Hot Days Can Raise Body Temperature and Strain the Heart

Key Takeaways

• Without body-cooling measures, heat pushes the heart to work harder to get oxygen.
• This is especially dangerous for older adults with existing heart disease, who are susceptible to myocardial ischemia and heat stroke.
• Staying in the shade, drinking plenty of water, and using a personal fan can help keep the body cool on hot days.

Even when people are young and healthy, heat exposure that causes a spike in body temperature can put stress on the heart, a new study suggests.

For the study, scientists exposed 20 healthy young adults, 21 healthy older adults, and 20 older adults with coronary artery disease to enough heat in a lab to increase their body temperatures by 1.5 degrees C (2.7 degrees F). That heat exposure caused myocardial blood flow - or the amount of blood circulating through coronary arteries - to increase, according to study results published in the Annals of Internal Medicine .

"During heat exposure, the body sends blood to the skin surface to exchange heat with the environment. This causes an increase in the work of the heart, by increasing heart rate and contractility (or the force of contractions) to maintain a stable blood pressure," says senior study author Daniel Gagnon, PhD , an associate professor at the Montreal Heart Institute and the University of Montreal.

The fact that the heart works harder is not necessarily a problem in and of itself, Dr. Gagnon says. It's actually part of the body's normal response to heat exposure. The problem is that conditions like coronary artery disease reduce the ability of the arteries to widen when the heart needs to receive more blood, which can make it harder for enough oxygen to reach everywhere in the heart where it's needed for the muscle to work effectively.

This type of oxygen deficit in the heart causes a condition known as myocardial ischemia, which can lead to chest pain, an irregular heart rhythm, or a heart attack , Gagnon says.

Symptoms of Myocardial Ischemia

Even though study participants didn't exhibit symptoms of myocardial ischemia, seven of the older adults with coronary artery disease showed evidence of this on imaging scans of their hearts taken after heat exposure.

It's possible that some people may have experienced subtle symptoms like mild chest discomfort or shortness of breath, says Sameed Khatana, MD, MPH , an assistant professor of medicine at the University of Pennsylvania in Philadelphia and a cardiologist at the Philadelphia Veterans Affairs Medical Center.

"If the ischemia gets more severe, it could result in ever more serious consequences, such as a heart attack or abnormal heart rhythms," says Dr. Khatana, who wasn't involved in the new study. "It is difficult to say how long this would take, as it depends on the level of heat, clothing, humidity levels, activity levels, and overall fitness."

However, the lab conditions weren't the same as what people might encounter outdoors or inside a building without air conditioning on a hot day. Participants were required to stop taking any medications - including drugs prescribed to manage coronary artery disease. They were also exposed to heat by wearing a special suit that prevented them from sweating, the body's natural cooling system, and they weren't allowed to drink water during the experiment.

What Happens to the Heart on Hot Days?

Under typical conditions, hot days would not be able to cause a rise in core body temperature to the extent created in the study, says Victor Ferrari, MD , the senior author of an editorial accompanying the study and a cardiologist and professor at the Hospital of the University of Pennsylvania.

"However, when heat is extreme and the humidity is high, the risks of elevated body temperatures and heat stroke increase, and these conditions could cause severe symptoms with exposures as short as 30 minutes," says Dr. Ferrari. "Be aware that symptoms such as dizziness, lightheadedness, and thirst can precede the more severe symptoms of heat stroke , such as slurred speech, disorientation, confusion, and passing out."

When heat is extreme, people with heart disease should avoid going outdoors during the hottest parts of the day, if possible, Ferrari advises. He also suggests the following precautions:

• Stay hydrated. Drink lots of water, and consider electrolyte-replenishing drinks if you don't have heart failure.
• Stick to the shade. Carry an umbrella to shield yourself from the sun if it's not shady.
• Carry a fan. Hand-held personal fans can help you cool down, especially if it has a mister or you wipe your face, arms, and legs with a moist cloth.

Beyond this, people need to know what symptoms to watch out for, and seek medical help immediately if these symptoms develop.

"The main symptoms to look out for would be chest pain, shortness of breath, a squeezing feeling on the chest, and dizziness or nausea," Gagnon says. "If someone feels these symptoms, they should call 911."

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Sources

1. Barry H et al. The Effect of Heat Exposure on Myocardial Blood Flow and Cardiovascular Function. Annals of Internal Medicine . June 11, 2024.
2. Guallar E et al. Feeling the Heat: Cardiovascular Consequences of Heat Exposure Under Controlled Experimental Conditions. Annals of Internal Medicine . June 11, 2024.