Understanding High Altitude Physiology
High altitude illness occurs when the body fails to acclimatize to the reduced atmospheric pressure and lower oxygen availability at elevation. At sea level, atmospheric pressure is 760 mmHg and the partial pressure of oxygen is approximately 160 mmHg. At 10,000 feet, atmospheric pressure drops to roughly 523 mmHg and the partial pressure of oxygen falls to approximately 110 mmHg. This reduced oxygen availability triggers a cascade of physiological responses that, when overwhelmed, lead to altitude illness.
While high altitude emergencies are most common in mountainous regions, EMS providers in any area should be familiar with these conditions. Patients may present to EMS after returning from high altitude travel, and some regions have ski resorts, hiking areas, or mountain passes where altitude illness occurs regularly.
Acute Mountain Sickness (AMS)
AMS is the mildest form of altitude illness and typically develops 6 to 12 hours after ascending above 8,000 feet. Symptoms include headache (the hallmark symptom), nausea, fatigue, dizziness, and difficulty sleeping. The Lake Louise Scoring System is used to diagnose AMS based on symptom severity. AMS is generally self-limiting and resolves within 1 to 3 days with acclimatization or descent. Treatment includes stopping ascent, rest, hydration, and over-the-counter analgesics for headache. If symptoms worsen or do not improve, descent is the definitive treatment.
High Altitude Cerebral Edema (HACE)
HACE is a life-threatening progression of AMS where fluid accumulates in the brain. It typically occurs above 12,000 feet but can develop at lower altitudes. The hallmark finding is ataxia (unsteady gait); test by having the patient walk heel-to-toe in a straight line. Other findings include severe headache unresponsive to analgesics, confusion, disorientation, hallucinations, and eventually coma. HACE progresses rapidly and can be fatal within 24 hours without treatment.
Treatment is immediate descent; even a descent of 1,000 to 3,000 feet can be lifesaving. Supplemental oxygen should be administered at high flow. Dexamethasone (8 mg initially, then 4 mg every 6 hours) is the pharmacological treatment if available per protocol. A portable hyperbaric chamber (Gamow bag) can simulate descent when actual descent is delayed.
High Altitude Pulmonary Edema (HAPE)
HAPE is the most common cause of death from altitude illness. It occurs when fluid leaks from pulmonary capillaries into the alveoli due to hypoxic pulmonary vasoconstriction. HAPE typically develops 2 to 4 days after ascent above 8,000 feet. Early symptoms include decreased exercise tolerance, dry cough, and dyspnea with exertion. As it progresses, the patient develops dyspnea at rest, productive cough with pink frothy sputum, tachycardia, tachypnea, crackles on lung auscultation, and cyanosis.
Field Management Priorities
For all high altitude emergencies, the priorities are the same: descent, oxygen, and supportive care. Descent is the single most effective treatment for all altitude illnesses. Administer supplemental oxygen to maintain SpO2 above 90%. Keep the patient warm, as cold exposure worsens altitude illness. Minimize patient exertion during descent; have the patient ride rather than walk if possible. Transport to a medical facility for evaluation, as altitude illness can worsen after initial improvement ("rebound"), and HAPE patients may require mechanical ventilation and intensive care.