No abstract

A 6-year-old female with congenital long QT syndrome (LQTS) presented to the hospital with recurrent syncope. She had frequent episodes of Torsade de pointes (TdP) that occurred at night during the hospital stay. The Figure shows interesting recordings from her telemetry monitor. In panel A, on first glance, the patient seemed to be in sinus bradycardia with a heart rate (HR) of 48 beats per minute (bpm) with abnormally long QT interval of 840 ms and bifid T waves. After a closer look, it became clear that the patient was actually in 2:1 AV block with P wave marching out regularly at a rate of 96 bpm, and ventricular rate was 48 bpm. When every other P wave arrived at the His-Purkinje system or ventricular myocytes, most likely they were still repolarizing and hence refractory, so the electrical impulse failed to propagate from the atria to the ventricles. When QT interval abbreviated to 680 ms due to spontaneous variation, refractory period of ventricular tissue shortened, thereby impulse was able to conduct normally (Panel B). With a HR of 125 bpm and rate-dependent shortening in the QT interval (Panel C), the T waves completely engulfed the P waves. The location of 2:1 AV block in this case is unlikely to be at the AV node, but it is rather at the His-Purkinje system or ventricular myocytes. Moreover, block is unlikely to be Figure.related to a disease of conduction system but is simply due to physiological block related to prolonged refractoriness as evident in panel B, where AV block disappears with abbreviation of QT interval. The 2:1 AV block related to conduction system abnormality has been reported in some infants with congenital LQTS, and many of these infants required permanent pacemaker implantation. 1 Our patient received an implantable cardioverter-defibrillator that was also programmed to prevent bradycardia. Reference