EMBARGO : 30 August 2015 at 09:30 BST
“Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia and it increases the risk of ischaemic stroke by four- to five-fold,” said Dr Chao. “During AF, the electrical activity of the left atrium is disordered and the contraction is ineffective which results in stasis of blood in the atrium. The blood pools and becomes sluggish and can result in the formation of blood clots. If a clot leaves the heart and travels to the brain, it can cause a stroke by blocking the flow of blood through cerebral arteries.”
The autumn/winter season has been associated witha higher incidence and mortality rate of myocardial infarction, stroke and congestive heart failure. A greater plasma fibrinogen level and factor VII clotting activity in the winter have been reported as possible mechanisms.
“The elevation and activation of these coagulation factors may lead to a pro-coagulant status in cool climates which may promote the formation of blood clots within the left atrium and increase the risk of stroke for AF patients,” said Dr Chao. “However until now it was unclear whether the risk of ischaemic stroke was higher in AF patients during cool seasons or on days with a lower temperature.”
The study investigated this issue in 289 559 new-onset AF patients from the “National Health Insurance Research Database” inTaiwan during 2000 to 2011. Average daily temperatures in six regions of Taiwan were obtained from the Central Weather Bureau and average temperatures of each month and season were calculated. Taiwan is a small island and the differences in temperature between different regions were not large (Figure 1). The researchers therefore averaged the temperatures in the six regions to calculate the country average. The risk of ischaemic stroke was estimated for each month and season.
Among the Taiwan AF cohort, 34 991 patients suffered from an ischaemic stroke during the mean follow up of three years. The risk of ischaemic stroke was higher among the months with a lower average temperature (Figure 2). In addition, the incidence of ischaemic stroke was highest in winter and lowest in summer (Figure 3). Compared to summer, the risk of ischaemic stroke increased by 10% in spring and 19% in winter. Stroke risk did not differ significantly between summer and autumn.
“Our study shows a clear association between temperature and risk of ischaemic stroke in patients with AF,” said Dr Chao. “Risk may increase in cold weather because of the pro-coagulant status.”
A 5 °C decrease in daily temperature within 14 days before the stroke occurred was associated with an increased risk of stroke with an odds ratio of 1.128 (95% confidence interval [CI] = 1.061–1.199, p<0.001). Dr Chao said: “This suggests that there may be an opportunity to predict strokes in AF patients before they happen and put preventative measures in place such as adequate anticoagulants and reducing cold exposure through protective clothing and heating homes in winter.”
When the average temperature was below 20 °C the risk of ischaemic stroke significantly increased compared to days with an average temperature of 30 °C. “This may be because of the increased coagulability and plasma viscosity,” said Dr Chao.
He concluded: “In this large-scale nationwide AF registry, the risk of ischaemic stroke in AF patients was higher during cool months and seasons. The results imply that the interaction between humans and the environment may play an important role in AF-related stroke and remind us that cool climate may be an underrated issue for health which deserves more attention.”
Figure 1: Average monthly temperatures in six regions and Taiwan as a whole
Figure 2: Distributions of mean temperature and incidence of ischaemic stroke by month
Figure 3: Incidence of ischaemic stroke by season