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Cold adaptation and the seasonal distribution of acute myocardial infarction.

Numerous studies have reported an increased mortality from coronary heart disease (CHD) during the winter.1–,5 Observational epidemiological data in England and Wales have shown that mortality from cardiovascular disease (CVD) increases linearly with decrease in diurnal minimum from 17 °C, accounting for about half of all excess cold-related mortality,1,,2 which is approximately 50 000 per year in Britain alone.3 This effect is particularly pronounced in the elderly, in whom there is a 30% increase in deaths from this cause. Mortality increases more with a given fall of temperature in regions with `warm' winters.6 These deaths may therefore represent a graded effect of mild to severe environmental cold, rather than a specific effect of severe cold stress. The short temporal relation between temperature drop and mortality observed in Taiwan, where ambient temperature fluctuates greatly, supports the hypothesis that temperature effect may be a major factor which contributes to the increased mortality in winter.7 Increased CVD mortality has been related to thrombosis due to haemoconcentration in the cold.8,,9 Mortality from CVD increased significantly with short-term falls in temperature. Short term falls in temperature also significantly increase blood pressure, haemoglobin (Hb), erythrocyte count, packed cell volume and serum albumin—changes that persist for 1–2 days.10

The secondary effects of winter respiratory infections may contribute to CVD deaths as observed during influenza epidemics,11 probably because of increases in blood fibrinogen during infections.12 However, most of the coronary deaths occur some hours after the exposure to cold, and before respiratory deaths increase.13 There is also epidemiological evidence for an increased incidence of strokes during winter, however, the data regarding the increase in case-fatality rate are somewhat equivocal.14

The decline in seasonality of coronary mortality in the US since 1970 …