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

Abstract

Mean systemic filling pressure (P sf ) is an indicator of the filling state of the systemic circulation. Cardiac output (Q) is related linearly to the difference between P sf and central venous pressure (P cv ), according to: Q = (P sf 9 P cv )/R sf , where R sf is the flow resistance downstream from the sites where blood pressure is equal to P sf . In 16 anaesthetized pigs we evaluated P sf , R sf and Q during baseline conditions, continuous endotoxin infusion and after subsequent fluid loading. P sf and R sf were determined from simultaneous measurements of Q and P cv at seven levels of lung inflation. The following results were obtained. P sf 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. R sf 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 R sf (P = 0.001). R sf changes were highly correlated with the changes in total systemic flow resistance (R s ) (P < 0.001). Endotoxin caused haemoconcentration and a decrease in plasma volume. The stability of P sf 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 nonstressed volume. The increase in R sf can be attributed to arteriolar vasoconstriction, venous vasoconstriction and haemoconcentration.

The effect of propofol on haemodynamics: cardiac output, venous return, mean systemic filling pressure, and vascular resistances

“…We defined MSFP by extrapolation to zero flow, assuming that airway pressure does not affect MSFP. We have previously validated this extrapolation in piglets [15][16][17] and described the technique in postoperative cardiac surgery patients. 9 Total systemic vascular resistance (R sys ) was calculated as the ratio of the pressure difference between mean P a and mean CVP and CO, as follows:…”

Driving forces of venous return

“…In 2018, Boer et al 3 emphasised this shortcoming in their review article in the British Journal of Anaesthesia. The current interest and wide international reach of the controversy related to intravenous fluid choice is evident from discussion in three articles from Europe and the United States [4][5][6] published in 2020 4,5 and 2021. 6 The rationale for new guidelines governing the use of an intravenous fluid should be based on its ability to maintain or restore the body's internal environment, the so-called…”

“…3,4 However, the main thrust of our review was not circulatory regulation but the behaviour of the venous system and the effect of vasopressors in the state of sepsis. In this regard, the relevance and applicability of the MCFP is quoted in different publications in septic patients, 5 as a tool to determine stressed versus unstressed volumes in sepsis, 6 and in surgical patients linking to fluid administration. 7 This high degree of clinical relevance and applicability led us, in our review, to use the MCFP to characterise the pathophysiological regulation playing at least some part in sepsis.…”

Ventilation

“…If the reason for the cardiac failure is an endotoxicosis then the vasculm factors are also changed in addition to the changes in contractility. Efforts to restore the decreased cardiac by volume infusion fail because the flow resistance to venous return is increased (8).…”