Changes in seasonal deaths from myocardial infarction.

BACKGROUND: Cardiovascular disease is the major contributor to excess morbidity and mortality in winter. With the rise in temperatures through global warming, and the use of central heating and air conditioning, this seasonal variation may be declining.

AIM: To study possible changes in seasonal variation in case-fatality rates of myocardial infarction (MI), in men and women, over a 20-year period and compare this with possible environmental influences.

DESIGN: Retrospective analysis of death certificate and climatological data.

METHODS: We analysed all monthly death certificate data from Northern Ireland, for death caused by MI from 1979 through 1998 (n=68 683). Mortality data were standardized to a single reference group for the whole period. Seasonal variation in mortality and in environmental variables was described using the cosinor model.

RESULTS: A total of 29 875 women and 38 808 men died from MI during the 20-year period. A significant decrease in mortality from MI was observed in both sexes, accompanied by a non-significant decline in the amplitude of the seasonal rhythm over the study period. Low temperature was associated with higher mortality rates from MI.

DISCUSSION: We have documented an overall decline in cardiovascular mortality from 1979 to 1998, together with a small but non-significant decrease in seasonal variation. While improvements in medical care, lifestyle, housing and diet may have contributed to the observed decline in mortality rate, seasonal fluctuations remain a significant problem.