Modelflow®, when applied to non-invasive fingertip pulse pressure recordings, is a poor predictor of cardiac output (Q, litre·min -1 ). The use of constants established from the aortic elastic characteristics, which differ from those of finger arteries, may introduce signal distortions, leading to errors in computing Q. We therefore hypothesized that peripheral recording of pulse pressure profiles undermines the measurement of Q with Modelflow®, so we compared Modelflow® beat-by-beat Q values obtained simultaneously non-invasively from the finger and invasively from the radial artery at rest and during exercise. Seven subjects (age, 24.0±2.9 years; weight, 81.2±12.6 kg) rested, then exercised at 50 and 100 W, carrying a catheter with a pressure head in the left radial artery and the photoplethysmographic cuff of a finger pressure device on the third and fourth fingers of the contralateral hand. Pulse pressure from both devices was recorded simultaneously and stored on a PC for subsequent Q computation. The mean values of systolic, diastolic and mean arterial pressure at rest and exercise steady state were significantly ( P <0.05) lower from the finger than the intra-arterial catheter. The corresponding mean steady-state Q obtained from the finger (Q porta ) was significantly ( P <0.05) higher than that computed from the intra-arterial recordings (Q pia ). The line relating beat-by-beat Q porta and Q pia was y =1.55 x -3.02 ( r 2 =0.640). The bias was 1.44 litre·min -1 and the precision was 2.84 litre·min -1 . The slope of this line was significantly higher than 1, implying a systematic overestimate of Q by Q porta with respect to Q pia . Consistent with the tested hypothesis, these results demonstrate that pulse pressure profiles from the finger provide inaccurate absolute Q values with respect to the radial artery, and therefore cannot be used without correction with a calibration factor calculated previously by measuring Q with an independent method.

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 (Q Modelflow ) and by means of the open-circuit acetylene uptake (Q C 2 H 2 ). Q values were correlated linearly ( r =0.784), but Bland–Altman analysis revealed that mean Q Modelflow -Q C 2 H 2 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 . Q Modelflow values were then multiplied by individual calibrating factors obtained by dividing Q C 2 H 2 by Q Modelflow for each subject measured at 150 W to obtain corrected Q Modelflow (Q corrected ) values. Q corrected values were compared with the corresponding Q C 2 H 2 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.