Relevance of the volume-axis intercept, V0, compared with the slope of end-systolic pressure-volume relationship in response to large variations in inotropy and afterload in rats

The end-systolic pressurevolume relationship (ESPVR) is proposed and used as a reliable index of left ventricular (LV) contractility despite the fact that its afterload independence has been challenged. Furthermore, the physiological relevance of its volume-axis intercept, V0, remains unclear. Systemic haemodynamics and pressurevolume loops obtained by inferior vena cava occlusion were recorded in 21 rats anaesthetized by isoflurane inhalation and instrumented with a conductance pressurevolume catheter in response to incremental i.v. doses of adrenaline, dobutamine, phenylephrine, metoprolol, papaverine and isoflurane inhalation. In conditions with large variations (+/- 100%) of both inotropy and afterload, infusion of negative inotropic drugs was associated with a dose-dependent rightward shift of ESPVR accompanied by a decrease in its slope (end-systolic elastance, Ees), whereas positive inotropic agents produced an isolated decrease in V0. With the predominant vasoactive drugs, there was a dose-dependent change in Ees without major horizontal shifts, demonstrating that this slope mainly represents LV afterload rather than inotropy. When contractility was altered, V0 was negatively correlated to the preload-adjusted contractility index, PAdP/dtmax, demonstrating that a reduced V0 provides a good reflection of increased LV contractility. From these results, we computed a logarithmically adjusted Ees/V0 ratio, which resulted in reasonably strong concordance with PAdP/dtmax, including all the investigated drugs and dosages [n= 288; bias, 0.8 +/- 16.2% (SD)]. Concordance with Ees (bias, 7.2 +/- 58.7%) or V0 (bias, -0.6 +/- 33.4%), used alone or with other commonly used contractility indices, was far less significant. In contrast to Ees, V0 provides a relatively good LV contractility index because it is much less sensitive to afterload.