Protective Responses to Freediving Reveal High-Altitude Tolerance

Abstract: High-altitude mountaineers - just as freedivers - are exposed to hypoxia. During freediving, the diving response leads to reduced oxygen consumption, and splenic contraction increases circulating hemoglobin concentration (Hb), which enhances freediving performance. It is unknown whether these responses relate with altitude-induced responses and what role the spleen has at high-altitude. My thesis aimed to explore whether associations exist between these apnea-induced responses and tolerance to high-altitude. In five studies, I investigated the diving response and splenic contraction during apnea in a range of groups, including recreational trekkers, elite climbers, indigenous Sherpa (living high and living low) and endurance athletes, at low-altitude and at high-altitude. My primary finding was striking: the diving response and splenic size were associated with tolerance to high-altitude; lowlanders with a strong diving response and large spleen showed less symptoms of acute mountain sickness at high-altitude. I also found that groups often exposed to high-altitude have larger spleens compared with groups who reside at sea-level. Interestingly, the Sherpa living high had larger spleens compared with Sherpa living low. Another important finding was that the spleen is reduced in size by ~14% per 1000 m of ascent in lowlanders, which was associated with enhanced baseline Hb. I also found that endurance athletes, who are dependent on efficient oxygen delivery, have larger spleens compared with untrained individuals. I conclude, that a strong diving response and a large spleen may be characteristics of high-altitude tolerant lowlanders, and could possibly be used to predict high-altitude sensitivity. Studies 1-4 suggest that a large spleen is a favourable trait in several groups to tolerate high-altitude hypoxia, likely by its ability to regulate circulating Hb. Sherpa had larger spleens compared with lowlanders, indicating that genetic factors influence splenic size, while the finding that Sherpa living high had larger spleens than Sherpa living low indicate that splenic size also is influenced by environmental exposure. Study 4 revealed a tonic splenic contraction in lowlanders at high-altitude, suggesting that the Hb regulating function may be important before EPO-induced red cell increase occurs, thereby aiding individual acclimatization.