Is resuscitation from hemorrhagic shock limited by blood oxygen-carrying capacity or blood viscosity?

Systemic and microvascular hemodynamic responses to volume restoration from hemorrhagic shock were studied in the hamster window chamber model to determine the significance of blood rheological and oxygen transport properties. Moderated hemorrhage was induced by means of arterial controlled bleeding of 50% of the blood volume. The hypovolemic shock state was maintained for 1 h before resuscitation. The animals were resuscitated by infusion of 25% of blood volume using either fresh plasma or blood and were studied for 90 min. Transfusion was performed with either oxygen-carrying fresh red blood cells (RBCs) or non-oxygen-carrying RBCs whose hemoglobin was converted to methemoglobin (MetHb). Systemic parameters, including cardiac output, vital organ blood flow distribution, microvascular hemodynamics, and capillary perfusion (functional capillary density [FCD]), were measured during the resuscitation period. Fluorescent-labeled microspheres were used to measure organ blood flow (brain, heart, kidney, liver, lung, spleen, and window chamber). The blood viscosities at the end of the 90-min period were 2.4 cP after resuscitation with plasma, and 2.9 to 3.0 cP after blood transfusion (baseline, 4.2 cP). Resuscitation with RBCs with or without oxygen-carrying capacity had greater mean arterial pressure than did the plasma resuscitation group. The FCD was substantially higher for RBC transfusions (0.56% +/- 7% of baseline) compared with plasma (46% +/- 7% of baseline), and the presence of MetHb in the fresh RBC did not change the FCD or the microvascular hemodynamics. Oxygen delivery and extraction levels were significantly lower for resuscitation with plasma and MetHb-loaded RBCs compared with oxygen-carrying RBCs. The curtailed recovery of systemic and microvascular conditions after volume restitution with plasma seems to be due to the decrease in blood viscosity. Conversely, the restoration of blood rheological properties improves resuscitation independently of the restitution of oxygen-carrying capacity.