Cold weather produces more heart attacks, researchers find

Professor Claeys said: "Air pollution and temperature changes are the most frequently reported environmental triggers for acute myocardial infarction (AMI). Epidemiologic studies have focused mainly on one environmental condition, but most environmental triggers are related to each other and may attenuate or reinforce the triggering effect of a single environmental factor."He added: "Better knowledge of the impact of environment on AMI will help medical care providers and policy makers to optimise prevention strategies for a target risk population." The present study evaluated the independent environmental triggers of AMI in a multifactorial nationwide environmental model. Weekly counts of AMI patients that underwent primary percutaneous coronary intervention (pPCI) during 2006-2009 in 32 Belgian PCI centres were extracted from the national PCI database.

AMI counts were correlated with average weekly meteorological data obtained from daily measurements in 73 meteorological sites, equally distributed in Belgium. The following meteorological measures were investigated: air pollution expressed as particulate matter both less than 10µM (PM10) and less than 2.5µM (PM2.5), black smoke, temperature and relative humidity.

During the study period a total of 15,964 AMI patients (mean age 63 years, 24.8% female) were admitted with a weekly average admission rate of 77 +/- 11 patients. Time series and univariate analysis revealed a significant positive correlation between AMI and air pollution and an inverse correlation between AMI and temperature.

Multivariate analysis showed that only temperature was significantly correlated with AMI, which increased by 7% for each 10°C decrease in minimal temperature (odds ratio [OR]=1.07, 95% confidence interval [CI]=1.04-1.11), and that there was no significant effect of air pollution (OR=1.01, 95%CI=1.00-1.02).

Professor Claeys said: "Additional analysis showed that the triggering effect of low temperature was also present outside the winter period. Apparently, smaller differences in temperature between indoor and outdoor can also precipitate AMI. In addition, below a minimal temperature of 10°C there is no additional effect of temperature decrease on the occurrence of AMIs." (see figure 1)

He added: "A potential mechanism to explain the increased risk of coronary events associated with decreasing temperature is the stimulation of cold receptors in the skin and therefore the sympathetic nervous system, leading to a rise in catecholamine levels. Moreover, increased platelet aggregation and blood viscosity during cold exposure promotes thrombosis and clot formation."

Professor Claeys concluded: "In a global environmental model, low temperature is by far the most important environmental trigger for AMI, whereas air pollution has a negligible effect. People at risk of AMI (for example elderly patients with diabetes and hypertension) can minimise their risk by avoiding big changes in temperature. This means wearing suitable clothes when going from the warm indoors to the colder outdoors, even beyond winter time."