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Jump to Key Points During heat waves, daily peak core temperature may rise in older adults, according to recently published data.
In a single-site, randomized crossover trial that included 18 participants, daily peak core temperature was 0.4 °C higher during a simulated heat wave than during a thermoneutral control.
“Core temperature was regulated, but at a cost to the body,” senior author Glen Kenny, PhD, director of the Human and Environmental Physiology Research Unit at the University of Ottawa, told Medscape News Canada. “Even though they can still regulate and maintain core temperature, the decrement and the stress that it induces on their body is significant, and that is what is really important.”
The study was published on September 30 in The Lancet Planetary Health.
- 3-day heat wave ↑ peak core temp 0.4 °C vs thermoneutral control.
- Older adults maintained core temp, but with physiologic strain.
- Heat exposure ↑ plasma volume change; suggests renal stress.
- Gastrointestinal injury marker ↑; cognitive scores and balance ↓.
- Standing SBP ↓ 27 mm Hg during heat exposure despite hydration.
Multisystem Effects
During the study, participants were exposed to a simulated heat wave and a thermoneutral control condition. The order of the sessions was random for all participants, and the two exposure sessions were separated by more than 10 days. Each session lasted for 3 days and 2 nights. During the simulated heat wave, the daytime temperature was 34 °C and the nighttime temperature was 26 °C. During the control, the daytime and nighttime temperature was 26 °C.
The length of the exposure points to the impact on the body after just a few days of hot weather, noted Kenny. “The body is being compromised, and that is after only 3 days. A heat wave is not 1 day or 2 days. It may last 7 or 10 days.”
The core temperature increase, while mild, has multiple other effects, such as impaired cognition, impaired orthostatic responses, kidney strain, and gastrointestinal distress, Kenny added.
Investigators observed that the change in plasma volume from preexposure baseline was higher during the heat wave simulation than during the control exposure (P = .00035), and this greater change suggested kidney strain, they said.
The study also revealed markers of gastrointestinal distress. Intestinal fatty acid-binding protein concentration (averaged across days) was 287 pg/mL higher during the heat wave simulation than during the control exposure (P < .0031).
Additionally, cognitive function was adversely affected during the heat wave simulation. The Stroop composite score was 8 points lower on day 2 and 7 points lower on day 3 of the heat wave simulation, compared with the corresponding days of the control condition.
The day-end balance composite score was also lower during the heat wave simulation than during the 26 °C controlled exposure. Investigators observed that the absolute standing systolic blood pressure was 27 mm Hg lower during the heat wave exposure than during the control condition (P < .0001).
“What we saw are consequences for their general physiology and their cognitive ability,” said Kenny. “Our older adults were hydrated, and despite that, you saw these reductions in blood pressure regulation. You also saw blood being diverted away from organs, resulting in real evidence that there is harm to the gastrointestinal system. There’s harm to the kidneys, even though they’re hydrating and have full access to water.”
Proposing Cooling Strategies
One of the study’s strengths is that it looked across several days of exposure, rather than one period of several hours, according to Samir K. Sinha, MD, a geriatrician and clinician scientist at the Sinai Health System and University Health Network, director of health policy research at the National Institute on Aging, and professor of medicine at the University of Toronto in Toronto.
“It’s the first study that I’m aware of that actually has looked at the cumulative effects of 2- to 3-day periods, showing that after continuous exposure, body temperatures rose almost half a degree, which may not sound like much, but that can be significant,” Sinha told Medscape News Canada. “They also saw the physiologic effects that it was having, for example, on cardiovascular systems, renal systems, and other systems.”
The researchers excluded patients with cardiac abnormalities and those who used medication that made study participation inadvisable. In real-world settings, many older adults have comorbidities and may be taking medications, making them even more vulnerable to the effects of a heat wave, Sinha noted. “These factors increase the negative impacts that extreme heat can have.”
The findings “highlight the need for evidence-based cooling strategies and other interventions to reduce physiologic strain during even moderate heat stress and not just during the most extreme conditions,” the authors noted.
Sinha echoed that the study’s data underscore an unmet need with respect to housing for older adults. These patients need housing with temperature controls in the warmer months.
Sinha cochaired the Canadian Red Cross/National Institute on Aging project called Advancing Emergency Preparedness, Response and Recovery for Older Adults, which recommended that there be “clear thresholds for temperature regulation, specifically, maximum and minimum temperatures permissible based on occupational and environmental health standards, and [clear] steps required to regulate temperatures and minimize fluctuations.”
The study was supported by Health Canada and the Canadian Institutes of Health Research. Kenny and Sinha reported no relevant financial relationships.
