Clinical Application of Cardiac Output During Ramp Exercise Calculated Using the Fick Equation

The purpose of this study was to clarify whether the direct Fick method is applicable to the measurement of cardiac output (Q) during ramp exercise. Twelve patients with chronic health failure underwent both a ramp exercise test and a steady-state exercise test. Oxygen intake (VO2), arterial oxygen saturation and mixed venous oxygen saturation were continuously measured using a pulse oximeter and a fiberoptic catheter, and the arteriovenous oxygen difference (a-v O2 diff) and Q were calculated. Both VO2 and a-v O2 diff were significantly lower in the ramp protocol than in the steady-state protocol when they were compared at the same workload. However, the VO2 vs Q relationship and the VO2 vs a-v O2 diff relationship were very similar in the 2 protocols. The difference between Q measured during steady-state exercise and Q calculated at the matched VO2 during ramp exercise was small (3.7 +/- 7.0% and 5.6 +/- 6.6%). The results indicate that, clinically, Q measured by the direct Fick method with simultaneous measurement of VO2 and a-v O2 diff during ramp exercise is a good substitute for the true Q measured by the steady-state protocol. We conclude that Fick Q is applicable to ramp exercise.