How a small drop in temperature can raise the risk of a heart attack

DROPPING temperatures lead to an increase in the risk of heart attacks, new research suggests.

The study found that each 1C reduction in outdoor temperature from one day to the next was linked to about 200 extra heart attacks in the UK compared to if it stayed the same.

The reasons behind the increased risk in cooler temperatures is not yet clear, but could be down to changes in blood pressure and blood thickness in colder weather.

Researcher Krishnan Bhaskaran and colleagues analysed data on 84,010 patients admitted to hospital with a heart attack in 2003-2006, compared with daily temperatures in England and Wales at the same time.

The results were adjusted to take into account other risk factors such as air pollution and rates of flu at the time.

The researchers found that a 1C drop in average daily temperature was linked to a 2 per cent increase in the risk of heart attack over the next 28 days. The risk was highest in the following two weeks.

The researchers, writing in the British Medical Journal, said while the risk appeared small, there were 146,000 heart attacks every year in the UK so even a small increase in risk translated into 200 extra heart attacks for each 1C drop on a single day.

Mr Bhaskaran said: "We found no increased risk of heart attacks during higher temperatures, possibly because the temperature in the UK is rarely very high in global terms. Our results suggest that even in the summer, the risk is increased by temperature reductions."

The researcher said a few studies exposing people to colder temperatures in the lab could help explain the increased risk. "These are experimental studies where volunteers have gone into lab and been hooked up and been exposed to temperature reductions to see what the effect is on their heart and circulation. From these studies you see an effect on blood pressure going up, the blood getting thicker and the heart working harder.

"Also the substances in the blood which help it to clot get a bit more concentrated.

"It might be that some combination of these factors is part of the reason for the effect we have observed, but more research is needed to home in on what the mechanisms are."