After effects of chronic hypoxia on cardiac output and muscle blood flow at rest and exercise.

Cardiac output (Q, by N2-CO2 rebreathing) and limb muscle blood flow (qm, from 133Xe clearance) were determined in eight male subjects at rest and during cycloergometric loads immediately before and 12 days after return from the 1981 Swiss Lhotse Shar (8,398 m) Expedition. Compared to control conditions, after exposure to hypoxia: 1) Q was unchanged at rest and at 75 watts (W) but was 18% less (P less than 0.01) at 150 W with constant heart rate (approximately 140 beats X min-1); 2) qm in the vastus lateralis was identical at rest but 26% and 39% less (P less than 0.05 and P less than 0.001, respectively) at two submaximal leg work loads (75 and 125 W); 3) qm in the biceps at 50 W was 34% less (P less than 0.01); 4) hemoglobin flow (QHb and qmHb), similarly to blood flow (Q and qm), was significantly reduced; 5) the qm adjustment rate, measured from the time required to attain a new steady state upon a square wave change of work load starting from rest, was slower, particularly at the lower work loads. From the above results as well as from corresponding morphometric findings showing in the same subjects: 1) a decrease of the ratio between fiber section and number of capillaries and 2) a rise of the mitochondrial to fiber volume ratio, it is concluded that during altitude acclimatization peripheral O2 delivery becomes more efficient.