Endless loop tachycardia among patients with devices having advanced preventive algorithms: A case series and brief review.

BACKGROUND: Endless loop tachycardia (ELT) is the commonest pacemaker mediated tachycardia (PMT) encountered among patients with cardiac implantable electronic devices (CIEDs). Despite being enabled with various preventive algorithms, we encountered several cases having recurrent, long, and symptomatic ELT. MATERIALS AND METHODS: We retrospectively analyzed consecutive device checkups at device clinic at a single center of eastern India between January 2019 to March 2023. RESULTS: There were 20 cases of confirmed and sustained ELT among 4520 device checks. Although mostly benign, in two cases ELT led to clinical worsening in patients having left ventricular (LV) systolic dysfunction. Even with good ventricular function, ELT resulted in improper atrioventricular (AV) synchrony leading to disabling symptom in one case. The differentiation of ELT from sinus tachycardia and atrial tachycardia (AT) was not always easy. Magnet application is certainly useful to differentiate. The situations that provoked ELT in this study were-long AV delays, VIP (ventricular intrinsic preferences)/MVP (managed ventricular pacing), atrial non-capture, atrial under/over sensing, premature ventricular contractions (PVCs)/couplets, premature atrial contractions (PAC) and slower ventriculo-atrial (VA) conduction. Rate responsive shortening of post-ventricular atrial refractory period (PVARP) also promoted its occurrence and hindered troubleshooting. When ELT occurred despite post-PVC extension of PVARP, lowering the atrial sensitivity, switching to bipolar sensing and manual setting of longer PVARP after measuring VA conduction time were useful. "Rate responsive PVARP" had to be turned off in a few cases to prevent ELT. On the contrary, an over aggressive prolongation of PVARP led to repetitive non-reentrant ventriculo-atrial synchrony (RNRVAS) in two cases. Checking VA conduction during implantation and noninvasive program stimulation (NIPS) during follow up were useful to check the tendency for ELT. CONCLUSION: Clinically significant ELT is rare but not uncommon among devices having in-built preventive algorithms. Manual adjustments are often useful to troubleshoot the same.

"On-Off" phenomenon in a case of concealed left lateral atrio-ventricular accessory pathway - What is the mechanism?

We report a case of symptomatic supraventricular tachycardia who had absent VA conduction during electrophysiology study. The interesting finding was appearance of VA conduction only at a specific cycle length of ventricular pacing which reproducibly induced a sustained orthodromic re-entrant tachycardia (ORT). We review the literature and conclude that supernormal AP conduction can explain such phenomenon.

Absent ventriculo-atrial conduction during right ventricular apical pacing but nodal response during para-Hisian pacing - What is the mechanism?

A 13-year-old-girl presented with one episode of pre-syncope while standing in a train. Her ECG was suggestive of preexcitation. Echocardiography revealed structurally normal heart without any ventricular hypertrophy. During electrophysiology study, her ventriculo-atrial (VA) conduction was absent even on isoprenaline. However, a para-Hisian pacing maneuver (PHP) revealed consistent VA conduction with a nodal response. This finding indicated that the VA dissociation at baseline was at infra-Hisian (VH) level and conduction at HA level was intact. In addition, this finding is coherent with a speculation of a fasciculo-ventricular pathway (FVP) resulting in such an ECG pattern in her. Pacing from various atrial sites (right atrium, coronary sinus) exhibited nearly fixed preexcitation and short non-varying HV interval confirmatory of FVP. Testing for a PRKAG mutation was advised for her.

Retrograde ventriculoatrial jump-What is the mechanism?

During ventricular extra stimuli(VES) protocol a VA jump was noted. In upper panel with VES @ 500/270 ms (Fig A) the His signal appeared after the A-EGM, ruling out pure nodal VA conduction and confirming presence of an accessory pathway (AP). The H signal was delayed due to probable retrograde RBBB. The atrium was already depolarised via AP and the VA traversing via left bundle-His-node got blocked with a H signal. In the lower panel with VES of 500/260 ms the retrograde AP conduction reached the effective refractory period and pure nodal VA conduction took place along with retrograde RBBB. The retrograde VA jump was hence due to a jump from AP to AV node.