Abstract
This study tests the hypothesis that volume loading during heat stress attenuates the reduction in regional blood volumes during an orthostatic challenge. 7 subjects underwent 30 mmHg lower-body negative pressure (LBNP) while normothermic, during heat stress (increased internal temperature ~1°C), and heat stress + colloid volume loading (12ml/kg). Relative thorax, heart, and liver blood volumes were determined by gamma camera imaging of 99mTc-labelled erythrocytes. Heat stress reduced blood volume in all regions, while subsequent volume infusion returned those values to normothermic levels. During LBNP, heat stress increased the magnitude of the reduction in blood volume (thorax=73±1, heart=72±1, liver=74±1%) compared to normothermia (thorax=22±3, heart=18±2, liver=14±63%; all P<0.001). Volume loading during heat stress did not alter the extent of the reduction in these blood volumes during LBNP (P>0.05). These data indicate that colloid volume loading during heat stress normalizes thorax, heart, and liver blood volumes but does not affect the magnitude of the subsequent reduction in blood volume in these regions during LBNP relative to heat stress alone. Supported by NIH-HL61388 & HL84072
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