Noninvasive techniques to estimate cardiac output (Qc) will be used during future space flight. This retrospective literature survey compared the Qc techniques of carbon dioxide rebreathing (CO2-R), CO2 single breath (CO2-S), Doppler (DOP), impedance (IM), and inert gas (IG: acetylene or nitrous oxide) to direct (DIR) assessments measured at rest and during exercise

Barrows, Linda H.

Moore, Alan D.

Rashid, Michael

Siconolfi, Steven F.

English

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Evaluati0n of Noninvasive
Cardiac Output Methods
During Exercise
Alan D. Moore,
Linda H. Barr0ws;-
Michael Rashid,
and Steven F. Siconolfi
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NASA
Technical
Paper
3174
1992
National Aeronautics and
Space Administration
Office of Management
Scientific and Technical
Information Program
Evaluation of Noninvasive
Cardiac Output Methods
During Exercise
Alan D. Moore
and Linda H. Barrows
I_ UG Life Sciences
Houston, Texas
Michael Rashid
and Steven F. Siconolfi
Lyndon B. Johnson Space Center
Houston, Texas

Introduction
Cardiac output is a measure of the ability of the heart to meet the
demands imposed by physical activity. The gold standard used for
determination of cardiac output is the direct Fick method. This
method requires sampling of both arterial and mixed venous (right
heart or pulmonary arterial) blood and is impractical to perform on
human subjects, particularly during activity. Several noninvasive
techniques of cardiac output estimation have been developed. These
techniques include:
• carbon dioxide rebreathing
• prolonged single breath carbon dioxide measurements
• rebreathing of a physiologically inert gas such as acetylene or
nitrous oxide
• impedance changes in the thoracic cavity due to fluid output of
the left ventricle
• measurement of ascending aortic blood flow velocity and aortic
diameter using Doppler and two dimensional echocardiography.
Measurement of cardiac output during exercise is proposed for
future studies during manned space flight missions. Each method is
associated with at least one drawback that makes it less than optimal
for use in flight. Two examples of viable methods and their
limitations are: (1) Doppler echocardiographic estimates of cardiac
output, which require a substantial amount of technician training for
the results to be reliable; and (2) rebreathing methods, which require
extra mass to store the rebreathing gas and a more complex gas
analysis system than that necessary for routine metabolic
monitoring. No one method has an advantage with regard to in-flight
technical feasibility. Thus, it is logical to compare each of the
techniques to determine if they are suited for use on the basis of
consistent measurement error. This comparison has not been
conducted to date.
The purpose of this investigation was to develop a data base from
published literature regarding cardiac output responses to graded
exercise. The data base will be used to determine which of the
noninvasive methods yielded values most comparable to those
obtained by the direct Fick technique.
Methods
A literature search was conducted to identify published data
regarding the use of noninvasive techniques to measure cardiac
output during exercise. Over 100 published papers were identified.
Studies reporting cardiac output and exercise values of each subject
are included in the data base. Studies with only group mean cardiac
output data are not included in the data base. Forty studies were
identified that met these criteria, representing 2,196 separate
estimates of steady-state exercise cardiac outputs. Exercise heart
rate responses were used to group exercise intensities. Three
exercise levels were defined as 33-64, 65-72, and 72-100% HRmax.
Examinations of the mean cardiac output responses to exercise levels
were conducted using a 3x6 ANOVA to compare all noninvasive
methods to the Fick technique. The correlation of each noninvasive
method to exercise level was compared to the relationship between
the direct Fick technique and exercise level. This comparison was
made to determine if the rate of increase in cardiac output for any of
the noninvasive methods was significantly different than the rate of
increase in cardiac output measured by the Fick technique.
Results/Discussion
None of the mean values for resting or exercise cardiac output
determined by the noninvasive methods differed significantly from
the values obtained with the direct Fick technique. This finding may
reflect that the methods estimated cardiac output well, or could be
due to the method by which exercise intensity was coded.
The expected increase in cardiac output with exercise intensity was
observed, regardless of the method.
The correlations between cardiac output and exercise intensity for
the impedance method (Figure 1) and the CO2 rebreathing method
(Figure 2) were significantly different (p<0.01) than the correlation
between the direct Fick technique and exercise intensity. The
impedance method overestimated cardiac output at lower levels of
exercise (33-64% HRmax) and underestimated cardiac output at
higher (74-100% HRmax)levels. The CO2 rebreathing method
2
overestimated cardiac output at all levels. These findings indicate
that impedance and CO2 rebreathing methods are not candidates of
choice.
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Exercise Intensity (%HRmax)
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Exercise Intensity (%HRmax)
Figure 2 - C02 Rebreathing
9O
The correlations between cardiac output and exercise intensity for
the CO2 single breath, Doppler, and inert gas methods were not
significantly different than the correlation between the direct Fick
technique and exercise intensity (Figures 3-5). The inert gas
estimates were most similar to those obtained by the direct Fick
technique with regard to slope and intercept of relationship. The
Doppler and CO2 single breath methods had the lowest sample sizes
of those in the literature data base (with 15 and 18 data points,
respectively). These small sample sizes may have introduced bias
(by violation of the central limit theorem) into the statistic used.
Thus, more information is needed on the Doppler and CO2 single
breath methods before firm conclusions can be drawn.
3
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Exercise Intensity (%HRmax)
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Exercise Intensity (%HRmax)
Figure 4 - Doppler Echocardiography
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Figure 5 - Inert Gas Methods
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90
Conclusions
The ANOVA results indicate no evidence to suggest that the
noninvasive methods used to estimate exercise cardiac output were
different than direct Fick measurements.
The inert gas method best expressed the expected relationship
(correlational analyses) between exercise intensity and cardiac
output. Both the CO2 single breath and Doppler methods were not
significantly different than the Fick technique; however, the small
sample sizes reported on these two methods make this finding
preliminary at best.
4
The impedance and CO2 rebreathing methods did not demonstrate a
similar relationship to exercise intensity as that obtained using the
direct Fick technique. Therefore, although the mean values obtained
by these methods did not significantly differ from those of the direct
Fick technique, refinement of these methodologies may be merited.
These methods currently cannot be recommended for use during
space flight.
Future Directions
A retrospective literature search is a useful procedure to address
methodology concerns in that it can identify inherent problems not
specific to individual laboratories. A stronger approach is to conduct
a study comparing the methods of cardiac output estimation to each
other and to the direct Fick method.
The literature data base will be continually updated to resolve which
of the noninvasive methods yield the best estimates of cardiac
output during exercise.
5
References
1. Capek, J. M. and Roy, R. J. Noninvasive measurement of cardiac
output using partial C02 rebreathing. IEEE Transactions on
BiomedicaI Engineering 35(9):653-661, 1988.
2. Christie, J., Sheldahl, L. M., Tristani, F. E., Sagar, K. B., Ptacin, M.
J., and Wann, S. Determination of stroke volume and cardiac output
during exercise: Comparison of two-dimensional and Doppler
echocardiography, Fick oximetry, and thermodilution. Circulation
76(3):539-547, 1987.
3. Daley, P. J., Sagar, K. B., and Wann, L. S. Doppler echocardio-
graphic measurement of flow velocity in the ascending aorta during
supine and upright exercise. Br. Heart J. 562-567, 1985.
4. Marks, C., Katch, V., Rocchini, A., Beckman, R., and Rosenthal, A.
Validity and reliability of cardiac output by C02 rebreathing. Sports
Med. 2:432-446, 1985.
5. Porter, J. M. and Swain, I. D. Measurement of cardiac output by
electrical impedance plethysmography. J. Biomedical Engineering
9:222-231, 1987.
6. Sackner, M. A., Greeneltch, D., Heiman, M. S., Epstein, S., and
Atkins, N. Diffusing capacity, membrane diffusing capacity, capillary
blood volume, pulmonary tissue volume, and cardiac output
measured by a rebreathing technique. Am. Review of Respiratory
Disease 111:157-165, 1975.
7. Smith, S. A., Russell, A. E., West, M. J., and Chalmers, J.
Automated noninvasive measurement of cardiac output: Comparison
of electrical bioimpedance and carbon dioxide rebreathing
techniques. Br. Heart J. 59:292-8, 1988.
8. Szlyk, P. C., Evans, K. C., and Sils, I.V. Validation of a modified
one-step rebreathing technique for measuring exercise cardiac
output. Aviat. Space Environ. Med. 59:1193-1197, 1988.
9. Triebwasser, J. H., Johnson, R. L., Burpo, R. P., Campbell, J. C.,
Reardon, W. C., and Blomqvist, C. G. Noninvasive determination of
cardiac output by a modified acetylene rebreathing procedure
6
utilizing mass spectrometer measurements.
Med. 48(3):203-209, 1977.
Aviat. Space Environ,
10. van Herwaarden, C. L. A., Binkhorst, R. A., Fennis, J. F. M., and
van't Laar, A. Reliability of the cardiac output measurement with the
indirect Fick-principle for C02 during exercise. Pflagers Archiv.
385:21-23, 1980.
1 1. Wilmore, J. H., Farrell, P. A., Norton, A. C., Cote, R. W., HI, Coyle,
E. F., Ewy, G. A., Temkin, L. P., and Billing, J. E. An automated, indirect
assessment of cardiac output during rest and exercise. J. Appl.
Physiol.: Respirat. Environ. Exerc. 52(6):1492-1497, 1982.

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Evaluation of Noninvasive Cardiac Output Methods During
Exercise
6 AUTHOR(S)
Alan D. Moore,
and Steven F.
Linda H. Barrows, Michael Rashid,
Siconolfi
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Space Biomedical Research Institute
National Aeronautics and Space Administration
Johnson Space Center
Houston, Texas 77058
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A. O. Moore and L. H. Barrows (KRUG Life Sciences, Houston, Texas)
M. Rashid and S. F. Siconolfi (NASA Johnson Space Center, Houston, Texas)
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13. ABSTRACT (Maximum 200 words)
Nonlnvasive techniques to estimate cardiac output (Qc) will be used during future space
flight. This retrospective literature survey compared the Qc techniques of carbon
dioxide rebreathing (C02-R), C02 single breath (C02-S), Doppler (DOP), impedance (IM)
and inert gas (IG: acetylene or nitrous oxide) to direct (DIR) assessments measured at
rest and during exercise.
14 SUBJE_ TERMS
Cardiac output, carbon dioxide rebreathing, C02 single breath,
Doppler, impedance, inert gas, and direct Fick
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Evaluation of noninvasive cardiac output methods during exercise

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