Evening Hours, Not 'Happy Hours' for Cardiac Arrest Victims

Based on records from the Seattle emergency system, scientists at the University of Washington found a high number of late afternoon calls reporting cardiac arrests in which the heart has stopped beating normally. Earlier studies 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 timing of cardiac arrests at certain hours of the day is a phenomenon of circadian rhythms. 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 the day progresses.

The explanation for the morning peak may be that chemicals that stimulate the heart are at higher levels in the morning than in the afternoon. Different factors may be at play in causing the afternoon peak. The study also 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.

This study also shows that the afternoon peak is as high as the morning peak and that the afternoon cardiac arrests were often due to ventricular fibrillation - the rapid, disordered contraction of the ventricles - the most "treatable" cause of cardiac arrest.

- From the American Heart Association's HeartStyle

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Courtesy of Texas Medical Center News

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©2006 Texas Medical Center

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URL: http://www.tmc.edu/health_briefs/02_01_99-evening.html