Ejection has both positive and negative effects on left ventricular isovolumic relaxation

In isovolumically beating hearts, the speed of left ventricular (LV) relaxation is uniquely determined by peak active stress (ςmax). In contrast, such a succinct description of relaxation is lacking for the ejection beats, although ejection is generally thought to hasten relaxation. We set out to determine how ejection modifies the relaxation-ςmax relationship obtained in the isovolumically beating hearts. Experiments were performed on five isolated rabbit hearts subjected to various loading conditions. Instantaneous LV pressure and volume were recorded and converted to active stress, from which isovolumic relaxation time (T r) was defined as the time for stress to fall from 75 to 25% of ςmax (isovolumic beats) or its end-ejection value (ejection beats). Steady-state and transient isovolumic beat and steady-state ejection beat data were used to develop a multiple regression model. This model identified stress, current beat ejection, and previous beat ejection history as independent predictor variables ofT...