When throughout the year is coronary death most likely to occur? A 12-year population-based analysis of more than 220 000 cases.

Background: Previous studies have suggested that there is an increase in cardiac events in the morning. Fewer data relate cardiac events to months of the year and season.

Methods And Results: We analyzed all monthly death certificate data from Los Angeles County, California, for death caused by coronary artery disease from 1985 through 1996 (n=222 265). The mean number of deaths was highest in December at 1808 and January at 1925; the lowest rates were in June, July, August, and September at 1402, 1424, 1418, and 1371, respectively. December and January had significantly higher rates than would be expected from a uniform distribution of monthly deaths (P=0.00001). The percent of yearly coronary deaths was defined by the quadratic U-shaped equation [percent=13.1198-1.5238(month)+0. 0952(month(2)), where January=1, February=2, etc]. When monthly deaths were plotted by year, there was a decrease from 1985 through 1996. Monthly mortality correlated inversely with temperature. During the months with the highest frequency of death (December, January), however, there was an increase in deaths that peaked around the holiday season and then fell, which could not be explained solely on the basis of the daily temperature change.

Conclusions: Even in the mild climate of Los Angeles County, there are seasonal variations in the development of coronary artery death, with approximately 33% more deaths occurring in December and January than in June through September. Although cooler temperatures may play a role, other factors such as overindulgence or the stress of the holidays might also contribute to excess deaths during these peak times.