Various Approaches to Define the Volume Intercept of the Ventricular End-Systolic Pressure-Volume Relationship: Implications for Statistical Analysis

Ventricular pump function is often characterized by the (non)linear end-systolic pressure-volume relationship (ESPVR). For each working point on that curve the tangent along with the intercept (Vo) reflect contractile state. Vo on the abscissa is an extrapolated point without physiological meaning, and may be negative. To obtain positive values for the intercept, investigators often choose a non-zero pressure level. Although this preference is mathematically sound, we demonstrate that statistical evaluations may yield different results, depending on the pressure level selected. Published data on 17 cardiac patients representing three diagnostic groups were analyzed, showing dicrotic notch pressure based values -14<Vo<119 mL/m2• At 20, 40, 60, 80 and 100 mmHg levels all intercepts were positive, but statistical outcomes varied when comparing the 3 groups, depending on the pressure level selected, e.g. ranging from P=0.03 to 0.28. Also, we found that the correlation between intercept value Vo and end-systolic volume varies with the choice made for the pressure level. However, as there is no need to explore alternatives for the intercept, we conclude that the traditional route for the estimation of Vo (i.e. extrapolated to zero pressure) is to be preferred and this practice warrants adequate comparison of outcomes generated by all studies. Clinical Relevance- Personal preference for an alternative intercept definition for the ESPVR does not serve any relevant goal but rather implies that the statistical outcomes are altered thus hampering interpretation of data when comparing patient groups or assessing the effect of clinical intervention.