Nonuniformity and volume loading independently influence isovolumic relaxation rates

We examined the influence of loading conditions and nonuniformity of left ventricular (LV) function on the rate of LV pressure fall in seven anesthetized dogs. Loading conditions were altered with vena cavae occlusions and/or intravenous infusions of dextran. Nonuniformity was produced by injecting 8-20 ng of isoproterenol into the mild left anterior descending coronary artery to produce an asynchronous and early onset of segment lengthening in the anterior wall. Temporal and regional nonuniformity were quantified with indexes derived by comparing segment lengths in the anterior and posterior walls, measured with midwall sonomicrometers. The rate of LV pressure fall was assessed with peak -dP/dt, the time constant, tau, and the duration of isovolumic relaxation. Volume loading decreased the rate of LV pressure fall without altering the nonuniformity indexes. Intracoronary isoproterenol produced nonuniformity and decreased the rate of LV pressure fall without altering global loading conditions. The effects of isoproterenol (in a constant dose) were similar irrespective of the LV volume. We conclude that in the intact, ejecting left ventricle, loading conditions and nonuniformity are important determinants of the rate of LV pressure fall, but these two factors exert their influence by mechanisms that are largely independent.