Dynamic fluid redistribution in hyperosmotic resuscitation of hypovolemic hemorrhage.

A mathematical description of blood volume restoration after hemorrhage with resuscitative fluids, particularly hyperosmotic solutions, is presented. It is based on irreversible thermodynamic transport equations and known physiological data. The model shows that after a 20% hemorrhage, the rapid addition of a hypertonic (7.5% NaCl)-hyperoncotic (6% Dextran 70) solution amounting to one-seventh of the shed blood volume reestablishes blood volume within 1 min. Measurements of systemic hematocrit, hemoglobin concentration, and plasma osmolality taken from 13 experiments on anesthetized rabbits verify this prediction. The model shows that immediately after hyperosmotic infusion, water shifts into the plasma first from red blood cells and endothelium and then from the interstitium and tissue cells. The increase in blood volume is transitory; however, it occurs in a fraction of the time compared with isoosmotic fluids at the same infusion rate and is partially sustained by Dextran 70. We theorize that the concurrent hemodilution and endothelial cell shrinkage during hyperosmotic infusion lead to a decreased capillary hydraulic resistance, an effect that is even more significant in capillaries with swollen endothelium. Our results support the significant role of an osmotic mechanism during hyperosmotic resuscitation in quickly restoring blood volume with the added benefit of improved tissue perfusion.