Cardiovascular disease (CVD) is the most common cause of death globally, remaining a considerable burden both in terms of health and costs. As in many countries, CVD mortality in the UK exhibits a marked seasonal variation; more people die during the winter months (December–March) than in other periods of the year and the majority of deaths occur among those aged 75 and over. This seasonal variation in death rates has been mainly attributed to cold weather and fall in temperature, which can alter vulnerability to specific diseases, in particular myocardial infarction, stroke and respiratory infection (especially influenza).
However, uncertainty still exists about the range in temperature which produces an increased risk of CVD and other health outcomes, since effects of both extremely cold days and moderately cold days on mortality have been demonstrated. To date, there is neither an established definition of a cold day nor a precise definition of the period for which a cold spell (e.g. two or more consecutive cold days) should last for detrimental health effects. Less frequently, cold spells in the UK can also occur during the non-winter months (May–November), with lowest minimum and maximum temperatures in England of −2 °C and 9 °C in August.
A much debated question is which people are more susceptible to cold temperature or cold spells, and the relative importance of individual characteristics such as age, previous chronic conditions, low income and cold homes. The elderly have been long considered more susceptible to cold weather, but the evidence is not consistent, For example, the odds of death in the elderly may be significant only if associated with cold spells, but not a linear decrease in temperature. In other studies the statistical power to examine evidence for effect modification was low and evidence for differences in effect of cold temperature on cardiovascular mortality according to obesity, smoking habit, alcohol intake, and hypertension was not found. Therefore, the aims of this study are threefold: To investigate the effect of cold spells on cardiovascular events during 1997–2012, to explore whether the effect of cold spells is modified by established cardiovascular risk factors and lastly, to explore whether the effect of cold spell is independent from average temperature over periods up to 6 days previously
To investigate effects of cold weather spells on incidence of cardiovascular disease (CVD), and potential effect modification of socio-demographic, clinical, behavioural and environmental exposures.
Data from two prospective studies were analysed: the British Regional Heart Study (BRHS), a population-based study of British men aged 60–79 years, followed for CVD incidence from 1998–2000 to 2012; and the PROSPER study of men and women aged 70–82 recruited to a trial of pravastatin vs placebo from 1997 to 9 (followed until 2009). Cold spells were defined as at least three consecutive days when daily mean temperature fell below the monthly 10th percentile specific to the closest local weather station. A time-stratified case-crossover approach was used to estimate associations between cold spells and CVD events.
921 of 4252men from BRHS and 760 of 2519 participants from PROSPER suffered a first CVD event during follow-up. More CVD events were registered in winter in both studies. The risk ratio (RR) associated with cold spells was statistically significant in BRHS (RR=1.86, 95% CI 1.30–2.65, p b 0.001), and independent of temperature level: results were similar whether events were fatal or non-fatal. Increased risk was particularly marked in BRHS for ever-smokers (RR of 2.44 vs. 0.99 for never-smokers), in moderate/heavy drinkers (RR 2.59 vs. 1.41), and during winter months (RR 3.28 vs. 1.25). No increased risk was found in PROSPER.
Although CVD risks were higher in winter in both BRHS and PROSPER prospective studies, cold spells increased risk of CVD events, independently of cold temperature, in the BRHS only.