Permanent His-Bundle pacing in a patient with advanced conduction system disease: What is the mechanism of QRS narrowing?

QRS narrowing during permanent His-bundle pacing is primarily thought to result from longitudinal dissociation within the His bundle. We present a case with an alternative mechanism, highlighting the likelihood that there are actually multiple explanations for this phenomenon. In addition, this case highlights the utility of His-bundle pacing even in the face of a wide QRS.