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Cardiac arrests do not occur randomly but follow certain temporal patterns

Out-of-hospital cardiac arrests occur most often on Monday (similar to heart attacks), happen more often in the winter months of December through March, and are least common in the summer (July, August, and September), says a new study supported in part by the Agency for Health Care Policy and Research (HS08197). It found that there were 31 percent more cardiac arrests in January (the highest month) than in August (the lowest month).

Previous studies have reported diurnal (time of day) variation in the onset of cardiac arrests, with high incidence in the morning and in the evening, lack of daily variation during the week, and some seasonal variation. This is the first study to report variation in rate of sudden cardiac deaths by day of the week, a pattern that may be explained by activity levels, note the researchers. For instance, people who are retired may be more active early in the week because that is when facilities are in least demand in communities where retirees live.

However, the seasonal variation is almost certainly from environmental influences. For instance, outdoor activity levels generally increase in the summer in the Seattle area (the study site) because the rains diminish, and the daylight hours are longer. It might be argued that the dreary and rainy winter months take a toll on a person's psyche, thus enabling emotional triggers of cardiac arrest.

Patterns of temporal variation in cardiac arrests may be important for understanding mechanisms leading to the onset of acute cardiovascular disorders, explain the University of Washington researchers. They explored weekly and seasonal temporal variation in 6,603 out-of-hospital cardiac arrests attended by the Seattle Fire Department between March 1985 and February 1993.

See "Weekly and seasonal variation in the incidence of cardiac arrests," by Monika Peckova, M.S., Ph.D., Carol E. Fahrenbruch, M.S.P.H., Leonard A. Cobb, M.D., and Alfred P. Hallstrom, Ph.D., in the February 1999 American Heart Journal 137, pp. 512-515.

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