Introduction: QRS width is commonly used as a maker of cardiac dyssynchrony and multisite pacing is employed for resynchronization therapy, however a substantial proportion of patients in functional class III-IV do not experience any clinical benefit and QRS duration is not correlated with mechanical improvement.

Hypothesis: We hypothesized that imaging with radionuclide ventriculography could help identify patients with dyssynchrony despite a "narrow" QRS, and assess the beneficial effect of carvedilol on contractile synchrony and mechanical function.

Methods: We measured QRS duration, LVEF, intra- and interventricular conduction delay by means of phase analysis in 164 patients of the CHRISTMAS Study, who underwent radionuclide ventriculography (RNVG) at the time of randomization and at the end of the maintenance phase of treatment with carvedilol or placebo (PL).

Results: QRS duration at baseline and at final examination was 107 ± 28 and 107 ± 27 ms, respectively, it was not different in the PL vs carvedilol group both at baseline (110±30 ms vs 104±5 ms) and at final examination (113±28 vs 107±26). The degree of interventricular dyssynchrony (difference in LV and RV mean phase angles) did not change from baseline to the final visit in the PL group (11 ±17 vs 12 ±9), while it improved in the carvedilol group (13 ±17 vs 9 ±14; p<0.01). The degree of intraventricular dyssynchrony (std deviation of the mean phase angle) significantly improved for LV in the carvedilol group (54 ±18 vs 51 ±19; p=0.04), but did not change for both the RV (41 ±12 vs 41 ±11) and the LV (57 ±14 vs 56 ±19) in the PL group, and for the RV in the carvedilol group (40 ±14 vs 30 ±10). No correlation existed between QRS width and interventricular dyssynchrony, both at baseline (R = 0.05, p=0.2) and at final exam (R = 0.16, p=0.01). LVEF (%) increased from 29 ±10 to 31 ±11 between basal and final examination, an improvement in EF (increase >= 5 units) was observed in 78% (40/51) of the patients who improved LV intraventricular synchrony.

Conclusion: Treatment with carvedilol in patients with ischemic cardiomyopathy improves both inter- and intra-ventricular synchrony, and mechanical function of the LV, independetly of QRS duration. Improvement in synchrony with carvedilol was not associated with a shortening of QRS.