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1 Department of Medicine, University of California, San Diego, La Jolla, California 92093; and 2 Department of Thoracic Medicine, Royal Adelaide Hospital, Adelaide, South Australia, Australia
Noninvasive measurement of cardiac output
(
T) is problematic during
heavy exercise. We report a new approach that avoids unpleasant
rebreathing and resultant changes in alveolar
PO2 or
PCO2 by measuring short-term
acetylene
(C2H2)
uptake by an open-circuit technique, with application of mass balance for the calculation of
T. The
method assumes that alveolar and arterial
C2H2
pressures are the same, and we account for
C2H2 recirculation by extrapolating end-tidal
C2H2
back to breath 1 of the maneuver. We
correct for incomplete gas mixing by using He in the inspired mixture.
The maneuver involves switching the subject to air containing trace
amounts of
C2H2
and He; ventilation and pressures of He,
C2H2,
and CO2 are measured continuously
(the latter by mass spectrometer) for 20-25 breaths. Data from
three subjects for whom multiple Fick
O2 measurements of
T were
available showed that measurement of
T by the
Fick method and by the
C2H2 technique was statistically similar from rest to 90% of maximal O2 consumption
(
O2 max). Data from
12 active women and 12 elite male athletes at rest and 90% of
O2 max fell on a
single linear relationship, with
O2 consumption
(
O2) predicting
T values of 9.13, 15.9, 22.6, and 29.4 l/min at
O2 of 1, 2, 3, and 4 l/min.
Mixed venous PO2 predicted from
C2H2-determined
T, measured
O2, and arterial
O2 concentration was ~20-25
Torr at 90% of
O2 max during air
breathing and 10-15 Torr during 13%
O2 breathing. This modification of
previous gas uptake methods, to avoid rebreathing, produces reasonable
data from rest to heavy exercise in normal subjects.
maximal exercise; new methodology; inert gas
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