Snow shoveling has long been blamed for the seasonal surge in heart attacks. So why do Sunbelters also experience an increase?

It's one of those classic scenes in emergency medicine: After the first big snowfall, hospital ERs fill up with men complaining of chest pain. The classic explanation has been that these heart attacks are triggered by sudden physical exertion of shoveling snow or the sudden shock of colder temperatures. However, epidemiological evidence shows that an increase in heart attacks in winter occurs not only in northern states, but also in sun belt states like Florida and southern California.

So if it's not the weather, what causes the surge in winter heart attacks?

• Inactivity — rather than activity — may play the biggest role. Americans are less active in wintertime, no matter where they live. About one in three persons engage in no physical activity in January, but by summer that rate of inactivity dropped to one in four persons. This peak of inactivity coincides remarkably well with the rise in heart attacks.
• Holiday stress and less sunlight can trigger depression in some people, which can induce more heart attacks. People who are depressed and also have heart disease are less likely to adhere to their doctor's recommendations to adopt a heart-healthy lifestyle — such as a low-fat diet, routine exercise, smoking cessation, stress reduction and regular socializing.

People who overreact to stressful situations are more prone to constriction of their heart arteries. This may put them at a higher risk of heart attack during physical activity, whether it's snow shoveling or carrying heavy suitcases during holiday travel.

Cold Does Play a Role

Thomas H. Lee, M.D., a cardiologist at Harvard Medical School, notes that cold does pose some problems for people with heart disease.

"In the cardiac catheterization laboratory, researchers use exposure to cold to cause coronary arteries to squeeze down," he said. "In a normal person, the constriction of the coronary artery wouldn't pose any problem. But for someone with an atherosclerotic plaque, the narrowing could lead to angina, or even a heart attack."

In fact, a 30% blockage of the coronary arteries — a level that wouldn't normally cause chest pain — could equate to a 70% blockage in the cold. Combining that with sudden physical stress may precipitate an original attack or a heart attack.

Reason: It turns out that tiny endothelial cells in normal arteries release a molecule called "nitric oxide" that causes arteries throughout the body to relax. In diseased arteries, nitric oxide isn't produced in adequate amounts, leaving arteries chronically constricted.

The bottom line: To protect yourself from heart attack during the winter, heed the temperature and act and dress accordingly. But also practice other heart-smart strategies that you should follow year-round. Get regular exercise, eat healthy and try to manage stress.