July 9, 1998 DALLAS, July 7 -- An analysis of telephone calls to an emergency medical services (EMS) system shows that cardiac arrests commonly occur during the afternoon, as well as in the morning, according researchers reporting in today's Circulation: Journal of the American Heart Association.
Leonard Cobb, M.D. and Alfred P. Hallstrom, Ph.D. at the department of biostatistics and the division of cardiology at the University of Washington, Seattle, used records from the Seattle EMS and found a high number of calls for cardiac arrests in the late afternoon. Studies, including this one, have shown that many cardiac arrests occur around 8 a.m. This is the first to show a definitive peak in the "happy hour" time of day from about 5 to 8 p.m. The fact that the arrests occur at certain times of the day is a phenomenon called circadian rhythm. Circadian rhythms are tied to the 24-hour cycle of the earth's rotation and are possibly a result of changing amounts of light and darkness as a day progresses.
The explanation for the morning peak may be that chemicals that stimulate the heart are at higher levels in the morning than afternoon. But different factors may be at play in causing the afternoon peak.
Over a 23-year period, scientists researched 5,248 adults who had cardiac arrests not caused by drug overdose or respiratory arrest. The 3,690 of these that were witnessed arrests were used to compare circadian variations among subgroups (male vs. female, race comparisons, and age comparisons).
During the study, 597 of the patients had more than one cardiac arrest Of these, 325 could provide a near-exact time of the second cardiac arrest. No difference in times of heart attacks was found between races, genders, or working days vs. weekend days.
The study also provided new information about individuals who have more than one cardiac arrest. The study found that individuals who had a cardiac arrest in the morning had about the same chance of having another morning arrest as an afternoon arrest. Researchers concluded that an environmental, rather than biological, factor explains the pattern of cardiac arrests.
"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? rather than some underlying characteristic the disease," says Hallstrom.
This study shows that the afternoon peak is as high as the morning peak and that the afternoon cardiac arrest were often due to ventricular fibrillation -- the rapid, disordered contraction of the ventricles -- which is the most "treatable" cause of cardiac arrest, says Hallstrom.
"Ventricular fibrillation can frequently be shocked, or defibrillated, back into a normal rhythm and the other types of cardiac arrest that typically occur in the morning usually can't be shocked effectively," Hallstrom says. Co-authors are Monika Peckova, M.S., Ph.D. and Carol E. Fahrenbruch, M.S.P.H.
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