Pulsed Doppler echocardiography was used to study the timing and dynamics of left ventricular filling in 14 patients with permanent dual-chamber programmable pacemakers. Pacemakers were programmed to atrial sensed (VDD) mode and atrial-ventricular sequential paced mode at low (DVI-L) and high (DVI-H) heart rates, and pulsed Doppler recordings of transmitral flow were analyzed at atrioventricular delays of 50 to 300 msec in each mode. There was a significant decrease in the one-third filling fraction in both VDD and DVI-L modes and a significant increase in DVI-H modes with increasing atrioventricular delay. The ratio of early filling area to atrial filling area was significantly lower at longer atrioventricular delays in both VDD and DVI-L modes. The time from pacemaker spike to mitral valve closure was highly significantly correlated with atrioventricular delay in VDD, DVI-L, and DVI-H modes (r = -0.92, p = 0.0001; r = -0.90, p = 0.0001; and r = -0.85, p = 0.0001, respectively) as was the diastolic filling time to a lesser extent (r = -0.73, p = 0.0001; r = -0.69, p = 0.0001; r = -0.61, p = 0.0001, respectively). Events reflecting atrial systole occurred at a later time in the cardiac cycle in the atrial paced vs the atrial sensed mode. Thus changes in atrioventricular delay and pacemaker mode in this group of patients are a strong determinant of the timing and dynamics of left ventricular filling.