Modelling of the nonlinear end-systolic pressure-volume relation and volume-at-zero-pressure in porcine experiments

The End-Systolic Pressure-Volume Relation (ESPVR) is generally modelled as a linear relationship between P and V as cardiac reflexes, such as the baroreflex, are typically suppressed in experiments. However, ESPVR has been observed to behave in a curvilinear fashion when cardiac reflexes are not suppressed, suggesting the curvilinear function may be more clinically appropriate. Data was gathered from 41 vena cava occlusion manoeuvres performed experimentally at a variety of PEEPs across 6 porcine specimens, and ESPVR determined for each pig. An exponential model of ESPVR was found to provide a higher correlation coefficient than a linear model in 6 out of 7 cases, and a lower Akaike Information Criterion (AIC) value in all cases. Further, the exponential ESPVR provided positive V₀ values in a physiological range in 6 out of 7 cases analysed, while the linear ESPVR produced positive V₀ values in only 3 out of 7 cases, suggesting linear extrapolation of ESPVR to determine V₀ may be flawed.