Electrocardiographic Imaging of Heart Failure Patients with Left Bundle Branch Block: Effects of Right Ventricular Pacing and Cardiac Resynchronization Therapy

Understanding of the electrical activation of the heart during pathological conditions and in response to pacing is limited.We used electrocardiographic imaging (ECGI), which maps epicardial ventricular excitation noninvasively, to evaluate intrinsic conduction during left bundle branch block (LBBB) in eight patients with left ventricular (LV) dysfunction and their responses to right ventricular (RV) pacing and cardiac resynchronization therapy (CRT). During intrinsic conduction with LBBB, LV activation was heterogeneous and unpredictable, with regions of delayed and/or absent conduction. Sites of terminal LV activation varied. The anterior LV was susceptible to block and slow conduction. RV pacing or CRT sometimes created regions of slow LV conduction, indicating functional electrical characteristics of local tissue. RV pacing prolonged RV activation. Interventricular electrical synchrony usually improved with CRT, especially with lateral LV lead positions, but this did not correlate with changes in QRS duration. We conclude that ECGI provided unique insight into ventricular activation during LV dysfunction. Complex electrophysiologic barriers, sometimes dependent on paced wavefront geometry and its direction of propagation, may determine the outcome of pacing therapies.