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By the way, doctor: Could swimming in cool water give me a heart attack?

MAY 2010

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Q. When I attempt to go into the outdoor pool at my beach club, I gasp for breath, get dizzy and light-headed, and have to get out. Several years ago I read an article that some people who are very sensitive to cold water may sustain a heart attack from submersion into cold water. Is this a possibility?

A. Cold temperatures can have important effects on your heart and circulation, so you're smart to be cautious and get out of the water when you start feeling winded and woozy.

Water conducts thermal energy 30 times more efficiently than air, so taking a plunge into cold water is going to have a more pronounced effect on body temperature than stepping out into air that's the same temperature. Of course, we also tend to be less covered when we swim, so we'll also lose more heat simply because there's more skin exposed.

The body's normal response to cold, whether it's water or air, is to go into a defensive posture and try to hold on to as much heat as possible. Less heat gets lost to the cold water if there's less warm blood circulating in your skin and through the parts of the body with a lot of surface area, such as your fingers and toes. So, blood gets shunted from the body's extremities to its core (the trunk and head), and blood vessels in the skin clamp down. Shivering, which creates body heat by cranking up metabolism, usually kicks in at temperatures a couple of degrees below those that put the circulatory system into heat-saving mode.

The neurological signals that constrict blood vessels also have cardiovascular effects. Blood pressure, heart rate, the amount of blood pumped with each heartbeat — they all go up. The result can be angina attacks from a heart that's working harder without enough oxygen to meet its needs. Heart rhythm abnormalities can also develop. Cold may also affect your breathing because cold air may cause airways to constrict. Some people get full-blown asthma attacks when they breathe in chilly air.

But there's another possibility that's unrelated to temperature. Walking in water or swimming can be hard work that puts additional strain on the heart. So there's a chance that the symptoms you're experiencing are related to some kind of heart problem that's apparent only when you are exerting yourself.

I'd let your doctor look you over, examine your electrocardiogram, and see if there's any evidence of heart disease. If you get a clean bill of health, then I'd conclude that maybe you're just more sensitive to cold temperatures than other people. I'd suggest that you continue to be appropriately careful — and use common sense and get out of the water if you feel any kind of discomfort at all.

— Thomas H. Lee, M.D.
Partners Healthcare System, Boston