Phase analysis to explore CRT mechanisms: Association of septal dyssynchrony with QRS duration

Objectives: Contemporary Cardiac Resynchronization Therapy (CRT) data indicate that QRS duration and left bundle branch block (LBBB) morphology predict outcome. We have previously shown that patients with LBBB have a septal to lateral wall delay in mechanical activation. However, the imaging correlate of QRS width is unknown.

Methods: We identified unselected patients with ejection fraction (EF) < 35%, on gated SPECT. QRS morphology and duration were determined from pre-SPECT ECG. Segmental phase of contraction and entropy (a measure of phase variability, greater value indicates more dyssynchrony) were determined from gated SPECT using a 17-segment model. Septal to lateral wall delay (SLD) was calculated as the difference between the mean phase of the earliest and latest activated segments. Classification and regression tree analysis (CART) was used to assess patterns of regional dyssynchrony as they relate to QRS duration.

Results: Among 128 patients (age 62.5 + 12 y, LVEF of 27.2 + 6%), SLD showed no correlation with QRS duration in both LBBB and non-LBBB patients (r = 0.2, p = 0.1 and r = 0.15, p = 0.32, respectively). However, CART modeling showed that patients with E > 28.5% in the mid anteroseptal segment (segment 8) had a significantly wider QRS (110ms vs 140ms, p <0.001). In addition, with E > 36.5% in the apical anterior segment (segment 13; mean QRS of 148ms) and E > 30.5% in the basal inferoseptal segments (segment 3; mean QRS of 156ms) was associated with progressively wider QRS complex. The combination of E - 8 > 28.5%, E - 13 > 36.5%, and E - 3 > 30.5% was predictive of QRS > 150ms.

Conclusion: Regional dyssynchrony in the septal wall, but not SLD, is associated with wide QRS duration. In combination with our prior data correlating delayed activation of the lateral wall with LBBB morphology, this finding offers insight into CRT mechanisms. Research Support: This study did not receive any kind of financial assistance.