Circadian variation of transient myocardial ischemia in the early out-of-hospital period after first acute myocardial infarction.

Circadian rhythms have been demonstrated in acute myocardial infarction (AMI) and in other clinical cardiac dysfunctions. The purpose of this study was to elucidate whether a circadian pattern of transient myocardial ischemia exists after first AMI. Prospectively, 24-hour ambulatory ST-segment monitoring was initiated at discharge on day 11 ± 5 in 123 consecutive survivors of first AMI. A total of 93 ischemic episodes (91 asymptomatic) occurred in 21 of the 123 patients (17%) (mean duration of 30 minutes, range 4 to 292). A significant circadian rhythm of transient myocardial ischemia was found with a peak activity occurring in the evening hours (p < 0.01). Thus, 43% of ischemic episodes and 42% of ischemic time occurred between 6 p.m. and 12 midnight. The characteristics of morning and evening episodes were similar, except for the heart rate at maximal ST-segment depression, which was significantly higher during morning episodes (p < 0.02). Patients with transient myocardial ischemia had a diurnal distribution similar to the circadian variation displayed during ischemic activity. Thus, 16 of the 21 patients had ischemic episodes from 6 p.m. to 12 midnight versus 10 patients from 6 a.m. to 12 noon (p < 0.01). The 24-hour mean minimal heart rate was significantly higher in patients with than without ischemic episodes (p < 0.02). In conclusion, this study has established a significant circadian peak of transient myocardial ischemia in the evening hours in survivors of first AMI. Whether the pattern displayed is due to endogenous biologic functions or cyclic variations, or both, in the external environment needs to be clarified.