Circadian variation in ischemic threshold. A mechanism underlying the circadian variation in ischemic events.

There is a circadian pattern in the occurrence of cardiac events in patients with coronary artery disease. Whether changes in coronary vascular tone contribute to these phenomena is unknown. We measured the ischemic threshold, defined as either the heart rate or rate-pressure product at 1-mm ST segment depression during treadmill exercise and used it as an index of the lowest coronary vascular resistance; the premise was that when ischemic threshold became lower, coronary vascular resistance was higher, and vice versa.