Regional venous return: nitroprusside effect in normal and chronically congested dogs.

The venous circulation is a key determinant of cardiac loading conditions. Although it may be altered by chronic congestive heart failure and vasodilating drugs, the mechanisms of these alterations have not been thoroughly studied, and their exact nature is unclear. We used venous return curves characterized by the pressure at zero flow (PZF) and resistance to venous return (RVR) to study the effect of nitroprusside and chronic congestion on regional venous return from the splanchnic and superior vena cava (systemic) venous beds in dogs. Chronic venous congestion was induced by prior creation of tricuspid insufficiency and pulmonic stenosis (TIPS). Nitroprusside lowered splanchnic PZF but not systemic PZF in both normal and TIPS animals. In normal animals systemic RVR increased during nitroprusside infusion but was unchanged in the congested animals. Resting values of PZF and RVR were different in the splanchnic and systemic venous beds in both normal and TIPS animals. We conclude that 1) nitroprusside works to selectively lower PZF in the splanchnic venous bed, 2) the regionally specific site of action of nitroprusside is not altered by TIPS, and 3) the independent behavior of the venous parameters of the two beds represents an important mechanism for the control of cardiac output.