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

PURPOSE: The mean systemic filling pressure (P(ms)) plays a central role for our understanding of the circulation. In a retrospective analysis of a clinical trial, we studied whether P(ms) indicates fluid responsiveness and whether P(ms) 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: P(msa) 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.