Stroke Risk Higher During Cold Weather for Patients with Atrial Fibrillation

New research suggests that cold weather may be an underappreciated hazard for patients with atrial fibrillation (AF).

Researchers in Taiwan combed through national insurance data from 2000 to 2011 and tracked outcomes for 289,559 AF patients from initial diagnosis until the end of the study period. They then correlated average daily temperatures with outcomes such as stroke.

In all, 34,991 patients suffered ischemic stroke during a mean follow-up period that lasted 3 years after the first AF diagnosis. Analysis of outcomes by season found that the patient stroke rate was 10% higher during springs than it was during summers and that it was 19% higher during winters than it was during summers. Analysis of stroke rate by month found similar differences.

Even weather than many people would consider far from cold was associated with higher stroke rates than the lowest risk weather — weather that was too hot to fit many other definitions of optimal. Indeed, temperatures just below a relatively pleasant 20 degrees C (68 degrees F) saw significantly more strokes than temperatures above 30 degrees C (86 degrees F).

Another interesting finding was that brief temperature fluctuations were also associated with a big change in outcomes. For example, a 5-degree C drop in daily temperature typically saw the stroke rate 2 weeks later increase more than 10% (odds ratio, 1.128; 95% confidence interval,1.061—1.199; p<0.001).

“In this large-scale nationwide AF registry, the risk of ischemic 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,” said Tze-Fan Chao, MD, a study author who is also a cardiologist at Taipei Veterans General Hospital and the National Yang-Ming University.

“This [also] 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,” said Chao, who presented the findings of the study before the European Society of Cardiology.

Other research has found that colder months consistently see higher rates of myocardial infarction and heart failure than warmer months — and that such events are more likely to prove fatal in winter than in summer. Some evidence suggests that plasma fibrinogen level and factor VII clotting activity, which both tend to rise, may explain such observations. Chao believes the same factors could also explain the findings of his team’s study.

“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 Chao. “However until now it was unclear whether the risk of ischemic stroke was higher in AF patients during cool seasons or on days with a lower temperature.”

The new study from Taiwan complements a 2-year-old Argentinian study, which looked at the effects of colder weather on people who had already had strokes and found that it was associated with higher rates of AF.

“We found a seasonal variation in the occurrence of atrial fibrillation, with a peak in winter and a valley in summer (23.1% versus 14.0%, P < .001),” the authors of that 899-patient study wrote in the Journal of Stroke and Cerebrovascular Diseases. “Atrial fibrillation showed a nonhomogeneous distribution across ranges of temperature (P < .001, goodness-of-fit test), with a peak between 5° C and 9° C.”