Comparative influence of load versus inotropic states on indexes of ventricular contractility: experimental and theoretical analysis based on pressure-volume relationships

We examined the quantitative influence of carefully controlled alterations in end-diastolic volume and afterload resistance on multiple simultaneously determined ejection and isovolumetric phase indexes of left ventricular contractile function in 23 isolated supported canine ventricles. The influence of load change on each index was compared with its sensitivity to inotropic stimulation, and this sensitivity was in turn contrasted to the response of the end-systolic pressure-volume relationship (ESPVR). Experimental data demonstrated various degrees of load sensitivity among the indexes, with a generally curvilinear relationship between load and index response for both preload and afterload alterations. The curvilinear nature of these relationships meant that over a select range of loading, many indexes demonstrated relative load independence. They also often displayed greater sensitivity to inotropic change than the ESPVR, and both factors help explain their enduring clinical utility. To further explore the influence of load and contractile state on several of the indexes, we developed a theoretical analysis, using variables common to pressure-volume relationships, in which these dependencies could be derived. The theoretical models fit very well with the experimental data, and reaffirmed the frequently curvilinear nature of the relationships. We conclude that while many clinical indexes of ventricular contractile function show significant load dependence, the information they provide can be reasonably interpreted within defined ranges of load and inotropic alteration. Any advantage of the ESPVR will derive not from the magnitude of its response to inotropic change, which is smaller than most other indexes, but from its relative insensitivity to load alteration over a wider range of load.