Circadian variations have been noted in the frequency of acute myocardial infarctions (MIs). The risk of acute MI appears to be higher between the hours of 6 a.m. and 12 noon than in any remaining six-hour period of the day. Because plaque rupture, and subsequent thrombosis, is a major cause of acute MI, it is presumed that plaque rupture also occurs in a similar rhythm.

Tanaka and associates studied patients with acute MI who had preinterventional intravascular ultrasonography (IVUS) within 12 hours of the onset of acute MI symptoms. This procedure allows a clear distinction to be seen between plaque rupture and nonrupture.

The authors studied 174 patients. Plaque rupture was noted in 81 (47 percent), while nonrupture was noted in the remaining 93 (53 percent). The onset of acute MI was significantly higher during the morning hours than during other periods. An associated increase in plaque rupture also was noted during the morning hours. In the nonrupture group, onset of acute MI was more frequent during sleep hours, between 12 midnight and 6 a.m.

The authors conclude that a morning increase in plaque rupture accounts for the characteristic circadian rhythm of acute MI. A variety of morning physiologic changes, including arterial pressure surge accompanied by an increase in heart rate and vascular tone, and decreased serum cortisol levels, may individually or in combination explain the increase in morning plaque rupture events. The authors note that acute MIs in their two study groups probably have different etiologies.