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).