Curvilinearity of LV end-systolic pressure-volume and dP/dtmax-end-diastolic volume relations

We investigated the effect of contractile state (CS) on the curvilinearity of the left ventricular (LV) end-systolic pressure-volume (Pes-Ves) and the dP/dtmax-end-diastolic volume (dP/dtmax-Ved) relations in eight anesthetized open-chest dogs. LV volume was determined from three orthogonal diameters measured by sonomicrometry. The Pes-Ves relation and dP/dtmax-Ved relation were assessed during transient vena caval occlusion and aortic constriction, while CS was altered with dobutamine. At all CS, both relations were linear when volumes were decreased by caval occlusion. However, at higher volumes obtained by aortic constriction, the relations became nonlinear. At enhanced CS, the nonlinearity of both the Pes-Ves relation and the dP/dtmax-Ved relation increased. The dP/dtmax-Ved relation began to deviate from linearity at larger volumes, but closer to baseline operating volume, than the Pes-Ves relation. The relation between end-systolic mean circumference stress and wall strain (sigma es-epsilon es) was linear at control CS when the Pes-Ves relation was nonlinear but became nonlinear with enhanced CS. We conclude that both the Pes-Ves relation and the dP/dtmax-Ved relation are nonlinear, saturating at higher volumes. With increased CS, the nonlinearity of both relations increases. This CS-dependent curvilinearity of the Pes-Ves relation is due to both CS-dependent nonlinearity of the sigma es-epsilon es relation (consistent with length-dependent activation) and geometric factors that alter the relation between Pes and sigma es.