"Pseudo-endless loop" tachycardia in an AV universal (DDD) pacemaker.

A 3-beat episode of ventricular arrhythmia was recorded in a patient with an AV universal (DDD) pacemaker. This arrhythmia mimicked "endless loop" tachycardia (ELT) because it appeared following a ventricular premature contraction (VPC), its rate approached the preset upper-rate limit and the regular P waves disappeared during the episode. However, as retrograde conduction was absent ELT could not exist. The mechanism of this arrhythmia was based on a combination of interference by VPC of the regular sequence of QRS complexes produced by P wave and the pacemaker commitment to maintain an upper rate limit by prolongation of the programmed pacemaker AV delay.