Cardiac output during exercise by the open circuit acetylene washin method: Comparison with direct Fick

An open-circuit (OpCirc) acetylene uptake cardiac output (Q̇T) method was modified for use during exercise. Two computational techniques were used. OpCirc1 was based on the integrated uptake vs. end-tidal change in acetylene, and OpCirc2 was based on an iterative finite difference modeling method. Six subjects [28-44 yr, peak oxygen consumption (V̇O₂) = 120% predicted] performed cycle ergometry exercise to compare Q̇T using OpCirc and direct Fick methods. An incremental protocol was repeated twice, separated by a 10- min rest, and subsequently subjects exercised at 85-90% of their peak work rate. Coefficient of variation of the OpCirc methods and Fick were highest at rest (OpCirc1, 7%, OpCirc2, 12%, Fick, 10%) but were lower at moderate to high exercise intensities (OpCirc1, 3%, OpCirc2, 3%, Fick, 5%). OpCirc1 and OpCirc2 Q̇T correlated highly with Fick Q̇T (R² = 0.90 and 0.89, respectively). There were minimal differences between OpCirc1 and OpCirc2 compared with Fick up to moderate-intensity exercise (<70% peak V̇O₂); however, both techniques tended to underestimate Fick at >70% peak V̇O₂. These differences became significant for OpCirc1 only. Part of the differences between Fick and OpCirc methods at the higher exercise intensities are likely related to inhomogeneities in ventilation and perfusion matching (R² = 0.36 for Fick - OpCirc1 vs. alveolar-to-arterial oxygen tension difference). In conclusion, both OpCirc methods provided reproducible, reliable measurements of Q̇T during mild to moderate exercise. However, only OpCirc2 appeared to approximate Fick Q̇T at the higher work intensities.