Effect of blood haemoglobin concentration on (V) over dot O2,max and cardiovascular function in lowlanders acclimatised to 5260 m

The principal aim of this investigation was to determine the influence of blood haemoglobin concentration ([Hb]) on maximal exercise capacity and maximal O-2 consumption (V-O2,V-max) in healthy subjects acclimatised to high altitude. Secondarily, we examined the effects of [Hb] on the regulation of cardiac output (CO), blood pressure and muscular blood flow (LBF) during exercise. Eight Danish lowlanders (three females and five males; 24 +/- 0.6 years, mean +/- S.E.M.) performed submaximal and maximal exercise on a cycle ergometer after 9 weeks at an altitude of 5260 m (Mt Chacaltaya, Bolivia). This was done first with the high [Hb] resulting from acclimatisation and again 2-4 days later, 1 h after isovolaemic haemodilution with Dextran 70 to near sea level [Hb]. After measurements at maximal exercise while breathing air at each [Hb], subjects were switched to hyperoxia (55 % O-2 in N-2) and the measurements were repeated, increasing the work rate as tolerated. Hyperoxia increased maximal power output and leg V-O2,V-max,V- showing that breathing ambient air at 5260 m, 1 02, a is limited by the availability of O-2 rather than by muscular oxidative capacity. Altitude increased [Hb] by 36 % from 136 +/- 5 to 185 +/- 5 gl(-1) (P < 0.001), while haemodilution (replacing 1 l of blood with 1 l of 6 % Dextran) lowered [Hb] by 24 % to 142 +/- 6 gl(-1) (P < 0.001). Haemodilution had no effect on maximal pulmonary or leg (P < 0.001). Haemodilution had no effect on maximal pulmonary or leg V-O2,V-max, or power output. Despite higher LBF, leg O-2 delivery Despite higher LBF, leg O-2 delivery was reduced and maximal VO2 was thus maintained by higher O-2 extraction. While CO increased linearly with work rate irrespective of [Hb] or inspired oxygen fraction (F-l,O-2), both LBF and leg vascular conductance were systematically higher when [Hb] was low. Close and significant relationships were seen between LBF (and CO) and both plasma noradrenaline and K+ concentrations, independently of [Hb] and F-I,F-O2. In summary, under conditions where O-2 supply limits maximal exercise, the increase in [Hb] with altitude acclimatisation does not improve maximal exercise capacity or VO2, max, and does not alter peak CO. However, LBF and vascular conductance are higher at altitude when [Hb] is lowered to sea level values, with both relating closely to catecholamine and potassium concentrations. This suggests that the lack of effect of [Hb] on VO2, a may involve reciprocal changes in LBF via local metabolic control of the muscle vasculature.