A UK study found that for each 1°C drop in outside temperature there were approximately 200 more heart attacks in a single day in Great Britain. The study, published in the BMJ (British Medical Journal) writes that there is increasing interest in the link between the weather and health in light of global climate change.
The authors write that previous studies had identified a link between outdoor ambient temperature and short term mortality risk – with both elevations and reductions in temperature having an effect. However, before this study, the effect of temperature on the risk of myocardial infarctions (heart attacks) was unclear.
Scientists from the London School of Hygiene and Tropical Medicine set out to determine whether there was a short-term relation between ambient temperature and heart attack risk.
They examined 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.
Adjustments were made for such factors as air pollution, flu activity, seasonality and long-term trends.
They found that a 1°C drop in average daily temperature was linked to a cumulative 2% increase in risk of heart attack for 28 days. The highest risk was within two weeks of exposure.
The increased risk may seem small, but the UK has approximately 146,000 heart attacks per year and 11,600 events in a 29 day period, so even a tiny increase translates to significant numbers of extra heart attacks, around 200 for each 1°C reduction in temperature nationwide on a single day.
Individuals most vulnerable to slight reductions in ambient temperature include those aged between 75 and 84 years, as well as patients with previous coronary heart disease.
People who had been taking long term aspirin were less vulnerable, the authors revealed.
No link was found between temperature rises and heart attack risk. However, this is probably because it never gets very hot in the UK. The researchers concluded:
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.
Further studies are required, which could examine the role of adaptive measures such as clothing and home heating, the authors said. More studies are also needed to 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, wrote:
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.
Krishnan Bhaskaran, Shakoor Hajat, , Emily Herrett, Paul Wilkinson, Liam Smeeth
BMJ 2010;341:c3823. doi:10.1136/bmj.c3823
Editorial:
Paola Michelozzi
BMJ 2010;341:c3720