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Department of Exercise Science and Sport Management, University of Tennessee, Knoxville, Tennessee 37996-2700
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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The accuracy of a
computerized metabolic system, using inspiratory and expiratory methods
of measuring ventilation, was assessed in eight male subjects. Gas
exchange was measured at rest and during five stages on a cycle
ergometer. Pneumotachometers were placed on the inspired and expired
side to measure inspired (
I) and expired ventilation
(E). The devices were connected to two systems
sampling expired O2 and CO2 from a single
mixing chamber. Simultaneously, the criterion (Douglas bag, or DB)
method assessed E and fractions of O2
and CO2 in expired gas (FEO2
and FECO2) for subsequent calculation of
O2 uptake (O2),
CO2 production (CO2), and
respiratory exchange ratio. Both systems accurately measured metabolic
variables over a wide range of intensities. Though differences were
found between the DB and computerized systems for
FEO2 (both inspired and expired systems),
FECO2 (expired system only), and
O2 (inspired system only), the
differences were extremely small
(FEO2 = 0.0004, FECO2 = 
0.0003,
O2 = 0.018 l/min). Thus a
computerized system, using inspiratory or expiratory configurations,
permits extremely precise measurements to be made in a less
time-consuming manner than the DB technique.
Douglas bag; oxygen uptake; carbon dioxide production; metabolism; pneumotachometer
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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THE MEASUREMENT
OF O2 consumption (O2)
by open-circuit spirometry is one of the fundamental measures in the
field of exercise physiology. Historically, gas exchange was measured
by the Douglas bag method. This involved the collection of exhaled air
in large, impermeable canvas bags and subsequent measurement of gas
fractions and expired volumes (4). The Douglas bag method
has served as the "gold standard" for gas exchange measurements for
over a century.
In the 1960s, with the development of the Parkinson-Cowan dry gas
meter, the measurement of inspired minute ventilation
(I) became common. Expired ventilation
(E) values were calculated using the Haldane
transformation of the Fick equation (5, 6, 13). In the
semiautomated method described by Wilmore and Costill (20), the measurement of FEO2
and FECO2 in expired air was achieved by
drawing representative gas samples from a mixing chamber into 2-liter
latex bags for subsequent analysis. In this application, the expired
gas was collected over the same time period as I was
measured to ensure matching of gas fractions and the ventilatory volumes. A later method involved pumping a continuous stream of exhaled
air from a mixing chamber directly into electronic gas analyzers
(1). The voltage output of the gas analyzers and inspired
ventilation meter was fed through an analog-to-digital converter into a
microcomputer, which carried out the metabolic calculations for
O2 uptake (O2) and
CO2 production (CO2)
(1). Because of the lag time associated with drying and
analysis of the gas, timing adjustments had to be made to assure
matching of the gas volume with its gas fractions (14).
Today, most computerized metabolic systems measure the ventilation rate
on the expired side. One advantage of this method is that the subject
can be connected to the metabolic cart by means of a single expired-gas
hose. A common method of measuring E is with the use
of the Hans Rudolf 3813 pneumotachometer (Kansas City, MO) that was
designed to have flow linearity in the range of 0-800 l/min (peak
flow rates). It consists of a series of three screens that create a
resistance to airflow. The drop in air pressure across the center
screen is used to compute the gas flow rate. However, the Hans Rudolf
pneumotachometer is nonlinear in the lower flow range (<80 l/min).
Hence, the Yeh algorithm (22, 23) is used to further
correct the linearity at low flow rates (<80 l/min) and to assess any
change in resistance created by the upstream geometry or changes in gas
viscosity (e.g., helium-O2 mixtures used in some studies).
The use of a pneumotachometer for measurement of E,
as opposed to I, has certain problems associated
with it. The principal concern is condensation of water vapor on the
screen, due to the moisture present in exhaled air. To eliminate this
concern, Hans Rudolf developed a heated pneumotachometer (model 3813)
that prevents condensation. However, this device increases the
temperature and therefore the volume of gas passing through it
(8). Various methods have been proposed to estimate the
temperature of the gas as it passes through the screen, so that
ventilation rates can be converted to reflect standard temperature and
pressure, dry (STPD) conditions. One method, derived from
the work of Kolkhorst et al. (8), is to average the
ambient temperature with body temperature (37°C). Little information
exists regarding the validity of this averaging method for determining
the expired gas temperature.
In recent years, indirect calorimetry has largely become an automated
procedure; hence, it is important to establish the accuracy with which
gas exchange measurements are made. The purpose of this study was to
validate the measurement of gas exchange using a computerized metabolic
system, with either I or E
measurement. Ventilatory and metabolic variables were compared with the
classical Douglas bag technique, which served as the criterion method.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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Participants. Eight male university students volunteered to participate in the study. The nature of the study was described, and they signed a written, informed consent statement in accordance with the policies of the university's institutional review board. Physical characteristics of the participants were recorded (means ± SD: age = 27.5 ± 5.6 yr, height = 181.8 ± 3.3 cm, weight = 74.9 ± 7.1 kg).
Experimental design. The exercise protocol was preceded by 10 min of seated rest on a Monark 818E cycle ergometer (Varburg, Sweden). Participants then performed a graded exercise test consisting of 5-min stages at power outputs of 50, 100, 150, 200, and 250 W. Before testing, the cycle ergometer was calibrated by placing it on a level surface and hanging known weights (1-4 kg) on the disconnected flywheel belt, while adjusting the position of the pendulum arm to reflect these settings. An electronic metronome was used to keep the participant's cadence at 51 rpm throughout the test.
Each subject was fitted with a rubber mouthpiece connected to a Hans Rudolf 2700 series two-way nonrebreathing valve (Kansas City, MO). A nose clip was worn to prevent nasal breathing. The breathing valve was connected to the metabolic systems on the inspired and expired sides with 2-m corrugated flexible plastic hoses with a 3.2-cm diameter. Continuous gas exchange measurements were made by using two TrueMax 2400 computerized metabolic systems purchased from the same manufacturer (ParvoMedics, Salt Lake City, UT). The software version was Consentius OUSW-3.3. Both systems utilized the Hans Rudolf 3813 pneumotachometer to measure ventilation. However, one of these systems was set up to measureI, whereas the other was set
up to measure E (see Fig.
1). The pneumotachometer on the expired
side was heated to a temperature of 37°C, while the heater on the
inspired side was turned off by unplugging the heater cable from the
back of the unit. The expired gas temperature was assumed to be the
average of body temperature (37°C) and ambient temperature. A
Y-connector was used to join the two gas sampling lines to the mixing
chamber. For each system, expired gas fractions were determined by
drawing a continuous sample of expired air from the mixing chamber
through a 61-cm Nafion Dryer (Permapure, Toms River, NJ) catheter into
a paramagnetic O2 analyzer and infrared CO2
analyzer.
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O2 was calculated
by using the respiratory Fick principle. Where E was
measured, I was computed from the so-called Haldane
transformation (5, 6): I = E × FEN2/FIN2,
where FEN2 and
FIN2 equal the fractional concentrations of nitrogen in the expired and inspired air, respectively. Likewise, in
the method in which inspired ventilation was measured, the expired
ventilation was computed using the same formula. These methods assume
that N2 is neither produced nor consumed by the body in
exercise, an assumption that had previously been questioned by Cissik
et al. (3) but was later examined by Wilmore and Costill
(19) and found to be valid.
Data analysis.
The dependent variables of E STPD,
FEO2,
FECO2,
O2, CO2,
and respiratory exchange ratio were examined at all power outputs, for
each of the three methods. Bland-Altman (2) plots were used to show the individual differences between the criterion method
(Douglas bag) and the inspired or expired computerized metabolic systems.
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RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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Table 1 shows the physiological
responses to a graded exercise test on a cycle ergometer. Data are
expressed as means ± SD. The computerized system (using
inspiratory or expiratory ventilation measures) showed close agreement
with the Douglas bag method for all of the gas exchange variables.
Where significant differences did exist, the magnitude of the
differences was very small. For instance,
FEO2 was slightly lower (by an average of
0.0004 or 0.04%) for both inspired and expired computerized systems,
compared with the Douglas bag method (P < 0.01).
O2 was an average of 0.018 l/min (or 18 ml/min) higher for the inspired system, compared with the Douglas bag
method (P < 0.05). FECO2
was slightly lower (by an average of 0.0003 or 0.03%) for the expired
system, compared with the Douglas bag method (P < 0.05). None of the other variables showed a significant difference
between the computerized systems and the Douglas bag method.
Significant interactions (power output × method) were found for
E, FECO2, and
CO2 for the expired system
(P < 0.05). However, because the magnitude of the
interaction was small, post hoc tests were not carried out.
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Figures 2 and
3 contain the Bland-Altman plots
illustrating the individual difference scores (Douglas bag minus
computerized system) for the inspiratory and expiratory methods.
Overall, the difference scores (expressed as mean and 95% CI) were
centered closely around zero, showing that both of the computerized
systems agreed closely with the Douglas bag method.
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Two of the subjects attained their maximum power output at 200 W. For the remaining six subjects, there were no statistical differences (at 250 W) among the Douglas bag method, inspired metabolic system, or expired metabolic system for any of the metabolic variables. Two other subjects could only complete 3 min of exercise at 250 W, so the collection period for their Douglas bag measurements for the final stage was over the second and third minute. Heart rate values achieved at the end of the test averaged 189 ± 16 beats/min (mean ± SD), and respiratory exchange ratio values assessed by the Douglas bag method averaged 1.14 ± 0.03, indicative of near-maximal exercise.
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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The main finding of the study was that the computerized system,
whether configured to measure inspiratory or expiratory ventilation, yielded gas exchange variables that were extremely close to those obtained by the Douglas bag method. For power outputs ranging from 0 to
250 W, there were only small differences in
O2, CO2, E or other variables. Even though some statistically
significant differences were found, the differences were so small as to
be not physiologically significant.
Because of the stability and linearity of recently developed
O2 and CO2 gas analyzers, the major source of
variation in assessing O2 comes from the
measurement of ventilation rates. The individual differences in
E between the Douglas bag method and ParvoMedics system never exceeded 1.6 l/min for any subject. This was true, for
both expired and inspired systems, for ventilatory rates
(STPD) ranging from 6 to 120 l/min. The accuracy of the
Hans Rudolf 3813 pneumotachometer (in conjunction with the Yeh
algorithm) was remarkable considering that our Douglas bag measurement
periods were done in real time. Wilmore and Costill (19)
noted that when ventilation is measured simultaneously on the inspired
and expired side, the accuracy depends on the subject being "switched
in" and "switched out" at the same phase of the tidal volume at
the start and end of the collection period. Although this was not done
in the present study, the use of 2-min collection periods minimized
this type of error.
Regardless of whether ventilation is measured on the inspired or
expired side, a "time lag" can occur when making a sudden transition between two power outputs. The increase in
I or E will be detected
immediately, but the change in FEO2 and
FECO2 takes longer to be seen. This delay
results from two components: 1) the time needed to wash out
the mixing chamber, and 2) the time for the gas analyzers to
detect the change in gas fractions within the mixing chamber. This
temporal mismatch is less of a concern during steady-state exercise
when I and expired gas fractions are relatively
stable but could be a factor during a graded exercise test.
Powers et al. (14) developed a method to correct for the
time lag problem during nonsteady-state exercise by holding
I data in memory for a user-specified time
period (usually 15-20 s) before combining it with the
FEO2 or FECO2
values. The first component of the time delay varied with the
ventilation rate. The second component of the delay was constant and
reflected a 15-second time lag for the gas analyzers to read the gas
concentrations in the mixing chamber. This second component
predominates at higher flow rates, as the first component diminishes.
The metabolic system (Rayfield Equipment, Waitsfield, VT) used by
Powers et al. (14) had a plastic drying tube containing
Drierite (W. A. Hammond Drierite, Xenia, OH) to dry the gas sample
before it entered the gas analyzers, and this increased the time needed
for stable gas fractions to be recorded.
The ParvoMedics software (inspired configuration) does not account for
the delay between the measurements of ventilation rate and
FEO2 or
FECO2, but this did not affect the
system's accuracy in measuring O2. One
reason is that, with the ParvoMedics system, the sample is transported
from the mixing chamber to the gas analyzers by small-bore Nafion
tubing (eliminating the need for a Drierite tube). Thus the response
time of the gas analyzers in detecting changes in gas fractions within
the mixing chamber is very short (~1 s). Furthermore, at moderate to
high ventilation rates, the time needed to flush out the volume of the
expired-gas hose and mixing chamber (combined total = 5.8 liters)
is brief. In addition, a mismatch between I (or
E) and gas fractions is minimized with the type of
experimental protocol we used, which approximated a steady state during
the last 2 min of each 5-min stage.
A major advantage to measuring E is that a single
expired-gas hose connects the subject to the metabolic system (as
opposed to two hoses for the alternative method). However, when
ventilation is measured with a heated pneumotachometer, one must
estimate the temperature of the gas as it moves through the screen. The averaging method is a simple method that satisfactorily describes the
temperature of the exhaled gas. Another method is to place a
temperature probe downstream of the heated pneumotachometer and
directly measure the gas temperature. Kolkhorst et al. (8) used this method and found that expired temperatures 1 cm downstream of
the heated pneumotachometer were stable at 30.2°C during 45 min of
steady-state exercise. This temperature was ~2.0°C higher than that
measured with the heater turned off. In their study, the probe
temperature was equal to the average of room temperature (23.5°C) and
body temperature (37°C).
We also placed an LM35 precision temperature probe (ParvoMedics, Salt
Lake City, UT) in the mixing chamber 1 cm above the downstream port of
the pneumotachometer to measure expired air temperature. However, this
method of measuring expired gas temperature resulted in
E being overestimated by 2%. Because the gas cools as it moves away from the heated pneumotachometer, the mixing chamber
temperature would have underestimated the actual gas temperature inside
the pneumotachometer. To express volumes under STPD
conditions, gas volumes must be corrected to reflect a temperature of
0°C (273°Kelvin), PB = 760 mmHg, and no water vapor.
This is done by multiplying E ATPS by an
STPD correction factor
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![<IT>=</IT>[273<IT>/</IT>(273<IT>+</IT>T<SUB>a</SUB>)]<IT> ∗ </IT>[(P<SC>b</SC><IT>−</IT>P<SUB>H<SUB>2</SUB>O</SUB>)<IT>/</IT>760 mmHg]](/content/vol91/issue1/fulltext/218/img002.gif)
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E STPD.
The average room temperature across all eight subjects was 21.4°C.
Thus the average temperature of the expired gas was estimated to be
29.2°C (i.e., the average of 21.4 and 37°C). The mixing chamber
values were 3-4° lower than those measured by the averaging method. Thus the averaging method yielded ventilation rates that were
more closely matched with the criterion method than did a mixing
chamber temperature probe. It should be noted that a 1.0°C difference in the estimated expiratory temperature from the actual temperature would result in only a 0.6% error in E
(see APPENDIX). Errors of this magnitude would have only a
minor effect on the calculation of O2 consumption.
The absolute accuracy of the computerized system used in the present
study was greater than observed with some other metabolic systems
(7, 11). For the Aerosport KB1-C, the individual E error scores (Douglas bag minus metabolic system)
had a 95% CI of approximately ±10 l/min (7). By
comparison, the ParvoMedics system error scores
(E) had a 95% CI range of ±1 l/min. Similarly, the
ParvoMedics had one-sixth the error in measuring
FEO2 and FECO2 compared with the Aerosport KB1-C,
indicating superior linearity and stability of the gas analyzers or
better gas sampling techniques. Peel and Utsey (11)
examined the Cosmed K2 system and reported a systematic underestimation
of O2 (by 12.5-17%) at all work rates. (It should be noted that the Cosmed and Aerosport systems are
portable and thus may have unique design features that do not allow for
a fair comparison to the ParvoMedics system.) Porszasz et al.
(12) examined the validity of the Medical Graphics CPX Express for minute ventilation and reported a level of accuracy similar
to that seen with the ParvoMedics system. The Medical Graphics system
uses a symmetrically disposed Pitot tube flow meter and adjusts for
nonlinearity using software correction. However, the Medical Graphics
system was not validated for metabolic variables such as
O2 and
CO2 in this study.
Although many other computerized metabolic systems have been validated in the literature, several studies used a previously validated metabolic system as the criterion (10, 11, 18, 21). In studies in which the Douglas bag method was used as the criterion, the gas exchange measurements were either nonsimultaneous (9, 16, 17) or nonsteady state (14), making direct comparisons with the present study difficult.
In conclusion, a computerized metabolic system (ParvoMedics) using the
Hans Rudolph 3813 pneumotachometer to measure ventilation rates
provides accurate gas exchange measurements, irrespective of whether
I or E is measured. The method of
averaging body temperature and room air seems to be adequate for
estimating the temperature of the expired gas moving through a heated
pneumotachometer. Minimal errors in gas volumes result from this
method, suggesting that direct measurement of the gas temperature is
not necessary when measuring ventilatory rates on the expired side.
Furthermore, a computerized metabolic system permits extremely precise
measurements to be made in a less time-consuming manner than the
Douglas bag technique.
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APPENDIX |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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The effect of temperature on STPD conversion does
not only consist of the temperature-volume relationship described by
Charles' law (1/303 = 0.33% per 1°C, at 30°C). It also
consists of a change in saturated water vapor pressure. At 30°C, the
saturated water vapor pressure is 31.5 mmHg. For any 1°C change, the
water vapor pressure changes by ~1.8 mmHg. The dry PB at
30°C = 760 31.5 = 728.5 mmHg. When the temperature
is around 30°C, the water vapor effect is 1.8/728.5 per 1°C = 0.25%, which is also small. The combined effect is ~0.6% per 1°C
in expiratory flow temperature.
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ACKNOWLEDGEMENTS |
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We thank Jason Langley and William O'Brien for assistance with data collection and subject recruitment and Cary Springer of the University of Tennessee Statistical Consulting Service for help with the data analysis.
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FOOTNOTES |
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The authors have no financial interest in any of the products mentioned in the text or in competing products.
Address for reprint requests and other correspondence: D. R. Bassett, Jr., Dept. of Exercise Science and Sport Management, Univ. of Tennessee, 1914 Andy Holt Ave., Knoxville, TN 37996-2700 (E-mail: DBassett{at}utk.edu).
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 22 September 2000; accepted in final form 9 February 2001.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
APPENDIX
REFERENCES
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