Swimming may be healthier in warm water• Dr. Komaroff

DEAR DOCTOR K: I'm an 80-year-old man who spends summers on the coast of Maine. I love to swim in the ocean for 20 or 30 minutes each day. The water is cold (55 degrees F), but it doesn't bother me. My real question is whether doing this is bad for my heart, because my heart rate is sometimes irregular. Are my cold-water swims OK?

DEAR READER: First of all, congratulations! Anyone who is 80 years old and is up to the challenge of a daily swim in the ocean deserves our admiration. You obviously have both courage and a zest for life.

Now, is it wise for you to swim in the cold ocean water? Swimming is an excellent exercise for your heart, arteries, lungs and muscles. If you enjoy swimming in cold water and have been doing it for some time with no ill effects, it's probably fine for you. But I do have three concerns.

First, immersing your body in water squeezes blood from your limbs into your chest. This makes your heart work harder and raises your blood pressure. When you have your face in the water and are holding your breath, your heart slows down and your blood pressure rises. And that is even more true as the water temperature gets colder.

Second, the shock of cold water against your skin automatically triggers a series of changes in your body called the fight-or-flight survival response. The most important physical change is that blood vessels supplying your skin narrow. This also makes your blood pressure rise.

Third, the fight-or-flight response can also disturb the heart's steady rhythm. Since you are already prone to an irregular heartbeat, it could spell trouble.

For these reasons, and because ocean water can be treacherous, I think it would be healthier for you to swim in a pool with temperatures in the 70s or low 80-degree range.

But if you want to continue to swim in the ocean, I surely wouldn't advise you to stop. Swimming daily is most likely better for your health than not doing so. I would suggest you always swim with someone close by and make sure that person knows CPR. Be on the lookout for warning symptoms: If you feel faint or notice irregular or "missed" heartbeats, get out of the water.

Here in Boston we have a fearless group of men who, by long tradition, take a swim in our salty harbor every New Year's Day. Then they try to get a suntan -- regardless of the weather. They enter the ocean at the foot of L Street, and they're called the L Street Brownies. Of course, the rest of us think they're crazy. But their pictures are in the paper every year, because we all admire their spirit, too.

• Dr. Komaroff is a physician and professor at Harvard Medical School. Go to his website to send questions and get additional information: www.AskDoctorK.com.