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Human Performance Laboratory, Department of Kinesiology and Health Education, The University of Texas at Austin, Austin, Texas 78712
We determined
whether the deleterious effects of dehydration and hyperthermia on
cardiovascular function during upright exercise were attenuated by
elevating central blood volume with supine exercise. Seven trained men
[maximal oxygen consumption
(
O2 max) 4.7 ± 0.4 l/min (mean ± SE)] cycled for 30 min in the heat
(35°C) in the upright and in the supine positions
(O2 2.93 ± 0.27 l/min) while maintaining euhydration by fluid ingestion or while being dehydrated by 5% of body weight after 2 h of upright exercise. When
subjects were euhydrated, esophageal temperature
(Tes) was 37.8-38.0°C
in both body postures. Dehydration caused equal hyperthermia during
both upright and supine exercise
(Tes = 38.7-38.8°C).
During upright exercise, dehydration lowered stroke volume (SV),
cardiac output, mean arterial pressure (MAP), and cutaneous vascular
conductance and increased heart rate and plasma catecholamines
[30 ± 6 ml, 3.0 ± 0.7 l/min, 6 ± 2 mmHg, 22 ± 8%, 14 ± 2 beats/min, and 50-96%, respectively;
all P < 0.05]. In contrast,
during supine exercise, dehydration did not cause significant
alterations in MAP, cutaneous vascular conductance, or plasma
catecholamines. Furthermore, supine versus upright exercise attenuated
the increases in heart rate (7 ± 2 vs. 9 ± 1%) and
the reductions in SV (13 ± 4 vs. 21 ± 3%) and cardiac output
(8 ± 3 vs. 14 ± 3%) (all P < 0.05). These results suggest that the decline in cutaneous vascular
conductance and the increase in plasma norepinephrine concentration,
independent of hyperthermia, are associated with a reduction in central
blood volume and a lower arterial blood pressure.
skin circulation; stroke volume; central blood volume; catecholamines
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