A heat wave can cause hearts to suffer an ‘oxygen debt’, laboratory experiments show STAT

WTemperatures rise, just like heart attacks. Now a laboratory experiment explains how temperatures soaring into the triple digits of Fahrenheit can cause ischemia and possible heart attacks, as international efforts to limit temperatures prolonged heating it seems like they are running out of time.

The experiment, which gradually exposed participants to higher temperatures, showed that exposure to heat, even while resting, made participants’ hearts work harder, speeding up blood flow through the hearts of healthy young and old people, but narrowed passages developed in a third of older people. trial participants who had existing coronary artery disease.

“Older people with heart disease are vulnerable to changes in weather — both extreme heat and extreme cold pose a risk for chest pain, heart attacks and sudden death,” said Erica Spatz, a Yale cardiologist and epidemiologist who was not involved in the study. is involved. told STAT via email. “A third of older people with heart disease developed ischemia which causes body temperature to rise – this is huge. This study not only provides clues about the mechanisms of this risk, but also a warning that we must be prepared with climate change to better protect our most vulnerable populations.”

For the experiment, three groups of people wore wetsuits with tubes, originally designed by NASA to cool astronauts, senior author Daniel Gagnon, an associate professor at the Université de Montréal and the Montreal Heart Institute, told STAT. In this case, after a rest period of 30 minutes, increasingly warmer water was poured into the packs. Core body temperatures, measured with rectal thermometers, were pushed up in three steps: 0.5 degrees, 1 degree and then 1.5 degrees Celsius, while PET scans revealed how blood flowed through the participants’ hearts.

What would that feel like in real life? These temperatures were chosen to approximate what would be a very warm environment outside the body for three hours, ranging from 38 to 47 degrees C (100 to 116 Fahrenheit) with humidity ranging from 10% to 60%. Once rare, such high temperatures are becoming increasingly common and frequent around the world Arizona Unpleasant India but also inside Canada And North Europe.

The experiment included 20 healthy people aged 18 to 40, 21 healthy people aged 60 to 80, and 20 participants also aged 60 to 80, but with coronary artery disease. During the trial, which lasted about an hour and forty minutes, no one was able to drink water and those taking heart medications such as beta blockers skipped their doses.

All of the people had increased blood flow, a sign that their hearts were working harder, partly to cool their bodies. Blood flow increased twice as much in the youngest, healthiest group compared to the oldest, least healthy group. A third of that oldest, least healthy group – 7 out of 20 – had blockages in their blood flow, despite feeling no symptoms of angina during the experiment. Their imaging resembled what would be seen in a stress test.

“Our hypothesis was that the reason why heat exposure might be bad is that it makes the heart work harder,” says Gagnon. “We didn’t know to what extent it works harder, or whether it works hard enough to think it could lead to anything, especially something like a heart attack.”

The participants’ hearts actually worked harder and faster than the researchers expected. Nearly half of the increase in myocardial blood flow occurred when body temperature rose by 0.5 degrees Celsius, which was the first increase. That’s a mild increase, which could normally occur over the course of a day without high heat, Gagnon said.

As the experiment progressed, the hearts of people with coronary artery disease had a reduced ability to open their blood vessels so that more oxygen-carrying blood could support the work. Their myocardial blood flow did not increase proportionally to the amount of work the heart had to do during the experiment. It’s as if their engines ran out of fuel, he said.

“It’s almost like oxygen debt,” Gagnon said. “It costs more energy than it produces.”

They were never in danger during the less than two hours of the experiment, Gagnon said, but the implications for longer heat exposure are clear.

“If we can imagine in a real heat wave that it’s hot for a day, two days, three days, and if someone suffers from myocardial ischemia for several hours or days, then that could potentially lead to something like a heart attack,” he said . .

Among the seven people with coronary artery disease whose arteries closed enough to be called ischemic, there were some differences. The blocks were mild for three participants, moderate for three participants and severe for one. In three of the seven, the constriction became progressively worse and rose in synchrony with their core temperature. Two participants whose imaging showed heat-induced ischemia also had abnormal electrocardiograms.

The small study defines a mechanism for what large studies have only been able to show as a link between high temperatures and heart problems, said Joel Kaufman, a family physician and professor of environmental and occupational sciences at the University of Washington in Seattle.

“I think the value of this study and the importance of this study physiologically demonstrates that in individuals with established coronary artery disease, ischemia can clearly be induced by heat,” he told STAT. As editor of the NIH-supported journal Environmental Health Perspectives, he was also not involved in the new one study, published Monday in the Annals of Internal Medicine. “That gives us an explanation for the risk of causing a heart attack during extreme heat.”

Gagnon suggested it may be time to broaden the focus to include not only keeping people from getting too hot, but also finding ways to reduce the heart’s function when it’s hot outside.

“When we talk about heat waves, we often say, OK, we have to make sure it doesn’t get too hot,” he said. “We don’t have to get that hot to potentially have other health problems that can occur as a result of the heat, without necessarily being extremely hot.”

Spatz echoed those concerns.

“We need to better advise elderly people with heart disease about heat exposure. Heat puts increased strain on the heart, and this can lead to ischemia with chest pain or even a heart attack,” she said. “Knowing this information can empower individuals to hydrate, wear cool clothing, seek air conditioning, and stay indoors when there is extreme heat. Our public health and social systems must ensure that older people with heart disease get the protection they need.”

These health problems could affect a large portion of the world’s population, Kaufman pointed out.

“If you can observe ischemia with an experimental intervention in a small group of people, you can assume that a lot of people are affected by this when you’re talking about millions of people who may have coronary artery disease and who have to experience heat,” he said . “People with established coronary artery disease are a vulnerable population during a heat wave and should consider cool environments during a heat wave, even if there are no symptoms.”

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