Transvascular fluid shift and redistribution of blood in hypothermia

The effect of hypothermia on circulating blood volume was studied using 9 dogs subjected to body surface and blood cooling. To analyze the movement of water between the vascular and interstitial fluid spaces, Ringer solution was infused during normothermia and hypothermia (30 degrees C core temperature). During body cooling and infusion experiments, blood volume, hematocrit, and colloid osmotic pressure were monitored continuously. When body temperature was lowered to about 30 degrees C within 2 hr, plasma volume decreased from 39.0 +/- 0.1 to 36.2 +/- 5.3 ml/kg and colloid osmotic pressure increased from 16.4 +/- 0.2 to 17.0 +/- 3.1 mmHg. The changes in blood volume, colloid osmotic pressure, and central venous pressure during the infusion of Ringer solution and recovery periods, were used for a simulation analysis based on a two-compartment model. The instantaneous and delayed compliances were determined on the vascular space and on the interstitial fluid space together with the transvascular filtration coefficient of water. Hypothermia to 30 degrees C increased vascular compliance, but decreased interstitial fluid space. The transvascular filtration coefficient of water decreased in hypothermia. These results suggest that the redistribution of blood due to hypothermia causes a reduction of capillary area available for fluid exchange. The transvascular filtration coefficient determined on the whole body decreases due to reduction of the compliances of the "effective" interstitial fluid space.