Tuesday, September 14, 2010

Altitude sickness and the role of acetazolamide

I am going to Kings Canyon National Park at the end of the month. I will leave Detroit, elevation 600 feet and will travel via planes, trains and automobiles to 9,000 feet for the first night. Then we will begin out hike and cross passes over 12,500 feet.

In the past, I have developed modest altitude sickness going from 600 to 8,000 feet. So, I am nervous about the same problem on this trip. Acetazolamide is supposed to ameliorate altitude sickness.

The body acclimates to decreased oxygen and is so effective that people can function at the top of Mt. Everest without supplemental oxygen. The partial pressure of oxygen at the summit is 43 mmHg which is equivalent to breathing 6% FiO2.
From NEJM 2009, 360: 140-9

The primary means of improving oxygenation is hyperventilation. Hypoxia stimulates ventilation. There is also an increased ventilatory response to carbon dioxide so that that the normal respiratory response to carbon dioxide is exaggerated so that one gets more ventilation at lower CO2 levels. The reason that increased ventilation improves oxygenation has to do with the effect carbon dioxide in the blood has on oxygen transfer in the alveoli. During respiration CO2 leaving the blood dilutes the incoming oxygen at the alveoli, increased respiration, lowers the pCO2 and hence minimizes this dilution.

Antagonizing the hyperventilatory response is respiratory alkalosis. Central chemoreceptors detect alkalosis in the CSF and slow respiration. This is one of the key factors preventing the essential hyperventilation.

Acetazolamide (Diamox) is a carbonic anhydrase inhibitor. Carbonic anhydrase catalyzes the reaction converting bicarbonate to carbon diaoxide and water:

This is the fundamental buffer reaction in the body and it is amazing to me that blocking this essential acid-base reaction is not lethal. Acetazolamide works in the proximal tubule by blocking the reabsorption of filtered bicarbonate.

Acetazolamide induces a proximal renal tubular acidosis (RTA 2). This results in metabolic acidosis. The metabolic acidosis stimulates compensatory hyperventilation. This metabolic acidosis antagonizes the respiratory alkalosis which normally occurs with hyperventilation.

Their maybe additional advantages of acetazolamide including decreased CSF production and antagonizing fluid retention.

Happy climbing.
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