Plasma volume expansion does not increase maximal cardiac output or VO2max in lowlanders acclimatized to altitude

With altitude acclimatization, blood hemoglobin concentration increases while plasma volume (PV) and maximal cardiac output ((Q) over dot(max)) decrease. This investigation aimed to determine whether reduction of (Q) over dot(max) at altitude is due to low circulating blood volume (BV). Eight Danish lowlanders ( 3 females, 5 males: age 24.0 +/- 0.6 yr; mean +/- SE) performed submaximal and maximal exercise on a cycle ergometer after 9 wk at 5,260 m altitude (Mt. Chacaltaya, Bolivia). This was done first with BV resulting from acclimatization (BV = 5.40 +/- 0.39 liters) and again 2 - 4 days later, 1 h after PV expansion with 1 liter of 6% dextran 70 (BV = 6.32 +/- 0.34 liters). PV expansion had no effect on (Q) over dot(max), maximal O-2 consumption ( (V) over dotO(2)), and exercise capacity. Despite maximal systemic O-2 transport being reduced 19% due to hemodilution after PV expansion, whole body (V) over dot O-2 was maintained by greater systemic O-2 extraction ( P < 0.05). Leg blood flow was elevated (P < 0.05) in hypervolemic conditions, which compensated for hemodilution resulting in similar leg O-2 delivery and leg (V) over dot O-2 during exercise regardless of PV. Pulmonary ventilation, gas exchange, and acid-base balance were essentially unaffected by PV expansion. Sea level (Q) over dot(max) and exercise capacity were restored with hyperoxia at altitude independently of BV. Low BV is not a primary cause for reduction of (Q) over dot(max) at altitude when acclimatized. Furthermore, hemodilution caused by PV expansion at altitude is compensated for by increased systemic O-2 extraction with similar peak muscular O-2 delivery, such that maximal exercise capacity is unaffected.