Hypothesis, aims and significance
We hypothesise that use of the pressure field method for managing perfusion in patients undergoing intermediate- to high-risk major abdominal surgery enables anaesthetists to better differentiate circulation phenotypes and the underlying cause of changes in blood pressure, enabling delivery of more targeted interventions, and resulting in greater haemodynamic stability, fewer hypotensive episodes, and fewer postoperative complications. The aims of this prospective randomised controlled feasibility trial are:
- To assess the feasibility of conducting a large definitive randomised controlled trial of the pressure field method for managing perfusion in patients undergoing intermediate- to high-risk major abdominal surgery.
- To provide an estimate of the incidence and magnitude of intraoperative hypotension experienced by patients undergoing intermediate- to high-risk major abdominal surgery to inform sample size calculation for a definitive clinical outcomes trial.
- To assess acceptability to anaesthetists of the pressure field as a method for managing perfusion, and to identify and address barriers to implementation of the pressure field method within clinical settings.
Endpoints will inform a future pragmatic study protocol suitable for scaling to a clinical outcomes trial.
The significance of this research is that the pressure field method appears to be a pragmatic, generalisable method for improving perfusion management during intermediate- to high-risk surgery, with potential to improve patient postoperative outcomes, particularly in the elderly (due to their change in circulation phenotype), and to reduce episode-of-care costs related to postoperative complications.
Background
Blood pressure is the key surrogate for perfusion, but intraoperative hypotension occurs frequently and negatively impacts postoperative outcomes. Blood pressure is a lumped parameter and there is a lack of clarity regarding when to administer fluid, cardioactive drugs, and/or vasoactive drugs, and how best to titrate these.
The pressure field is a software-based method for managing perfusion with real-time visualization of the beat-to-beat contributions of the heart and vasculature (stroke volume and systemic elastance) to the generation of blood pressure. The method draws heavily on the work of Kenji Sunagawa, who argued that elastance is a better measure of ventricular afterload than systemic vascular resistance, especially in that it is independent of heart rate. The pressure field visualization enables clinicians to quickly identify a patient’s circulatory phenotype, which changes with age and which has important implications for management: in youth, the cardiovascular system has a cardiac (type 1) phenotype and is more fluid-responsive, while in old age there is a change to a vascular (type 2) phenotype that is more responsive to vasoactive drugs, with fluid having limited benefit in the absence of haemorrhage. Most importantly, sequential changes in the pressure field visualization then help to identify the underlying cause of a change in pressure, and so deliver specific and targeted interventions to guide the treating anaesthetist.
CI-Woodford’s doctoral thesis outlines the method, and a study of 480 patients undergoing cardiac surgery concluded the method reduces transfusion requirements and better preserves haemoglobin, as it reduced haemodilution from inappropriate fluid therapy. The method has also assisted in the management of an additional 1,000 patients by study investigators.
Methods
We propose a two-centre, prospective, two-arm, randomised controlled feasibility trial with parallel process evaluation. Eligible patients are adults scheduled for elective intermediate- to high-risk abdominal surgical procedures. Patients in the pressure field group will be managed by an anaesthetist trained in the pressure field method using the pressure field software with the goal of care to maintain mean arteriovenous pressure (MAVP) within 15% of a patient’s pre-induction baseline, and stroke volume and systemic elastance within 15% of baseline values. The control group will represent standard care with the goal to maintain MAVP within 15% of a patient’s pre-induction baseline with haemodynamic monitoring and without access to the pressure field software.
The primary outcome is the percentage of patients for whom complete haemodynamic and intervention data is available. Additional outcomes relate to further trial feasibility measures, clinical outcome measures including the incidence and magnitude of intraoperative hypotension, acceptability of the pressure field as a method for managing perfusion, and identification and resolution of barriers to implementation. The study will be run at two University of Melbourne affiliated academic hospitals, where the required validated monitoring technology is available and in use.
Likely benefits
We anticipate that the pressure field will be a ground-breaking method for improving perfusion management. This study will inform design of a definitive clinical outcomes trial of the pressure field method.
Dr Stephen Woodford, Professor Laurence Weinberg, Austin Health, Melbourne;
Dr Sanne Peter, University of Melbourne.
The project was awarded A$57,386 funding through the ANZCA research grants program for 2024.