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High-Altitude Recreation


Acute mountain sickness and poor sleep are common complaints of travelers participating in high altitude recreation. In this article, the authors share in an effort to bolster awareness of the challenges and physiological responses associated with these symptoms and its causes.

Evangelista Torricelli, an Italian physicist and mathematician who is credited with inventing the barometer, once said, “we live submerged at the bottom of an ocean of the element air, which by unquestioned experiments is known to have weight”. At sea level, the barometric pressure of the air is approximately 760 mm Hg, but is reduced to approximately 522 mm Hg at an altitude of 3,000 m. Because air contains the same percentage of oxygen (21%) as one moves to higher altitude, the partial pressure of oxygen is reduced proportionally. Thus, less oxygen is diffusing from the lungs into circulation and the body experiences hypoxia. Peripheral chemoreceptors in the body signal the brain to stimulate ventilation when arterial oxygen content is extremely low, a reflex commonly known as the hypoxic ventilatory response (HVR). The rate and depth of breathing increases in proportion to the level of hypoxia, and it is likely that a strong HVR improves the tolerance to acute exposure to high altitude.


When there is a deficiency in the biological adjustment to altitude-induced hypoxia, an individual may experience symptoms of AMS, a syndrome characterized by shortness of breath, nausea, lassitude (e.g., lack of energy), dizziness, and headaches. Among recreational climbers hiking the Swiss Alps, the prevalence of AMS increased from 9% at 2,850 m, to 34% at 3,650 m, to 53% at 4,559 m, indicating that AMS occurrence is positively related with altitude attained. Symptoms usually occur within 4 to 12 hours after ascending to an altitude of 2,000 m or greater and commonly intensify during periods of physical exertion because the metabolic requirements for oxygen increase substantially. Because prevention of AMS is preferable to treatment, much research has been dedicated toward identifying the nonmodifiable (e.g., age and sex) and modifiable factors that impact the development of AMS. In mountaineers ascending from 2,000 m to 4,559 m, there is an almost linear reduction in AMS prevalence with an increasing number of days needed for ascent. In contrast, a recent meta-analysis of existing data suggests that a history of AMS from previous exposures is not necessarily a reliable predictor of AMS.

The severity of AMS can be assessed using the Lake Louise Scale. A positive diagnosis is based on 1) a rise in altitude within the last four days, 2) presence of a headache, plus 3) presence of at least one other symptom, and 4) a total score of three or more on the Lake Louise Score for Diagnosis of Acute Mountain Sickness. A score of six or more indicates severe AMS. For most individuals, AMS disappears within one to two days with ongoing acclimatization. However, in some cases, AMS can progress into highaltitude pulmonary edema (HAPE) and highaltitude cerebral edema (HACE), conditions resulting from leakage of blood plasma and cells into the lungs and brain, respectively. These serious conditions present as AMS along with dyspnea on exertion, dry cough, decreased exercise performance, fatigue, and fever (HAPE); or worsening headache, ataxic (e.g., lack of muscular coordination) gait, altered mental state, drowsiness, stupor, and coma (HACE). Descent to a lower altitude is critical for successful treatment. Despite the potential detrimental effects, it is important to recognize that the incidences of these conditions vary according to altitude reached and, therefore, are rare in the recreational hiker. For example, 1 in 10,000 skiers in the Rocky Mountains (maximum altitude, 3,500 m) show HAPE, whereas 4% of Swiss climbers reaching an altitude of 4,559 m experience this condition.

Another challenge faced by the highaltitude adventurer is poor sleep quality. As one moves from sea level to high altitude, there is a shift from deeper to lighter sleep stages and a trend toward more time spent awake. This may be related to periodic breathing, a pattern characterized by hyperpnea (i.e., increased depth of breathing due to hypoxia) followed by apnea (i.e., cessation of breathing). Once breathing has stopped, carbon dioxide levels in the body accumulate and hypoxia increases. These signals stimulate ventilation and a cyclical pattern ensues. It has been argued, somewhat paradoxically, that arousals from sleep serve as protection from severe oxygen deprivation. Among climbers with pronounced symptoms of AMS at 2,760 m to 2,920 m, periodic breathing tends to be replaced with irregular nocturnal breathing, although it is difficult to determine whether abnormal breathing is the cause or effect of AMS.


Equipped with the aforementioned knowledge and enthusiastic about meeting these challenges head on, we joined a group of colleagues on a highaltitude expedition across the Peruvian Andes in June 2014. Our travel to Peru was part of a study abroad experience designed by the lead author of this article. The overarching objective of the program was to provide students with experiential knowledge of the basis of altitude stress and the challenges, responses, and factors of exertion affected by high altitude. Peru was chosen for its rich history and culture, accessibility to altitudes higher than 2,500 m, and because it is the location of the famed lost city of the Incas — Machu Picchu. rediscovered more than a century ago by the young Yale University history lecturer Hiram Bingham, it has become a popular tourist destination.

Before leaving, 14 student members of the expedition (11 women; 3 men) underwent a maximal exercise test on a treadmill to evaluate aerobic capacity (V˙O2max) and an evaluation of body composition using dual-energy X-ray absorptiometry. The students were trained to recognize AMS using the Lake Louise Scale. Heart rate during the trek was monitored using Polar E30 heart rate monitors (Polar Electro). Moreover, because we were curious about how sleep would be affected by acute altitude exposure, three women were fitted with a Morpheus Ox (WideMed Ltd., Herziliya, Israel) portable single-channel (pulse oximeter), photoplethysmography (PPG)-based device at sea level, and later at higher altitudes. PPG is obtained through the use of pulse oximetry to detect blood volume changes in the fingertip and correlated with oxygen saturation to detect the oxygen desaturation index (ODI).

The group was young (mean age, 22 years) and had an average (women: mean V˙O2max of 38.5 mL/kg per minute) to above-average (men: mean V˙O2max of 50.8 mL/kg per minute) fitness level according to sex- and age-specific normative data published in ACSM’s Guidelines for Exercise Testing and Prescription. Eleven students were classified as normal weight (body mass index (BMI): 18.5–24.9), two were overweight (BMI: 25.0–29.9), and one was obese (BMI > 30). We emphasize these points because neither young age nor higher fitness levels decrease susceptibility to AMS. On the contrary, obesity seems to be associated with AMS development. Suffice it to say, we were eager to compare our own experiences with the existing literature.

We awoke at 5:30 a.m. the following day to board a bus that transported us to the town of Tambomachay (3,750 m). There, we met our guides and porters from a licensed touring company who were responsible for hauling our gear, setting up camp, and providing meals. (The Peruvian government tightly regulates hiking in Peru because of concerns about overuse and erosion. Most treks need to be booked six months in advance.) The journey began at 8 a.m., and it took approximately five hours to reach our first checkpoint (4,100 m), where we stopped to eat lunch. Because adequate carbohydrate intake is generally regarded as critical for maintaining health and performance at altitude, rice or potatoes were staples at each meal. Yet, it is currently unknown what effect fat adaptation has on these outcomes.

Our climb continued for two hours until we reached Huchuy Qosqo pass (4,300 m), the highest point of our trek. After which, we descended to Paucarcancha (3,800 m), where the porters had set up camp for the evening. We covered a total distance of 8 miles in approximately 8 hours, and our rate of ascent to the first checkpoint was approximately 75 m per hour. The hike elicited heart rate responses of between 60% and 80% of age-predicted maximum, a range commensurate with a moderate to high intensity of effort. However, we remain cautious in the interpretation of these data because altitude exposure alters resting and submaximal heart rates.

By the time we had arrived at camp, 10 of the 14 students (71%) reported a Lake Louise score of 3 or more. Four students had severe AMS, but fortunately showed no signs of HACE or HAPE. Moreover, we noted that the average abdominal visceral fat for these four students was 295 grams (as estimated by dual-energy X-ray absorptiometry [DEXA] and expressed per total body fat mass). By comparison, the average abdominal visceral fat of the remaining students who did not experience AMS or who had mild symptoms was 55 grams. Although these observations are by no means definitive, we find them interesting when viewed in the context of the existing literature.

For example, Ri-Li et al.  reported that after 24 hours at a simulated altitude of 3,658 m, seven of nine obese men (78%) compared with 4 of 10 (40%) nonobese men reported AMS scores of four or more. More recently, it was found that obese men working on the Tibetan railway had higher AMS scores than nonobese counterparts. These data suggest that obesity is an important factor in the development of AMS. It has been theorized that obesity-related respiratory function abnormalities, such as sleep-disordered breathing and nocturnal hypoxia play a role.

Indeed, all three women on our trek who underwent PPG monitoring experienced disturbed nocturnal breathing and complained of poor quality sleep the following morning. Yet, the highest ODI (52 events/hour) at altitude was observed in the woman with the highest percentage of both abdominal visceral and total body fat (43%), whereas the lowest ODI (9 events/hour) at altitude was observed in the leanest of the three women (23% total body fat). The lean woman was symptom free throughout the first day of the hike, whereas the obese woman reported a Lake Louise score of 8 at 4,100 m.

Abdominal visceral fat is an abundant source of proinflammatory cytokines, which, when released, may lead to depression of central nervous system (CNS) activity and upper airway neuromuscular control. Thus, an obese individual may be more susceptible to disturbed nocturnal breathing. Alternatively, obese individuals may exhibit higher HVR at low altitude, and therefore more pronounced periodic breathing during sleep at altitude. Regardless of the mechanism, our observations suggest that obese individuals might consider prophylactic measures to reduce AMS risk.

If the first day of the hike was the challenge, then the second day was the reward. After a day of mostly uphill hiking, we enjoyed marvelous views of the Sacred Valley as we descended on a series of switchbacks en route to the town of Lamay, located at 2,900 m. It took us approximately five hours to hike a very technical six-mile descent. We arrived early in the afternoon, at which point we were greeted by the porters who had prepared an exceptional meal to help us celebrate our efforts. At this point, all the students completed the Lake Louise Scale, and only three students reported mild AMS, a sign that most of us had acclimatized to high altitude. For others, the drop in 900 m may have been what was needed to alleviate any lingering symptoms. After enjoying lunch, we took a bus to Ollantaytambo, where a train transported us to Aguas Calientes, the town that serves as the principal access point to Machu Picchu. We spent the evening in a hotel and awoke the next morning to board a bus that would transport us to the main gates. At approximately 7 a.m., we enjoyed seeing the sun rise above the mountains and illuminate the Lost City. The scene was spectacular and it served as a reminder to us that despite some challenges, highaltitude recreation, and physical activity in general, are means to most of life’s pleasures and achievements.

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