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Institute for Exercise and Environmental Medicine, Presbyterian Hospital of Dallas 75231, and University of Texas Southwestern Medical Center, Dallas, Texas 77231
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
ACKNOWLEDGEMENTS
FOOTNOTES
REFERENCES
ABSTRACT 
Williams, J. S., and T. G. Babb. Differences between
estimates and measured PaCO2 during rest
and exercise in older subjects. J. Appl.
Physiol. 83(1): 312-316, 1997.
Arterial
PCO2 (PaCO2) has been estimated during
exercise with good accuracy in younger individuals by using the Jones
equation
(PJCO2)
(J. Appl. Physiol. 47: 954-960,
1979). The purpose of this project was to determine the utility of
estimating PaCO2 from end-tidal PCO2
(PETCO2) or
PJCO2
at rest, ventilatory threshold (
Th), and maximal
exercise (Max) in older subjects. PETCO2 was determined from
respired gases simultaneously (MGA 1100) with arterial blood gases
(radial arterial catheter) in 12 older and 11 younger subjects at rest
and during exercise. Mean differences were analyzed with paired
t-tests, and relationships between the
estimated PaCO2 values and the actual
values of PaCO2 were determined with
correlation coefficients. In the older subjects, PETCO2 was not significantly
different from PaCO2 at rest (
1.2 ± 4.3 Torr), Th (0.4 ± 2.5), or Max
(0.8 ± 2.7), and the two were significantly
(P < 0.05) correlated at
th (r = 0.84) and
Max (r = 0.87) but not at
rest (r = 0.47).
PJCO2
was similar to PaCO2 at rest (1.0 ± 3.9) and th (1.3 ± 2.3) but significantly lower at Max (3.0 ± 2.6), and the two were
significantly correlated at th
(r = 0.86) and Max
(r = 0.80) but not at rest (r = 0.54).
PETCO2 was significantly
higher than PaCO2 during exercise in the
younger subjects but similar to PaCO2 at rest.
PJCO2
was similar to PaCO2 at rest and
th but significantly lower at Max in younger
subjects. In conclusion, our data demonstrate that
PaCO2 during exercise is better
estimated by PETCO2 than by
PJCO2
in older subjects, contrary to what is observed in younger subjects.
This appears to be related to the finding that
PETCO2 does not exceed
PaCO2 during exercise in older subjects,
as occurs in the younger subjects. However,
PaCO2 at rest is best estimated by
PJCO2
in both younger and older subjects.
aging; blood gases; arterial end-tidal carbon dioxide difference; arterial partial carbon dioxide pressure
INTRODUCTION THE MEASUREMENT OF END-TIDAL
PCO2
(PETCO2) has been
used to estimate arterial
PCO2
(PaCO2) in young subjects during
rest and exercise (7, 12, 16). Although PETCO2 may underestimate
PaCO2 at rest, most studies of younger
subjects have concluded that
PETCO2 represents a good
index of PaCO2 at rest. However, it has
been shown that PETCO2 may
significantly overestimate PaCO2 during
exercise. The instantaneous alveolar
PCO2
(PACO2) fluctuates cyclically with breathing, and the
PETCO2 is higher than the
average PACO2 over the
complete breathing cycle, particularly during exercise (3). To predict
PaCO2 from
PETCO2 and tidal volume
(VT), Jones et al. (7)
developed a regression equation
(PJCO2)
that corrects for the overestimation of
PaCO2 by
PETCO2
where
VT is in liters.
Results demonstrated that PaCO2
was predicted to within 1.04 (±SD) Torr between 25 and
58 Torr (r = 0.96) in a group of
younger subjects (n = 56). This
noninvasive estimation of PaCO2
from PETCO2 facilitates the
use of repeated measures where the risks associated with an arterial
puncture are eliminated. The purpose of the present study was to
determine the utility of
PJCO2
in older subjects by comparing differences between
PJCO2
and measured PaCO2 during rest and
exercise. Changes in respiratory function secondary to the aging
process, such as increased dead space and ventilation-perfusion (A/
)
inhomogeneity, may alter the relationship between
PETCO2 and
PaCO2. The Jones equation is designed to
correct for the overestimation of PaCO2
by PETCO2, and during
conditions of increased dead space and
A/
mismatching, which may occur with aging, PJCO2
may not be an applicable estimate of
PaCO2.
METHODS
O2) and
CO2 production
(CO2) were determined with
a customized gas-exchange system (NEC 486DX) on a breath-by-breath basis and averaged over 20-s intervals. Gas samples were drawn continuously at 60 ml/min from a mouthport and analyzed with a mass
spectrometer (model 1100, Marquette Electronics). Expired volume was
measured at the mouth with a turbine flowmeter (Interface Associates)
that was calibrated before each test with a 3-liter calibration
syringe. Subjects breathed through a two-way valve (model 2700, Hans
Rudolph) attached distally to the turbine flowmeter. Total system dead
space was 170 ml, and system resistance was <1
cmH2O · l
1 · s
up to 6 l/s expiration. Validation of the automated gas-exchange system
is provided by regularly performed comparisons with values obtained
through the expired collection bag technique. The ECG was monitored
continuously (model CS-100, Schiller), and blood pressure was monitored
via an automated system (model 4240, SunTech). PETCO2 values were
determined and averaged from the breaths occurring during the middle 20 s of each 1-min workload up to maximal exertion. At maximal exercise,
PETCO2 values were
also averaged over 20 s before the cessation of exercise. Standard
reference gases were used to calibrate the respired gas analyzer before
all testing. Care was taken to ensure that the total lung capacity
(TLC) maneuver was obtained after the
PETCO2 determination or
blood sampling period. The dead space-to-tidal volume ratio
(VDS/VT)
was calculated by using standard procedures and
PaCO2 and averaged expired
PCO2 from the same time period.
Breathing mechanics.
End-expiratory lung volume (EELV) was estimated at rest and during
exercise from measurement of inspiratory capacity (IC). Measurement of
IC was performed by having the subject, on cue from the investigator,
inhale maximally to TLC during the last few seconds of each exercise
stage. The subjects in our study were able to perform the procedure
without difficulty. EELV was estimated to determine the
VT/EELV ratio (in %). EELV was
expressed as a percentage of TLC (EELV/TLC%).
Blood samples.
Measurements of arterial blood gases were made via an arterial catheter
placed in the radial artery. The catheter was connected to an extension
tube. After drawing of sample-line dead space, samples were drawn into
a heparinized syringe at rest and during the middle 20 s of each
exercise level during the same time frame in which the
PETCO2 determinations were
made. At maximal exercise, the blood samples were obtained as close as
possible to the time frame corresponding to the
PETCO2 determinations before
the cessation of exercise. The samples were immediately placed in an
ice bath and were subsequently analyzed (model 282, Instrumentation
Laboratories). Reference gases and commercial standards were used to
calibrate the blood-gas analyzer before all testing. Routine
calibration of the blood-gas analyzer is also provided by participation
in an external quality- control program, and the laboratory meets all
standards of a clinical blood-gas laboratory.
Exercise protocol.
Testing began with the subjects seated on an electronically braked
cycle ergometer (model CPE 200, MedGraphics). After 3 min of baseline
measurements, exercise began at 10 W for the women and 20 W for the
men, with 10- or 20-W increments until exhaustion.
Data analysis.
The data were analyzed with a paired
t-test to determine whether
significant differences existed between measured
PaCO2 and estimated
PaCO2
(PETCO2 and
PJCO2)
at rest, ventilatory threshold (Th) as described by
Davis (2), and maximal exercise (Max) in both younger and older
subjects. The relationship between measured and estimated
PaCO2 was determined by correlation
coefficients. P values less than 0.05 were considered to be significant. Data were analyzed with the use of
SAS-PC statistical software (13).
RESULTS
The physical characteristics and selected pulmonary function values of
the study population are shown in Table 1.
Each individual subject had normal pulmonary function as determined by
a forced vital capacity (FVC) and forced expiratory volume in 1 s
(FEV1) >80% of predicted and
a TLC >90% of predicted. Mean differences ± SD between measured
and estimated PaCO2 values for both
older and younger subjects are shown in Fig.
1. In older subjects,
PETCO2 was not different
from PaCO2 at rest (1.2 ± 4.3 Torr), th (0.4 ± 2.5 Torr), or Max
(0.8 ± 2.7 Torr).
PJCO2
was similar to PaCO2 in
older subjects at rest (1.0 ± 3.9 Torr) and
th (1.3 ± 2.3 Torr), but it was
significantly lower (P < 0.05) at
Max (3.0 ± 2.6 Torr). In younger subjects,
PETCO2 was not different from PaCO2 at rest (0.7 ± 2.0 Torr) but was significantly higher (P < 0.05) at th (2.5 ± 2.6 Torr) and Max (2.2 ± 1.3 Torr). PJCO2
was not significantly different from
PaCO2 in younger subjects at rest
(0.8 ± 1.8 Torr) or th (0.5 ± 1.3 Torr). At Max,
PJCO2 was significantly lower (P < 0.05)
than PaCO2 in younger subjects (1.4 ± 1.9 Torr).
|
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th,
ventilatory threshold; Max, maximal exercise; 
,
PETCO2 PaCO2; 
,
PJCO2
PaCO2. Values are means ± SD.
The relationship between the estimated and the actual
PaCO2 values for both older and younger
subjects is shown at rest, th, and Max in Fig.
2. In older subjects,
PETCO2 was correlated with
PaCO2
(P < 0.05) at th
(r = 0.84) and Max
(r = 0.87) but not at rest
(r = 0.47).
PJCO2
in older subjects was also correlated with
PaCO2
(P < 0.05) at th
(r = 0.86) and Max
(r = 0.80) but not at rest
(r = 0.54). In younger subjects,
PETCO2 was correlated
(P < 0.05) at rest
(r = 0.85), th
(r = 0.81), and Max
(r = 0.96).
PJCO2
was correlated with PaCO2
(P < 0.05) at rest
(r = 0.87), th
(r = 0.95), and Max
(r = 0.92) in younger subjects. PaCO2 values were not
significantly different between older and younger subjects at rest or
Max; however, a difference was detected at th
(P < 0.05).
VDS/VT
was significantly higher (P < 0.01)
at rest (0.35 ± 0.07 vs. 0.24 ± 0.04), th
(0.25 ± 0.07 vs. 0.14 ± 0.04), and Max (0.26 ± 0.07 vs. 0.17 ± 0.08) for the older compared with the younger
subjects, respectively. The change in
VDS/VT
from rest to exercise was similar for both groups.
DISCUSSION
Our results indicate that, in the older subjects we tested during exercise, PETCO2 provides a better estimate of PaCO2 than does PJCO2. In contrast, as has been previously shown (7, 12), PJCO2 appears to be superior in predicting actual PaCO2 during exercise in younger subjects. Furthermore, at rest, both PJCO2 and PETCO2 provide reasonable estimates of PaCO2 in both groups (Fig. 1). However, in the older subjects, PJCO2 was better correlated to PaCO2 than was PETCO2, and both the estimates had similar correlation coefficients at rest in the younger subjects (Fig. 2).
Differences between PETCO2 and PaCO2 during exercise are well documented in younger subjects (7, 12, 17). PETCO2 has been shown to rise to a significantly higher value than PaCO2 during exercise. Our findings among younger subjects are consistent with these reported observations. At rest, our data showed that PETCO2 was lower, though not significantly different from PaCO2 in younger subjects, also in agreement with previous reports (10, 12).
Consistent with the findings of previous investigations (7, 14) our
results demonstrated that
PJCO2
was not significantly different from
PaCO2 at rest or during submaximal
exercise in younger subjects. At Max,
PJCO2
significantly underestimated PaCO2 in
younger subjects. However, the mean difference was only
1.4 ± 1.9 Torr, and the two variables were
strongly correlated (r = 0.92;
P < 0.05).
A comparison between estimates of PaCO2 and actual PaCO2 in older adults was recently reported by St. Croix et al. (14). PaCO2 was estimated from PJCO2 and PETCO2 at rest and during submaximal exercise (25-50 W). In contrast to our findings, these authors reported that both PETCO2 and PJCO2 significantly overestimated PaCO2 at rest during all experimental conditions. However, measurements were made with various gas mixtures used to force PETCO2 to desired values. The authors attributed these unexpected findings in part to the inspiration of hypercapnic gas mixtures, which have been shown to mask the diluting effect of the alveolar dead space on PETCO2 measurements (10). Our findings at rest in older subjects revealed no significant differences between PETCO2 or PJCO2 and PaCO2, although both estimates were lower than measured PaCO2. Our findings, although contrasting with those of St. Croix et al. (14) in subjects at rest, are consistent with the dilutional effects of underperfused lung apexes due to gravitational forces acting on blood flow, which causes PETCO2 to be lower than PaCO2 at rest in younger subjects (10, 11). PETCO2 and PJCO2 may even be lower at rest in an older population than values observed in younger subjects, because the "normal" decline in lung function that occurs with aging has been shown to increase alveolar dead space (15). Our findings revealed that, although neither PETCO2 nor PJCO2 was significantly different from PaCO2 at rest in either experimental group, a slightly lower value occurred in the older subjects for both estimates.
Our findings are in partial agreement with those of St. Croix et al. (14) at submaximal exercise. During exercise, they found PJCO2 produced estimates that were higher than PaCO2, although not significantly different, and PETCO2 continued to significantly overestimate PaCO2. We found no significant difference between PJCO2 and PaCO2 at this level of exertion in our older subjects. Our findings also revealed that PETCO2 was not significantly different from PaCO2, and, in our older subjects at submaximal exercise, PETCO2 provided a better estimate of PaCO2 than did PJCO2.
To our knowledge, estimates of PaCO2 from PJCO2 have not been reported in older subjects at maximal exercise when greater CO2 delivery to the lung results in an increased slope of the alveolar phase of the expiratory cycle (16). At Max, PJCO2 significantly underestimated PaCO2, although PETCO2 was not different from PaCO2 in our group of older subjects.
Several factors have been reported to determine the extent to which
PETCO2 differs from
PaCO2 during rest and exercise (9). The fluctuation of
PACO2 during the breathing cycle is one such factor (3). As
VT and
CO2 increase during exercise, the
variation in PACO2 during
the respiratory cycle is magnified. The increased
CO2 production and decreasing lung
volume as expiration continues result in
PETCO2 being higher than
PaCO2 during exercise in young healthy
subjects. Comparisons between CO2
production for younger and older subjects in the present study revealed
nonsignificant differences at rest and th
(P > 0.05). However,
CO2 was significantly higher for younger subjects compared with the older subjects at Max
(P < 0.05). The greater
CO2 production at Max in the
younger subjects could account for the higher
PET-aCO2
due to the effect CO2 excretion
has on the slope of the expired
PACO2.
Our observation that VT/EELV%
was greater in the younger subjects appears to support the finding that
PETCO2 exceeds PaCO2 in younger subjects more than in
the older (Fig. 3), as not only the size of
the VT but the volume in which
the VT mixes will affect the
expired gases. Further support is provided by the finding that
EELV/TLC% is greater in the older subjects and decreases less during
exercise. In their study of younger subjects, Jones et al. (7) have
demonstrated that VT was the
single most important determinant of the differences between
PETCO2 and
PaCO2. We found no significant
differences in VT or
VT/FVC% between our older and
younger subjects, although
VT/EELV% was significantly
higher at th and Max for younger subjects compared with the older group. Also, in older subjects,
PETCO2 may not exceed
PaCO2 to the same degree as occurs in
younger subjects, as it has been suggested that the
PET-aCO2
is most dependent on the
VDS/VT
ratio (6).
th, and Max. ,
VT/EELV% in older subjects;
, VT/EELV% in
younger subjects; 
, EELV/TLC% in older subjects; 
,
EELV/TLC% in younger subjects. * Significantly different;
younger compared with older subjects
(P < 0.05).
PaCO2 values have been shown to
exceed PETCO2 values at rest
and exercise during conditions of
A/
inequality or increased alveolar dead space (16), both of which have
been shown to increase as a consequence of aging (6, 15). Johnson et
al. (6) have shown, via direct measures of
PaCO2,
VDS/VT ratios in older fit individuals that were 30% higher than those of
younger subjects, both at rest and during heavy exercise.
VDS/VT provides a valuable estimate of the degree of matching of ventilation to perfusion in the lung during rest and exercise (16). Our data are in
agreement with Johnson et al. (6), as we found significantly higher
VDS/VT
values for the older subjects compared with the younger at rest,
th, and Max. Previously, Liu et al. (9) reported
significantly lower differences between
PETCO2 and
PaCO2 in patients with obstructive lung
disease. Their data are essentially consistent with our findings,
comparing differences between
PETCO2 and
PaCO2 in both our younger and older subjects. Though direct comparisons between patients with known lung
disease and our apparently normal older subjects cannot validly be
made, it is tempting to speculate that a similar
mechanism of
A/
mismatching in our older subjects contributed to the lower differences
observed between PETCO2 and
PaCO2 compared with our younger
subjects. Thus it appears that the Jones equation (7) overcorrects in
situations where PETCO2 does
not rise to a significantly higher value than
PaCO2, as may occur under conditions of
A/
inequality or increased alveolar dead space. On the other hand, the
differences noted between
PJCO2 and PaCO2 at Max in our older subjects
could be related to the lower CO2
production and its influence on the slope of the
PACO2 that would result in a
lower PETCO2 and subsequent
overcorrection by the Jones equation.
From our data, we conclude that
PETCO2 provides the
best estimate of PaCO2 at
th and Max in older subjects. Both PJCO2
and PETCO2 yield reasonable
estimates of PaCO2 in older subjects at
rest. However, in younger subjects,
PJCO2 provides a reasonable estimate of PaCO2
at th but significantly underestimates
PaCO2 at Max.
ACKNOWLEDGEMENTS
The authors thank Joseph O'Kroy, Rebecca Morrow, Robyn Etzel, Stacy Blaker, Julie Zuckerman, and Angela Chen for technical assistance throughout this project; acknowledge the assistance of Penny Palumbo with data reduction and graphics; and express their appreciation to Dr. Benjamin Levine of the medical staff for support of this project.
FOOTNOTES
Address for reprint requests: T. G. Babb, Institute for Exercise and Environmental Medicine, 7232 Greenville Ave., Dallas, TX 75231.
Received 7 June 1996; accepted in final form 17 March 1997.
REFERENCES
| 1. | American Thoracic Society. Evaluation of impairment/disability secondary to respiratory disorders. Am. Rev. Respir. Dis. 133: 1205-1209, 1986[Medline]. |
| 2. | Davis, J. A. Anaerobic threshold: review of the concepts and directions for future research. Med. Sci. Sports Exercise 18: 143-155, 1985[Medline]. |
| 3. |
DuBois, A. B.,
A. G. Britt,
and
W. O. Fenn.
Alveolar CO2 during the respiratory cycle.
J. Appl. Physiol.
4:
535-548,
1952 |
| 4. | Enright, P. L., R. A. Kronmal, M. Higgins, M. Schenker, and E. F. Haponik. Spirometry reference values for women and men 65 to 85 years of age. Cardiovascular health study. Am. Rev. Respir. Dis. 147: 125-133, 1993[Medline]. |
| 5. | Gardner, R. M., J. L. Hankinson, J. P. Clausen, R. O. Crapo, R. L. J. Johnson, and G. R. Epler. Standardization of spirometry-1987 update. Statement of the American Thoracic Society. Am. Rev. Respir. Dis. 136: 1285-1298, 1987[Medline]. |
| 6. | Johnson, B. D., W. G. Reddan, D. F. Pegelow, K. C. Seow, and J. A Dempsey. Flow limitation and regulation of functional residual capacity in a physically active aging population. Am. Rev. Respir. Dis. 143: 960-967, 1991[Medline]. |
| 7. |
Jones, N. L.,
D. G. Robertson,
and
J. W. Kane.
Difference between end-tidal and arterial PCO2 during exercise.
J. Appl. Physiol.
47:
954-960,
1979 |
| 8. | Knudson, R. J., R. C. Slatin, M. D. Lebowitz, and B. Burrows. The maximal expiratory flow-volume curve: normal standards, variability and effects of age. Am. Rev. Respir. Dis. 113: 587-600, 1976[Medline]. |
| 9. |
Liu, Z.,
F. Vargas,
D. Stansbury,
S. A. Sasse,
and
R. W. Light.
Comparison of the end-tidal arterial PCO2 gradient during exercise in normal subjects and in patients with severe COPD.
Chest
107:
1218-1224,
1995 |
| 10. | Matell, G. Time-course of changes in ventilation and arterial gas tensions in man induced by moderate exercise. Acta Physiol. Scand. 58, Suppl. 206: 1-47, 1963. |
| 11. | Overend, T. J., D. A. Cunningham, D. H. Patterson, and W. D. F. Smith. Physiological responses of young and elderly men to prolonged exercise at critical power. Eur. J. Appl. Physiol. Occup. Physiol. 64: 187-193, 1992. [Medline] |
| 12. |
Robbins, P. A.,
J. Conway,
D. A. Cunningham,
S. Khamnei,
and
D. J. Patterson.
A comparison of indirect methods for continuous estimation of arterial PCO2 in men.
J. Appl. Physiol.
68:
1727-1731,
1990 |
| 13. | SAS Institute. SAS-PC Software: Version 6.03. Cary, NC: SAS Institute, 1987. |
| 14. |
St. Croix, C. M.,
D. A. Cunninghan,
J. M. Kowalchuk,
A. K. McConnell,
A. S. Kirby,
B. W. Scheuermann,
R. J. Petrella,
and
D. H. Patterson.
Estimation of arterial PCO2 in the elderly.
J. Appl. Physiol.
79:
2086-2093,
1995 |
| 15. |
Tenny, S. M.,
and
R. M. Miller.
Dead space ventilation in old age.
J. Appl. Physiol.
9:
321-327,
1956 |
| 16. | Wasserman, K., J. E. Hansen, D. Y. Sue, and B. J. Whipp. Principles of Exercise Testing and Interpretation. Philadelphia, PA: Lea & Febiger, 1987, p. 40-41. |
| 17. |
Whipp, B. J.,
and
K. Wasserman.
Alveolar-arterial gas tension differences during graded exercise.
J. Appl. Physiol.
27:
361-365,
1969 |
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