Mechanical determinants of left ventricular relaxation in isovolumically beating hearts

Both pressure and volume have been proposed to determine the speed of left ventricular (LV) relaxation, but their relative importance is not known. Accordingly, we used isolated, buffer-perfused, isovolumically beating ferret hearts to study the effects of maximal developed pressure (Pdmax) and LV volume (V) on the speed of LV relaxation. Experiments were performed at 30 degrees C, and the hearts were paced at a baseline interbeat interval (BI) of 800 ms. Pdmax was varied independently of V by use of seven BI (75 to 133% of baseline BI), which resulted in test beats that developed a range of Pdmax due to varying degrees of restitution. Pdmax was also varied by setting V at five levels (80 to 120% of baseline V) during the test beats. Speed of relaxation was quantified as the time period of pressure decay from 75 to 25% Pdmax (T75-25). Data were analyzed by multiple linear regression. Increases in both Pdmax and V independently prolonged T75-25, and T75-25 was 1.45 times more sensitive to Pdmax than to V. However, when Pdmax and V were combined to estimate maximal wall stress (sigma max), the effects of Pdmax and V, as well as relative circumferential muscle length (estimated by V1/3), were not important determinants, and T75-25 depended on sigma max alone. Thus we conclude that 1) Pdmax and V are both determinants of the speed of LV relaxation and that Pdmax is approximately 1.5 times more important than V, and 2) the effects of Pdmax and V on relaxation act via the common mechanism of sigma max.