Coronary vasodilator reserve and flow distribution during near-maximal exercise in dogs

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Abstract
In 10 chronically instrumented dogs, coronary blood flow was studied during a progressive treadmill test. Total left ventricular coronary flow increased from 91 .+-. 6 ml/min per 100 g at rest (heart rate (HR) 81 .+-. 6) to 424 .+-. 30 ml/min per 100 g during steady-state near-maximal exercise (HR 271 .+-. 8). Regional flow became redistributed away from the subendocardium as the endocardium-to-epicardium (endo/epi) ratio dropped from 1.29 .+-. 0.06 to 1.03 .+-. 0.05. Flow also became redistributed away from the subendocardium when resting heart rate was more than 100 beats/min (endo/epi 1.08 .+-. 0.05). The increase in left ventricular flow utilized only 63% of the total dilator reserve capacity, as dipyridamole increased flow an additional 194 ml/min per 100 g during near-maximal exercise. The endo/epi ratio following maximal dilatation with dipyridamole was 1.15 .+-. 0.08 indicating dilator reserve in both the subepicardial and subendocardial layers. Myocardial O2 supply/demand balance (diastolic pressure time index/tension time index (DPTI/TII)) dropped from 2.53 .+-. 0.22 at rest to 0.90 .+-. 0.08 during near-maximal exercise which is still higher than the critical level for subendocardial ischemia. A left ventricular, transmural electrocardiogram showed significant ST depression following isoproterenol injection but was normal during near-maximal exercise. These observations of substantial dilator reserve, an adequate O2 supply/demand balance, and normal transmural ECG all show that subendocardial muscle does not become ischemic during near-maximal exercise.