The beat-by-beat non-invasive assessment of cardiac output (Q, litre·min-1) based on the arterial pulse pressure analysis called Modelflow® can be a very useful tool for quantifying the cardiovascular adjustments occurring in exercising humans. Q was measured in nine young subjects at rest and during steady-state cycling exercise performed at 50, 100, 150 and 200 W by using Modelflow® applied to the Portapres® non-invasive pulse wave (QModelflow) and by means of the open-circuit acetylene uptake (QC2H2). Q values were correlated linearly (r=0.784), but Bland–Altman analysis revealed that mean QModelflow-QC2H2 difference (bias) was equal to 1.83 litre·min-1 with an S.D. (precision) of 4.11 litre·min-1, and 95% limits of agreement were relatively large, i.e. from -6.23 to +9.89 litre·min-1. QModelflow values were then multiplied by individual calibrating factors obtained by dividing QC2H2 by QModelflow for each subject measured at 150 W to obtain corrected QModelflow (Qcorrected) values. Qcorrected values were compared with the corresponding QC2H2 values, with values at 150 W ignored. Data were correlated linearly (r=0.931) and were not significantly different. The bias and precision were found to be 0.24 litre·min-1 and 3.48 litre·min-1 respectively, and 95% limits of agreement ranged from -6.58 to +7.05 litre·min-1. In conclusion, after correction by an independent method, Modelflow® was found to be a reliable and accurate procedure for measuring Q in humans at rest and exercise, and it can be proposed for routine purposes.

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