The pressure field defined by stroke volume and systemic elastance as a novel paradigm for perfusion management of the circulatory system
I propose a novel precision medicine approach for anaesthetists and intensivists to understand, measure, and manage how oxygen is delivered to metabolising tissues in the intact circulation. This approach is based on more than 10 million haemodynamic data points collected using minimally invasive haemodynamic monitoring in 922 surgical and intensive care patients, and more than 100,000 data points collected noninvasively in 3,255 surgical and medical patients. The approach proposes that oxygen delivery be conceptualised as the product of oxygen availability and perfusion, with oxygen availability measured in terms of flow and its constituents, and perfusion measured in terms of a pressure field. That is, pressure alone is insufficient to understand perfusion. Perfusion must be understood in terms of how pressure is generated on a beat-to-beat basis: the beat-to-beat contribution of the heart and the beat-to-beat contribution of the vasculature to pressure. The pressure field equation asserts that perfusion pressure, calculated as the mean arteriovenous pressure difference (MAP-CVP), is the appropriate measure of pressure within the systemic circulation. Perfusion pressure can be understood as the product of stroke volume (SV) and systemic elastance (Es), where Es is the afterload of the systemic circulation (combined arterial and venous systems) calculated on a beat-to-beat basis. When perfusion pressure and SV are simultaneously measured and Es is calculated for a patient, this constitutes the patient’s pressure field. This pressure field varies with age, sex, weight, and disease. In conjunction with blood flow measurement, the pressure field provides anaesthetists and intensivists with information critical to understanding when to administer intravenous fluid and vasoactive drugs, and in what doses. The consequences appear to be smaller and earlier interventions, decreased fluid loading, greater haemodynamic stability, and the possibility of better patient outcomes. A retrospective analysis of 480 patients undergoing cardiac surgery confirmed that patients managed using the pressure field approach receive less fluid. Of these, 67 patients managed using the pressure field approach experienced a -13 ± 7 g/L change in pre- to post-operative haemoglobin levels compared to -52 ± 14 g/L for the 413 patients managed conventionally (p < 0.0001). Using blood flow and the pressure field to guide management of the circulation constitutes a significant step on the path to individualising care.