Puzzle 2006 11 393 Answer

Author(s)	A.A.M. Wilde, L.R.C. Dekker
NHJ edition:	2006:11,393
These Rhythm Puzzles have been published in the Netherlands Heart Journal and are reproduced here under the prevailing creative commons license with permission from the publisher, Bohn Stafleu Van Loghum.
The ECG can be enlarged twice by clicking on the image and it's first enlargement

An otherwise healthy 57-year-old lady presented with palpitations without dizziness. The symptoms had been present for a couple of years but the number of episodes had increased recently. Onset and termination were always sudden without specific triggers. Physical examination on admission revealed no specific abnormalities except for a rapid pulse (150 beats/min). Blood pressure was normal. The ECG is shown in figure 1 and a few minutes later figure 2 was recorded without any specific intervention.

Figure 1. The ECG on admission

Figure 2. The ECG shortly thereafter

What is your diagnosis?

Answer

Figure 3. Detail from the aVR, aVL and aVF leads during the tachycardia. The arrows point to the (retrograde) P top.

Figure 1 shows a narrow-complex tachycardia with an RR interval of 390 msec (rate 155 beats/min). In the latter part the arrhythmia is slightly slower (RR interval 410). In the differential diagnosis of any narrowcomplex tachycardia, the position of the P wave (and its number) is critical for the correct diagnosis.

A close look for the P waves reveals a P wave in every other T wave in the left part of the ECG. After the 15th QRS complex, the P wave is present in every T wave (see rhythm strip lead II at the bottom of the ECG and figure 3, arrows). Comparison of the T waves with and without superimposed P waves reveals a negative morphology of the P wave in lead II. In lead aVR the morphology is clearly positive. The timing of the P wave is between 40 and 80 ms after the QRS complex.

The number of P waves indicates a dissociation between the atrial and ventricular rate and the morphology indicates retrograde (VA) conduction. The presence of VA dissociation is only compatible with AV-nodal reentrant tachycardia and it excludes AVreentry tachycardia (orthodromic circus movement tachycardia), atrial tachycardia and atrial flutter, the most important differential diagnostic entities for a narrow-complex tachycardia with the present rate. A rare alternative diagnosis would be a His-bundle tachycardia with 2:1 VA conduction. Actually the position of the P wave would be in favour of this diagnosis.

An invasive electrophysiological study revealed the presence of dual nodal characteristics and, under isoprenaline, the induction of an AV-nodal reentrant tachycardia. Slow pathway ablation successfully modified the arrhythmogenic substrate.