Men who die of a sudden cardiac event are less likely to do so on the first day of mountain activities if they sleep at higher elevations the night before, according to a new study.
"The recommendations now are if you're an active, healthy person above about 8,000 or 9,000 feet, you should spend some time acclimatizing before you're active. You should have a day of rest or more," said Bruce Johnson, a professor at the Mayo Clinic in Rochester, Minnesota.
The findings don't prove that sleeping at a higher altitude will prevent sudden cardiac deaths. But acclimatizing - spending some time getting used to altitude - already falls in line with recommendations from mountain researchers, said Johnson, who did not participate in the study.
In their report in the American Heart Journal, the researchers write that 25 to 30 percent of all mountain sport-related deaths are from sudden cardiac death, and that mountain hikers have a four-fold risk of sudden cardiac death compared to the general population.
Still, the risk of dying from mountain sports is quite low - on the order of one sudden cardiac death for every 802,000 hours of mountain hiking.
Based on the results, study co-authors Dr. Martin Burtscher, a researcher at the University of Innsbruck in Austria, and Dr. Benjamin Levine, a professor at the University of Texas Southwestern Medical Center at Dallas, said altitude is a trigger for sudden cardiac death.
At higher altitudes, there is less pressure and therefore less oxygen in the blood stream.
"To make up for it the heart has to work a little harder to deliver oxygen to our active muscles," said Johnson.
The authors sent questionnaires to the relatives of more than 550 men over the age of 34 who died on hiking trips in the Austrian Alps.
Most of the men were hikers or skiers, and half of them died on the first day of their trips.
The researchers found that men who slept below 700 meters (about 2,300 feet) were more than five times as likely to die on the first day in the mountains than men who slept above 1,299 meters (4,261 feet).
"We guess that sleeping at altitude was protective - those who slept at altitude acquired sufficient amounts of acclimatization to reduce their risk," Levine and Burtscher told Reuters Health by email.
Johnson called the data "somewhat surprising" and said he would not have expected to see such a large difference between the groups at moderate altitudes.
"Some people would consider 3,000 feet or 1,000 meters as minimal altitude exposure," he told Reuters Health.
Johnson said the results are interesting and "hypothesis-generating," meaning they need to be explored further before any definitive statements can be made about the benefits of sleeping at altitude and sudden cardiac death.
In addition, "unaccustomed physical activity is probably a more important trigger than altitude per se," Burtscher and Levine said.