Early Morning Reduction in Ischemic Threshold in Patients With Unstable Angina and Significant Coronary Disease

Abstract

Background The objective of this study was to investigate in patients with unstable angina and significant coronary stenosis (>70%) whether or not the morning peak of myocardial ishemia is associated with a reduction in the ischemic threshold. The morning increased incidence of ischemic episodes in stable angina appears to be attributable to a coincidence of several factors. Patients with unstable angina who remain at bed rest, however, also present a similar morning increased incidence of ischemia, but its mechanisms are not completely understood. Methods and Results The ischemic threshold was assessed by atrial pacing at 7 to 8 am and at 12 to 1 pm in 46 patients. In the 34 with a positive pacing response (ST segment shift >1.0 mm), ischemic threshold was lower at 7 to 8 am than at 12 to 1 pm (131±16 versus 139±15 beats per minute, P <.001), whereas in the remaining 12 patients, the pacing response was negative. Moreover, 4 patients presented ST segment elevation during pacing in the morning but only 1 at noon and at a higher threshold. Baseline heart rate and diastolic blood pressure were higher at noon than in the morning (81±16 versus 76±13 beats per minute, P <.01, and 87±11 versus 82±10 mm Hg, P <.05). Conclusions The morning lowering of ischemic threshold in the absence of increases in baseline blood pressure or heart rate suggests that a reduced coronary vasodilator capacity or an increased coronary tone may favor the increased incidence of ischemic events during this interval.