LAUSR

In a recent study Wijnberge and colleagues assessed the ability of mean systemic filling pressure (MSFP) determined with the inspiratory-hold method (MSFPinsp_hold) to track fluid boluses in 20 sedated and ventilated patients after coronary artery bypass grafting (Wijnberge et al., 2022). Since the method has been available for a decade, the authors implicitly raised the question why it has not been implemented in routine clinical use? The authors are to be commended to include a clinical feasibility assessment in their well performed study. Still, we think the main reason for the method not being widely used is not that it may be perceived as cumbersome, but rather that it has been proved inaccurate. We share the opinion of the authors that bedside knowledge of stressed vascular volume and driving pressure for venous return (VRdP) could aid in managing haemodynamic therapy. We respect the impetus to find a practical solution, but experimental evidence suggests that it does not come in the form of MSFPinsp_hold. The development of the concept to assess MSFP by recording pressure-flow data pairs during changing inspiratory pressure is a fascinating story about applied physiology and heart-lung interactions and has deepened the understanding of venous return (VR) physiology. Already from the start, MSFPinsp_hold produced surprisingly high values of MSFP and VRdP [the record being 33 and 24 mmHg respectively (Persichini et al., 2012)] as compared to zero-flow measurements in animal models and patients [clinical testing of implantable cardioverter-defibrillator devices typically report MSFP and VRdP of 10–13 and 4–4.5 mmHg respectively (Jellinek et al., 2000; Schipke et al., 2003)]. The hope of being able to measure the underlying MSFP with clinically useful accuracy was severely questioned when we demonstrated that MSFPinsp_hold not only overestimated reference MSFP— but did so with a bias that varied with the volume state (Berger et al., 2016). Our initial porcine study was small but well-conducted, with intact circulation and reference method MSFP measured during intermittent right atrial balloon-occlusion. An additional physiological/mathematical OPEN ACCESS