Exercise limb blood flow response to acute and chronic hypoxia in Danish lowlanders and Aymara natives

Aim:  The femoral artery blood flow response to submaximal, one‐legged, dynamic, knee‐extensor exercise was determined in acute and chronic hypoxia to investigate the hypotheses that with adaptation to chronic hypoxia blood haemoglobin increases, allowing preservation of blood flow as in normoxia. Methods:  Sixteen Danish lowlanders participated, in groups of six to eight, in the experiments at sea level normoxia (FiO 2  ≅ 0.21) and acute hypoxia (FiO 2  ≅ 0.11), and chronic hypoxia after ∼7 and 9–10 weeks at ∼5260 m altitude breathing ambient air (FiO 2  ≅ 0.21) or a hyperoxic gas (FiO 2  ≅ 0.55). The response was compared with that in six Aymara natives. Results:  The haemoglobin and haematocrit increased ( P  < 0.003) in the lowlanders at altitude vs. at sea level by ∼39 and 27% respectively; i.e. to a similar ( P  = ns) level as in the natives. At rest, blood flow was the same ( P  = ns) in the lowlanders at sea level and altitude, as in the natives at altitude. During the onset of and incremental exercise, blood flow was the same ( P  = ns) in the lowlanders at sea level and altitude, as in the natives at altitude. Acute hypoxia increased ( P  < 0.05) blood flow by ∼55% during exercise in the lowlanders at sea level. Acute hyperoxia decreased ( P  < 0.05) blood flow by ∼22–29% during exercise in the lowlanders and natives at altitude. Conclusion:  In chronic hypoxia, blood haemoglobin increases, allowing normalization of the elevated exercise blood flow response in acute hypoxia, and preservation of the kinetics and steady‐state exercise blood flow as in normoxia, being similar as in the natives at altitude.