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

Noninvasive measurement of cardiac output (Q̇T) 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 Q̇T. 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 Q̇T were available showed that measurement of Q̇T by the Fick method and by the C₂H₂ technique was statistically similar from rest to 90% of maximal O₂ consumption (V̇O(2max)). Data from 12 active women and 12 elite male athletes at rest and 90% of V̇O(2max) fell on a single linear relationship, with O₂ consumption (V̇O₂) predicting Q̇T values of 9.13, 15.9, 22.6, and 29.4 l/min at V̇O₂ of 1, 2, 3, and 4 l/min. Mixed venous PO₂ predicted from C₂H₂-determined Q̇T, measured V̇O₂, and arterial O₂ concentration was ~20-25 Torr at 90% of V̇O(2max) 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.