The role of alpha 1- and alpha 2-adrenoceptors in canine systemic capacitance vessels--a study with measurement of mean circulatory pressure

Autonomic control of the venous system in health and disease: effects of drugs

The venous system contains approximately 70% of the blood volume. The sympathetic nervous system is by far the most important vasopressor system in the control of venous capacitance. The baroreflex system responds to acute hypotension by concurrently increasing sympathetic tone to resistance, as well as capacitance vessels, to increase blood pressure and venous return, respectively. Studies in experimental animals have shown that interference of sympathetic activity by an alpha1- or alpha2-adrenoceptor antagonist or a ganglionic blocker reduces mean circulatory filling pressure and venous resistance and increases unstressed volume. An alpha1- or alpha2-adrenoceptor agonist, on the other hand, increases mean circulatory filling pressure and venous resistance and reduces unstressed volume. In humans, drugs that interfere with sympathetic tone can cause the pooling of blood in limb as well as splanchnic veins; the reduction of cardiac output; and orthostatic intolerance. Other perturbations that can cause postural hypotension include autonomic failure, as in dysautonomia, diabetes mellitus, and vasovagal syncope; increased venous compliance, as in hemodialysis; and reduced blood volume, as with space flight and prolonged bed rest. Several alpha-adrenoceptor agonists are used to increase venous return in orthostatic intolerance; however, there is insufficient data to show that these drugs are more efficacious than placebo. Clearly, more basic science and clinical studies are needed to increase our knowledge and understanding of the venous system.

Effects of endotoxin infusion on mean systemic filling pressure and flow resistance to venous return

Mean systemic filling pressure (Psf) is an indicator of the filling state of the systemic circulation. Cardiac output (Q') is related linearly to the difference between Psf and central venous pressure (Pcv), according to: Q' = (Psf - Pcv)/Rsf, where Rsf is the flow resistance downstream from the sites where blood pressure is equal to Psf. In 16 anaesthetized pigs we evaluated Psf, Rsf and Q' during baseline conditions, continuous endotoxin infusion and after subsequent fluid loading. Psf and Rsf were determined from simultaneous measurements of Q' and Pcv at seven levels of lung inflation. The following results were obtained. Psf was 8.1 +/- 1.8 mm Hg (mean +/- SD) during baseline conditions, increased after endotoxin infusion to 9.9 +/- 3.2 mm Hg (P = 0.04) and remained the same after infusion of 18 ml kg-1 of Ringer s lactate. Rsf increased from 0.34 +/- 0.07 to 0.80 +/- 0.34 mm Hg ml-1 s by endotoxin and decreased after fluid infusion to 0.58 +/- 0.14. Q' changed inversely proportional to Rsf (P = 0.001). Rsf changes were highly correlated with the changes in total systemic flow resistance (Rs) (P < 0.001). Endotoxin caused haemoconcentration and a decrease in plasma volume. The stability of Psf during endotoxin infusion and after volume loading indicate that the stressed volume was well maintained and changes in blood volume are compensated by changes in non-stressed volume. The increase in Rsf can be attributed to arteriolar vasoconstriction, venous vasoconstriction and haemoconcentration.