The Threshold for Myocardial Ischemia Varies in Patients with Coronary Artery Disease Depending on the Exercise Protocol

Abstract

It is generally accepted that angina pectoris and, presumably, myocardial ischemia occur at a fixed heart rate-systolic blood pressure product in a given patient. This concept of a fixed threshold has recently been challenged. To evaluate the effects of varying exercise intensity on the ischemic threshold, 33 patients with coronary artery disease and provokable myocardial ischemia, documented by thallium-201 myocardial perfusion imaging, underwent two exercise tests 2 to 7 days apart. A symptom-limited incremental treadmill exercise test was followed by a 20 min submaximal treadmill test at an intensity approximating 70% of the peak heart rate attained during the incremental test. During the incremental exercise test, angina pectoris developed in 16 patients and 17 patients were asymptomatic. At least 0.1 mV of ST segment depression developed in all subjects during the incremental exercise test at a mean exercise duration of 5.3 ± 2.6 min, a rate-pressure product of 19,130 ± 5,735 and oxygen uptake of 19.6 ± 7.0 ml/kg per min. During the submaximal exercise test, 28 (85%) of the 33 patients had significant ST segment depression. Of these patients, 24 (86%) were asymptomatic, including 10 patients who had previously reported anginal symptoms during the incremental test. The average time to onset of 0.1 mV ST segment depression during the submaximal test was 8.1 ± 4.5 min. These changes occurred at a rate-pressure product of 15,250 ± 3,705 and an oxygen uptake of 14.3 ± 5.9 ml/kg per min, and were significantly (p These results demonstrate that myocardial ischemia, whether or not accompanied by angina pectoris, can occur at a lower rate-pressure product and oxygen uptake during submaximal, steady state exercise compared with symptom-limited incremental exercise. It may be concluded that the ischemic threshold varies under different exercise conditions.