Size Matters: Normalization of QRS Duration to Left Ventricular Dimension Improves Prediction of Long-Term Cardiac Resynchronization Therapy Outcome.

Background: In patients with left bundle branch block (LBBB), QRS duration (QRSd) depends on left ventricular (LV) dimension. Previously, we demonstrated that normalizing QRSd to LV dimension, to adjust for variations in LV size, improved prediction of hemodynamic response to cardiac resynchronization therapy (CRT). In addition, sex-specific differences in CRT outcome have been attributed to normalized QRSd. The present study evaluates the effect of normalization of QRSd to LV dimension on prediction of survival after CRT implantation.

Methods: In this 2-center study, we studied 250 heart failure patients with LV ejection fraction ≤35% and QRSd ≥120 ms who underwent cardiac magnetic resonance imaging before CRT implantation. LV end-diastolic volumes were used for QRSd normalization (ie, QRSd/LV end-diastolic volumes). The primary end point was a combined end point of death, LV assist device, or heart transplantation.

Results: During a median follow-up of 3.9 years, 79 (32%) patients reached the primary end point. Using univariable Cox regression, unadjusted QRSd was unrelated to CRT outcome ( P=0.116). In contrast, normalized QRSd was a strong predictor of survival (hazard ratio, 0.81 per 0.1 ms/mL; P=0.008). Women demonstrated higher normalized QRSd than men (0.62±0.17 versus 0.55±0.17 ms/mL; P=0.003) and showed better survival after CRT (hazard ratio, 0.52; P=0.018). A multivariable prognostic model included normalized QRSd together with age, atrial fibrillation, renal function, and heart failure cause, whereas sex, diabetes mellitus, strict left bundle branch block morphology, and LV end-diastolic volumes were expelled from the model.

Conclusions: Normalization of QRSd to LV dimension improves prediction of survival after CRT implantation. In addition, sex-specific differences in CRT outcome might be attributed to the higher QRSd/LV end-diastolic volumes ratio that was found in selected women, indicating more conduction delay.