physiology of altitude

Last reviewed 01/2018

• as altitude increases, barometric pressure decreases and hence the pressure of oxygen (PO2) falls
  • on Ben Nevis (4406ft or 1344m high) the partial pressure is approximately 90% of that at sea level
  • on Mont Blanc (4260 metres (13976 feet)) the partial pressure is approximately half that at sea level
  • on Mount Everest (8850 metres, or 29035 feet) the PO2 is approximately 1/3 that at sea level
  • air contains 20.9% oxygen whether at sea level or on top of Everest. However the PO2 (density of oxygen molecules) drecreases in direct proportion to the decrease in barometric pressure upon ascending
  • the inherent nature of the oxyhaemoglobin dissociation (S or sigmoid shaped curve) shows that only a small change in haemoglobin's percent saturation occurs with decreasing PO2 until about 3048m, when there is a rapid fall off in % haemoglobin saturation and a more marked impact on aerobic activities. For example at 4300m an unacclimatised individual would experience a 32% reduction in aerobic capacity. To tolerate such physiological changes the body needs to adapt, commonly known as altitude acclimitisation
    • at high altitude the partial pressure of oxygen is reduced in the inspired air. Various adaptive processes occur if there is chronic exposure to oxygen levels at high altitude. Acclimatisation is the process of the body adjusting to the decreasing availability of oxygen. Acclimatisation (biochemical changes) usually occurs from about 1500m (5000ft)
      • acute adaption
        • increased respiratory rate } mediated by peripheral
        • hypocapnia } chemoreceptors which are sensitive to oxygen (respiratory alkalosis)
        • arterial oxygen saturation drops
        • hyperventilation is reversible if oxygen is administered during the first 5-10 days. After this period hyperventilation is unaffected by inspiration of increased partial pressure oxygen
          • hyperventilation as noted above also causes the adverse effect of respiratory alkalosis, inhibiting the respiratory center from enhancing the respiratory rate as much as would be required. Gradually, the body compensates for the respiratory alkalosis by renal excretion of bicarbonate (diuresis), allowing adequate respiration to provide oxygen without risking alkalosis. It takes about 4 days at any given altitude and is greatly enhanced by acetazolamide
          • inability to ventilatory acclimatizize can be caused by inadequate carotid body response or pulmonary or renal disease
      • other changes
        • polycythaemia - may take months to develop fully
        • oxygen dissociation curve is shifted to the right - thus the oxygen affinity for haemoglobin is reduced and this facilitates oxygen delivery to the tissues. The shift of the oxygen dissociation curve is mediated by the increased levels of 2,3 diphosphoglycerate in red cells produced as a result of hypoxia