Pushing the Limits of Hypoxia

So, what happens?

Dramatic decreases in partial pressure of oxygen (PaO2) are directly proportional to the decrease in barometric pressure associated with an increase in altitude. The barometric pressure at the peak of Mt. Everest is 253 mm Hg. However, subjects studied by Grocott et al. were able to maintain arterial oxygen saturation (SaO2) levels relatively well (group mean of 54%) in spite of a group mean PaO2 of 24.6mm Hg. It was theorized that this was due to the characteristics of the oxygen-hemoglobin dissociation curve and respiratory acclimatization (increase in hypoxic ventilatory response) causing a decrease in partial pressure of carbon dioxide (PaCO2) (group mean 13.3 mm hg). The mean pH of the group studied was 7.53, indicating a respiratory alkalosis. This, coupled with increases in hemoglobin concentration (group mean 19.3 g/dl), assured that arterial oxygen content (CaO2) was maintained until subjects reached over 23,000 ft. 

The alveolar-arterial oxygen difference was calculated as a mean of 5.4 mm Hg. This represented near normal differential and minimal diffusion impairment. Despite chronic hypoxic conditions, none of the subjects exhibited hyperlactatemia. The group mean was 2.2 mmol/liter. This suggests that increased lactate may have been used as a fuel source due to extreme conditions. 

Conclusion 

Understanding the numerous mechanisms by which the human body adapts and compensates for extreme conditions can be insightful for those treating critically ill patients. Studying the limits of hypoxic tolerance can give guidance to directed interventions that may better benefit patients.