Uneven hypoxic pulmonary vasoconstriction overperfuses some capillaries
Expansion
The mechanism is uneven hypoxic pulmonary vasoconstriction:
- Global alveolar hypoxia constricts pulmonary arterioles, but not uniformly
- Blood is diverted into the regions that constrict least
- Those capillary beds are overperfused at high pressure
- Capillary stress failure allows a protein-rich, sometimes bloody, exudate into alveoli
It is therefore non-cardiogenic: left atrial and wedge pressures are normal, but pulmonary artery pressure is high.
Presentation is at altitudes above about 2500 metres, typically on the second or third night, with breathlessness at rest, cough, reduced exercise tolerance and pink frothy sputum.
Management follows the physiology: descent is the definitive treatment, with oxygen and a portable hyperbaric bag if descent is delayed. Nifedipine helps by reducing pulmonary artery pressure, and phosphodiesterase-5 inhibitors act similarly. Diuretics are unhelpful, since the patient is not volume overloaded.
Prevention is by gradual ascent, with acetazolamide and nifedipine for susceptible individuals.