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A 30 year old woman with a three year history of intermittent palpitations presented to the emergency department following a typical episode. She was known to have β thalassaemia minor and took no regular medication. A 12 lead ECG on presentation demonstrated sinus rhythm with a PR interval of 112 ms but without evidence of ventricular pre-excitation. During examination, syncope occurred and transient asystole was observed on a bedside monitor. Subsequent ambulatory ECG monitoring demonstrated a single episode of asymptomatic complete atrioventricular block of 10 seconds duration occurring in the late afternoon, and preceded by Wenckebach PR interval prolongation (fig 1). Accordingly, a pacemaker (Medtronic Minuet 7108, Medtronic Ltd, Watford, UK) was implanted and programmed to DDD mode.
24 hour ambulatory ECG demonstrating onset of complete atrioventricular block with preceding Wenckebach PR interval prolongation.
Palpitations continued following discharge, and during clinic follow up a non-paced regular narrow complex tachycardia with a cycle length of 260 ms occurred; this was terminated with intravenous adenosine. Her symptoms continued despite both oral verapamil and flecainide. An electrophysiological study was subsequently performed and a typical atrioventricular nodal reentrant tachycardia (AVNRT) with a cycle length of 328 ms was easily initiated following extrastimulus pacing (fig 2). Successful radiofrequency ablation of the slow pathway was performed, and AVNRT remained non-inducible following atropine and isoprenaline provocation. The patient remained in sinus rhythm following the procedure with a PR interval of 129 ms. Antiarrhythmic treatment was discontinued and at 18 months’ follow up she was asymptomatic.
Electrophysiological study indicating typical AVNRT following extrastimulus pacing. MAP, mapping electrode; HRA, high right atrium; HBp, proximal His bundle; HBd, distal His bundle; CSp, proximal coronary sinus; CSd, distal coronary sinus.
Discussion
Tritto and Calabrese recently described a young woman with paroxysmal first and second degree atrioventricular block, and AVNRT inducible at transoesophageal electrophysiological study.1 To our knowledge, however, this appears to be the first report of intermittent complete atrioventricular block during sinus rhythm associated with AVNRT. Episodes of atrioventricular block were documented during waking hours, suggesting that high vagal tone was unlikely to account for these episodes, and the patient was taking no concomitant medication. This case illustrates that although a patient may have impaired atrioventricular conduction, they may still be able to sustain AVNRT.