Effects of Altitude on Respiration

Expansion

• Fractional oxygen concentration stays at 21 per cent at all altitudes
• Barometric pressure falls, so the partial pressure of inspired oxygen falls
• At about 5500 metres barometric pressure is halved

Sequence of adaptation:

1. Hypoxic ventilatory drive via the carotid bodies causes hyperventilation
2. This produces a respiratory alkalosis, which itself restrains further hyperventilation
3. Over 24 to 72 hours the kidney excretes bicarbonate, correcting the pH and permitting more hyperventilation
4. 2,3-DPG rises, shifting the dissociation curve right
5. Erythropoietin rises over days to weeks, increasing haemoglobin

Acetazolamide accelerates acclimatisation by inducing a renal bicarbonate diuresis, producing a metabolic acidosis that stimulates ventilation from the outset.

Failure of adaptation causes acute mountain sickness, and in severe cases high altitude pulmonary oedema (from uneven hypoxic vasoconstriction) or cerebral oedema. The definitive treatment for all is descent.