Arterial versus venous changes in vascular capacitance during nitroprusside infusion: a vascular modelling study

The distributions of nitroprusside (NP) induced changes in vascular capacitance, arterial versus venous, are unknown. We measured canine ileal arterial and venous pressures and total (isolated loop) vascular volumes (scintigraphy), before and during NP infusion. NP sufficient to decrease perfusion pressure by 30% increased total vascular volume to 111 +/- 3% (+/- SEM) of control (p < 0.01). Increasing flow to restore perfusion pressure increased volume 4% more (p < 0.01). Assuming a two-compartment model and on the basis of the literature data, changes in venous capacitance were estimated and compared with arterial capacitance. During constant-flow perfusion, NP increased venous volume by 10.0% (vs. 18.1%, arterial). When flow was increased to restore pressure, venous volume increased by another 3.7% (vs. 2.6%, arterial). Assuming an original arterial to venous volume ratio of 133/1033, the final, constant-pressure increase in venous volume was almost 4 times the arterial increase. In conclusion, the increase in vascular volume during NP infusion was due primarily to similar-magnitude, active increases in venous and arterial capacitances (i.e., rightward shifts in pressure-volume relations). However, as venous volume is so much larger than arterial, the NP-induced increase in venous volume was greater.