Background: Cardiac resynchronization therapy (CRT) is effective in many symptomatic patients with wide QRS complexes. However, whether CRT is effective in patients with narrow QRS complexes and dyssynchrony is unknown. We evaluated the efficacy of CRT in patients with narrow QRS complexes who were evaluated using real time three-dimensional echocardiography (RT3DE) before the implant. Method and Results: In a total of 18 out of 34 patients, we evaluated the mechanical dyssynchrony using 2D echocardiography and RT3DE. LV dyssynchrony was defined as a standard deviation of the time to reach the minimum systolic volume in 16 LV segments, expressed as the percentage of the cardiac cycle (systolic dyssynchrony index, SDI). A positive response to CRT was defined as a persistent reduction in the NYHA class during the long-term follow-up. An SDI score in the upper 10% was positive. Fourteen of 18 patients (78%) were responders. Six of 14 patients had narrow QRS complexes (QRS duration &lt
130ms). There was no significant difference in the SDI before the CRT implantation between the patients with narrow or wide QRS complexes (SDI: narrow 11.9±4.1% vs. wide 8.9±3.8%, p=0.09). Conclusion: The evaluation of the dyssynchrony using RT3DE might be useful to decide indication for CRT in patients with a narrow QRS complex. © 2011, Japanese Heart Rhythm Society. All rights reserved.