Vail Dr. Larry Gaul remembers running with a longtime training partner, and noticing the abnormally fast beeping of his friend's heart rate monitor.
Finally, on hour four of the run, the friend told Gaul, a cardiologist, that his heart had been racing while on runs. Gaul urged him to get checked out, and tests showed that the friend had an unusual heart-rhythm disorder.
And because the condition was discovered, he was able to successfully treat the problem.
Heart problems aren't uncommon among endurance athletes in the mountains, said Gaul.
The strains of intense cardiovascular activities such as running, biking and Nordic skiing at high altitude, especially at elevations above 7,000 feet, can bring out existing heart problems, he said, citing Avon's Mike Janelle as one of the more well-known cases.
Janelle, 40, was a professional mountain biker who died unexpectedly in 2007 of heart failure.
"People think that because they exercise like that, you're not at risk. But you can't run from genetics - the difference is that more often, you'll live to talk about it," Gaul said.
Gaul has served as a team physician for the U.S. Nordic Team for a decade, and as an endurance athlete himself - he's run the Leadville 100 and is a former Nordic skiing instructor - he knows the challenges that come with higher altitude.
At high altitudes, the heart has to work harder because there's less oxygen available in the air.
Often, people tout the benefits of high-altitude training - the body compensates for the change by producing more oxygen-carrying red blood cells, getting more oxygen to the working muscles.
However, the increased strain also means that problems that might not show up at lower altitudes, such as a small blockage in a blood vessel, will cause problems at high altitude.
"You need to have less of a blockage to show symptoms up here," Gaul said. "Any problems will show up faster."
So, who should be concerned?
"You do risk assessment," Gaul said. "People who are told they have a heart murmur, people with high cholesterol or people with a family history (of heart problems.) I always ask, has anyone in your family died young (for health reasons?)"
Your own body will usually tip you off if something is amiss.
"Have you had a sudden change in your exercise performance? Is there anything you could do before that you can't now?" Gaul asked. "A lot of people say they've just felt 'off,' and they don't know why. But your body knows when you shouldn't be doing something."