Mean systemic filling pressure indicates fluid responsiveness and anaesthesia-induced unstressed blood volume

Purpose
The mean systemic filling pressure (Pms) plays a central role for our understanding of the circulation. In a retrospective analysis of a clinical trial, we studied whether Pms indicates fluid responsiveness and whether Pms can indicate an anaesthesia-induced increase of the unstressed blood volume, which is the volume that does not increase the transmural pressure.

Methods
An analogue to P_ms based on cardiac output, the mean arterial pressure and the central venous pressure, abbreviated to P_msa, were calculated in 86 patients before induction of general anaesthesia and before 3 successive bolus infusions of 3 mL kg^–1 of colloid fluid. An increase in stroke volume of ≥ 10% from a bolus infusion indicated fluid responsiveness. Receiver operator characteristic (ROC) curves were used to find the optimal cut-off for P_msa to indicate fluid responsiveness. Changes in blood volume were estimated from anthropometric data and the haemodilution.

Results
Pmsa was lower in fluid responders than in non-responders before induction (13.2 ± 2.2 vs. 14.7 ± 2.7 mmHg; mean ± SD, P < 0.01) and after induction of general anaesthesia (11.4 ± 2.1 vs. 12.8 ± 2.1 mmHg; P < 0.006). ROC curves showed that 14 mmHg before anaesthesia and 12 mmHg after anaesthesia induction served as optimal cut-offs for P_msa to indicate fluid responsiveness. A linear correlation between P_msa and blood volume changes suggested that the anaesthesia increased the unstressed blood volume by 1.2 L.

Conclusions
P_msa was lower in fluid responders than in non-responders. General anaesthesia increased the need for blood volume by 1.2 L.