Respiratory · Year 1 · from Respiratory
Case 2: High Altitude Pulmonary Edema
Case Presentation
A 28-year-old previously healthy man presents to a mountain clinic at 4,200 meters (13,800 feet) in the Himalayas. He ascended rapidly from sea level over 3 days. He reports progressive dyspnea at rest for the past 12 hours, dry cough, and fatigue. He notes his symptoms worsened significantly overnight. On examination, he appears dyspneic and mildly cyanotic. Vital signs show respiratory rate 32/min, heart rate 120 bpm, blood pressure 100/70 mmHg, temperature 37.8C, and oxygen saturation 72% on room air. Lung examination reveals bilateral crackles. He has no peripheral edema.
The alveolar gas equation explains his hypoxemia. At 4,200 meters, barometric pressure is approximately 450 mmHg. Alveolar PO2 = FiO2 x (Patm - PH2O) - (PaCO2/RQ) = 0.21 x (450 - 47) - (30/0.8) = 47 mmHg. This profound alveolar hypoxia triggers hypoxic pulmonary vasoconstriction (HPV), which is normally a protective mechanism to optimize V/Q matching. However, in susceptible individuals, exaggerated HPV leads to uneven pulmonary vasoconstriction, regional overperfusion of areas with less vasoconstriction, capillary stress failure, and high-altitude pulmonary edema (HAPE).
Arterial blood gas shows pH 7.48, PaCO2 30 mmHg, PaO2 38 mmHg. The A-a gradient is elevated (47 - 38 = 9 mmHg, which is significant at altitude where the normal gradient is smaller). Chest X-ray shows patchy bilateral alveolar infiltrates.
The patient is treated with supplemental oxygen (improving saturation to 92%), nifedipine (to reduce pulmonary artery pressure), and immediate descent to lower altitude. Within 24 hours of descent to 2,500 meters, his symptoms resolve and chest X-ray normalizes.
Key Learning Points
- The alveolar gas equation shows how reduced barometric pressure at altitude decreases PAO2
- Hypoxic pulmonary vasoconstriction is normally protective but can cause HAPE in susceptible individuals
- HAPE is a form of non-cardiogenic pulmonary edema from pulmonary hypertension and capillary leak
- The A-a gradient is elevated in HAPE due to V/Q mismatch and shunt from alveolar flooding
- Treatment includes supplemental oxygen, descent, and pulmonary vasodilators (nifedipine)