The effect of dipyridamole on transmural blood flow gradients

The effect of dipyridamole (DP) on subendocardial (ENDO) and subepicardial (EPI) blood flow in stenosed and normal regions was determined with radioactive microspheres before and after intravenous infusion of 0.08 mg/kg.min-1 of DP in two groups of dog hearts in situ. Group 1 (n = 6) had coronary stenoses that did not reduce distal perfusion at rest, but did attenuate hyperemic response, and group 2 (n = 6) had stenoses which reduced resting blood flow. In nonstenosed zones, a doubling in perfusion during DP infusion induced proportionate changes in ENDO and EPI flows (unchanged ENDO/EPI ratio) in group 1 hearts, but increased EPI more than ENDO flows in group 2 hearts (decreased ENDO/EPI ratio). In mildly stenosed regions, a 47 percent increase in perfusion during DP was associated with an increase in only EPI flow, which thereby reduced the ENDO/EPI ratio. In severely stenosed zones, the ENDO/EPI ratio was already significantly reduced at rest and a further reduction in ENDO flow during DP caused a significant decline in the ENDO/EPI ratio. Therefore, during pharmacologic vasodilation, a redistribution of coronary transmural flow occurs (relative ENDO to EPI steal) in non- and mildly stenosed regions while an absolute decrease or steal in ENDO flow was noted only in regions distal to more severe stenoses. These studies suggest that an interaction between the stenosis severity and vasodilation is a determining factor in ENDO/EPI ratios with dipyridamole.