Myocardial Blood Flow

Current methods of measuring myocardial blood flow using 133xenon have failed to separate normals from patients with ischemic heart disease at rest. In the present study such a separation was attempted by utilizing pacing-induced tachycardia (PIT) to stress the myocardium. 133Xenon was injected into the left coronary artery to measure blood flow in 27 patients at rest and during pacing-induced tachycardia. Oxygen content was simultaneously measured in the aorta and coronary sinus of 13 of these patients. Pacing rates of 150 beats/min or greater were obtained in 21 patients. In 10 patients (group I), who developed ischemia, manifested either by typical angina or ischemic electrocardiographic changes during PIT, myocardial blood flow increased 32 ml/min/100 g of tissue (P < 0.001). In 17 patients (group II), who did not develop ischemia, myocardial blood flow increased 12 ml/min/ 100 g of tissue (P <0.02). The increase in group I patients was significantly greater than in group II (P < 0.02). Myocardial oxygen consumption increased 4.63 ml/min/100 g of tissue (P < 0.01) in ischemic responders (group I) and 2.62 ml/min/100 g of tissue in nonischemic responders (group II). It is concluded that patients with an ischemic response to pacing-induced tachycardia had a greater increase in myocardial blood flow and myocardial oxygen consumption than patients who did not develop an ischemic response. This unexpected finding is best explained by an increased myocardial blood flow in the nonischemic areas of the myocardium which may result from a vasodilator response to ischemia.