Two-to-one AV block associated with the congenital long QT syndrome

Introduction: Conduction abnormalities associated with long QT syndrome (LQ TS) have been reported as "pseudo 2:1 AV block" due to sinus intervals shor ter than ventricular refractoriness. Method and Results: We report the electrophysiologic characteristics of a p atient suffering from congenital LQTS with episodes of true 2:1 AV block. I nduction of 2:1 infra-Hisian blocks and return to 1:1 conduction were obser ved using single atrial and ventricular extrastimuli. The block was located in the Purkinje network but not in the myocardium. Conclusion: The His-Purkinje system of our LQTS patient displayed dynamic p roperties with a strong increase in refractoriness for short-long sequences and a decrease for long-short sequences that triggered intermittent 2:1 AV blocks.