Supernormal phase of conduction

A 93-year-old man with previously known left bundle branch block (LBBB) underwent transcatheter aortic valve replacement for severe aortic stenosis. Two days later, during sinus rhythm at cycle lengths ranging from 940-980ms with LBBB, an atrial premature beat P4 (P3P4 = 680ms) is shown to also conduct with an LBBB. However, the following atrial beat (P5), although also premature (P4P5 = 560ms) is unexpectedly conducting with a QRS complex (B) different from the previously conducted LBBB beat (A). QRS complex B has a morphology of right bundle branch block (RBBB), with a left axis deviation compatible with a left anterior fascicular block (LAFB). In addition, a prominent R wave is observed in V2 suggesting the presence of an additional left septal fascicular block (LSFB). 1 Pérez-Riera A.R. Barbosa-Barros R. Andreou A.Y. et al. Left septal fascicular block: Evidence, causes, and diagnostic criteria. Heart Rhythm. 2023; 20: 1558-1569 Google Scholar The last 4 QRS complexes have a LBBB morphology including a junctional escape rhythm for the last 3 beats. Considering that QRS complex B originates from P5, we postulate that this conduction occurs during the supernormal phase of conduction 2 Elizari M.V. Schmidberg J. Atienza A. Paredes D.V. Chiale P.A. Clinical and experimental evidence of supernormal excitability and conduction. Curr Cardiol Rev. 2014; 10: 202-221 Google Scholar in the left posterior fascicle (LPF) whereas conduction is blocked in the RBB, LAF and the LSF. In the clinical setting, supernormality of conduction is defined as conduction that is better than expected. 2 Elizari M.V. Schmidberg J. Atienza A. Paredes D.V. Chiale P.A. Clinical and experimental evidence of supernormal excitability and conduction. Curr Cardiol Rev. 2014; 10: 202-221 Google Scholar In the presence of moderate hypopolarization in an injured fascicle, anodic stimulation generated by ventricular repolarization (T wave) generates a hyperpolarizing current that causes removal of inactivation allowing excitation and conduction of the impulse 3 Hoffman BF & Cranefield PF. Electrophysiology of the heart. New York: McGraw Hill, Chapter 8, pp 211-256. Google Scholar . The presence of junctional escape rhythm on the last 3 complexes of the tracing may suggest an increase in the automaticity of the RBB. Both increased automaticity and supernormal conduction are known to represent electrophysiologic changes related to injured tissues. In the absence of intracardiac recordings of the His-Purkinje system, however, we do recognize that alternative explanations to supernormal conduction, especially different types of gaps as detailed by Elizari et al. 2 Elizari M.V. Schmidberg J. Atienza A. Paredes D.V. Chiale P.A. Clinical and experimental evidence of supernormal excitability and conduction. Curr Cardiol Rev. 2014; 10: 202-221 Google Scholar , cannot be ruled out. Finally, marked changes in ventricular repolarization are observed in the supernormal conducted beat, because of electrotonic T-wave modulation and cardiac memory.