IV. Oxygen transport system before and after exposure to chronic hypoxia

Maximal VO2 on the treadmill (VO2max) and on the bicycle ergometer (VO2peak), maximal cardiac output (Qmax), by a CO2 rebreathing method, maximal heart rate (HRmax), blood hemoglobin concentration (Hb), and hematocrit (Hct) were measured on six subjects before (B) and 3 weeks after (A) prolonged exposure to chronic hypoxia. It was observed that after high-altitude exposure VO2max, VO2peak, and Qmax were lower (P < 005) than before [A: 4.13 ± 0.67; 3.28 ± 0.41 and 16.89 ± 2.49 (1/min ± SD); B: 4.39 ± 0.39; 3.53 ± 0.34 and 21.81 ± 1.27, respectively}, whereas Hb and Hct were larger (A: 162 ± 8 g/l and 0.46 ± 0.02; B: 142 ± 7 and 0.41 ± 0.02) and HRmax was unchanged (178 ± 7 vs 175 ± 9 bts/min). Thus, we calculated stroke volume of the heart and the Hb flow at VO2 peak were lower in A than in B (95 ± 15 vs 124 ± 7 ml and 2,723 ± 307 vs 3,129 ± 196 g/min) (P < 0.05, respectively), whereas the arteriovenous O2 difference was greater in A than in B (195 ± 16 vs 162 ± 19 ml O2/l; P < 0.05). At any given submaximal work load, VO2 and HR were the same in B and in A, whereas Q was lower in A by ~ 2-3 l/min. However, because of the increased Hb, leading to a higher arterial O2 content, at any work load the O2 flow remained unchanged.