Calbet, J. A., Radegran, G., Boushel, R., Sondergaard, H., Saltin, B., & Wagner, P. D., (2002). Effect of blood haemoglobin concentration on VO2max and cardiovascular function in lowlanders acclimatized to 5260 m. Journal of Physiology, 545(Part 2), 715-728.

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This study determined the influence of blood hemoglobin concentration on maximal exercise capacity and maximal O2 consumption (VO2max) in Ss acclimatized to high altitude. Examination was made also of the effects of blood hemoglobin concentration on the regulation of cardiac output (CO), blood pressure, and muscular blood flow during exercise. Danish lowlanders (M = 5; F = 3) performed submaximal and maximal exercise on a cycle ergometer after nine weeks at an altitude of 5,260 m (Mt Chacaltaya, Bolivia). This was done first with the high blood hemoglobin concentration resulting from acclimatization and again 2-4 days later, one hour after isovolaemic hemodilution with Dextran 70 to near sea-level blood hemoglobin concentration. After measurements at maximal exercise while breathing air at each blood hemoglobin concentration, Ss were switched to hyperoxia (55 % O2 in N2), the measurements repeated, and the work rate WAS increased as tolerated.

Hyperoxia increased maximal power output and leg VO2max, showing that breathing ambient air at 5,260 m, VO2max is limited by the availability of O2 rather than by muscular oxidative capacity. Altitude increased blood hemoglobin concentration by 36 % while hemodilution (replacing one liter of blood with one liter of 6 % Dextran) lowered blood hemoglobin concentration by 24 %. Hemodilution had no effect on maximal pulmonary or leg VO2max, or power output. Despite higher muscular blood flow, leg O2 delivery was reduced and VO2max was thus maintained by higher O2 extraction. While cardiac output increased linearly with work rate irrespective of blood hemoglobin concentration or inspired oxygen fraction, both muscular blood flow and leg vascular conductance were systematically higher when blood hemoglobin concentration was low. Close and significant relationships were seen between muscular blood flow (and cardiac output) and both plasma noradrenaline and K+ concentrations. They occurred independently of blood hemoglobin concentration and inspired oxygen fraction.

Under conditions where O2 supply limits maximal exercise, the increase in blood hemoglobin concentration with altitude acclimatization does not improve maximal exercise capacity or VO2max, and does not alter peak cardiac output. However, muscular blood flow and vascular conductance are higher at altitude when blood hemoglobin concentration is lowered to sea level values, with both relating closely to catecholamine and potassium concentrations. This suggests that the lack of effect of blood hemoglobin concentration on VO2max may involve reciprocal changes in muscular blood flow via local metabolic control of the muscle vasculature.

Implication. Blood and blood flow factors at altitude are governed by peripheral cellular metabolic control factors and not the characteristics of the blood. [Forced changes of the blood through the use of erythropoietin or other chemicals to increase hemoglobin concentration are unlikely to alter the performance characteristics of demanding exercise.]

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