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.