Influence of left ventricular regional nonuniformity on afterload-dependent relaxation in intact dogs.

The time course of systolic loading conditions is considered an important factor in affecting left ventricular (LV) relaxation. However, the exact underlying mechanisms remain controversial. The goal of this study was to evaluate the role of the change in loading sequence on LV regional wall motion and LV relaxation. Early and late systolic loading were produced by constricting the ascending aorta and the descending aorta, respectively, in seven anesthetized dogs. In two pairs of cardiac beats with a comparable peak LV pressure during the mild and moderate constrictions of the ascending or descending aorta, we measured LV time constant (T) of LV pressure decay and determined regional nonuniformity in basal and apical segmental motion during the isovolumic relaxation period (by Lew's method). Constriction of the descending aorta led to a significant delay of the timing of peak LV pressure, associated with an increase in T, end-systolic pressure, and the regional nonuniformity between the two segments. T was correlated well with the amount of the nonuniformity, suggesting that LV regional nonuniformity between basal and apical regions might be involved in the complex mechanism of afterload-dependent relaxation.