BACKGROUND Although moderate hemodilution is usually well tolerated in coronary artery surgery patients, this may not be the case when myocardial oxygen demand is increased. We hypothesized that, in these patients, hemodilution in the presence of an increased heart rate could be associated with an impairment of myocardial function. METHODS Forty coronary surgery patients were randomly assigned to two groups (n=20), according to the rate of atrioventricular pacing [70 bpm (Group 70) or 90 bpm (Group 90)]. While paced at the fixed heart rate, hemodilution was performed before the start of cardiopulmonary bypass. Data were obtained from a pulmonary artery, a PiCCO catheter and a left ventricular pressure catheter. Measurements were obtained in steady-state conditions before and after isovolemic hemodilution. RESULTS Hemodilution from 40%+/-2% to 30%+/-1% in Group 70, and from 39%+/-4% to 30%+/-2% in Group 90 resulted in a decrease in systemic vascular resistance and an increase in end-diastolic volume in both groups. This was associated with an increase in stroke volume in Group 70 but not in Group 90. In this latter group, the maximal rate of pressure development decreased significantly after hemodilution [from 856+/-93 to 716+/-80 mm Hg/s (P<0.01)], whereas it remained unchanged in Group 70 (843+/-86 mm Hg/s before and 832+/-79 mm Hg/s after hemodilution). CONCLUSIONS In the conditions of the present study, increased heart rate during moderate hemodilution was associated with a depression of myocardial function.