Last November, 54-year-old Conrad Anker had a heart attack at 20,000 feet while climbing in Nepal. He was roped to an ice wall at the time of the attack and had to rappel down to base camp before a helicopter could take him to the hospital. Speaking with National Geographic after the incident (he made a full recovery), Anker said:
At about 9:30 a.m., I was seconding [following] with a pack on, and I felt tired and out of breath. Initially I thought, ‘The sun will come, I’ll warm up, I’ll feel good, and then we’ll finish the next two pitches to get to our bivouac site.’ Then all of a sudden it was like, bam, what the heck is that?…Every time you go into the doctor’s office, they always have these charts: How to recognize a heart attack, how to recognize stroke, how to perform the Heimlich maneuver. You’re not committing them to memory, but with repeated exposure you understand them, so that was sort of how I self-diagnosed.
Altitude wreaks havoc on our bodies in many ways, but can it increase the chance of a heart attack? Was the altitude a factor in Anker’s experience?
In a strange twist of irony, Anker has been working with the Mayo Clinic for the past five years on a series of studies investigating altitude’s effects on the human body. Mayo’s medical scientists have been looking into how altitude affects everything from breathing patterns to gene expression. In the past, researchers have struggled to show through epidemiological studies that extreme altitudes can cause a heart attack. This is because it’s tough, both ethically and statistically, to design something like a randomized controlled trial, where you’d transport large groups of people from low to high altitude and wait for one to have a heart attack. Also, there aren’t large populations of people who live at extreme altitudes to examine in natural experiments. (A study of Sherpas would be incredibly interesting, and a few have been done, but studying those who were born at and are genetically adapted to altitude creates a sampling bias.) The Mayo Clinic’s work has not yet been peer reviewed, and currently there is no published research conclusively linking heart attacks to altitude exposure, but we do know a few things about altitude’s effects on our bodies.
Altitude exposure causes changes in our blood—specifically, it affects our hematocrit, or the ratio of red blood cells to the total volume of blood. As altitude increases, the total volume of air above your head decreases. As this happens, air pressure—including oxygen—decreases as well. Think of it like coming up from the bottom of a swimming pool. Less oxygen gas pressure reduces how much oxygen is absorbed in the blood. So to compensate, the body adds more oxygen-carrying red cells to the blood. Think: 20 half-full boxcars can carry as much as ten full ones. (This, in short, is how athletes dope—erythropoietin, or EPO, is a hormone that can push red blood cell count far higher than training at altitude.)
But there is a cost to adding more red blood cells. “The blood is thicker and doesn’t flow as smoothly and may tend to clot easier, particularly if it runs up against a partial obstruction,” says Erik Swenson, a professor of medicine, physiology, and biophysics at the University of Washington in Seattle and editor of the academic journal High Altitude Medicine and Biology. Anker told National Geographic that, at 54 years old, after a lifetime of mountaineering, he has thick blood with a high red blood cell and hematocrit level. He usually takes an aspirin while he’s climbing as a prophylactic blood thinner, but Anker forgot to on the day of his heart attack.
There isn’t a specific altitude threshold at which these changes start coming into play. Red blood cells gradually increase with height: the higher you go, the more your body makes to compensate, and everyone responds a bit differently. At what elevation you’ll notice symptoms depends on what you’re acclimated to. If you live at sea level, for example, you’ll notice some differences if you drive to the Rockies and ride the chairlift to the top of Peak 8 at Breckenridge (13,000 feet) the same day.
So it’s hard to finger altitude as the cause of Anker’s heart attack—there are so many other risk factors present in mountain sports. Cold weather constricts blood vessels, and thinner vessels are more susceptible to clotting. Climbing is also an intense upper-body workout guaranteed to increase heart rate and further tax the cardiovascular system.
High places—and the pursuits of reaching them—are generally stressful. These stressors change the balance of hormones, like adrenaline and glucocorticoids in the blood, leading to responses like narrowing arteries and blood sugar changes. The link between stress and heart disease is well established. Anker’s attack also occurred early in the morning, when stress hormone levels are naturally high. Then there’s the unavoidable risk of aging. No matter how healthy you are, plaque accumulates in your arteries over time. Anker was 54 years old at the time of the arrest—not old by any stretch but well into the age where risks start to increase. All of these factors greatly overshadow the role of altitude in Anker’s experience.
One study, however, gets us close to understanding what might cause a heart attack at altitude. In 1993, physiologist Martin Burtscher and colleagues published a letter to the editor in the New England Journal of Medicine that described his research tracking how many people experienced sudden cardiac death (a fatal heart attack) in the Australian Alps between 1985 and 1991. Then, by analyzing how many people used the ski lifts during the same time period, they calculated the odds of experiencing sudden cardiac death while exercising in the mountains compared to sea level. “As compared with the overall risk of sudden cardiac death, the risk during mountain hiking for men over the age of 34 was increased by a factor of 4.3, and the risk during downhill skiing was increased by a factor of 2.1.” A 4.3-fold increase for death might sound like a lot, but overall the risk was low either way: “one sudden cardiac death per 780,000 hiking hours and one sudden cardiac death per 1,630,000 skiing hours.”
The biggest predictor of a heart attack wasn’t the altitude at which they occurred, but rather the age and sex of the individual and whether they’d taken time to acclimate. “No increase in risk was found for men who participated regularly in mountain sports,” Burtscher wrote.
“If ever there was a heart that was adapted to low oxygen, it was Conrad’s heart. I’m sure the heart attack he had would’ve gotten rid of me quickly up there,” says Bruce Johnson, a researcher at the Mayo Clinic who researches altitude’s effects on the heart and has conducted studies (some including Anker) on Everest and Kilimanjaro. “But he’ll probably have a complete and full recovery.”
Even though Swenson and Johnson both agree that risk of a heart attack from altitude exposure is relatively small for healthy people, it can be minimized by following the classic advice for adapting to altitude. They both emphasize acclimating slowly, which, in addition to minimizing stress on the heart, will allow the rest of body to gradually adapt. It will also help to stave off other noncardiac issues associated with climbing, like altitude sickness and swelling in the head and lungs. Also, staying hydrated is always a good idea when you’re aiming to keep blood viscosity down.
“The reality is the human body is amazing and adapts to these extremes,” says Johnson. “There’s probably added risk at extreme altitudes, but for the average guy in the continental U.S. who takes a moderate approach to altitude, life is pretty safe.”