Subendocardial ischemia provoked by tachycardia in conscious dogs with coronary stenosis.

We investigated the effect that mild coronary stenosis exerts on the ability of the coronary circulation to compensate for the increased extravascular compression that occurs in the subendocardium during tachycardia. An electromagnetic flowmeter transducer and balloon cuff occluder were implanted on the left circumflex coronary artery in seven dogs, and experiments were performed 1 week later with the dogs under sedation but conscious. Stenosis of the left circumflex artery was produced by partial inflation of the cuff occluder. We determined coronary blood flow distribution by the radioactive microsphere technique, injection 200,000 15mu spheres into the left ventricular cavity during (1) a control period, (2) stenosis of the left circumflex artery and a normal heart rate, and (3) stenosis of the left circumflex artery and tachycardia. When the heart rate was normal, the degree of stenosis used caused no change in myocardial microsphere distribution but eliminated postocclusion reactive hyperemia. Thus, reserve coronary vasodilation compensated for the stenosis. With the degree of stenosis kept constant, an increase in heart rate to 196 beats/min caused a marked transmural shift in distribution of microspheres from subendocardium into subepicardium within the region of the left ventricle supplied by the left circumflex artery. There was no significant transmural shift in the region supplied by the uninvolved left anterior descending coronary artery. Myocardial lactate extraction decreased. These results suggest that when reserve coronary vasodilation has already been utilized to compensate for coronary stenosis, the increased extravascular coronary compression from tachycardia causes subendocardial ischemia and hypoxia.