Because oxygen is essential for cellular metabolism, when only an acutely diminished amount is available (hypoxia), survival is at risk—paradoxically, more so at sea level than at high altitude. Extremely low arterial oxygen tensions (Pao2) are quite well tolerated at high altitude. For example, at 3600 m in La Paz, Bolivia, patients can present with Pao2 between 30 mm Hg and 40 mm Hg. Recovery is uneventful within a few days after the efficient treatment of the underlying cause. At this altitude, the normal acid-base values are Pao2 60 ± 2 mm Hg, arterial carbon dioxide partial pressure (Paco2) 30 ± 2 mm Hg, pH 7.40 ± 0.02, and Spo2 91% ± 1%, giving course to completely normal cellular function. Conversely, at sea level, a patient presenting with a similar Pao2 of 60 mm Hg acutely is sent to an intensive care unit as that person’s life is in peril.
Immediately upon arrival at high altitude, arterial oxygen and carbon dioxide partial pressures decrease. The persistent low levels of Paco2, due to not only hyperventilation but also to high altitude per se, induce changes in the acid-base balance. The pH is inextricably linked critically with hemoglobin (Hb), carbon dioxide, and oxygen status, all crucial at high altitudes. The alkaline pH during acute high altitude exposure shifts the oxygen dissociation curve to the left, allowing more capture and transport of oxygen. Another variable that can allow for the tolerance to extremely low Pao2 values is a normal low (relative to sea level) Paco2. The acid-base balance in the human body is calculated using the Van Slyke equation based on sea level measurements.
The maintenance of blood pH within a fairly strict range at approximately pH 7.4, with due consideration of the effect of hyperventilation, is essential for cellular function at any altitude. That is because various chemical processes occurring in the body, for example, those involving proteins and enzymes, are pH dependent. Therefore, for a more precise recalculation of the titratable hydrogen ion difference that should use Hb and Hco3− values for a particular altitude, we previously derived our modified Van Slyke equation. An adequate acid-base balance is probably one of the fundamental metabolic adaptations that allow for mountaineers to tolerate extreme hypoxia and even reach the summit of Mount Everest. Furthermore, the increase in Hb and decrease in arterial carbon dioxide tension (Paco2) are 2 essential changes that occur with high altitude chronic exposure. We propose the following formula: tolerance to hypoxia = Hb/Paco2 × 3.01. In conclusion, we present evidence that the direct relationship between Hb and the inverse relationship with Paco2 concomitantly linked to the high altitude acid-base status explains the tolerance to hypoxia at high altitude.
