Introduction
Endless loop tachycardia (ELT) 1,2 and repetitive nonreentrant VA synchrony (RNRVAS) 1,2 are the two types of pacemaker-mediated tachycardias that are seen in patients with retrograde ventriculoatrial (VA) conduction. Thanks to ever improving technology and algorithms associated with the implantable cardiac electronic devices (CIED), it is possible to prevent or stop ELT, which is a well-studied and characterized condition. The data on RNRVAS is limited due to its lower occurrence rates however studies suggest that RNRVAS may trigger atrial fibrillation 3,6,8,9.
Atrial fibrillation, atrial tachycardia and related atrial rhythms recorded by CIEDs are classified as ‘atrial high-rate events’ (AHRE) and increased rate of strokes were shown to correlate with the presence of AHRE during CIED controls3-5. However, some studies have shown that RNRVAS incidences are misinterpreted as AHRE attacks by the CIED which leads to the under diagnosis of RNRVAS occurrences.
Data on the factors that trigger RNRVAS are limited. The only known significant trigger until now is ventricular ectopy1,2,8,10. The underlying mechanisms of RNRVAS and ELT are similar, therefore we think that the triggers may be similar too and RNRVAS may actually be more frequent and more significant than it is reported. Atrial premature beats and atrial capture problems are known to trigger ELT. We hypothesized that under certain predisposing conditions loss of atrial capture during atrial threshold test, as well as increased atrial threshold or an atrial premature beat causing the atrial pace stimulus to fall in the myocardial refractory period may result in VA conduction, triggering RNRVAS as a result. In order to test this hypothesis, we planned to do atrial threshold test in DDD mode and evaluate if RNRVAS develops upon loss of capture in patients with DDD pacemakers, DDD implantable Cardioverter-defibrillators (ICD) and cardiac resynchronization therapies (CRT).