As altitude increases, atmospheric pressure decreases while the percentage of oxygen in air remains constant; thus, the partial pressure of oxygen decreases with altitude and, at 5800 m (19,000 ft), is about one half that at sea level, while on the summit of Mt. Everest (8848 m, or 29,032 ft), it is roughly one third that of sea level.
Most people can ascend to 1500 to 2000 m (5000 to 6500 ft) in one day without problems, but about 20% of those who ascend to 2500 m (8000 ft) and 40% of those who ascend to 3000 m (10,000 ft) develop some form of altitude illness, most commonly AMS. Rate of ascent, maximum altitude reached, and sleeping altitude influence the likelihood of developing these disorders.
Risk factors
Effects of high altitude vary greatly among individuals. But generally, risk is increased by the following:
History of previous altitude illness
Living near sea level
Going too high too fast
Overexertion
Sleeping at too high an altitude
Disorders such as asthma, hypertension, diabetes, coronary artery disease, and mild chronic obstructive pulmonary disease are not risk factors for altitude illness, but hypoxemia may exacerbate symptoms caused by these disorders (1). Physical fitness is not protective.
General reference
1. Luks AM, Hackett PH: Medical conditions and high-altitude travel. N Engl J Med 386(4):364-373, 2022. doi: 10.1056/NEJMra2104829
Pearls & Pitfalls
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Pathophysiology of Altitude Illness
Whereas severe acute hypoxemia can cause altered central nervous system function within a matter of minutes, acute altitude illness develops anywhere from 1 to 5 days after ascent to a given elevation.
The pathogenesis of acute mountain sickness (AMS) and high-altitude cerebral edema (HACE) remain unclear despite considerable research in this area. The diseases likely develop as a result of the same underlying pathophysiology and fall on a continuum of severity, with HACE representing the extreme of the spectrum. AMS may involve activation of the trigeminovascular system due to a variety of stimuli, while HACE involves blood-brain barrier leakage and increased intracranial pressure.
High-altitude pulmonary edema (HAPE) is caused by hypoxia-induced elevation of pulmonary artery pressure which causes interstitial and alveolar pulmonary edema, resulting in impaired oxygenation. Small-vessel hypoxic vasoconstriction is patchy, causing elevated pressure, capillary wall damage, and capillary leakage in less constricted areas. Other factors, such as sympathetic overactivity, may also be involved.
Long-time high-altitude residents can develop HAPE when they return after a brief stay at low altitude, a phenomenon referred to as reentry pulmonary edema. Recent reports suggest that long-term high-altitude residents can also develop noncardiogenic pulmonary edema—referred to as high-altitude resident HAPE—even if they do not descend to and then return from a lower elevation.
Acclimatization
Acclimatization is an integrated series of responses that allow the body to tolerate the hypoxia at high altitude. Most people acclimatize reasonably well to altitudes of up to 3000 m (10,000 ft) within a few days. The higher the altitude, the longer acclimatization takes. However, no one can fully acclimatize to long-term residence at altitudes > 5100 m (> 17,000 ft).
Features of acclimatization include sustained hyperventilation, which raises the alveolar and arterial PO2 but also causes respiratory alkalosis. Blood pH tends to normalize within days as bicarbonate is excreted in urine; as pH normalizes, ventilation can increase further. Cardiac output increases initially and returns to baseline values over time; red blood cell mass and tolerance for aerobic work also increase over a period of several weeks.
Symptoms and Signs of Altitude Illness
Acute mountain sickness is by far the most common form of altitude illness.
Acute mountain sickness (AMS)
AMS is unlikely unless altitude is above 2440 m (8000 ft), but it can develop at lower elevations in some highly susceptible people. It is characterized by headache plus at least one of the following: fatigue, gastrointestinal symptoms (anorexia, nausea, vomiting), or persistent dizziness. Poor sleep was previously considered a symptom of AMS but is no longer considered one of the diagnostic criteria. Symptoms typically develop 6 to 10 hours after ascent and, in most cases, subside in 24 to 48 hours. AMS is common at ski resorts, and some people affected by it mistakenly attribute it to excessive alcohol intake (hangover) or a viral illness.
High-altitude cerebral edema (HACE)
HACE occurs rarely, anywhere from 1 to 5 days following ascent. Marked cerebral edema manifests as headache and diffuse encephalopathy with confusion, drowsiness, stupor, and coma. Gait ataxia is a reliable early warning sign. Seizures, focal deficits (eg, cranial nerve palsy, hemiplegia), fever, and meningeal signs are uncommon and should prompt concern for other diagnoses. Papilledema may be present but is not necessary for diagnosis. Coma and death may occur within a few hours unless HACE is treated promptly.
High-altitude pulmonary edema (HAPE)
HAPE typically develops 24 to 96 hours after rapid ascent to > 2500 m (> 8000 ft) and is responsible for most deaths due to altitude illness. It may be preceded by AMS, but can also develop in isolation in people who were not manifesting other evidence of altitude illness.
Initially, patients have dyspnea on exertion, decreased exertion tolerance, and dry cough. Later, dyspnea is present with simple activities or at rest. Pink or bloody sputum and respiratory distress are late findings. On examination, cyanosis, tachycardia, tachypnea, and low-grade fever (< 38.5° C) are common. Focal or diffuse crackles (sometimes audible without a stethoscope) are usually present. HAPE may worsen rapidly; coma and death may occur within hours unless HAPE is treated promptly.
Other manifestations
Peripheral and facial edema is common at high altitude even in the absence of altitude illness.
Headache, without other symptoms of AMS, is also common.
Retinal hemorrhages can develop at altitudes as low as 2700 m (9000 ft) and are common at > 5000 m (> 16,000 ft). They are usually asymptomatic unless they occur in the macular region, in which case they typically present as painless vision loss; they resolve over weeks without sequelae. With symptomatic hemorrhages, descent is indicated and further ascent is contraindicated until hemorrhages have resolved. Ophthalmology evaluation is warranted following descent for all patients with symptomatic high-altitude retinal hemorrhages.
People who have had radial keratotomy or LASIK may have significant visual disturbances at altitudes > 5000 m (> 16,000 ft). These symptoms disappear rapidly after descent.
Chronic mountain sickness
Diagnosis of Altitude Illness
Clinical evaluation
Prompt diagnosis and initiation of treatment of high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE) are essential to prevent coma and death.
Diagnosis of altitude illness is usually clinical; laboratory tests are usually unnecessary and may be unavailable depending on the setting in which illness develops (eg, remote mountain valley versus mountain resort community). However, imaging, when available, may be useful for HAPE and sometimes HACE.
In HAPE, hypoxemia is often severe, with pulse oximetry showing 40 to 70% saturation, depending on the elevation at which the individual becomes ill. Plain chest radiography shows a normal-sized heart and patchy lung edema.
HACE can usually be differentiated from other causes of headache and coma (eg, infection, brain hemorrhage, uncontrolled diabetes) by history and clinical findings; if not, CT or MRI imaging of the head may help rule out other causes. MRI findings in HACE, such as microhemorrhages, can persist for many months after illness and thus help confirm HACE after a return from a trip to high altitude.
The Lake Louise acute mountain sickness score is a tool that can be used in diagnosis of AMS but is primarily designed for the purpose of research studies rather than clinical practice (1).
Diagnosis reference
1.Roach RC, Hackett PH, Oelz O, et al: The 2018 Lake Louise acute mountain sickness score. High Alt Med Biol 19(1):4-6, 2018. doi: 10.1089/ham.2017.0164
Treatment of Altitude Illness
For high-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE), immediate descent and treatment with oxygen, drugs, and pressurization
Acute mountain sickness (AMS)
Patients should halt ascent and reduce exertion until symptoms resolve (1, 2
High-altitude cerebral edema (HACE) and high-altitude pulmonary edema (HAPE)
Patients should descend to low altitude immediately. Helicopter evacuation may be life-saving (1). If descent is delayed, patients should rest and be given oxygen. If descent is impossible, oxygen (to raise the O2 saturation to > 90%), drugs, and pressurization in a portable hyperbaric bag help buy time but are not substitutes for and should not delay descent.
For HACE (and severe AMS)
For HAPE
Some patients with HAPE in areas with adequate medical resources (eg, a ski resort community) and family or friends who can adequately monitor them can be discharged with supplemental oxygen. People who have had one episode of HAPE are likely to have another and should be so warned.
Pearls & Pitfalls
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Treatment references
1. Luks AM, Auerbach PS, Freer L, et al: Wilderness Medical Society practice guidelines for the prevention and treatment of acute altitude illness: 2019 Update. Wilderness Environ Med. doi.org/10.1016/j.wem.2019.04.006
2. Bartsch P, Swenson ER: Clinical practice: Acute high-altitude illnesses. N Engl J Med 368:2294-2302, 2013. doi: 10.1056/NEJMcp1214870
Prevention of Altitude Illness
Slow ascent
Although physical fitness enables greater exertion at altitude, it does not protect against any form of acute altitude illness. Maintaining adequate fluid intake does not prevent acute mountain sickness (AMS) but does protect against dehydration, the symptoms of which closely resemble AMS. Opioids, benzodiazepines, and heavy alcohol consumption, particularly shortly before sleep, should be avoided.
Ascent
The most important measure is a slow ascent (1, 2). Graded ascent is essential for activity at > 2500 m (> 8000 ft). Above 3000 m (10,000 ft), climbers should not increase their sleeping altitude by more than 300-500 m (1000 to 1600 ft) per day and should include a rest day (ie, sleep at the same altitude) every 3 to 4 days. If a single day's ascent must exceed 500 m (eg, due to logistics or terrain features), they should add in rest days to achieve an average rate of < 500 m (1600 ft) per day over the entire ascent. During rest days, climbers can engage in physical activity and ascend to higher altitudes but should return to the lower level for sleep. Climbers vary in ability to ascend without developing symptoms; a climbing party should be paced for its slowest member.
Acclimatization is gradually lost after a few days at low altitude, and climbers returning to high altitude after this duration should once more follow a graded ascent.
Drugs
125 mg orally every 12 hours reduces the incidence of altitude illness. Acetazolamide should be started the night before ascent; it acts by inhibiting carbonic anhydrase and thus increasing ventilation. Acetazolamide 125 mg orally at bedtime reduces the amount of periodic breathing (almost universal during sleep at high altitude), thus limiting sharp falls in blood oxygen.
Patients allergic to sulfa drugs have a small risk of cross-reactivity to acetazolamide; a supervised trial of acetazolamide should be considered for these patients before they make a trip to altitudes remote from medical care. Acetazolamide should not be given to patients with a history of anaphylaxis to sulfa drugs. Acetazolamide may cause numbness and paresthesias of the fingers; these symptoms are benign but can be annoying. Carbonated drinks taste flat to people taking acetazolamide.
2 mg orally every 6 hours (or 4 mg orally every 12 hours) is an alternative to acetazolamide.
Low-flow oxygen during sleep at altitude is effective but inconvenient and may pose logistic difficulties.
3).
Prevention references
1. Luks AM, Auerbach PS, Freer L, et al: Wilderness Medical Society practice guidelines for the prevention and treatment of acute altitude illness: 2019 Update. Wilderness Environ Med. doi.org/10.1016/j.wem.2019.04.006
2. Bartsch P, Swenson ER: Clinical practice: Acute high-altitude illnesses. N Engl J Med 368:2294-2302, 2013. doi: 10.1056/NEJMcp1214870
3. Maggiorini M, Brunner-La Rocca HP, Peth S, et al: Both tadalafil and dexamethasone may reduce the incidence of high-altitude pulmonary edema: A randomized trial. Ann Intern Med 145(7):497-506, 2006. doi: 10.7326/0003-4819-145-7-200610030-00007
Key Points
About 20% of people who ascend to 2500 m (8000 ft) and 40% of those who ascend to 3000 m (10,000 ft) in one day develop some form of altitude illness, the most common form of which is acute mountain sickness (AMS).
AMS causes headache plus fatigue, gastrointestinal symptoms (anorexia, nausea, vomiting), or persistent dizziness.
High-altitude cerebral edema (HACE) causes headache, ataxia, and encephalopathy.
High-altitude pulmonary edema (HAPE) causes dyspnea, decreased exertion tolerance, and dry cough initially which can progress to dyspnea with simple activities or at rest, profound fatigue, cyanosis and a cough productive of blood sputum in the more severe stages.
Diagnose altitude illness based on clinical findings.
Arrange immediate descent for patients with HACE, HAPE, or very severe AMS.
