The clinical effects of active pacing were distinctly different between patients in the long QRS group (Table 3) and those in the short QRS group (Table 4). With active pacing, the long QRS group exhibited significant improvements over inactive pacing in peak Vo2 (2.46 ml/min/kg) (Fig. 2B), Vo2 at the anaerobic threshold (1.55 ml/min/kg), distance walked in 6 min (47 m), and the quality-of-life score (8.1 points), whereas the short QRS group did not have an improvement in peak Vo2 (Fig. 2C) nor any other end point measure. Considering individual results, 71% of patients in the long QRS group and 38% of patients in the short QRS group had increased peak Vo2 by >1 ml/min/kg with active pacing.