CIRCADIAN VARIATIONS IN THE OCCURRENCE OF CARDIAC ARRESTS - INITIAL AND REPEAT EPISODES

Background-Patterns of temporal variation of cardiac arrests may be im portant for understanding mechanisms leading to the onset of acute car diovascular disorders. Previous studies reported diurnal variation of the onset of cardiac arrests, with high incidence in the morning and i n the evening, lack of daily variation during the week, and some seaso nal variation. The association between the time of day and recurrent c ardiac arrests has not been previously examined.Methods and Results-We explored temporal variation in 6603 out-of-hospital cardiac arrests a ttended by the Seattle Fire Department. The data exhibit diurnal varia tion, 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 attribut ed primarily to the patients found in ventricular fibrillation, wherea s arrests that show other rhythms exhibit mainly a morning peak. Cardi ac arrests associated with survival have more pronounced diurnal varia tion than episodes in which survival did not occur. This difference pe rsists 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 tim es of day of the first and second arrests were closer to each other th an one would expect if the times were entirely unrelated. Conclusions- Cardiac arrests do not occur randomly during the day, but rather follo w certain periodic patterns. These patterns are probably associated wi th patterns of daily activities. The hypothesis that cardiac arrests a re triggered by a person's activity rather than by some underlying cha racteristics of his or her disease is supported by the lack of associa tion between the times of the first and second arrests in the patients with recurrent arrests.