Chronic mountain sickness: Recent studies of the relationship between hemoglobin concentration and oxygen transport

Although an increase in hemoglobin concentration [Hb] in high altitude residents assists oxygen transport, excessive polycythemia ([Hb] ≥ 21 g/100 mL) may cause the syndrome of chronic mountain sickness (CMS). A recent theoretical analysis has suggested that increasing [Hb] above 18 g/100 mL provides no further benefit in oxygen transport at rest. To test this hypothesis, we examined oxygen transport at rest for given arterial oxygen saturations (Sa_O2, in classes at intervals of 5%) as reported in 206 residents of various altitudes. For Sa_O2 of 97% versus 87%, [Hb] and a-v oxygen content difference increased (respectively, 14.5 to 17.5 g/100 mL and 4.11 to 5.03 volume %). As Sa_O2 fell further to 66%, a-v progressively decreased to 3.77 volume %, despite an increase in [Hb] to 24.2 g/100 mL. Over the Sa_O2 range of 97% to 66%, the a-v difference changed little (-8%) compared to other subjects made acutely hypoxic (-33%), for Sa_O2 change from 97% to 75%. The results suggest that increasing [Hb] allows greater oxygen extraction (a cardiac output sparing effect), which is maximal at Sa_O2 of 87% and a [Hb] of 17.5 g/100 mL. For more severe hypoxemia, even to Sa_O2 of 66%, both increasing [Hb] and increasing output are utilized for oxygen transport.