Effects of arterial load changes on systolic pump functions: internal resistance or inotropic factor?

The influence of different loading interventions on the end-systolic pressure-volume relation (ESPVR) of the left ventricle was investigated in anesthetized dogs. A pressure intervention (INp) by aortic occlusion and a volume intervention (INv) by venous infusion were applied. The mean slope, Ees, of the ESPVR of the INp was twice as large (1.37±0.45 kPa/ml) as Ees (INv) (0.73±0.32 kPa/ml). This difference was found also in a model study with an internal resistance in series with time varying elastance. An afterload dependence similar to that of Ees emerged for the slope of the dP/dt_max vs. EDV relation. The results can be explained in part by shortening or flow deactivation. But because dP/dt_max behaves like Ees, it is suggested that INp leads to an increased inotropism w.r.t. INv