Quick Recap
This completes the Environmental System - all 4 protocols now have Quick Recap sections.
Environmental System, Protocol 4/4 — completing the Environmental System.
1. Definition & Risk Factors
High-altitude illness (HAI) = a spectrum including acute mountain sickness (AMS), high-altitude cerebral edema (HACE), and high-altitude pulmonary edema (HAPE), resulting from hypoxic stress at high altitude — the shared underlying mechanism is EXTRAVASCULAR FLUID ACCUMULATION, in the brain for AMS/HACE and in the lungs for HAPE.
Threshold altitude: commonly 2,500m, though some develop HAI as low as 1,500-2,100m. Affects individuals of ALL ages/genders regardless of fitness level — susceptibility CANNOT be determined before exposure, a genuinely important point for pre-travel counseling (fitness does not confer protection).
Risk factors: past history of HAI; rapid ascent (<24h) from low altitude to a sleeping altitude >2,750m; rapid ascent >500-1,000m/day from altitudes above 2,750m; substances interfering with acclimatization (alcohol, sedatives); comorbidities interfering with respiration (neuromuscular disease, severe OSA, moderate-severe COPD, cystic fibrosis) or circulation (angina, CHF, sickle cell, pulmonary hypertension); overexertion prior to acclimatization.
Onset: 6-12 hours to 5 days after arrival at high altitude.
2. Acute Mountain Sickness (AMS)
The most common HAI — prevalence and severity increase with altitude: 10-25% of unacclimatized persons ascending to 2,500m; 50-85% ascending to 4,500-5,500m.
Cardinal symptom: HEADACHE, accompanied by any combination of fatigue, dizziness, malaise, nausea, anorexia, sleep disturbance.
Course: symptoms usually resolve within 1-2 days if there is NO further ascent. Symptoms do NOT recur at the SAME altitude, but CAN reappear with ascent to a HIGHER altitude — a useful piece of patient counseling: prior tolerance at a given elevation does not guarantee tolerance further up.
Treatment (severity-based):
- Mild: symptomatic treatment (analgesics, antiemetics), AVOID further ascent, limit exercise
- Moderate-severe: dexamethasone 4mg PO/IM every 6 hours for 1-2 days, supplemental oxygen, or descent
- If symptoms do NOT respond to treatment or WORSEN, suspect HACE and initiate descent — AMS and HACE represent a SPECTRUM of the same underlying pathophysiology, though AMS is not a REQUIRED precursor to HACE (HACE can occur without preceding recognized AMS).
3. High-Altitude Cerebral Edema (HACE)
Hallmark symptoms: ENCEPHALOPATHY, TRUNCAL ATAXIA, and usually a mild fever — plus AMS-overlapping symptoms (lassitude, irritability) with a PROGRESSIVE DECLINE in mental function/consciousness. Truncal ataxia specifically is a key, relatively specific bedside sign — have the patient perform a tandem gait test if HACE is suspected.
Course: unpredictable, can develop in 12 HOURS to 3 DAYS, and tends to develop FASTER in patients with CONCURRENT HAPE — co-occurring HAPE should raise vigilance for accelerated HACE progression.
Treatment — DESCENT is the definitive therapy:
- Immediate descent is required; a descent of just 1,000m is USUALLY LIFESAVING
- Dexamethasone (8-10mg PO/IV/IM loading, then 4mg every 6 hours) is used as a RESCUE medication UNTIL descent can be achieved — it is NOT a substitute for descent, a critical distinction: do not delay or avoid descent because dexamethasone has been given and symptoms seem to be improving
- Oxygen (2-4 L) to maintain SpO2 >90%
- Portable, manually-inflated hyperbaric chamber ("Gamow bag") is LIFESAVING if the patient cannot be immediately transported to lower elevation — simulates descent physiologically when actual descent is not immediately feasible (weather, terrain, logistics)
- If untreated or allowed to progress to an irreversible stage before descent, HACE IS FATAL — the mortality risk of delayed action is absolute, not just morbidity
4. High-Altitude Pulmonary Edema (HAPE)
A form of NONCARDIOGENIC pulmonary edema, developing 2-4 days after rapid ascent above 2,500m, RARELY occurring after 1 week at the same altitude. The most common cause of death among the HAIs. Often preceded by AMS, can occur alone or with AMS/HACE, and can RECUR with repeat ascent to the same elevation in susceptible individuals.
Mechanism: hypobaric hypoxia -> increased sympathetic tone, inadequate endothelial nitric oxide production, endothelin overproduction -> UNEVEN pulmonary vasoconstriction and pulmonary hypertension (mean PA pressure usually >35-40 mmHg is essential for HAPE development) -> regional overperfusion in the less-constricted areas -> alveolar-capillary barrier leakiness -> PATCHY pulmonary edema.
Clinical course: initial nonproductive cough and exertional dyspnea -> progression from dyspnea-with-exertion to dyspnea-AT-REST is the cardinal clinical feature signaling advancing severity -> worsening cough/dyspnea, orthopnea, PINK FROTHY SPUTUM production. Exam: cyanosis, tachypnea, tachycardia, low-grade fever, crackles FIRST heard in the RIGHT MIDDLE LOBE (a specific, somewhat unusual early auscultation finding).
Imaging: patchy alveolar infiltrates PREDOMINANTLY in the right central hemithorax initially, becoming bilateral/confluent over time; lung ultrasound (if CXR unavailable) shows B-lines consistent with edema.
A DISTINCTIVE, genuinely counterintuitive diagnostic clue: the patient does NOT appear as clinically ill as the imaging (or the oxygen numbers) would suggest, AND shows STEADY IMPROVEMENT with oxygen therapy — SpO2 can range 50-75% and PO2 25-40, severely abnormal by any standard critical care threshold, yet the patient often remains relatively well-appearing and improves RAPIDLY (within 1-2 hours) with supplemental oxygen — this specific pattern (severe hypoxemia + relatively preserved clinical appearance + rapid O2 response + recent altitude gain history) is itself diagnostically supportive of HAPE over other causes of severe hypoxemic respiratory failure.
Treatment (severity- and resource-dependent):
- Oxygen is FIRST-LINE therapy where available
- Rest and warmth — both independently DECREASE pulmonary artery pressure, a genuine adjunct therapy, not just comfort care
- If oxygen is NOT available, begin DESCENT as soon as HAPE is suspected
- Hyperbaric therapy (portable chamber) as an alternative if oxygen is unavailable
- Nifedipine (30mg slow-release every 12 hours) can be used if oxygen is unavailable AND safe descent is not possible — a pharmacologic bridge option, not first-line when oxygen/descent are accessible
- Descent is MANDATORY at elevations above 4,000m
- Oxygen and descent (or hyperbaric chamber) are SUPERIOR to pharmacologic treatment — nifedipine is a resource-limited-setting adjunct/bridge, not an equivalent alternative to definitive management
5. Prevention
Gradual/staged ascent is the primary prevention strategy for all three HAI entities.
Pharmacologic prophylaxis reserved for HIGH or MODERATE risk individuals (prior AMS/HACE history) when rapid ascent cannot be avoided:
- Acetazolamide 125mg every 12 hours — reduces AMS symptoms by ~75%; start the DAY BEFORE ascent, continue for 48 hours; can also be used at any point to speed acclimatization
- Dexamethasone 2mg every 6 hours OR 4mg every 12 hours — alternative for those allergic to acetazolamide
- Nifedipine 30mg every 12 hours — the drug of choice SPECIFICALLY for HAPE prophylaxis in high-risk individuals when rapid ascent cannot be avoided; start 24 hours prior to ascent, continue for 5 days at the destination altitude
- Tadalafil and sildenafil have shown promise in small studies but lack established optimal dosing; dexamethasone for prophylaxis (as opposed to rescue treatment) needs further study
6. Immediate Stabilization (ABCDE)
Checklist:
7. Investigations
Pulse oximetry (severity marker, understanding the counterintuitive well-appearing-despite-severe-hypoxemia HAPE pattern), CXR or lung ultrasound (B-lines) for HAPE, neurologic exam including tandem gait/truncal ataxia assessment for HACE, standard workup to exclude alternative causes of headache/altered mental status/hypoxemic respiratory failure if the altitude-exposure history or clinical pattern is atypical.
8. Organ Support
Supplemental oxygen (central to both HACE rescue and HAPE first-line therapy); descent as the definitive intervention for HACE/HAPE; hyperbaric chamber as an alternative when descent is not immediately feasible; standard ICU supportive care for severe presentations requiring evacuation to definitive care.
9. Consultation Matrix
Consultation | Trigger | Timing |
Critical Care / Pulmonology | HAPE requiring ICU-level respiratory support | Immediate once evacuated to a facility |
Neurology | HACE with persistent/worsening neurologic deficit despite descent | As needed |
Wilderness/Travel Medicine | Prevention counseling, recurrent HAI history | Pre-travel, non-urgent |
10. Monitoring Framework
Serial neurologic exam (truncal ataxia, mental status trend) for HACE, continuous pulse oximetry and respiratory status trend for HAPE, symptom trend with ascent/descent altitude changes, response to oxygen therapy as a diagnostic/monitoring tool for HAPE specifically.
11. Complications
HACE progression to coma/death if descent is delayed, HAPE progression to severe respiratory failure, recurrent HAPE with repeat ascent, combined HACE+HAPE with accelerated deterioration. Prevention: gradual/staged ascent, appropriate pharmacologic prophylaxis in high-risk individuals, early recognition and prompt descent at the first sign of progression beyond mild AMS. Rescue: immediate descent, hyperbaric chamber, dexamethasone as a bridge for HACE, oxygen/nifedipine as a bridge for HAPE when descent is delayed.
12. Escalation & De-escalation
Escalate: AMS not responding to symptomatic treatment or worsening -> suspect HACE, initiate descent; HAPE progressing from exertional to resting dyspnea -> escalate oxygen/descent urgency.
De-escalate: symptoms resolving with descent/treatment, no further ascent -> continue supportive care at lower altitude, reassess before any further ascent attempt.
13. ICU Discharge Criteria
Symptoms resolved with descent to a tolerated altitude, no ongoing neurologic deficit (HACE) or respiratory compromise (HAPE), oxygen requirement resolved or minimal, counseling provided on recurrence risk and prevention strategy for future high-altitude travel.
14. Documentation & Medicolegal Checklist
15. Key Guidelines
Bartsch P, Swenson ER. Acute high-altitude illnesses. N Engl J Med. 2013;368:2294-2302.
16. Controversies
Optimal dexamethasone dosing for AMS/HAPE PREVENTION (as opposed to established HACE rescue dosing) remains under active study, with acetazolamide and nifedipine having more firmly established prophylactic roles for AMS and HAPE respectively. Tadalafil/sildenafil dosing for HAPE prophylaxis is not yet standardized despite promising small-study signals.
17. References
- Nguyen N. High-Altitude Illness. Washington Manual of Critical Care, 4th ed, 2025 (Ch. 35).
- Bartsch P, Swenson ER. Acute high-altitude illnesses. N Engl J Med. 2013;368:2294-2302.
This completes the Environmental System (4/4 protocols).