Circadian variations in the occurrence of cardiac arrests: initial and repeat episodes.

BACKGROUND Patterns of temporal variation of cardiac arrests may be important for understanding mechanisms leading to the onset of acute cardiovascular disorders. Previous studies reported diurnal variation of 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. The association between the time of day and recurrent cardiac arrests has not been previously examined. METHODS AND RESULTS We explored temporal variation in 6603 out-of-hospital cardiac arrests attended by the Seattle Fire Department. The data exhibit diurnal variation, with a low incidence at night and two peaks of approximately the same size (at 8 to 11 AM and 4 to 7 PM). The evening peak is attributed primarily to the patients found in ventricular fibrillation, whereas arrests that show other rhythms exhibit mainly a morning peak. Cardiac arrests associated with survival have more pronounced diurnal variation than episodes in which survival did not occur. This difference persists after adjustment for rhythm. For 597 patients who had at least two separate cardiac arrests, we found no overall association between the times of day of the recurrent arrests. For women, however, the times of day of the first and second arrests were closer to each other than one would expect if the times were entirely unrelated. CONCLUSIONS Cardiac arrests do not occur randomly during the day, but rather follow certain periodic patterns. These patterns are probably associated with patterns of daily activities. The hypothesis that cardiac arrests are triggered by a person's activity rather than by some underlying characteristics of his or her disease is supported by the lack of association between the times of the first and second arrests in the patients with recurrent arrests.