Effect of volume status on the estimation of mean systemic filling pressure.

Various methods for indirect assessment of mean systemic filling pressure (MSFP) produce controversial results as compared to MSFP at zero blood flow. We recently reported that the difference between MSFP at zero flow, measured by right atrial balloon occlusion (MSFP) and MSFP estimated using inspiratory holds depends on the volume status. We now compare three indirect estimates of MSFP with MSFP in Euvolemia, Bleeding, and Hypervolemia, in a model of anesthetized pigs (n=9) with intact circulation. MSFP was estimated using instantaneous beat-to-beat venous return during tidal ventilation (MSFP), right atrial pressure-flow data-pairs at flow nadir during inspiratory holds (MSFP), and using a dynamic model analog adapted to pigs (MSFP). MSFP was underestimated by MSFP and MSFP in all volume states. Volume status modified the difference between MSFP and all indirect methods (method*volume state interaction; p≤0.020). All methods tracked changes in MSFP concordantly, with the lowest bias seen for MSFP [bias (CI): -0.4 (-0.7 to -0.0) mmHg]. We conclude that indirect estimates of MSFP are unreliable in this experimental setup