Twice in the past two weeks I have seen the morning sun glinting off frost on the grass. “Heart attack season,” I said to myself as I drove to work. When I lived in Pennsylvania -where the winters are truly cold – I tracked the day-by-day hospital census for my cardiology group. In my first year there, the low of 69 hospital inpatients occurred in August, and the high of 176 happened in January. So, what was going on, and should we be concerned? After all, this is South Carolina, and we get off easy with the weather.
Cold weather alters the activity of the autonomic nervous system (that part of the nervous
system that regulates such things as blood pressure and heart rate). One of the tests we do to evaluate the autonomic nervous system is the cold pressor test: one hand is immersed in ice water for one minute. In some people, this causes a significant jump in blood pressure of 15 mmHg or more.
“Canadian researchers also learned that cardiovascular events occur less frequently on extremely cold days than on moderately cold days. Perhaps when it is frigid people don’t go outside to shovel snow but stay inside where it is warm. Or maybe we are not as safe as we thought.”
In a cold snap, the freezing weather jacks up the blood pressure. Further, strenuous work in the cold increases heart rate. One of the measures of the workload on the heart is the “double product,” the product of blood pressure and heart rate (BP x HR). So, it is easy to see why winter is the season of heart attacks. A Canadian study showed that 4.46% of heart attacks were caused by winter cold, and only 0.54% by summer heat. And this is just heart attacks. Studies have also linked the winter season with increased numbers of deep vein thromboses (clots in deep veins), pulmonary emboli (clots in the lung), strokes, aortic ruptures, and even heart rhythm problems.
You might be thinking, “But we are far south of Canada, so this doesn’t apply to me.” But the Canadian researchers also learned that cardiovascular events occur less frequently on extremely cold days than on moderately cold days. Perhaps when it is frigid people don’t go outside to shovel snow but stay inside where it is warm. Or maybe we are not as safe as we thought.
My subjective impression is that there is much less seasonal variation in cardiovascular disease in the warmer South than in the frozen North. I have wanted to plot cardiovascular admissions to the hospital against temperature here in South Carolina, but the the confounding factor of Covid makes a meaningful analysis impossible at this time. This is a study I will put on the shelf until we arrive in a Covid-free future.
Until then, bundle up and stay safe.
Dr. Ian Smith
