Lab experiment shows exactly how heat waves can put hearts into ‘oxygen debt’

When temperatures soar, so do heart attacks. Now, a lab experiment explains just how temperatures climbing into Fahrenheit’s three-digits can cause ischemia and potential heart attacks, all while international efforts to limit long-term warming seem like they’re 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, ramping up blood flow through the hearts of healthy young and old people, but hitting narrowed passages in one-third of older trial participants who had existing coronary artery disease.

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

For the experiment, three groups of people donned tube-equipped wetsuits originally designed by NASA to cool down astronauts, senior author Daniel Gagnon, an associate professor at the Université de Montréal and the Montreal Heart Institute, told STAT. In this case, progressively warmer water was infused into the suits after a 30-minute rest period. Core body temperature as measured by rectal thermometers was pushed up by three notches: 0.5 degree, 1 degree, then 1.5 degrees Celsius while PET scans revealed how blood was flowing through participants’ hearts.

What would that feel like in real life? Those temperatures were chosen to approximate what a very hot environment outside the body would be over three hours, climbing from 38 to 47 degrees C (100 to 116 Fahrenheit) with 10% to 60% humidity. Once rare, temperatures that high are becoming more common and more frequent around the world, from Arizona to India but also in Canada and northern Europe.

Taking part in the experiment were 20 healthy people 18 to 40 years old, 21 healthy counterparts age 60 to 80, and 20 participants also age 60 to 80, but with coronary artery disease. During the trial, which took about an hour and 40 minutes, no one could drink water and those who were on heart drugs like beta-blockers skipped their doses.

All of the people had increased blood flow, a sign that their hearts were working harder, in part to cool their bodies. Blood flow rose twice as much in the youngest, healthiest group compared to the oldest, least healthy group. One-third of that oldest, least healthy group — 7 out of 20 — had blood flow blockages, despite feeling no angina symptoms during the experiment. Their imaging looked like what’s seen in a stress test.

“Our hypothesis was that the reason why heat exposure might be bad is because it makes the heart work harder,” Gagnon said. “We didn’t know to what extent does it work harder, and does it work sufficiently hard to think that it could lead to something, especially something like a heart attack.”

The participants’ hearts were in fact working harder, and sooner than the researchers expected. Almost half of the increase in myocardial blood flow occurred when body temperature went up by 0.5 degree C, which was the first bump up. That’s a mild increase, one that might happen normally over the course of a day without high heat, Gagnon said.

As the experiment progressed, the hearts of those with coronary artery disease had impaired ability to open up their vessels to allow more oxygen-carrying blood to sustain the work. Their myocardial blood flow did not increase proportionately to the amount of work that the heart needed to do during the experiment. It’s like their engines were running out of fuel, he said.

“It’s almost like oxygen debt,” Gagnon said. “It’s spending more energy than it’s bringing in.”

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

“If we can imagine during an actual heat wave when it’s hot for a day, two days, three days, and if somebody has 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 were closing enough to be called ischemic, there were some differences. Blockages were mild for three participants, moderate for three participants, and severe for one. For three of the seven, the narrowing got progressively worse, climbing in sync 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 been able to show only as a correlation between high heat and heart problems, said Joel Kaufman, a primary care doctor 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 is showing physiologically that in individuals with established coronary artery disease, clearly ischemia can be induced by heat,” he told STAT. Also an editor of the NIH-supported journal Environmental Health Perspectives, he was not involved in the new study, published Monday in the Annals of Internal Medicine. “That provides us an explanation for the risk of triggering myocardial infarction during an extreme heat event.”

Gagnon suggested it might be time to widen attention to include not just making sure people don’t get too hot, but also finding ways to reduce the work of the heart when it’s hot outside.

“A lot of times when we talk about heat waves, we say, OK, we need to make sure we don’t get too hot,” he said. “We don’t need to get that hot to potentially have other health problems that might occur because of the heat without necessarily being extremely hot.”

Spatz echoed that concern.

“We need to do a better job about counseling older people with heart disease about heat exposure. Heat poses an increased stress to the heart, and this may 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 out air conditioning, and stay indoors when there is extreme heat. Our public health and social systems need to ensure that older people with heart disease have the protection they need.”

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

“If you’re able to observe ischemia in a small group of people with an experimental intervention, you can assume that there’s a lot of people affected by this when you’re talking about millions of people who might have coronary artery disease who are going to experience heat,” he said. “People with established coronary artery disease are a vulnerable population during a heat wave that should be thinking about cool environments to get to during a heat wave, even in the absence of symptoms.”

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