Spleen Size and Function in Sherpa Living High, Sherpa Living Low and Nepalese Lowlanders

Abstract

High-altitude (HA) natives have evolved some beneficial responses leading to superior work capacity at HA compared to native lowlanders. Our aim was to study two responses potentially protective against hypoxia: the spleen contraction elevating hemoglobin concentration (Hb) and the cardiovascular diving response in Sherpa highlanders, compared to lowlanders. Male participants were recruited from three groups: (1) 21 Sherpa living at HA (SH); (2) seven Sherpa living at low altitude (SL); and (3) ten native Nepalese lowlanders (NL). They performed three apneas spaced by a twomin rest at low altitude (1370 m). Their peripheral oxygen saturation (SpO 2), heart rate (HR), and spleen volume were measured across the apnea protocol. Spleen volume at rest was 198 ± 56 mL in SH and 159 ± 35 mL in SL (p = 0.047). The spleen was larger in Sherpa groups compared to the 129 ± 22 mL in NL (p < 0.001 compared to SH; p = 0.046 compared to SL). Spleen contraction occurred in all groups during apnea, but it was greater in Sherpa groups compared to NL (p < 0.001). HR was lower in Sherpa groups compared to NL both during rest (SL: p < 0.001; SH: p = 0.003) and during maximal apneas (SL: p < 0.001; SH: p = 0.06). The apnea-induced HR reduction was 8 ± 8% in SH, 10 ± 4% in SL (NS), and 18 ± 6% in NL (SH: p = 0.005; SL: p = 0.021 compared to NL). Resting SpO 2 was similar in all groups. The progressively decreasing baseline spleen size across SH, SL, and NL suggests a role of the spleen at HA and further that both genetic predisposition and environmental exposure determine human spleen size. The similar HR responses of SH and SL suggest that a genetic component is involved in determining the cardiovascular diving response.

Spleen contraction elevates hemoglobin concentration at high altitude during rest and exercise

“…The difference between the results in these two similar studies could possibly be caused by a general catabolism in the "Death Zone" above 8000 m, not allowing spleen enlargement. It was also recently observed that top climbers had larger spleens and spleen contraction during apnea, compared to recreational high altitude trekkers (Schagatay et al 2020). Taken together these studies suggest that also the spleen itself may be subject to altitude acclimatization, which is in line with the current study's observations of a significant spleen function in elevating Hb at altitude.…”

“…However, in that study, the spleen contraction during apnea at 1370 m was not significantly associated with AMS symptoms (P = 0.121; Holmström et al 2019). It was however recently found that spleen baseline volume was greater in experienced mountaineers going to climb Mt Everest, than in Mt Everest Base Camp trekkers (Schagatay et al 2020), suggesting that spleen function is important for successful climbing at high altitude. It was also recently reported that larger spleens and more powerful contractions were present in the Sherpa population of high altitude origin, compared to Nepali lowlanders (Holmström et al 2020).…”

“…It was also recently reported that larger spleens and more powerful contractions were present in the Sherpa population of high altitude origin, compared to Nepali lowlanders (Holmström et al 2020). A difference between Sherpas currently residing high and low, respectively, was also observed, suggesting that not only genetic factors but also exposure to hypoxia may determine spleen size (Holmström et al 2020).…”

“…It was however recently found that spleen baseline volume was greater in experienced mountaineers going to climb Mt Everest, than in Mt Everest Base Camp trekkers (Schagatay et al 2020), suggesting that spleen function is important for successful climbing at high altitude. It was also recently reported that larger spleens and more powerful contractions were present in the Sherpa population of high altitude origin, compared to Nepali lowlanders (Holmström et al 2020). A difference between Sherpas currently residing high and low, respectively, was also observed, suggesting that not only genetic factors but also exposure to hypoxia may determine spleen size (Holmström et al 2020).…”

“…This seems to contradict the finding in a longitudinal study that spleen contraction during apnea and exercise was enhanced after a climb to the summit of Mt Everest (Engan et al 2014), suggesting an important role of the spleen at altitude. It was also recently observed that elite high altitude climbers, going to the summit of Mount Everest at 8848 m, had larger spleens than trekkers to Mount Everest Base Camp at 5300 m (Schagatay et al 2020).…”