Research has found that heart attacks peak during the winter months, and the prevailing hypothesis has been that cold temperatures stress the heart. But in 2004, researchers analyzed 12 years of Los Angeles County death certificates and found that heart attack deaths also rise in the balmy Los Angeles winters. What's more, cardiac deaths peak on Christmas and New Year's Day in L.A. County.

Could it be that the weather is not the most important factor behind the seasonal increase in heart attacks? Cardiologist Robert Kloner discusses his research in Episode 28 of the podcast, Life Lines. You can find that episode here.

Dr. Kloner and his colleagues found that cardiac deaths in Los Angeles County rose 33% during the months of November through January and peaked on Christmas and New Year's Day. Because the temperatures were mild and stable during the three-month period, they concluded that factors other than temperature must play a role in the increase in cardiac deaths.

Why would heart attack deaths spike during the holidays? The researchers considered a variety of possibilities, but concluded that emotional stress associated with the holidays is a primary factor.

How stress affects the cardiovascular system

Stress stimulates the sympathetic nervous system, causing the release of catecholamines, which increase heart rate, blood pressure, and the force of the heart's contractions. This can intensify the pressure of the blood against plaque in the arteries and can cause the plaque to rupture and possibly block blood flow.

Ruptured plaque may also release tissue factors which stimulate formation of a blood clot. The blood clot could shut off blood flow in the coronary artery, contributing to a heart attack.

In addition to increasing the possibility of plaque rupture, the stimulation of the sympathetic nervous system may trigger arrhythmias, in which the heart beats irregularly or rapidly.

Physiology is the study of how molecules, cells, tissues and organs function to create health or disease. The American Physiological Society (APS) has been an integral part of this scientific discovery process since it was established in 1887.

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American Physiological Society (APS)