Heat is hard on the heart; simple precautions can ease the strain

Heat waves are unpleasant for healthy folks. For people with cardiovascular trouble, hazy, hot, humid days can be downright dangerous.

Your body shouldn't get too hot (or too cold). If your temperature rises too far, the proteins that build your body and run virtually all of its chemical processes can stop working. The human body sheds extra heat in two ways, both of which stress the heart:

Radiation. Like water flowing downhill, heat naturally moves from warm areas to cooler ones. As long as the air around you is cooler than your body, you radiate heat to the air. But this transfer stops when the air temperature approaches body temperature.

Radiation requires rerouting blood flow so more of it goes to the skin. This makes the heart beat faster and pump harder. On a hot day, it may circulate two to four times as much blood each minute as it does on a cool day.

Evaporation. Every molecule of sweat that evaporates from your skin whisks away heat. On a dry day, the evaporation of a teaspoon of sweat could cool your entire bloodstream by 2 degrees F. But as the humidity creeps above 75% or so, there's so much water vapor in the air that evaporation becomes increasingly difficult.

Evaporation also strains the cardiovascular system. Sweat pulls more than heat from the body—it also pulls out sodium, potassium, and other minerals needed for muscle contractions, nerve transmissions, and water balance. To counter these losses, the body begins secreting hormones that help the body hold onto water and minimize mineral losses.

Problems coping

Most healthy people tolerate these changes without missing a beat. People with damaged or weakened hearts, or older people whose bodies don't respond as readily to stress as they once did, have a much harder time, and may succumb to heat stroke. For example:

• Damage from a heart attack can keep the heart from pumping enough blood to get rid of heat.
• Cholesterol-narrowed arteries can limit blood flow to the skin.
• Medications interfere with heat regulation. Beta blockers slow the heartbeat, and so limit the heart's ability to circulate blood fast enough for effective heat exchange. Diuretics (water pills) make dehydration worse by increasing urine output. Some antidepressants and antihistamines can block sweating.
• A stroke, Parkinson's disease, Alzheimer's disease, diabetes, and other conditions can dull the brain's response to dehydration. So, it may fail to send thirst signals.

Hot, humid weather can be especially hard for people with heart failure, or those on the verge of it. The extra work for the heart, compounded by the loss of sodium and potassium and the internal flood of stress hormones, can push some people into trouble. The combination of increased blood flow to the skin and dehydration may drop blood pressure enough to cause dizziness or falls.

Beat the heat

Some simple choices can help you weather the weather and keep heat from overstressing your heart and spoiling your summer.

Take it easy. Turn procrastination from a vice to a virtue by putting off exercise or other physical activity until things cool down. Evening and early morning are the best times to get out. If you do exercise, drink more than you usually do.

Cool is cool. Chilled air is the best way to beat the heat. Fans work, but only to a point — when the air is as warm as you are, sitting in front of a fan is about as helpful as sitting in front of a blow dryer. If you don't have an air conditioner, spending an hour or two in a movie theater, at a store, or with an air-conditioned neighbor can help. So can a cool shower or bath, or putting a cold, wet cloth or ice pack under your arm or at your groin.

Drink to your health. The lower your coolant level, the greater your chances of overheating. Unfortunately, staying hydrated isn't always easy. Stomach or bowel problems, diuretics, a faulty thirst signal, or low fluid intake can all interfere. On dangerously hot and humid days, try downing a glass of water every hour. (If you have congestive heart failure, check with your doctor or nurse first.) Go easy on sugary soda and full-strength fruit juice since they slow the passage of water from the digestive system to the bloodstream. And don't rely on caffeinated beverages or alcohol for fluid because they can cause or amplify dehydration.

Eat light. Stick with smaller meals that don't overload your stomach. Cold soups, salads, and fruits can satisfy your hunger and give you extra fluid.

Warning signs of heat illness

Heat-related trouble ranges from irritating problems such as prickly heat (also known as heat rash) to heat exhaustion and the potentially deadly heat stroke. It can be hard to tell where heat exhaustion ends and heat stroke begins. Both can be mistaken for a summer "flu," at least at first. Be on the lookout for:

• nausea or vomiting
• fatigue
• headache
• disorientation or confusion
• muscle twitches

If you think you are having heat-related problems, or if you see signs of them in someone else, getting to an air-conditioned space and drinking cool water are the most important things to do. If these don't help or the symptoms persist, call your doctor or go to a hospital with an emergency department.

Adapted from a Harvard Health Blog post by Patrick J. Skerrett.