Scientists mostly agree that acute mountain sickness (AMS) is caused by hypoxia at high altitude and a failure to adapt to that hypoxia (hence occurring more rapidly at rapid ascent because of less time for adaptation). Hypoxia leads to hyperventilation that causes respiratory alkalosis. This respiratory alkalosis limits the rise in ventilation rate and leads to symptoms of acute mountain sickness.
Carbonic anhydrase inhibitors like Acetazolamide, by inhibiting carbonic anhydrase in the choroid plexus, cause decreased secretion of bicarbonate into the CSF, thereby causing decreased pH of the CSF (CSF acidosis). There is also generalized metabolic acidosis due to excess loss of bicarbonate from the kidneys.
This MAINTAINS hyperventilation that is useful in mountain sickness, because normally hyperventilation produced without acetazolamide leads to respiratory alkalosis; By giving acetazolamide, it causes metabolic acidosis; Metabolic acidosis attenuates the inhibitory effects of hypoxia-induced respiratory alkalosis; so that now increase in ventilation is not limited-metabolic acidosis balances the respiratory alkalosis

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