Change in the H–V Interval Without a Change in QRS Morphology During Atrial Pacing in a Case of Wolff–Parkinson–White Syndrome—What is the Mechanism?

In the background of an accessory pathway (AP), the H–V interval can vary during atrial/coronary sinus pacing, but only with a concomitant change in the QRS morphology and the degree of pre-excitation. In an interesting case of a 62-year-old woman, the H–V interval varied during coronary sinus pacing despite a fixed pre-excitation. This appears to have happened due to infra-Hisian complete atrioventricular dissociation, which resulted from iatrogenic mechanical bumping of the left anterior fascicle in the background of right bundle branch block and left posterior hemiblock.