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

An open-circuit (OpCirc) acetylene uptake cardiac output ((Q)over dot T) me thod was modified for use during exercise. Two computational techniques wer e 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 ((V)over dot O-2) = 120% predicted] performed cycle ergometry exercise to compare (Q)over dot T using OpCirc and direct Fick methods. An incremental protocol was repeat ed twice, separated by a 10-min rest, and subsequently subjects exercised a t 85-90% of their peak work rate. Coefficient of variation of the OpCirc me thods and Fick were highest at rest (OpCirc1, 7%, OpCirc2, 12%, Fick, 10%) but were lower at moderate to high exercise intensities (OpCirc1, 3%, OpCir c2, 3%, Fick, 5%). OpCirc1 and OpCirc2 (Q)over dot T correlated highly with Fick T (R-2 = 0.90 and 0.89, respectively). There were minimal differences between OpCirc1 and OpCirc2 compared with Fick up to moderate-intensity ex ercise (<70% peak Vet); however, both techniques tended to underestimate Fi ck at >70% peak (V)over dot O-2. These differences became significant for O pCirc1 only. Part of the differences between Pick and OpCirc methods at the higher exercise intensities are likely related to inhomogeneities in venti lation and perfusion matching (R-2 = 0.36 for Fick OpCirc1 vs. alveolar-to- arterial oxygen tension difference). In conclusion, both OpCirc methods pro vided reproducible, reliable measurements of (Q)over dot T during mild to m oderate exercise. However, only OpCirc2 appeared to approximate Pick (Q)ove r dot T at the higher work intensities.