Time interval from left ventricular stimulation to QRS onset is a novel predictor of nonresponse to cardiac resynchronization therapy.

<bold>Background: </bold>Left ventricular (LV) lead placement at the late activation site (LAS) has been proposed as an optimal LV pacing site (ie, Q-LV interval). However, LAS may be relevant to local electrical conduction, measured as an interval from LV pacing stimulation to QRS onset (S-QRS interval).<bold>Objective: </bold>The purpose of this study was to evaluate the prognostic value of S-QRS for reverse remodeling and the impact of S-QRS on pacing QRS configuration in patients undergoing cardiac resynchronization therapy (CRT).<bold>Methods: </bold>Sixty consecutive heart failure patients with a wide QRS complex underwent CRT. A site with Q-LV ≥95 ms was targeted for LV lead placement. A responder was defined as one with >15% reduction in LV end-systolic volume 6 months after CRT.<bold>Results: </bold>LV lead placement with Q-LV ≥95 ms was achieved in 52 of 60 patients (86.7%). Thirty-two of 52 patients (61.5%) were responders. S-QRS was significantly shorter in responders than nonresponders (P <.01), whereas Q-LV was not significantly different. A cutoff value of 37 ms for S-QRS had sensitivity and specificity of 81% and 90%, respectively. Shorter S-QRS (<37 ms) showed significantly narrower LV pacing QRS width and biventricular pacing QRS width compared to longer S-QRS. After multivariate analysis, PQ interval (odds ratio 0.97; P = .01) and long S-QRS ≥ 37ms (odds ratio 0.014; P <.01) were independent predictors of response to CRT.<bold>Conclusion: </bold>In addition to a sufficient Q-LV, S-QRS can be a useful indicator of optimal LV lead position to achieve reverse remodeling. S-QRS contributes to the pacing QRS configuration associated with CRT response. [ABSTRACT FROM AUTHOR]