Low haemoglobin concentration in Tibetan males is associated with greater high-altitude exercise capacity

Abstract

Tibetans living at high altitude have adapted genetically such that many display a low erythropoietic response, resulting in near sea-level haemoglobin (Hb) concentration. We hypothesized that absence of the erythropoietic response would be associated with greater exercise capacity compared to those with high [Hb] as a result of beneficial changes in oxygen transport. We measured, in 21 Tibetan males with [Hb] ranging from 15.2 g dl(-1) to 22.9 g dl(-1) (9.4 mmol l(-1) to 14.2 mmol l(-1) ), [Hb], ventilation, volumes of O2 and CO2 utilized at peak exercise (V̇O2 and V̇CO2), heart rate, cardiac output and arterial blood gas variables at peak exercise on a cycle ergometer at ∼4200 m. Lung and muscle O2 diffusional conductances were computed from these measurements. [Hb] was related (negatively) to V̇O2 kg(-1) (r = -0.45, P< 0.05), cardiac output kg(-1) (QT kg(-1) , r = -0.54, P < 0.02), and O2 diffusion capacity in muscle (DM kg(-1) , r = -0.44, P<0.05), but was unrelated to ventilation, arterial partial pressure of O2 (PaO2) or pulmonary diffusing capacity. Using multiple linear regression, variance in peak V̇O2 kg(-1) was primarily attributed to QT, DM, and PCO2 (R(2) = 0.88). However, variance in pulmonary gas exchange played essentially no role in determining peak V̇O2. These results (1) show higher exercise capacity in Tibetans without the erythropoietic response, supported mostly by cardiac and muscle O2 transport capacity and ventilation rather than pulmonary adaptations, and (2) support the emerging hypothesis that the polycythaemia of altitude, normally a beneficial response to low cellular PO2, may become maladaptive if excessively elevated under chronic hypoxia. The cause and effect relationships among [Hb], QT, DM, and PCO2 remain to be elucidated.

Figure 1

Haemoglobin concentration ([Hb]) versus peak Relationships between haemoglobin concentration ([Hb]) and peak ( kg^–1) in 21 male Tibetan subjects at 4200 m.

Figure 2

Haemoglobin concentration ([Hb]) compared to components of oxygen transport at 4200 m Components of the oxygen transport cascade compared to haemoglobin concentration ([Hb]) in Tibetan and Han Chinese males. Cardiac output (QT kg^–1; A) and diffusing capacity in muscle (DM kg^–1; B) are significantly related to [Hb]; arterial (, ventilation) and lung-related (alveolar–arterial difference) components are not associated with [Hb] in Tibetans. Relationships between [Hb] and QT kg^–1 (P < 0.04; C) and (P < 0.02; D) are also shown for 9 Han Chinese.

Figure 3

Components of oxygen transport associated with exercise capacity Relationships observed between ventilation (, P <0.01; A), QT kg^–1 (P < 0.001l; B), and DM kg^–1 (P < 0.001; C), with  kg^–1 in Tibetans; and A-a difference did not exhibit a relationship with peak kg^–1. Relationships between  kg^–1 and (P < 0.02; D), and DM kg^–1 (P < 0.03; E) are also shown for 9 Han Chinese.

Figure 4

Measured versus predicted peak VO2 in 21 Tibetan males at 4200 m. Highly significant relationship (r = 0.95; p < 0.001) between measured VO2peak/kg (y axis) compared to that predicted by linear regression; standardized predictors include PaCO2 (b = −0.18), cardiac output (QT)/kg (b = 0.38), diffusion capacity in muscle (DM)/kg (b = 0.63).