Measurement of cardiac output during exercise by acetylene uptake

Non-invasive measurement of cardiac output (QT) is problematic during heavy exercise. We report a new approach avoiding intolerable rebreathing by using short-term acetylene (C₂H₂) uptake and applying mass balance, assuming that alveolar and arterial C₂H₂ pressures are the same. To account for C₂H₂ recirculation, end tidal [C₂H₂] is extrapolated back to breath 2 of the maneuver, which involves simply switching the subject to air containing trace amounts of C₂H₂ (and helium to correct C₂H₂ for incomplete mixing). The two gases are measured continuously by mass spectrometer for 20-25 breaths. Results for two subjects for whom Fick O₂ measurement of QT were available revealed statistical similarity from rest to 90% V̇O₂MAX. Data from 9 active women at rest and 90% of V̇ O₂MAX, and from 12 elite athletes also at rest and 90% V̇O₂MAX fell on a single, linear relationship with V̇O₂ predicting QT of 8.8, 15.4, 22.0, and 28.7 1/min at V̇O₂ of 1,2, 3 and 4 1/min. Mixed venous PO₂ predicted from C₂H₂-QT, measured V̇O₂ and arterial [O₂] was about 20-25 Torr at 90% V̇ O₂MAX (room air) and 10-15 Torr breathing 13% O₂. This modification of previous gas uptake methods to avoid rebreathing produces reasonable data from rest to heavy exercise in normal subjects.