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The present study analyzed coronary sinus blood flow alterations after dipyridamole induced coronary vasodilation in seven patients whose endomyocardial biopsies evidenced no sign of rejection (group 1) and in five patients with histologic signs of rejection (group 2) after orthotopic heart transplantation. All patients were treated with cyclosporine and prednisone and some with azathioprine and had normal coronary arteriograms. Coronary sinus blood flow and coronary resistance were measured before and after intravenous dipyridamole (0.18 mg/ kg/min over 4 minutes). Basal values were similar in groups 1 and 2 for coronary sinus blood flow (166+34 compared with 181±39 mUmin, respectively), coronary resistance (0.62±10 compared with 0.52±13 mm Hg/ml/min, respectively), coronary sinus blood oxygen content (5.7±1.6 compared with 4.5±0.9 ml/100 ml, respectively) and arterial-coronary sinus blood oxygen difference (10.6±1.3 compared with 10.3±1.8 ml/100 ml, respectively). After dipyridamole, patients with heart rejection had lower coronary sinus blood flow and coronary sinus blood oxygen content than did patients without heart rejection (263±76 in group 2 compared with 505±96 mVlmin in group 1,p <0.001, and 7.6±0.9 in group 2 compared with 13.3± 1.8 ml/ 100 ml in group 1, p<0.001 respectively) and had higher coronary resistance and arterialcoronary sinus blood oxygen difference than did patients without heart rejection (0.34±0.15 in group 2 compared with 0.16±0.02 mm Hg/ml/min in group 1, p <0.02, and 7.0±1.4 in group 2 compared with 3.3 ±0.8 ml/100 ml in group 1, p <0.001, respectively). Coronary flow reserve evaluated by the dipyridamole:basal coronary sinus blood flow ratio and resistance reserve evaluated by the basal:dipyridamole coronary resistance ratio were significantly reduced in patients with heart rejection (1.56±0.09 in group 2 compared with 3.09±0.44 in group 1, p<0.001, and 1.63±0.30 in group 2 compared with 3.89±0.82 in group 1, p<0.001, respectively). These data indicate that capacity of the intramyocardial coronary circulation to vasodilate is severely impaired during heart rejection. (Circulation 1989;79:59-

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