substantial mixing of the gas does not take place. Thus, if at one instant the breath leaving the user's mouth has an oxygen content of twenty percent and the oxygen content then falls to eighteen percent over the next half second, as that gas is directed to and passes the oxygen sensor, the output of the oxygen sensor will show the gas with an oxygen content substantially at twenty percent dropping to an oxygen content substantially at eighteen percent over a half second interval. In other words, the output of the sensor will substantially track the actual values of the parameters being measured as they are upon exhalation of the breath. This does not happen with a mixing chamber where the exhaled gas over an entire exhaled breath or substantial portion of an exhaled breath is mixed so as to be averaged or equilibrated so that only an average oxygen content reading for the breath is obtained by the sensor. This means that the diameter of the exhalation pathway of the device has to be small enough to prevent substantial mixing of the gas as it is passing through the pathway before reaching the sensors, and that the sensors used have a fast response time so that the response can follow changes of the parameter in the breath. However, some delay in sensor response and some small mixing of the sample will take place so the sensor readings are only approximate.

While various sensors have been described to measure oxygen content and flow rate, as indicated, the sensors used should be fast sensors meaning that the T₁₀-T₉o response time of the sensor is less than about 500 msec, and preferably less than 300 msec. This means that output response of the sensor to a step change in input to the sensor, with output changing from ten percent to ninety percent, occurs in less than 500 msec. Thus, a preferred oxygen sensor would have a Tι₀-T₉₀ response time of less than 500 msec and preferably less than 300 msec, for — tracking-the intra-breath oxygen partial-pressure value in the exhalation gas stream.

Whereas this invention is here illustrated and described with reference to embodiments thereof presently contemplated as the best mode of carrying out the invention in actual practice, it is to be realized that various changes may be made in adapting the invention to different embodiments without departing from the inventive concepts disclosed herein.

Claims

1. An indirect calorimeter for measuring a user's metabolic rate, comprising: an exhalation gas flow pathway through which all exhalation gas from a user is directed in one way flow; an exhalation flow transducer associated with the exhalation gas flow pathway to produce a signal representative of the instantaneous exhalation gas flow rate of the gas passing the flow transducer; an oxygen sensor associated with the exhalation gas flow pathway to produce a signal representative of the instantaneous oxygen content of the exhalation gas passing the oxygen sensor; and a microprocessor for receiving the signals representative of the instantaneous exhalation gas flow rate and the instantaneous oxygen content of the exhalation gas and calculating the user's resting metabolic rate.

2. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a display for displaying to the user the calculated resting metabolic rate.

3. An indirect calorimeter for measuring a user's metabolic rate according to Claim 2, wherein the microprocessor also calculates the user's oxygen consumption.

4. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a measurement flow pathway extending from the exhalation gas flow pathway, a vacuum pump in gas, flow communication with the measurement flow pathway to draw exhalation gas from the exhalation gas flow pathway through the measurement flow pathway at a substantially constant rate, and wherein the oxygen sensor is located in the measurement flow pathway.

5. An indirect calorimeter for measuring a user's metabolic rate according to Claim 4, additionally including pneumatic damping means in the measurement flow pathway.

6. An indirect calorimeter for measuring a user's metabolic rate according to Claim 5, wherein the pneumatic damping means includes an accumulator and a flow restrictor.

7. An indirect calorimeter for measuring a user's metabolic rate according to Claim 5, wherein the pneumatic damping means is located downstream of the oxygen sensor.

8. An indirect calorimeter for measuring a user's metabolic rate according to Claim 5, additionally including a valve in the measurement flow pathway upstream of the oxygen sensor for selectively connecting the measurement flow pathway with the exhalation flow pathway or to the atmosphere so that atmospheric air is drawn into the measurement flow pathway for calibration of the oxygen sensor.

9. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a barometric pressure sensor for sensing atmospheric barometric pressure and producing a signal representative of atmospheric barometric pressure, and wherein the microprocessor is adapted to receive the signal representative of atmospheric barometric pressure.

10. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a temperature sensor for sensing ambient temperature and producing a signal representative of ambient temperature, and wherein the microprocessor is adapted to receive the signal representative of ambient temperature.

11. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including an humidity sensor for sensing ambient humidity and producing a signal representative of ambient humidity, and wherein the microprocessor is adapted to receive the signal representative of ambient humidity.

12. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a carbon dioxide partial pressure sensor for sensing instantaneous partial pressure of carbon dioxide in the exhalation breath and producing a signal representative of carbon dioxide partial pressure, and wherein the microprocessor is adapted to receive the signal representative of carbon dioxide partial pressure.

13. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, wherein the exhalation gas flow pathway has an inlet adapted to removably receive an outlet of an interface with the user to direct the user's exhalation gas into the exhalation gas flow pathway.

14. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including an interface with the user to be used by the user to collect and direct the user's exhalation gas to an interface exhalation gas outlet, and wherein the exhalation gas flow pathway has an inlet adapted to removably receive the outlet of the interface to direct the user's exhalation gas into the exhalation gas flow pathway.

15. An indirect calorimeter for measuring a user's metabolic rate according to Claim 14, wherein the interface includes an inhalation air inlet in gas flow communication with the atmosphere and an inlet one way valve which allows flow of atmospheric air into the interface upon inhalation by the user and blocks flow of exhalation air through the air inlet.

16. An indirect calorimeter for measuring a user's metabolic rate according to Claim 15, wherein the interface includes an outlet one way valve which allows flow of exhalation gas from the interface through the interface exhalation gas outlet and blocks flow of gas through the interface exhalation gas outlet into the interface.

17. An indirect calorimeter for measuring a user's metabolic rate according to Claim 16, wherein the interface is a mask adapted to cover the user's mouth and nose.

18. An indirect calorimeter for measuring a user's metabolic rate according to Claim 17, wherein the mask includes contour portions to fit closely around the user's face to minimize air space in the mask.

19. An indirect calorimeter for measuring a user's metabolic rate according to Claim 18, wherein the mask includes a filter through which the exhalation gas passes.

20. An indirect calorimeter for measuring a user's metabolic rate according to Claim 14, wherein the interface is a mask adapted to cover the user's mouth and nose.

21. An indirect calorimeter for measuring a user's metabolic rate according to Claim 20, wherein the mask includes contour portions to fit closely around the user's face to minimize air space in the mask.

22. An indirect calorimeter for measuring a user's metabolic rate according to Claim 14, wherein the interface additionally includes a bar code indicator on a portion of the exhalation gas outlet that is inserted into the exhalation gas inlet, and a bar code reader in the exhalation gas inlet to read the bar code and produce signals representative of the bar code read, and wherein the microprocessor is adapted to receive the signal representative of the bar code read.

23. An indirect calorimeter for measuring a user's metabolic rate according to Claim 22, wherein the interface is designated for a single use, wherein the bar code represents a serial no for the interface, and wherein the microprocessor is programmed to block operation of the calorimeter if an interface is inserted into the exhalation gas inlet more than once.

24. An indirect calorimeter for measuring a user's metabolic rate according to Claim 14, wherein the interface includes a filter through which the exhalation gas passes.

25. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, wherein the oxygen sensor has a Tι₀-T₉₀ response time of less than about 500 msec.

26. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, wherein the flow sensor has a T₁₀-T₉o response time of less than about 500 msec.

27. An indirect calorimeter for measuring a user's metabolic rate according to Claim 1, additionally including a user input to input information to the microprocessor.

28. An indirect calorimeter for measuring a user's metabolic rate, comprising: a portable, hand held calorimeter body; an exhalation gas flow pathway through the body and having an inlet through which all exhalation gas from a user is directed in one way flow; an exhalation flow transducer associated with the exhalation gas flow pathway to produce a signal representative of the instantaneous exhalation gas flow rate of the gas passing the flow transducer; an oxygen sensor associated with the exhalation gas flow pathway to produce a signal representative of the instantaneous oxygen content of the exhalation gas passing the oxygen sensor; a microprocessor for receiving the signals representative of the instantaneous exhalation gas flow rate and the instantaneous oxygen content of the exhalation gas and calculating the user's resting metabolic rate; a user input to input information to the microprocessor; and a display for displaying to the user the calculated resting metabolic rate.

29. A face mask for use with an indirect calorimeter for measuring a user's metabolic rate, comprising: a face mask exhalation gas outlet configured to be removably connected to an indirect calorimeter; a face mask body forming a mask that fits over a user's mouth and nose to collect and direct the user's exhalation gas to the face mask exhalation gas outlet; an inhalation air inlet in gas flow communication with the atmosphere; an air inlet one way valve which allows flow of atmospheric air into the mask upon inhalation by the user and blocks flow of exhalation air through the air inlet; an outlet one way valve which allows flow of exhalation gas from the interface through the interface exhalation gas outlet and blocks flow of gas through the interface exhalation gas outlet into the interface; and a filter through which exhalation gas passes.

30. A face mask for use with an indirect calorimeter for measuring a user's metabolic rate according to Claim 29, additionally including a bar code indicator on a portion of the exhalation gas outlet that is inserted into the exhalation gas inlet.

31. A face mask for use with an indirect calorimeter for measuring a user's metabolic rate according to Claim 30, wherein the mask is designated for a single use, and wherein the bar code represents a unique identification for the mask.

32. A face mask for use with an indirect calorimeter for measuring a user's metabolic rate according to Claim 29, wherein the mask includes contour portions to fit closely around the user's face to minimize air space in the mask.

33. A method for determining a user' s metabolic rate, comprising: determining a user's instantaneous exhalation gas flow rate during an exhalation breath; determining instantaneous oxygen content of the exhalation breath during the breath; and using a Haldane Transform modified to require input of only instantaneous flow rates of exhalation gas and instantaneous oxygen content of exhalation gas during a breath in combination with a Weir Equation calculating the user's metabolic rate.