Abstract 1470: A Circadian Periodicity in the Time of Onset of ST-segment Elevation Myocardial Infarction Disappears in Obese Subjects
Background It is well known that the time of onset of ST-elevation myocardial infarction has a pronounced circadian periodicity, with peak incidence of events between 6AM and noon. The circadian rhythm of blood pressure, heart rate, vascular tonus, and prothrombotic factors could generate the peak incidence. Plasma type-1 plasminogen activator inhibitor (PAI-1) activity also has the diurnal variation and is thought to generate the prothrombotic circadian rhythm. Adipose tissue is one of the main sources of circulating PAI-1. Therefore, we hypothesized that obese subjects had more distinct periodicity than those of non-obesity do in the time of STEMI onset.
Methods and Results We reviewed data from 2,644 acute myocardial infarction patients who admitted to our institute from January 1998 to April 2009. Of those, 1,962 cases showed ST elevation in their ECG on admission. After exclusion of the cases with ambiguous onset time and missing demographic data, 1,719 patients (female 28%, mean age 68.2y) were analyzed. Mean body mass index (BMI) was 23.7 kg/m2 and obesity (BMI≥25) accounted for 23.0% of the total cases. There were strong circadian periodicity, with peak incidence of STEMI onset between 6AM and noon in all (chi-square 49.887, p<0.0001; 19.1% in midnight-6AM, 30.7% in 6AM-noon, 26.8% in noon-6PM, 23.4% in 6PM-midnight) and non-obese (chi-square 59.201, p<0.0001; 17.2% in midnight-6AM, 32.4% in 6AM-noon, 27.4% in noon-6PM, 23.0% in 6PM-midnight) cases. On the other hand, in obese cases, circadian rhythm in the time of onset of STEMI disappeared totally (chi-square 1.592, p<0.6613; 23.2% in midnight-6AM, 26.9% in 6AM-noon, 25.6% in noon-6PM, 24.3% in 6PM-midnight).
Conclusions and Discussion Contrary to our hypothesis, in obese cases, the onset time of STEMI dispersed evenly in a day. One possible explanation of this unexpected finding is that obesity is not a cause of loss of a circadian variation but could be one of results induced by preexistent circadian dysfunction, which affects diurnal rhythm of physiological and pathological conditions including STEMI onset time. Results from a recent molecular study, circadian Clock mutant mice are hyperphagic and obese, and develop a metabolic syndrome (Science 2005;308:1043), may support this speculation.