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O2 max after altitude
acclimatization still reduced despite normalization of arterial
O2 content?
1 Department of Physical Education, University of Las Palmas de Gran Canaria, 35017 Las Palmas de Gran Canaria, Spain; 2 The Copenhagen Muscle Research Centre, Rigshospitalet, 2200 Copenhagen N, Denmark; 3 Department of Exercise Science, Concordia University, Montreal, Quebec, Canada H4B 1R6; and 4 Department of Medicine, Section of Physiology, University of California San Diego, La Jolla, California 92093
Acute hypoxia (AH) reduces maximal
O2 consumption (
O2 max),
but after acclimatization, and despite increases in both hemoglobin
concentration and arterial O2 saturation that can normalize arterial O2 concentration ([O2]),
O2 max remains low. To determine why,
seven lowlanders were studied at O2 max (cycle ergometry) at sea level (SL), after 9-10 wk at 5,260 m [chronic hypoxia (CH)], and 6 mo later at SL in AH
(FIO2 = 0.105) equivalent to
5,260 m. Pulmonary and leg indexes of O2 transport were
measured in each condition. Both cardiac output and leg blood flow were
reduced by ~15% in both AH and CH (P < 0.05). At
maximal exercise, arterial [O2] in AH was 31% lower than
at SL (P < 0.05), whereas in CH it was the same as at
SL due to both polycythemia and hyperventilation. O2
extraction by the legs, however, remained at SL values in both AH and
CH. Although at both SL and in AH, 76% of the cardiac output perfused
the legs, in CH the legs received only 67%. Pulmonary
O2 max (4.1 ± 0.3 l/min at SL)
fell to 2.2 ± 0.1 l/min in AH (P < 0.05) and was
only 2.4 ± 0.2 l/min in CH (P < 0.05). These
data suggest that the failure to recover O2 max after acclimatization despite
normalization of arterial [O2] is explained by two
circulatory effects of altitude: 1) failure of cardiac
output to normalize and 2) preferential redistribution of
cardiac output to nonexercising tissues. Oxygen transport from blood to
muscle mitochondria, on the other hand, appears unaffected by CH.
cardiac output; fatigue; performance; exercise; cardiovascular physiology; maximal oxygen consumption
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