Study design
The study was performed at nine investigational centers, following local ethics committee approval. Enrollment began September 1998 and ended January 2001. The study was designed as a six-month, randomized crossover comparing three months of atrial-synchronized univentricular (active) pacing with three months of ventricular-inhibited (inactive) pacing at a lower rate of 40 beats/min, with the two pacing periods occurring in random order for each patient (Fig. 1). After a baseline evaluation, patients were stratified prospectively into two groups according to their QRS duration, as assessed by 12-lead surface electrocardiography and measured by the electrocardiographic core center. Patients with QRS >150 ms were assigned to the long QRS group, and those with QRS between 120 and 150 ms to the short QRS group. Then patients underwent an invasive procedure to evaluate their acute hemodynamic response to pacing and to select the best site for permanent ventricular pacing. The device was implanted in a second procedure, and proper device operation subsequently was verified within five days. Then patients were randomly assigned to a treatment sequence (active pacing first or inactive pacing first) following randomization in blocks of size 8, which was separate for each QRS group. Randomization was followed by the two crossover periods during which patients were switched between active and inactive pacing while blinded as to treatment. After the crossover periods, devices were programmed to active pacing in all patients, who were followed until one year after implantation. Only the six-month crossover study results are reported here.