Vascular viscoelasticity of perfused rat hindquarters

To determine viscoelastic features of the rat hindquarters vasculature, we measured pressure-volume curves. Male Wistar rats were transected at the lumbar level, and the perfused hindquarters were oxygenated with a hollow fiber artificial lung. The blood volume was measured by counting 51Cr-labeled red cells led to a gamma counter through an extracorporeal circuit at a constant rate. With continuous monitoring of the venous pressure and circulating blood volume, saline was infused into the circuit from a venous branch for 5 min [1.2 +/- 0.3% (SD) of tissue weight] followed by a 10-min recovery phase. In the recovery phase, the venous pressure promptly declined to the preinfusion level, whereas the circulating blood volume decreased more slowly. This implied vascular stress relaxation of the hindquarters. Maxwell's viscoelastic model, consisting of a spring component and a viscous component, was applied to analyze the venous pressure-volume diagram. With a curve-fitting method, the calculated vascular compliance and relaxation time (a time constant of stress relaxation) were 1.31 +/- 0.14 ml.mmHg-1.kg-1 and 15.7 +/- 4.0 min (means +/- SE), respectively. The value of compliance of the hindquarters was smaller than those of visceral organs reported. In addition, the value for relaxation time suggests that the viscous response of the vasculature simultaneously overlaps change in blood volume due to extravascular fluid shift during the postinfusion period.