Systemic and splanchnic hemodynamics following hemorrhage and volume restitution with Haemaccel in portal hypertensive rats: the effect of propranolol

Portal hypertension and blood viscosity

Previous studies, exploring the effect of blood viscosity on portal pressure in portal hypertensive humans and animal models, have shown conflicting results. In a series of studies, in portal vein constricted rats, we investigated effects of reduced blood viscosity on the hyperdynamic circulation, portal pressure, and vascular geometry. Blood was withdrawn at a rate of 0.3 mL/min for 15 minutes followed by 15 minutes of stabilization. The shed blood or Haemaccel was infused at the same rate and volume as used for withdrawal. Hemodynamic measurements were performed using radioactive microspheres. Blood viscosity was measured with an Ostwald viscometer. Vascular hindrance (reflecting vessel geometry) was calculated as resistance/viscosity ratio. In normal and portal hypertensive rats, acute volume replacement with Haemaccel, induced increase in systemic and splanchnic blood flows reflecting mainly changes in viscosity and not in blood vessel geometry. However, 24 hours later, in Haemaccel treated animals, an increased splanchnic arteriolar and porto-collateral vascular hindrance were observed. This indicated vasoconstriction in the porto-collateral vascular bed. The increase in portal venous inflow after acute volume restitution with Haemaccel was prevented by pretreatment with propranolol. Although, caution should be taken in extrapolating these results to humans, we would like to speculate that during a portal hypertensive-related bleeding episode: (1) volume replacement with low viscosity plasma expanders may aggravate the hyperdynamic circulation of portal hypertension. (2) Slow rate volume replacement enables hemodynamic adaptation to occur. (3) Volume replacement maybe more safe in a subject pretreated with propranolol.

Blood substitutes

Blood substitutes are solutions intended to replace transfusion of banked red blood cells. Several variations of products based on either hemoglobin (animal or human) or perfluorocarbon emulsions are in advanced stages of clinical development. The need for such products is pressing as shortages of banked blood worsen and awareness of the dangers of blood transfusion increases. Animal and human studies with these cell-free oxygen carriers have led to new concepts of how oxygen is delivered to tissue and how the microcirculation is regulated. Although development of products is exciting and timely, understanding how they function to perfuse and oxygenate tissue could be at least as important. Because cell-free oxygen carriers will perfuse every organ of the body, their effects are far-reaching, and the transition from the laboratory to the bedside can be expected to be slow and deliberate. Comparison of oxygen carriers with more traditional starch-based products provides new insight into the interaction of oxygen transport, microvascular perfusion, and blood volume expansion.