With the thermodilution technique, repeated determination of cardiac index (CI) has become a rapid, easily accomplished, safe procedure. We paired 77 CI determinations measured by direct Fick and thermodilution techniques in 21 patients who underwent myocardial revascularization. Commercially available thermistor-tipped catheters and a cardiac output computer were used to determine CI according to the manufacturer's instructions. Oxygen consumption and arterial-venous oxygen content differences were measured directly to determine CI by the Fick method. Comparisons were made during mechanical ventilation with and without positive end-expiratory pressure (PEEP) and during spontaneous ventilation with and without PEEP. Cardiac indices measured by the two techniques were within +/- 0.5 L/min/m2 of each other only 76% of the time and within +/- 1.0 L/min/m2 96% of the time, if CI greater than or equal to 4.0 L/min/m2 were omitted. Ventilatory pattern had no apparent effect on results.