A Rare Cause of 2:1 AV Block: Long QT Syndrome

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

Electrophysiological studies of transgenic long QT type 1 and type 2 rabbits reveal genotype-specific differences in ventricular refractoriness and His conduction

“…However, a few case reports also described true infranodal AV block in these patients (16,31). Despite the relatively infrequent overall occurrence of AV conduction abnormalities (30), they are clearly associated with a poorer prognosis. In neonates, AV block has been associated with an elevated risk of arrhythmia and is more frequent in patients with mutations in HERG, particularly within the pore region (18,27).…”

Calmodulin mutation in long QT syndrome 15 associated with congenital heart defects further complicated by a functional 2:1 atrioventricular block: Management from foetal life to postpartum

“…‘Pseudo’ or functional AVB in LQTS is primarly due to markedly prolonged QTc interval. When every other P wave reached the His-Purkinje system or ventricular myocytes, they were most likely still in repolarisation and thus refractory, so that the electrical impulse could not propagate from the atria to the ventricles [ 17 ]. In 1999, Pruvot et al demonstrated electrophysiological features of a patient with congenital LQTS with episodes of true 2:1 AV block, in which the block was seemingly associated with a general disturbance of electrogenesis that caused prolongation of the refractoriness of muscle and conduction structures.…”

2:1 Atrioventricular Conduction due to Congenital Long QT Syndrome

“…Electrocardiographic (ECG) manifestations of long QT syndrome are prolonged QT more than 470 milliseconds, T wave alternans, polymorphic ventricular tachycardia (VT), and, rarely, 2:1 atrioventricular (AV) conduction. [1][2][3] Case Report A 3-month-old child was referred to us with a history of shortness of breath for cardiac evaluation. She was evaluated with ECG revealing prolonged QT with QTc of 440 milliseconds and 2:1 AV conduction (►Fig.…”