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 press ure (P-dmax) and LV volume (V) on the speed of LV relaxation. Experime nts were performed at 30 degrees C, and the hearts were paced at a bas eline interbeat interval (BI) of 800 ms. P-dmax, was varied independen tly of V by use of seven BI (75 to 133% of baseline BI), which resulte d in test beats that developed a range of P-dmax due to varying degree s of restitution. P-dmax was also varied by setting V at five levels ( 80 to 120% of baseline V) during the test beats. Speed of relaxation w as quantified as the time period of pressure decay from 75 to 25% P-dm ax, (T-75-25) Data were analyzed by multiple linear regression. Increa ses in both P-dmax, and V independently prolonged T-75-25, and T-75-25 was 1.45 times more sensitive to P-dmax than to V. However, when P-dm ax and V were combined to estimate maximal wall stress (sigma(max)), t he effects of P-dmax and V, as well as relative circumferential muscle length (estimated by V-1/3), were not important determinants, and T-7 5-25 depended on sigma(max) alone. Thus we conclude that 1) P-dmax and V are both determinants of the speed of LV relaxation and that P-dmax is similar to 1.5 times more important than V, and 2) the effects of P-dmax and V on relaxation act via the common mechanism of sigma(max).