Measurement of Oxygen Uptake with Portable Equipment
John F. Patton1
The energy balance of an organism is a function of energy intake and energy expenditure. Energy intake, the amount of food consumed in a given period, is theoretically easy to assess through careful quantification of food intake. In contrast, the measurement of energy expenditure under free-living conditions has been one of the more elusive goals of research scientists. Energy expenditure is an important measurement in many nutritional, epidemiological, and ergonomic studies, whether it be in determining daily energy requirements, calculating energy balance, measuring habitual physical activity, or quantifying energy cost of physical task performance.
Generally, two approaches have been used to assess energy expenditure: indirect estimates and direct measurement of oxygen uptake. Indirect methods include such procedures as activity questionnaires (Montoye, 1971),
heart-rate recording (Spurr et al., 1988), pedometers and accelerometers (Kashiwazaki et al., 1986; Montoye, et al., 1983), and doubly labeled water (Stager et al., 1995). In general, many of these suffer from various practical and/or theoretical limitations, and some have not been evaluated to determine their reliability or validity.
The validity of the direct measurement of oxygen uptake as a basis for measuring energy expenditure, however, has been well established and has been used to determine the energy cost of a great variety of human activities. In field studies, the classical method has been to collect expired air in Douglas bags (DB) carried by the subject or the experimenter. While this technique has been widely used, it is cumbersome, limited to fairly short collection periods, and requires timed collections of expired gas and subsequent analysis of expired oxygen and carbon dioxide concentrations using conventional laboratory techniques.
The modified Kofranyi-Michaelis (KM) respirometer (Consolazio, 1971), which also has been used extensively for energy exchange studies in a field environment, represents an early portable system to measure expired volume. The KM meter is a simple, compact, lightweight unit consisting of a dry gas meter for measuring the total volume and temperature of the expired air. An aliquoting device continuously removes a small percentage of each breath into a bag that is then analyzed for oxygen and carbon dioxide concentrations in the laboratory.
A truly portable system, however, is one that not only is capable of measuring the volume of the expired or inspired air for determination of minute ventilation (˙VE or ˙VI), but also is capable of measuring gas concentrations so that oxygen uptake (˙VO2) can be calculated by the instrument. In addition, such a system must be light enough to be easily transported and must be battery operated, and the analysis of both gas volume and concentration must be rapid and integrated with a high-speed microprocessor. Currently, there are three systems available that possess these technological advancements and allow for the measurement of ˙VO2 in humans under free-living conditions. This paper will describe the specifications of each of these and present data on their validity, reliability, and operability in the laboratory and, where applicable, in a field environment.
PORTABLE SYSTEMS FOR MEASURING OXYGEN UPTAKE
The three portable systems capable of providing a continuous measurement of ˙VO2 and ventilation for long periods are (1) the Total Energy Expenditure Measurement system (TEEM 100, AeroSport, Ann Arbor, Mich.), (2) the COSMED K2 (Vacumed, Ventura, Calif.), and (3) the Oxylog (P. K. Morgan, Andover, Mass.). Each of these has unique features but contains the basic technological advances previously mentioned to allow for the rapid measure-
ment and integration of ventilation and gas concentrations and, thus, the calculation of oxygen uptake.
The TEEM 100 is the newest of the portable systems and has been available commercially for approximately 4 years (TEEM 100 Operator's Manual, 1993). The principal features of this instrument are depicted in Table 13-1.
Briefly, the TEEM 100 uses an open-circuit continuous sampling system for the measurement of oxygen uptake (˙VO2) and carbon dioxide production (˙VCO2). A simple orifice plate pneumotachometer located in the face mask measures expired flow. A high-frequency pulse-modulated valve extracts proportional samples of expired gas over the flow wave form. These samples are introduced into a closed-loop mixing chamber and then passed into a gas analysis chamber, both of which are located in the base unit. An electronic variable sampling system provides a constant flow of gas independent of the expired flow rate. This means equilibration time is constant over a wide range of ˙VE. The oxygen and carbon dioxide percentages are measured using an absolute oxygen (O2) sensor (galvanic fuel cell) and a nondispersive, infrared CO2 detector, respectively. The microprocessor integrates ˙VE with expired O2 and CO2 percentages to calculate ˙VO2, ˙VCO2, ˙VE, and respiratory exchange ratio
TABLE 13-1 TEEM 100 Specifications
(RER). The measurement of CO2 and calculation of the RER are unique features of the TEEM 100 and represent important variables in many nutrition studies.
Available data evaluating the TEEM 100 for the measurement of ˙VO2 are limited. Segal et al. (1994) compared the TEEM 100 to the Sensormedics 2900 (S2900) Metabolic Analysis System during low-intensity cycle ergometer exercise (0, 25, and 50 W) and found no significant differences in either ˙VO2 or ˙VE at any intensity. Also, correlations between the two systems ranged from r = 0.85 to r = 0.96 (p < 0.001), with no significant differences between the regression line and the line of identity. The authors concluded that the findings support the validity of the TEEM 100 for low-intensity exercise and suggest that it is an acceptable alternative to the more cumbersome, expensive systems for gas exchange and metabolic measurements in the laboratory.
In a study to evaluate the TEEM 100 at higher exercise intensities, Clure et al. (1995) compared it to the S2900 during maximum exercise testing of well-conditioned athletes. At peak ˙VO2, ˙VE (liters · min-1) was significantly lower with the TEEM 100 compared to the S2900 (146 ± 15 vs. 135 ± 16, p < 0.05) but no difference was seen in ˙VO2 (60.7 ± 6.2 vs. 58.0 ± 6.5 ml · kg-1 · min-1). The correlation between the two systems for ˙VO2 at all levels of exercise was 0.71, suggesting that while the TEEM 100 appears to provide valid data for maximum performance testing, considerable individual variability may occur.
Because of its portability, apparent ease of use, and validity at low-exercise intensities compared to the SensorMedics 2900 system, it can be concluded that the TEEM 100 has potential for use in the areas of exercise science, nutrition, and rehabilitation and athletic medicine. Certainly more validity studies need to be performed using different exercise protocols and under conditions where the instrument is worn by the subject to establish its robustness. Also its suitability for use in environments other than the laboratory must be determined.
The COSMED K2 is a relatively new, integrated, portable, telemetric oxygen uptake system (K2 Operator's Manual, 1991). Its attractiveness lies in its capability to obtain ˙VO2 and ˙VE from individuals performing work in the field or in conditions such as space flight where large, more cumbersome equipment would be unmanageable. The basic features of this system are depicted in Table 13-2.
TABLE 13-2 COSMED K2 Specifications
The COSMED K2 consists of a specially designed facemask that contains a photoelectric turbine-type flowmeter (response range over 300 liters· min-1) and a capillary tube for sampling expired air. An extra valve located at the bottom of the face mask reduces inspiratory resistance and helps eliminate condensation for the comfort of the subject.
The system is equipped with an FM radio transmitter that broadcasts signals to a receiver unit. The receiver includes a microcomputer that processes, archives, and displays calculated data (on screen and in print) in real time. The receiver is equipped with an interface for downloading to a computer. The range of the transmitter in an open field is 600 m (1,967 ft), using the small antenna supplied.
The transmitter is composed of two subunits carried by the subject in a specially designed harness, with the front unit containing the gas-sampling and transmitter apparatus on the subject's chest and the rear unit containing the battery pack on the subject's back. Both the receiver and transmitter are powered by rechargeable nickel-cadmium (NiCd) batteries.
As ambient air passes through the facemask, it is sampled by the capillary tube at a rate proportional to the ventilatory rate by way of a dynamic sampling pump. It then passes through a desiccant and into a miniature mixing chamber located inside the transmitter, which also contains the oxygen polarographic electrode.
The transmitter sends data for ˙VO2 and ˙VE to the receiver, which displays and prints the ˙VO2, ˙VE, respiratory rate (Rf), tidal volume, and fractional concentration of oxygen in expired air (FeO2). Because the COSMED K2 does not analyze CO2 in the expired air, it is not able to determine the RER. It assumes that RER equals 1.00 for every value of ˙VO2. The equation used to calculate ˙VO2 at standard temperature and pressure, dry gas (STPD) is:
˙VO2 = ˙VE × F1O2 -FeO2),
where fractional concentration of oxygen in inspired air (FIO2) is assumed to be 20.93 percent.
The COSMED K2 has been evaluated by a number of researchers over the past few years (Crandall et al., 1994; Kawakami et al., 1992; Lucia et al., 1993). Kawakami et al. (1992) compared the COSMED to the DB method during continuous cycle ergometry exercise of gradually increasing loads and found, with a few exceptions, little difference between systems over ranges of 0.5 to 2.5 liters · min-1 for ˙VO2 and 10 to 90 liters · min-1 for ˙VE. These authors also used the COSMED during such sporting events as rowing and soccer and found it to be useful in assessing training.
Crandall et al. (1994) compared the COSMED to a breath-by-breath (BBB, the criterion method) metabolic measurement system during a maximal graded exercise test (GXT) using the Bruce protocol (treadmill exercise test) (Bruce et al., 1973). As expected with a system that lacks the capacity to measure CO2, at the lower exercise intensities the COSMED tended to underestimate the ˙VO2 measured by the BBB system and at higher intensities tended to overestimate the BBB ˙VO2. However, there were no significant differences between systems at any stage of the GXT. COSMED ventilation volumes, however, were significantly higher than those measured with the BBB system, but this did not have any apparent effect on the validity of the ˙VO2 data.
In the most comprehensive evaluation of the COSMED, Lucia and coworkers (1993) compared it to the DB technique during submaximal progressive treadmill exercise consisting of six stages of 3 minutes each after which a maximal test was performed following a rest period. In addition, a comparison also was made between two different COSMED systems. Tables 13-3, 13-4, and 13-5 present average values for ˙VE, FeO2, and ˙VO2, respectively, for three of the exercise stages and at maximal exercise.
All comparisons between mean values of ˙VE (Table 13-3) measured by the two COSMEDs or DB indicated no significant differences and, indeed, were remarkably close at all stages with the exception of maximal exercise. At this latter intensity, the difference was less than 6 percent. Correlation coefficients among the three systems were significant (p < 0.01) and above 0.90 for all intensities.
Comparisons among mean values of FeO2 measured by COSMED1, COSMED2, and the DB showed significant differences for the submaximal stages (Table 13-4). Correlation coefficients among testing sessions were significant (p < 0.01) but relatively low for most exercise intensities, consistently below 0.80.
TABLE 13-3 Average Values of Minute Ventilation (liters · min-1, BTPS)
TABLE 13-4 Average Values of Fractional Concentration of Oxygen in Expired Air (%)
TABLE 13-5 Average Values of Oxygen Uptake (liters · min-1, STPD)
Mean values of ˙VO2 among systems showed significant differences only at Stage 6 of the submaximal exercise test (Table 13-5). The percent variation among mean ˙VO2 levels over the three testing sessions was consistently below 5 percent across all exercise intensities. Correlation coefficients among COSMED1, COSMED2, and the DB were significant at all intensities (p < 0.01) and relatively high, always above 0.86.
Because the COSMED K2 does not measure CO2, it is predisposed to an inherent error in calculating ˙VO2. However, Lucia et al. (1993) found that if the average RER obtained from the DB technique is used to correct the ˙VO2 measured by the COSMED, this value does not differ significantly from that of the actual value obtained across all exercise intensities. Thus, the results indicate that the COSMED K2 is a reliable and valid instrument for the measurement of ˙VO2 during laboratory exercise testing at submaximal and maximal intensities. Also the assumption made by the COSMED of a constant respiratory gas exchange ratio of 1.00 did not have a significant influence on ˙VO2 measurements.
The Oxylog, initially reported on by Humphrey and Wolff (1977), was the first of the portable systems able to provide a continuous, direct measure of ˙VO2 and ˙VE for long periods.
The specifications for the Oxylog are presented in Table 13-6. In 1994, the instrument was completely updated with respect to its electronics, data acquisition, and storage capability and the method used to measure O2 concentration in inspired and expired gases (Oxylog Operator's Manual, 1994).
The instrument is equipped with an oronasal mask that is held against the face by an elastic head harness. On the inspired side of the mask, a turbine flowmeter is attached for measurement of inspiratory volume, a unique feature of this instrument. Expired air passes through flexible respiratory tubing connected to the Oxylog. The older model measured the inspired ventilation over a range of only 6 to 80 liters · min-1, which, due to the limitations of the system, limited the measurement of ˙VO2 to 3.0 liters · min-1. The new model, with a larger flowmeter and greater ventilatory range, can measure ˙VO 2 up to 10 liters · min-1.
The O2 fraction of the inspired air is measured separately from that of the expired air by using two fuel cells (the older model used polarographic electrodes) in samples dried by passing through a desiccant. The use of two fuel
TABLE 13-6 Oxylog Specifications
cells to determine the difference in O2 concentrations (between inspired air vs. expired air) is also a unique feature among portable systems. The TEEM 100 and COSMED K2 both assumed an inspired O2 concentration of 20.93 percent.
The Oxylog uses the same formula as the COSMED K2 for calculating ˙VO2 and is, therefore, subject to the same error when RER is other than 1.00. At RERs of 0.8 and 0.9, this underestimation is approximately 3.5 percent and 1.8 percent, respectively (Harrison et al., 1982).
The Oxylog is powered by NiCd batteries that provide a timed use per charge of up to 12 hours. This represents a major advantage over the previous systems. ˙VO2 and ˙VE data are stored internal each minute during use and may be retrieved by serial link when connected to an IBM-compatible computer. The total inclusive storage time is over 2,000 minutes.
Over the past few years, several authors have evaluated the accuracy of the Oxylog for use in both the laboratory and the field environment for measurement of ˙VO2 (Ballal and MacDonald, 1982; Harrison et al., 1982; Louhevaara and Ilmarinen, 1985).
Harrison et al. (1982) compared steady-state values of minute ventilation and ˙VO2 measured by the Oxylog and a standard laboratory system comprising a dry gas meter and mass spectrometer during cycle ergometer exercise at intensities ranging from 75 to 150 W. They found that mean differences between ˙VI (Oxylog) and ˙VE (standard) were significant (p < 0.05) only at the
highest intensity (4.9% lower for Oxylog), while Oxylog values for ˙VO2 were 4.4 percent lower (p < 0.01) than standard values for all intensities.
In the study by Louhevaara and Ilmarinen (1985), similar results were found in a comparison between the Oxylog and the DB technique during both dynamic exercise (walking) and combined static (lifting) and dynamic exercise at various intensities. While the correlation coefficients between these two systems were 0.99 and 0.91 for walking and lifting, respectively, the Oxylog underestimated the ˙VO2 by an average of 4.1 percent for walking and 6.4 percent for lifting.
The Oxylog also was evaluated in this laboratory (Unpublished data, J. F. Patton, M. M. Murphy, R. P. Mello, T. Bidwell, and M. Harp, U.S. Army Research Institute of Environmental Medicine, Natick, Mass., 1992) by comparing it to the DB method in 12 men during steady-state treadmill walking at 3.5 mph at grades of 0 percent, 5 percent, and 10 percent. The data for inspired ventilation and ˙VO2 are presented in Tables 13-7 and 13-8, respectively.
As seen, these data agree quite closely with previous reports that found the Oxylog to underestimate ˙VO2 by 4 to 5 percent. Again, if the correction factor for RER is applied (1.8% for an RER of 0.9; 3.5% for 0.8), the underestimation is reduced to approximately 2 to 3 percent since the RER at the above intensities was between 0.83 and 0.90.
The correlation coefficient for ˙VO2 between the DB technique and the Oxylog was 0.98 (p < 0.001), indicating a very good relationship over the range of exercise intensities studied.
Other Studies Using the Oxylog
In a report by the World Health Organization (WHO) on energy and protein requirements (1985), emphasis was placed on the importance of relying on
TABLE 13-7 Ventilation, liters · min-1 (mean ± SE)
TABLE 13-8 Oxygen uptake, liters · min-1 (mean ± SE)
measures of energy expenditure rather than energy intake as a basis for arriving at possible estimates of energy requirements of various individuals and populations. As a result, a number of investigators have reported on the use of the Oxylog for the direct measurement of metabolic rate to validate daily energy expenditure estimates and to standardize methodology.
In a study by Chiplonkar et al. (1992), the Oxylog was used to measure resting metabolic rate (RMR) in both men and women to validate the FAO/WHO/UNU (Food and Agriculture Organization/World Health Organization/United Nations University) equation for RMR which, in turn, was applied using FAO/WHO/UNU factors for energy cost of physical activities as multiples of RMR to calculate daily energy expenditure estimates.
Soares et al. (1989) measured the basal metabolic rates (BMRs) of 34 healthy individuals with the Oxylog and compared these values to other procedures, such as the Hartmann and Braun Metabolator, ventilated tent and hood, and whole-body indirect calorimeter, and found no significant differences among instruments. This comparison was made because it is important to know whether errors in methodology could account for differences in BMR when collating worldwide measurements.
McNeill et al. (1987), in another metabolic study, modified the Oxylog turbine flowmeter to operate at low flow rates and then compared its use to the DB method for the measurement of RMR. They reported a correlation coefficient of 0.94 between systems, with an underestimation of ˙VO2 of 4 to 5 percent by the Oxylog, similar to that previously seen during exercise, and concluded that it was sufficiently accurate for field studies of energy expenditure.
In other applications of the Oxylog, Ikegami et al. (1988) described the development of a telemetry system for the Oxylog and then applied it to the measurement of ˙VO2 during a doubles tennis game lasting 80 minutes. This probably represented the first reported successful continuous measurement of ˙VO2 during actual sports activity.
Patton et al. (1995) also used the Oxylog to quantify the increase in energy cost for both men and women of performing physical tasks (such as load
carriage, lifting, lift and carry, and obstacle course navigation) in chemical protective clothing. These tasks were conducted in both laboratory and field conditions. It was concluded that the Oxylog provided a very accurate measure of ˙VO2 and represented the only practical way such information could be obtained.
Finally, Riley et al. (1992) have employed the Oxylog to assess the functional capacity of patients with chronic cardiac failure during corridor walk testing.
It is evident from these reports that the Oxylog has proven to be an acceptable instrument for the measurement of ˙VO2 in nutritional, physiological, and physical rehabilitation type studies.
Author's Conclusions and Recommendations
The portable systems described herein feature state-of-the-art technology and represent considerable improvements to previously available systems for the direct measurement of ˙VO2.
Available data suggest that all three systems are valid and reliable for the measurement of ventilation and oxygen uptake under laboratory conditions. However, only the Oxylog has been thoroughly tested under a variety of field scenarios and been found sufficiently accurate for reliable determinations of ˙VO2.
The technology in these systems has matured sufficiently for practical use and is being used in numerous laboratories throughout the world. Further advances in technology will undoubtedly continue, but it is not expected that any significant changes will be made in these instruments in the immediate future since all have been recently developed or updated.
The major drawback to the wide-scale use of these portable systems is their cost (the Oxylog and TEEM 100 are approximately $8,000 and the COSMED K2 is approximately $35,000). This limits the use of these systems to relatively few subjects. This cost, however, should be weighed against that of techniques that estimate only energy expenditure, that is, a cost/benefit (accuracy) analysis must be considered.
The use of Department of Defense funds for further technological developments of portable systems does not seem warranted since these systems encompass the latest in available technology.
The technology utilized in these systems is very practical and requires trained personnel to operate the equipment, but it is not inordinately complex or exotic. The techniques can be learned readily, and the data provided are easily analyzed and interpreted.
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KARL FRIEDL: Are those underestimations something that can be fixed with just a screw or something?
JOHN PATTON: The only way you can really fix them is to know what your respiratory exchange ratio is at that level of exercise. If you knew that, and you never do when you are using these systems, you could correct it, because there are correction factors for that.
For example, if the RER of exercise is 0.9, then the Oxylog and the COSMED will underestimate oxygen uptake about 1.8 percent, so if your underestimation is 4 percent, about half of that is due to the fact that you are not measuring carbon dioxide and using that in the equation to calculate oxygen uptake. That is a problem.
JOHANNA DWYER: John, I did not see a clear advantage, and I saw a big cost of the COSMED versus the Oxylog. Did I miss something?
JOHN PATTON: Advantage between them?
JOHANNA DWYER: Yes, for the COSMED.
JOHN PATTON: The cost difference is the big factor, I think.
DOUGLAS WILMORE: Why? That one was out of line with the other two.
JOHN PATTON: It is telemetry.
JOHANNA DWYER: I just wondered what the advantage was in terms of …
JAMES DeLANY: But it is not lighter.
JOHN PATTON: Oh, yes, it is. The COSMED K2 is very light.
JOHANNA DWYER: Then that is what it is. Sorry. It is lots lighter.
JOHN PATTON: The data you get out of all these systems appear to be very similar in terms of the underestimation. I kind of am wedded to the Oxylog. I have used it, I know it, I feel comfortable with it. I am not that knowledgeable with the other systems, but that is not to say that the other systems are not good, too.
WENDY KOHRT: What about calibration procedures, and how long is the calibration stable?
JOHN PATTON: The Oxylog is very stable. I cannot really comment on the other two systems, I really do not know. We calibrate the Oxylog at the beginning of the experiment, and it usually holds its calibration for at least an hour or two without any problem—you might want to check it. You can usually tell. You start seeing a little drop-off in the oxygen uptake, but it is pretty stable for 3 or 4 hours.
WENDY KOHRT: And you just calibrate it to room oxygen?
JOHN PATTON: Yes, and barometric pressure. That is all you are going to need. Well, we can also run gases through it to calibrate the fuel cell. You can do that, too. And volume, you can put volume through it, too, sure. But it is fairly stable.
DOUGLAS WILMORE: John, what do you do with these data? Do you convert them to energy equivalencies? What do you do with them?
JOHN PATTON: As far as oxygen uptake, we do not really, at least in my lab, convert it very frequently to kilocalories. You can calculate the data in terms of oxygen uptake consumed per kilogram body weight per minute.
DOUGLAS WILMORE: So you use it in a relative way, basically?
JOHN PATTON: Right.
DOUGLAS WILMORE: In other words, you say, here [at this time point] they are at rest, and here [at another time point] they are doing a task.
JOHN PATTON: Yes, just to quantify the level of intensity of the activity or whatever they are doing.
DOUGLAS WILMORE: Have you ever used just heart rate or something like that?
JOHN PATTON: Oh, sure.
DOUGLAS WILMORE: Does that get you to the same place?
JOHN PATTON: Sure, you can do a heart rate-oxygen uptake relationship. Is that what you mean? You can do that on the treadmill. But when you go out to the field, I do not think that heart rate remains reliable. If you did a load carriage study on the treadmill and got a heart rate-oxygen uptake relationship, and then you were to go out in the field and do load carriage, you could see some relationship there between heart rate and oxygen uptake, but very seldom do soldiers go out and do just one type of exercise.
DOUGLAS WILMORE: So thermal loads and things [affect the results]?
JOHN PATTON: That is another thing that will affect it, certainly, the length of activity; all kinds of variables will affect the heart rate. I just do not think heart rate has been too reliable as a measure of energy expenditure.
KARL FRIEDL: John, as an example of the application that we used this for, didn't you do a big MOPP [Military Oriented Protective Posture] study? Did you talk about that?
JOHN PATTON: No, I did not, really.
KARL FRIEDL: That is a beautiful example. I think that is what he is asking. How do you use this?
JOHN PATTON: I did not present any of those data. Certainly one thing that we used it for is to look at the difference in energy cost between performance of physical tasks in the MOPP-4 condition2 compared with a MOPP-0 condition,3 looking at changes and at the effect of the MOPP gear itself on physical task performance, and this entailed all kinds of things, from load carriage to lifting and carrying to litter carrying to obstacle course and all kinds of militarily relevant tasks, so we could get quantification of the MOPP gear per se.
Yes, thank you, Karl, that is one way in which we have used it.
ARTHUR ANDERSON: I have a question from the perspective of a scuba diver concerned about oxygen utilization while diving. Apropos of the MOPP gear comment, I felt it was important to ask the question of whether or not you tested this equipment in a MOPP gear environment in Riyadh during Operation Desert Storm, when the extreme emotional stress of having false alarms go off caused people to put their MOPP gear on. Would the hyperventilation associated with stress interfere with the benefit of the data that you get out of that oxygen utilization versus breathing in a nonstressful laboratory?
JOHN PATTON: You mean in a masked condition? I do not think it is going to change the oxygen uptake too much. Sure, your ventilation will change, and the amount of oxygen that actually is extracted is going to change. But when you
apply it to the equation for oxygen uptake, there is going to be little change, little effect on oxygen uptake per se. I mean with just the masked condition itself, you tend to hypoventilate a little bit, but you extract more oxygen, so when you make the calculation for oxygen uptake there is little change.
ARTHUR ANDERSON: Hyperventilation, where you are inefficiently ventilating and blowing off air and not necessarily …
JOHN PATTON: Okay, then, your extractions would be much less, so when you calculate oxygen uptake, you are quite likely to get similar values.
JOHN VANDERVEEN: Did they have any evaluation of how well the carbon dioxide analyzer works in the TEEM 100?
JOHN PATTON: I have not seen any data on that, no. All I presented were the data on ventilation and the oxygen uptake. That is a good question. I know that is being looked at right now, as a matter of fact, both the oxygen and the carbon dioxide.