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How hot weather can affect your heart when you exercise

May 12, 2022 1990

Summer heat — especially during exercise — can be hard on the heart.

That’s because your heart plays a big role in keeping you cool. The added workload of exercise only increases the demands on your cardiovascular system.

The load on the heart increases with activity and exercise, especially in hot weather. For every degree the body’s internal temperature rises, the heart beats about 10 beats per minute faster. The result is a dramatic increase of stress on your heart.

Keeping your cool
Your body sheds extra heat in two ways: by perspiring and by radiating warmth, says exercise physiologist Michael Crawford, MS, who works in cardiac rehabilitation and preventive cardiology.

You radiate heat through the blood flowing in your veins and arteries. On a hot day, your cardiovascular system ramps up its efforts to radiate heat to cool you down.

Your heart beats faster and pumps harder, and may circulate two to four times as much blood each minute as it does on a cool day.

When temperatures soar, perspiring can put a strain on your cardiovascular system, too. When you sweat, you excrete sodium, potassium and other minerals that your body needs to contract muscles, transmit information via nerves and maintain a balance of fluid. Your body takes steps to hold onto its fluid and minimize the loss of important elements.

“The demands are much greater during the hot weather,” Mr. Crawford says. “Your body needs to get rid of excess heat and it does that is by dilating the blood vessels peripherally. To get the blood around, your heart works harder.”

Keep your heart cool while you exercise
Mr. Crawford recommends exercising in the cooler morning or evening hours to minimize thermal stress, as well as these other tips for safer hot weather exercise:

Slow down on hot, humid days. Reduce your exercise pace on hot days with high humidity. If the temp is above 80 degrees and humidity above 80 percent, it’s best to postpone your activity until it cools off.

Keep extra-hydrated if exercising more than 30 minutes. Drink 8 to 12 ounces of water 20 to 30 minutes before exercise, plus 6 to 12 ounces more every 30 minutes of exercise to prevent dehydration.

Wear heat-appropriate clothing. Don’t wear clothing like rubber suits or long-sleeved sweatsuits that prevent sweat evaporation and interfere with your body’s ability to cool itself. Wear loose-fitting cotton T-shirts, shorts and a brimmed hat while exercising outside.

Take caution even for normal activities
Mr. Crawford says take precautions even if you’re simply out in the sun for an extended period.

“If you’re in the heat and begin to feel fatigued or your heart rate going up, find some shade or go indoors for air conditioning and stay hydrated,” Mr. Crawford says. “It’s important to work to get your core body temperature down.”