Right ventricular pacing reduces the rate of left ventricular relaxation and filling

Right ventricular pacing alters left ventricular synchrony and loading conditions, each of which may independently influence left ventricular relaxation. Addition of a properly timed atrial contraction by using sequential atrioventricular (AV) pacing minimizes changes in left ventricular loading conditions, but ventricular asynchrony persists. To separate the effects of altered loading from those of asynchrony, the effects of right ventricular pacing and sequential AV pacing on the rate of isovolumic pressure decline (relaxation time constant), myocardial (segment) lengthening rate and chamber (minor axis dimension) filling rate were examined. In 12 open chest anesthetized dogs, left ventricular pressure (micromanometer) and either left ventricular free wall segment length transients (n = 6) or minor axis dimension transients (n = 6) were measured during right atrial, right ventricular and sequential AV pacing; length and dimension were measured using ultrasonic crystals.Compared with right atrial pacing, right ventricular pacing produced a decrease in systolic pressure, a reduction in fractional shortening, a prolongation of the relaxation time constant (23.5 ± 0.7 to 29.8 ± 0.8 ms, p < 0.05), slower peak segment lengthening rate (6.2 ± 0.6 to 4.6 ± 0.8 s−1, p < 0.05) and a slower rate of increase in chamber dimension (3.5 ± 0.1 to 2.7 ± 0.1 s p < 0.05). In contrast, systolic pressure and fractional shortening did not change during sequential AV pacing; however, the relaxation time constant remained prolonged (23.5 ± 0.7 to 30.0 ± 0.8 ms, p < 0.05) and peak lengthening and filling rates (5.9 ± 0.6 versus 5.3 ± 0.8, p < 0.05 and 3.5 ± 0.1 versus 3.0 ± 0.1 s−1, p < 0.05, respectively) showed small but statistically significant declines.Thus, pacing-induced left ventricular asynchrony caused a decrease in the global and regional indexes of left ventricular relaxation rate that is not closely related to left ventricular loading conditions or shortening