Colder days raise the risk of more heart attacks

A drop in the average temperature outside is linked to a higher risk of people having heart attacks, according to a new study published on bmj.com today.

UK researchers found that each 1 C reduction in temperature on a single day is associated with around 200 extra heart attacks.

In the light of global climate change, the relations between weather and health are of increasing interest. Previous studies have shown that ambient outdoor temperature is linked to mortality risk in the short term, with both hot and cold days having an effect, but the effect of temperature on the risk of myocardial infarctions (heart attacks) is unclear.

Researchers from the London School of Hygiene and Tropical Medicine carried out a study to examine the short term relation between ambient temperature and risk of heart attack.

They analysed data on 84,010 hospital admissions for heart attack recorded in the Myocardial Ischaemia National Audit Project (MINAP) during 2003-2006, and daily temperatures from the British Atmospheric Data Centre, focusing on 15 geographical areas in England and Wales.

The results were adjusted to take into account factors such as air pollution, influenza activity, seasonality and long term trends.

The researchers found that a 1 C reduction in average daily temperature was associated with a cumulative 2% increase in risk of heart attack for 28 days. The highest risk was within two weeks of exposure.

The heightened risk may seem small, but the UK has an estimated 146,000 heart attacks a year and 11,600 events in a 29 day period, so even a small increase in risk translates to substantial numbers of extra heart attacks, around 200 for each 1 C reduction in temperature nationwide on a single day.

Older people between the ages of 75 and 84 and those with previous coronary heart disease seemed to be more vulnerable to the effects of temperature reductions, while people who had been taking aspirin long-term were less vulnerable.

The researchers found no increase in the risk of heart attacks at higher temperatures, possibly because temperature in the UK is rarely very high in global terms.

In conclusion, they say 'our study shows a convincing short term increase in risk of myocardial infarction associated with lower ambient temperature, predominantly operating in the two weeks after exposure.'

They call for further studies to help shed light on the role of adaptive measures such as clothing and home heating, and further clarify which groups are likely to be the most vulnerable.

In an accompanying editorial, Dr Paola Michelozzi and Manuela De Sario, of the Lazio Region Department of Epidemiology in Rome, write: Heat and cold exposure affect people with cardiovascular diseases and increase the incidence of coronary events with high impact on short term mortality. Moreover, while the effect of cold on myocardial infarction is well documented, the short-term effect of heat is still contradictory but cannot be disregarded.

This is even more relevant under climate change scenarios that predict a decrease of cold related mortality that will be outweighed by an increase in cardiovascular mortality and morbidity associated with increased frequency and intensity of heat waves.

Clinicians should be aware that exposure to environmental heat and cold is a risk factor for cardiovascular disease and should consider this in risk prevention and management, and efforts should be especially directed towards most vulnerable individuals identified by a multiplicity of risk factors.