Imperial College London
A narrow complex tachycardia with variable R-R intervals: What is the mechanism?Authors:Vishal Luther MRCP PhDIan Wright BScDavid Lefroy MA FRCPFu Siong Ng MRCP PhDIntuitional affiliation: Imperial College Healthcare NHS Trust & Imperial College LondonCorresponding Author: Dr Fu Siong NgEmail: f.ng@imperial.ac.ukAddress:4th Floor, Imperial Centre for Translational and Experimental Medicine,Imperial College London, Hammersmith Campus,Du Cane Road, London W12 0NN, United Kingdom.Case PresentationA 46-year-old man was referred for an invasive electrophysiological study with a view to ablation, for a history of classic sudden onset-offset palpitations. The patient’s son had recently survived an out of hospital cardiac arrest, and was found to have an accessory pathway (details unknown) at another institute, which was ablated. Our patient’s 12 lead electrocardiogram (ECG) showed sinus rhythm with no evidence of pre-excitation. Echocardiography revealed a structurally normal heart. An electrophysiological study was performed with a quadripolar catheter positioned at the high right atrium (HRA), a steerable decapolar catheter in the coronary sinus and quadripolar catheters along the His bundle and at the right ventricular apex. Baseline atrio-His (AH) and His-ventricular (HV) intervals measured 60ms and 40ms respectively. Programmed atrial extra-stimulus testing revealed decremental AH intervals, before tachycardia was reproducibly induced. Figure 1 shows the tachycardia on a 12-lead ECG. Figure 2 shows the induction of tachycardia with atrial pacing. Based on the findings within the figures, what is the mechanism of the tachycardia?Discussion:The 12 lead ECG demonstrates an irregular narrow complex tachycardia which might suggest a diagnosis of Atrial Fibrillation. However, there are negative P waves in the inferior leads and a 1:1 atrio-ventricular relationship (best appreciated in Lead II). The differential diagnoses include an atrioventricular nodal re-entrant tachycardia (AVNRT), an atrial tachycardia (AT) with low-to-high atrial activation, and an orthodromic atrio-ventricular re-entrant tachycardia (AVRT). The variable R-R intervals might occur with AV nodal Wenkebach behaviour during an atrial tachycardia or alternating slow/fast AV nodal pathway conduction to the ventricle. Importantly, despite changes in the R-R interval, the RP interval appears fixed. This suggests that the ventricle is part of the circuit, which makes orthodromic AVRT most likely.Following a 600ms atrial drive from the high right atrium (HRA), a single 280ms extra-systole initiates the narrow complex tachycardia. Tachycardia begins with prolongation of the AH interval, consistent with antegrade slow pathway conduction. Subsequent atrial activation is eccentric with the earliest retrograde atrial activation seen at HRA, suggesting an orthodromic AVRT utilising a right sided accessory pathway. Right ventricular entrainment revealed a post pacing interval of 102ms, whilst His-synchronous ventricular paced beats delivered during tachycardia were seen to advance atrial activation, both of which are consistent with a diagnosis of orthodromic AVRT.There were two sources of variation in the tachycardia cycle length. Firstly, because of dual atrioventricular nodal pathways, variations in AH intervals could been seen during tachycardia, due to conduction via the fast or slow AV nodal pathway (the 4th and 5th beats of tachycardia respectively), resulting in variations in RR intervals. Secondly, with intermittent right bundle branch block (6th beat of tachycardia), i.e. bundle branch block ipsilateral to the accessory pathway, there was prolongation of the VA time as ventricular activation must travel a further distance down the left bundle and across the ventricular septum before reaching the accessory pathway, which also prolongs the R-R interval. The alternating anterograde conduction via the fast and slow AV nodal pathways and the intermittent RBBB led to the variable R-R intervals seen during the tachycardia.The location of the accessory pathway was mapped at 9-oclock in the tricuspid annulus, with a balanced and fused atrio-ventricular signal seen on the ablation catheter. Radiofrequency ablation resulted in accessory pathway block within 6.00 seconds.Figure 1: The 12-lead ECG of the tachycardia induced during the EP study.Figure 2: The induction of tachycardia with atrial pacingFigure 2 (annotated): The first 6 QRS beats following an atrial drive and early extra-stimulus are labelled. The His bundle EGM is highlighted throughout the tracing. A dashed black line highlights the earliest retrograde atrial activation in HRA following the first QRS, via a right sided accessory pathway. Changing AH intervals down the fast pathway (FP; 99ms; QRS 4) and the slow pathway (SP; 174ms; QRS 5), as well changing VA intervals (159ms with narrow QRS; 216ms in the presence of RBBB (QRS 6)) are shown. The alternating anterograde conduction via the fast and slow AV nodal pathways and the intermittent RBBB led to the variable R-R intervals seen during the tachycardia. ................
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