Previous work has reported that coronary
vasodilator reserve may persist in myocardium rendered ischemic by hypoperfusion. This study investigated the presence and extent of residual coronary vasomotor tone in myocardial regions made acutely ischemic by a flow-limiting
coronary stenosis during exercise.
METHODS AND RESULTS: Studies were done in chronically instrumented dogs undergoing treadmill exercise in the presence of a
coronary stenosis that decreased distal left circumflex coronary artery perfusion pressure to approximately 40 mm Hg. Measurements of myocardial blood flow were made with radioactive
microspheres during exercise (6.5 km/hr, 6% grade) before and during intracoronary infusion of the potent coronary
vasodilator adenosine (40 micrograms/kg/min). Distal coronary perfusion pressure was held equal before and during intracoronary
adenosine infusion (43 +/- 5 versus 42 +/- 5 mm Hg) by adjusting the hydraulic coronary occluder. During exercise in the presence of a
coronary stenosis, myocardial blood flow (milliliter per minute per gram) was significantly reduced in all layers of the ischemic posterior region compared with the nonischemic anterior region. During intracoronary
adenosine infusion, with no change in coronary perfusion pressure, myocardial blood flow was significantly increased compared with preadenosine flows for both the subendocardial layer flow (1.03 +/- 0.74 versus 0.66 +/- 0.50; p less than 0.05) and mean transmural flow (1.54 +/- 0.59 versus 1.16 +/- 0.36; p less than 0.05). In the presence of a
coronary stenosis, regional myocardial segment shortening in the ischemic region during exercise fell significantly to 49 +/- 8% of shortening in the absence of a
coronary stenosis but improved modestly during
adenosine infusion (65 +/- 7 versus 49 +/- 8%; p less than 0.05).
CONCLUSIONS: