high altitude pulmonary oedema

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High-Altitude Pulmonary Oedema (HAPE)

HAPE is the most lethal form of altitude illness. It is a non-cardiogenic pulmonary oedema that develops within 2-4 days after rapid ascent to high altitude, and is a leading cause of death in mountaineers.

Definition and Context

HAPE occurs in the spectrum of altitude-related illness, which includes:

Acute Mountain Sickness (AMS) - mild, benign neurological symptoms
High-Altitude Cerebral Oedema (HACE) - severe neurological end of the spectrum
High-Altitude Pulmonary Oedema (HAPE) - primarily a pulmonary disorder, not necessarily preceded by AMS

HAPE rarely develops beyond 4-5 days at the same altitude, likely due to pulmonary vascular remodeling and adaptation. It is rare below 3,600 m (12,000 ft).

Harrison's Principles of Internal Medicine, 22nd Edition
Park's Textbook of Preventive and Social Medicine

Risk Factors

Risk Factor	Notes
Rapid rate of ascent	Most important modifiable factor
Prior history of HAPE	Strongest individual predictor
Male sex	Men more susceptible than women
Cold environmental temperatures	Exercise + cold increase pulmonary vascular pressure
Respiratory tract infections	Predispose even constitutionally resistant individuals
Mitral stenosis, primary pulmonary hypertension	Cardiopulmonary abnormalities causing baseline pulmonary HTN
Unilateral absence of pulmonary artery
Patent foramen ovale	4x more common in HAPE-susceptible individuals (causal link unproven)

Harrison's Principles of Internal Medicine, 22nd Edition

Pathophysiology

HAPE is fundamentally driven by hypoxia-induced exaggerated pulmonary vasoconstriction, with several interacting mechanisms:

1. Patchy Hypoxic Pulmonary Vasoconstriction

Severe hypoxia causes pulmonary arteriolar constriction, but this is uneven - some regions constrict more than others. Blood is redirected into fewer unconstricted vessels, causing markedly elevated capillary pressure (>18 mmHg) in those areas. This leads to capillary stress failure and fluid leakage into alveoli. The pulmonary artery wedge pressure remains normal (confirming non-cardiogenic nature).

Guyton and Hall Textbook of Medical Physiology

2. Impaired Nitric Oxide Availability

Endothelial dysfunction from hypoxia impairs the release of nitric oxide (NO), a key pulmonary vasodilator. HAPE-prone individuals have reduced exhaled NO levels at high altitude. This explains why phosphodiesterase-5 (PDE-5) inhibitors (e.g., tadalafil, sildenafil) - which potentiate NO - are effective in HAPE prevention.

3. Sympathetic Overactivation

Hypoxia triggers increased sympathetic drive, causing pulmonary venoconstriction and extravasation from pulmonary capillaries into alveoli. Alpha-adrenergic blockade (phentolamine) improves HAPE haemodynamics more than other vasodilators. HAPE-susceptible individuals display enhanced sympathetic activity even during short-term hypoxic breathing at low altitude.

4. Elevated Endothelin-1

The endothelium also produces endothelin-1, a potent vasoconstrictor. Concentrations of endothelin-1 are higher than average in HAPE-prone mountaineers, amplifying vasoconstriction.

5. Impaired Alveolar Fluid Clearance

Beta-adrenergic agonists upregulate transepithelial sodium and water clearance from alveoli. HAPE may partly result from impaired alveolar fluid clearance, which is why inhaled salmeterol reduces HAPE incidence by ~50%.

6. Inflammation (Secondary Role)

Patients often have fever, leukocytosis, and raised ESR - but evidence suggests inflammation is an epiphenomenon rather than the primary driver. Viral respiratory infections predispose to HAPE even in normally resistant individuals.

Harrison's Principles of Internal Medicine, 22nd Edition

Clinical Features

Symptoms (progressive)

Reduced exercise tolerance (often the earliest sign, disproportionate to altitude)
Dry, persistent cough (nearly universal in mountain climbers - not specific alone)
Blood-tinged or frothy sputum (as oedema develops)
Dyspnoea at rest
Mental confusion, hallucinations, stupor, seizures, coma (advanced)
Cheyne-Stokes breathing and oliguria may develop

Signs

Tachypnoea and tachycardia at rest - important markers of progression
Crackles on auscultation (not diagnostic alone)
Cyanosis
May have concurrent signs of HACE (ataxia, altered consciousness)
Harrison's Principles of Internal Medicine, 22nd Edition
Park's Textbook of Preventive and Social Medicine

Investigations

Chest X-ray

Classic findings: patchy or localized opacities, often asymmetric, commonly in the right middle and lower zones. Streaky interstitial oedema may also be seen. Importantly:

Kerley B lines are NOT seen (unlike cardiogenic pulmonary oedema)
Bat-wing appearance is NOT seen
Can mimic pneumonic consolidation - historically misdiagnosed as pneumonia

Chest radiograph showing HAPE: opacity in the right middle and lower zones simulating pneumonic consolidation, which cleared almost completely in 2 days with descent and supplemental oxygen.

Chest X-ray of HAPE showing right middle and lower zone opacification. - Harrison's, 22nd Edition

ECG

Right ventricular strain or hypertrophy pattern

ABG / Oximetry

Hypoxaemia - consistently present
Respiratory alkalosis - consistently present (unless on acetazolamide, where metabolic acidosis may supervene)
Pulse oximetry is generally sufficient; formal ABG not mandatory

Echocardiography

Recommended when HAPE develops at relatively low altitudes (<3,000 m) or when underlying cardiopulmonary abnormalities are suspected

Ultrasound

Comet-tail scoring (B-lines) is sensitive but not specific for HAPE - many individuals without HAPE also show B-lines at altitude

Differential Diagnosis

Pneumonia
Pulmonary embolism
Pneumothorax
Anxiety attack / hyperventilation
Cardiogenic pulmonary oedema

Treatment

1. Immediate Descent (Priority #1)

Descent is the single most effective treatment - symptoms respond rapidly. It is mandatory when HAPE is diagnosed.

2. Supplemental Oxygen

High-flow oxygen reverses hypoxic vasoconstriction and usually leads to dramatic improvement within hours.

3. Portable Hyperbaric Chamber (Gamow bag)

Simulates descent by increasing ambient pressure
Provides "spectacular improvement" and buys time when descent is impossible
Lightweight and used in remote locations

4. Pharmacological Treatment

Drug	Role	Dose
Nifedipine (calcium channel blocker)	Treatment and prevention of HAPE	30 mg SR every 12h OR 20 mg SR every 8h
Tadalafil (PDE-5 inhibitor)	Prevention of HAPE	10 mg twice daily
Salmeterol (beta-2 agonist)	Prevention of HAPE	125 mcg inhaled twice daily
Dexamethasone	Prevention of HAPE (78% reduction in incidence)	8 mg once, then 4 mg every 6h for treatment

Goldman-Cecil Medicine, International Edition
Harrison's Principles of Internal Medicine, 22nd Edition

Note on PDE-5 inhibitors: Tadalafil prophylaxis reduces HAPE risk by 65%. Dexamethasone reduces it by 78% - possibly via both NO enhancement and sympathetic dampening. However, PDE-5 inhibitors have no role in treating established AMS or HACE.

5. Position

Sitting upright reduces venous return and pulmonary capillary pressure.

Prevention

Slow, graded ascent - most important preventive measure (limit gain in sleeping altitude to ~300-500 m/day above 3,000 m, with a rest day every 3 days)
Avoid over-exertion in the first 48 hours at new altitude
Pharmacological prophylaxis in HAPE-susceptible individuals: nifedipine, tadalafil, or salmeterol (see table above)
Acetazolamide (125-250 mg twice daily) is primarily for AMS/HACE prevention, not HAPE - but is an important overall prophylactic drug for altitude illness

Key Distinguishing Points (vs. Cardiogenic Pulmonary Oedema)

Feature	HAPE	Cardiogenic PE
Pulmonary artery wedge pressure	Normal	Elevated
Kerley B lines	Absent	Present
Bat-wing appearance	Absent	Common
Cardiomegaly on CXR	Absent	Often present
Context	High altitude	Cardiac disease
ECG	RV strain	LV strain/ischaemia

Recent Evidence (2024-2026)

A 2024 review in Internal Medicine examines clinical and pathophysiological features of HAPE, including data from high-altitude studies in the Japanese population - highlighting reduced exercise tolerance and exaggerated hypoxic pulmonary vasoconstriction as the core mechanism.
A 2026 review in Signal Transduction and Targeted Therapy (PMID 41571626) covers altitude hypoxia and hypoxaemia pathogenesis and management, including molecular targets.
A 2025 pharmacological review (PMID 40516140) covers molecular and clinical insights into high-altitude physiology, including pharmacological interventions.

No recent meta-analyses or RCTs contradict standard management; descent + oxygen remain the cornerstone of treatment.

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