King of the Mountains – Part I

The following is a summary, presented by Gabby and Courtney, of the first half of “King of the Mountains: Tibetan and Sherpa Physiological Adaptations for Life at High Altitude Edward T. Gilbert-Kawai, James S. Milledge, Michael P.W. Grocott and Daniel S. Martin” (Physiology 29:388-402, 2014)

Tibetans and Sherpas have lived at over 13,000 feet of elevation for over 500 generations, thus giving them plenty of time to develop an evolutionary advantage to the hypoxic environment in which they live. The purpose of this review was to identify the physiological differences between Sherpa/Tibetan populations living at high altitude, compared to lowlanders who ascend to and acclimate to high altitude. For the purpose of the review, Tibetans and Sherpas were considered as a single population and will be referred to as Sherpas for the rest of this summary.

The typical primary response to ascent to high altitude – in order to compensate for the lower oxygen content of the high altitude air – is to hemoconcentrate; that is, to increase red blood cell production and thus increase the number of red cells per ml of blood, thereby raising oxygen carrying capacity. Because hemoconcentration also increases blood viscosity, this compensatory response would, on a chronic basis, represent a cardiovascular risk factor by increasing cardiac workload.

Through evolutionary pressures, Sherpas have developed alternative adaptations to high altitude. Sherpas are found not to hemoconcentrate at altitude. Instead, they increase their blood flow rate; they develop larger chest circumference and lung volumes to increase surface area for diffusional exchange; and they have a lower ventilatory recruitment threshold during exercise (since they work on an oxygen sensing mechanism rather than the CO2 dependent sensing mechanism of lowlanders). Sherpas also do not show hypoxic pulmonary vasoconstriction, which is common in lowlanders who ascend. This prevents in Sherpas the increased cardiac afterload that is typical in chronically adapted lowlanders; consequently, Sherpas are less likely to develop myocardial hypertrophy, a risk factor for cardiovascular dysfunction. Sherpas also show an increased ability to metabolize glucose as a substrate in place of fatty acids (glucose yields more ATP per molecule of oxygen than do fatty acids).

Pregnant Sherpa women also show the ability to divert a larger amount of blood flow and blood volume to the uterine artery, lessening premature births and miscarriages compared to chronically adapted lowlanders. On the other hand, the review found that there was no difference between Sherpas and lowlanders in the hypoxic ventilatory response to ascent and no differences in arterial oxygen saturation. With continued human hypoxic research, it is hoped that advances in the prevention and care of hypoxemic critically ill patients can be achieved.

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