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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 two-min rest at low altitude (1370 m). Their peripheral oxygen saturation (SpO2), 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 SpO2 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.
Highlights
High altitude (HA) is an extreme environment posing stress on the human body, due to the hypobaric hypoxia which reduces arterial oxygen saturation (SaO2; Grocott et al, 2009)
Baseline spleen volume was 129 ± 22 mL in Nepalese lowlanders (NL), which was smaller than both Sherpa groups [p < 0.001 compared to Sherpa living at HA (SH), effect size (ES) = 1.43 (0.57–2.3); p = 0.046 compared to SL, and ES = 1.07 (0.0–2.0) Figure 3A]
We found that the apnea-induced bradycardia was less pronounced in both Sherpa groups compared to NL, a novel finding which may seem contradictory to the superior hypoxia tolerance and work capacity with limited oxygen in Sherpa
Summary
High altitude (HA) is an extreme environment posing stress on the human body, due to the hypobaric hypoxia which reduces arterial oxygen saturation (SaO2; Grocott et al, 2009). One means to enhance the oxygen-carrying capacity is to increase the amount of circulating red blood cells (RBC) and thereby hemoglobin concentration (Hb) and oxygen content. This can be achieved both through the long-term effects of hypoxia on erythropoiesis (Siebenmann et al, 2017) and in the short term by spleen contraction (Schagatay et al, 2001)
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