Exercise-induced myocardial ischemia. Importance of coronary blood flow.

T he onset of ischemia is thought to occur when myocardial oxygen demand exceeds supply. In an individual patient the ischemic threshold might be a reproducible measure that could be used in the short term to establish the efficacy of therapy. In clinical studies the operational definition of the onset of ischemia has been 0.1 mV of ST depression on the electrocardiogram. In earlier investigations it was assumed that coronary supply was relatively fixed in patients with chronic ischemic heart disease and that changes in the onset of ischemia during exercise were mainly determined by changes in demand. Traditionally demand has been estimated by heart rate or the rate-pressure product. Clinical investigations have shown relatively constant rate-pressure products at the time of onset of ischemic ST segment changes; these observations confirmed the theory that supply was relatively fixed and explained the efficacy of agents that lowered demand, such as /3-blockers.' However, in individual patients the test-to-test variability of rate-pressure product at the time of ischemia onset may be great.2