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O2 and leg blood flow
during submaximal exercise
John Rankin Laboratory of Pulmonary Medicine, Department of Preventive Medicine, University of Wisconsin, Madison, Wisconsin 53705
The work of
breathing (Wb) normally incurred
during maximal exercise not only requires substantial cardiac output
and O2 consumption (
O2) but also causes
vasoconstriction in locomotor muscles and compromises leg blood flow
(
leg).
We wondered whether the Wb normally incurred during submaximal exercise would also reduce leg.
Therefore, we investigated the effects of changing the Wb on
leg via
thermodilution in 10 healthy trained male cyclists [maximal
O2
(O2 max) = 59 ± 9 ml · kg
1 · min1]
during repeated bouts of cycle exercise at work rates corresponding to
50 and 75% of
O2 max.
Inspiratory muscle work was 1)
reduced 40 ± 6% via a proportional-assist ventilator,
2) not manipulated (control), or
3) increased 61 ± 8% by
addition of inspiratory resistive loads. Increasing the
Wb during submaximal exercise caused O2 to
increase; decreasing the Wb was
associated with lower
O2
(
O2 = 0.12 and 0.21 l/min at 50 and 75% of
O2 max, respectively,
for ~100% change in Wb).
There were no significant changes in leg vascular resistance (LVR),
norepinephrine spillover, arterial pressure, or
leg when
Wb was reduced or increased. Why
are LVR, norepinephrine spillover, and
leg influenced
by the Wb at maximal but not
submaximal exercise? We postulate that at submaximal work rates and
ventilation rates the normal Wb
required makes insufficient demands for
O2 and cardiac output to
require any cardiovascular adjustment and is too small to activate
sympathetic vasoconstrictor efferent output. Furthermore, even a
50-70% increase in Wb during
submaximal exercise, as might be encountered in conditions where
ventilation rates and/or inspiratory flow resistive forces are higher
than normal, also does not elicit changes in LVR or leg.
blood flow distribution; sympathetic vasoconstriction; thermodilution; work of breathing
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