Cardiac output during exercise: A comparison of four methods

Several techniques assessing cardiac output ( Q ) during exercise are available. The extent to which the measurements obtained from each respective technique compares to one another, however, is unclear. We quantified Q simultaneously using four methods: the F ick method with blood obtained from the right atrium ( Q F ick‐ M ), I nnocor (inert gas rebreathing; Q I nn ), P hysioflow (impedance cardiography; Q P hys ), and N exfin (pulse contour analysis; Q P ulse ) in 12 male subjects during incremental cycling exercise to exhaustion in normoxia and hypoxia ( F i O 2  = 12%). While all four methods reported a progressive increase in Q with exercise intensity, the slopes of the Q /oxygen uptake ( VO 2 ) relationship differed by up to 50% between methods in both normoxia [4.9 ± 0.3, 3.9 ± 0.2, 6.0 ± 0.4, 4.8 ± 0.2 L/min per L/min (mean ±  SE ) for Q F ick‐ M , Q I nn , Q P hys and Q P ulse , respectively; P  = 0.001] and hypoxia (7.2 ± 0.7, 4.9 ± 0.5, 6.4 ± 0.8 and 5.1 ± 0.4 L/min per L/min; P  = 0.04). In hypoxia, the increase in the Q / VO 2 slope was not detected by N exfin. In normoxia, Q increases by 5–6 L/min per L/min increase in VO 2 , which is within the 95% confidence interval of the Q / VO 2 slopes determined by the modified F ick method, P hysioflow, and N exfin apparatus while I nnocor provided a lower value, potentially reflecting recirculation of the test gas into the pulmonary circulation. Thus, determination of Q during exercise depends significantly on the applied method.