Determinants of systolic and diastolic flow in coronary bypass grafts with inotropic stimulation

Using implanted pulsed Doppler microprobes sutured on saphenous bypass grafts in ten patients we studied, 6 h after cardiac surgery, the effects of 5 and 10 micrograms.kg-1.min-1 of dobutamine on mean (Qm), systolic (Qs), and diastolic (Qd) coronary bypass graft flows, as well as on coronary systolic (integral of Qs) and diastolic (integral of Qd) blood volumes entering the myocardium per cardiac beat. Qm increased during the inotropic stimulation from 61.8 +/- 19.2 to 81.1 +/- 21.8 ml.min-1 (P less than 0.001) and resulted from an unchanged Qs and from a large increase in Qd (P less than 0.01). Qd increased more than did diastolic arterial pressure and was related to rate pressure product taken as an index of myocardial oxygen consumption (r = 0.76, P less than 0.001). Despite the dobutamine-induced increase in heart rate (P less than 0.01), integral of Qs, and integral of Qd, the systolic and diastolic inflow volumes per cardiac beat were unchanged. We conclude that increased myocardial blood supply through the saphenous vein bypass graft during inotropic stimulation by dobutamine resulted from different systolic and diastolic events. The oxygenated blood volume entering the coronary vascular bed per beat was unchanged despite tachycardia.