Why are heart attacks more common in winter? A cardiologist explains
Cold weather is a challenge in itself, with travel disruptions, snow and ice making even the simplest tasks take twice as long. But winter also poses a risk to physical health for those who are at a higher risk of heart attack, angina or high blood pressure, because of the ways the body responds to the cold.
A 2021 study in the journal Medicine found that overall, U.S. cardiac arrests peak during December and January, although the exact statistics is difficult to determine. Live Science spoke to experts to find out why.
How does cold weather affect the heart?
So how does cold weather, and the body’s response to it, affect the heart?
Dr. Oliver Guttmann, a consultant cardiologist at St Bart’s Hospital in the U.K., told Live Science that cold weather decreases body heat, meaning the heart needs to work harder to keep the body warm. This can increase the chance of a cardiac event in people already at a heightened risk, such as people who smoke and the elderly, he said.
Cold weather also causes blood vessels to narrow around extremities such as fingers and toes, a process known as vasoconstriction, Guttmann said. The result is that the body’s core remains warm and the organs continue working. “With less blood going to extremities like your fingers and toes, there is more pressure to pump blood around the rest of the body,” he said. “[However] this means that your heart has to work harder, increasing both heart rate and blood pressure.”
The additional pressure put on the heart due to vasoconstriction can cause fatty plaques that have built up inside arteries to rupture, Guttman said, which can block the blood supply to the heart.
In addition, colder weather changes the viscosity of blood, Guttman said. “When we are cold our blood thickens, which can also increase the chance of developing blood clots and consequently, having a heart attack,” he said.
Blood coagulability is also increased, said Dr. Deborah Lee, a medical writer for Dr Fox Online Pharmacy in the U.K. Blood coagulability is the speed at which blood forms a clot and later a scab to prevent blood loss. “Cold temperatures have been found to cause an increase in blood fibrinogen levels – this is the major protein involved in blood clot formation,” she told Live Science. Blood clots increase the risk of heart attack.
Another possible cause for the seasonal increase in heart attacks is vitamin D deficiency, which is more prevalent over the winter months due to a lack of sunlight. Vitamin D deficiency has been linked to several cardiovascular risk factors. However, more research is needed to understand the underlying mechanisms, according to a 2018 article in the journal Clinical Hypertension.
A 2013 review in Current Treatments in Cardiovascular Medicine indicates that vitamin D may have a protective effect against cardiovascular disease, although the authors also said that more research is needed.
It’s difficult to quantify exactly how much the risk of cardiovascular disease increases in winter, according to a 2003 review in the journal BMC Cardiovascular Disorders, because cardiovascular and respiratory diseases are often related disorders, but either could be listed as a cause of death. Depending on what is listed, there can be a wide variation in statistics, from a 5% to 30% risk of death from cardiovascular disease across different studies carried out in different countries.
The wide range of increased risk in winter may also be because cold weather is experienced differently in different countries. For instance, the BMC Cardiovascular Disorders review found that winter mortality from cardiovascular disease is lower in Scandinavian countries compared with the U.K., which may be due to socioeconomic factors and healthcare systems. It could also be because Scandinavian countries are better set up to deal with cold weather.
A 2017 study in the journal BMJ Open looked at deaths and daily temperatures in Finland , and found that cardiac death increased by approximately 19% on "unusually cold days."
In the BMC Cardiovascular Disorders review, older patients admitted to hospital in November, December, January and February were 30% to 50% more likely to die from cardiovascular disease than those admitted in May. However, for the younger age group, there was no difference in death rates whichever month of the year they were admitted. Therefore, age may also be a risk factor.