Impaired Restitution of Blood Volume after Large Hemorrhage

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Abstract
Increases in extracellular osmolality mediate a shift of intracellular fluid into the interstitium following 10% hemorrhage. This unidirectional fluid movement is required for full restitution of blood volume (BV) and for cardiovascular stabilization. The extent to which this mechanism acts in larger hemorrhages was examined in 42 dogs studied 3 wk after splenectomy; 20 were anesthetized with pentobarbital, the others were studied awake. Hemorrhages of 10 (7.5 ml/kg, 15 dogs), 20 (15 ml/kg, 13 dogs) and 30% (22.5 ml/kg, 15 dogs) were carried out over 3 min. Patterns of changes in osmolality, total plasma protein content and blood volume were identical in the 2 groups, although osmolality was significantly greater during the first 2 h in anesthetized dogs. Osmolality increased with increasing degrees of hemorrhage, providing an increasing driving force for fluid shifts. Restitution of BV was significantly greater after 20% hemorrhage than after 10% hemorrhage. Restitution of BV after 30% hemorrhage was no greater than after 10% hemorrhage. Patterns of protein restitution paralleled changes in BV. Fluid shifts into cells were reported after a 30% hemorrhage associated with a fall in membrane potential. A fluid shift out of cells in response to increased osmolality apparently is offset by shifts into cells as the magnitude of hemorrhage becomes large. This failure of homeostatic restoration of BV with consequent failure of cardiovascular stabilization may represent an early event in the development of hemorrhagic shock.

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