Measurement of cardiac output during exercise by open-circuit acetylene uptake

Abstract

Noninvasive measurement of cardiac output (Q˙t) is problematic during heavy exercise. We report a new approach that avoids unpleasant rebreathing and resultant changes in alveolar[Formula: see text] or[Formula: see text] by measuring short-term acetylene (C2H2) uptake by an open-circuit technique, with application of mass balance for the calculation ofQ˙t. The method assumes that alveolar and arterial C2H2pressures are the same, and we account for C2H2recirculation by extrapolating end-tidal C2H2back 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 C2H2and 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 ofQ˙t were available showed that measurement ofQ˙t by the Fick method and by the C2H2technique was statistically similar from rest to 90% of maximal O2 consumption (V˙o 2 max). Data from 12 active women and 12 elite male athletes at rest and 90% ofV˙o 2 max fell on a single linear relationship, with O2 consumption (V˙o 2) predictingQ˙t values of 9.13, 15.9, 22.6, and 29.4 l/min atV˙o 2 of 1, 2, 3, and 4 l/min. Mixed venous [Formula: see text] predicted from C2H2-determinedQ˙t, measuredV˙o 2, and arterial O2 concentration was ∼20–25 Torr at 90% ofV˙o 2 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.