Simple Measures Ease Cardiac Strain in 100-Degree Weather

In the absence of air conditioning, simple measures proved effective at lowering heat-induced cardiac strain in older people.

Randomized crossover studies showed that electric fans and skin wetting both worked better than no cooling in a hot and humid environment — and in people with or without coronary artery disease (CAD) — to reduce the mean change from baseline to hour 3 in rate-pressure product (heart rate multiplied by systolic blood pressure):

• Electric fan: -517 bpm x mm Hg (P=0.004)
• Skin wetting: -468 bpm x mm Hg (P=0.01)
• Fan use plus skin wetting: -750 bpm x mm Hg (P<0.001)

In a very hot and dry environment, however, there was harm observed with fan use. Heat-induced cardiac strain actually increased with fan use relative to controls (+2,139 bpm x mm Hg) and these tests had to be stopped early after 43% of participants started feeling unwell, had a high heart rate, or had abnormal findings on electrocardiography. Notably, this set of experiments was only conducted in people without CAD.

In contrast, skin wetting did manage to reduce the cardiac strain in the very hot and dry environment (-478 bpm x mm Hg), CAD or not, reported Ollie Jay, PhD, of University of Sydney in Australia, and colleagues in a brief correspondence in the New England Journal of Medicine.

“Health hazards from extreme heat are becoming increasingly common because of climate change. Older adults, especially those with heart disease, are at greater risk due to the strain that hot temperatures put on the heart. Understanding the impacts of different cooling strategies on the heart is important to help vulnerable people stay well during hot summer weather,” said Jay in a press release.

“Importantly, the study has shown that the weather conditions affect the type of cooling strategy that should be used — a vital piece of information that will help older people to stay safe in heatwaves,” added study co-author Daniel Gagnon, PhD, of the Montreal Heart Institute.

The CDC warns that older people are particularly sensitive to sudden changes in temperature and may have chronic medical conditions or take prescription medications that affect their body’s ability to control temperature and sweat. Air conditioning is generally advised to lower the risk of health problems from heat, while fans are said to backfire in temperatures above 90°F and actually raise body temperatures.

Jay’s group said the present study contradicts that last recommendation, given the observed efficacy of electric fans in the hot and humid setting.

“While air conditioning is an effective way of staying cool, it’s not available to everyone, especially those most vulnerable to the heat such as the elderly and people with heart disease — so it’s positive news that low-cost alternatives are effective,” Gagnon said.

Study authors recruited older participants to test the low-cost cooling strategies at high temperatures with a high or low humidity. The randomized crossover studies were conducted at the University of Sydney and the Montreal Heart Institute. Each person completed up to eight exposures, 3 hours at a time, each session separated by at least 72 hours.

The people without CAD were 31 individuals averaging 70 years old, 17 of them women. The 27 people with CAD averaged 66 years of age and had only two women.

The experiments were set up so that the hot and humid environment averaged 100.4°F (38.0°C) and a mean relative humidity of 60%. The very hot and dry environment had a mean temperature of 113.0°F (45.0°C) and a relative humidity of 15%.

Jay’s group reported that rate-pressure product increased during exposure to either environment despite attempts at cooling.

Study findings may not be generalizable to exposures longer than 3 hours. They may also not apply to people with unmanaged coronary artery disease or other coexisting conditions, the authors cautioned.

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