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Introduction: QRS duration in patients with left bundle branch block (LBBB) predicts adverse clinical outcomes and response to cardiac resynchronization therapy (CRT). Changes in LBBB QRS duration over time have not been reported. Hypothesis: QRS duration in patients with LBBB changes over time and may be predicted by patient demographics and comorbidities. Methods: We retrospectively identified 450 patients with LBBB on an index ECG (QRS ≥120 msec) recorded between 1992 and 2000 and examined any follow-up ECGs recorded after approximately 1, 2, 3, 5, and 10 years. Piecewise linear interpolations were performed, and separate generalized linear models (GLM) with Schwartz Bayesian Criterion effect selection were used to relate baseline clinical variables to changes in QRS duration at follow-up. Results: The cohort was 53.1% male, average age 66.7 yr, and mean index ECG QRS duration 144.7 msec. Although there was significant variability, the duration of the mean QRS increased as a tapering exponential function of time (p<0.0001) with progressively less increase at later follow-up. Three additional variables strongly and independently predicted progressive lengthening of the QRS within the overall GLM: heart failure with reduced ejection fraction (HFrEF) (p<0.0001), index ECG QRS duration (neg) (p<0.0001), and male gender (p=0.0063). In a subgroup of patients with an index ECG QRS <150 msec, these four GLM model variables also predicted the increase in QRS: time (p=0.0129), index QRS duration (p=0.0042), HFrEF (p=0.0001), and male gender (p=0.0084). (Equation, Figure below) ΔQRS(t;i) = 50.22 - 9.71*e -0.29*t - 0.344*QRS(0;i) + 7.62* 1 HFrEF (i) +5.12* 1 Male (i) Adjusted R 2 = 0.103 Conclusions: QRS duration progressively prolongs with time in patients with LBBB especially in patients with relatively narrow QRS and can be predicted by the presence of HFrEF and male gender. These predictors may be useful in better informing clinical prognosis and decisions regarding CRT.

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