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| [[Puzzle_2006_11_393_Answer|Answer]] | | [[Puzzle_2006_11_393_Answer|Answer]] |
| [[Image:Puzzle_2006_11_393_fig2.jpg|Figure 3. Detail from the aVR, aVL and aVF leads during the tachycardia. The arrows point to the (retrograde) P top.|thumb]]
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| Figure 1 shows a narrow-complex tachycardia with an
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| RR interval of 390 msec (rate 155 beats/min). In the
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| latter part the arrhythmia is slightly slower (RR interval
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| 410). In the differential diagnosis of any narrowcomplex
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| tachycardia, the position of the P wave (and
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| its number) is critical for the correct diagnosis.
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|
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| A close look for the P waves reveals a P wave in
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| every other T wave in the left part of the ECG. After
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| the 15th QRS complex, the P wave is present in every
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| T wave (see rhythm strip lead II at the bottom of the
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| ECG and figure 3, arrows). Comparison of the T waves
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| with and without superimposed P waves reveals a
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| negative morphology of the P wave in lead II. In lead
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| aVR the morphology is clearly positive. The timing of
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| the P wave is between 40 and 80 ms after the QRS
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| complex.
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| The number of P waves indicates a dissociation
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| between the atrial and ventricular rate and the morphology
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| indicates retrograde (VA) conduction. The
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| presence of VA dissociation is only compatible with
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| AV-nodal reentrant tachycardia and it excludes AVreentry
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| tachycardia (orthodromic circus movement
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| tachycardia), atrial tachycardia and atrial flutter, the
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| most important differential diagnostic entities for a
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| narrow-complex tachycardia with the present rate. A
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| rare alternative diagnosis would be a His-bundle tachycardia
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| with 2:1 VA conduction. Actually the position
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| of the P wave would be in favour of this diagnosis.
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| An invasive electrophysiological study revealed the
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| presence of dual nodal characteristics and, under
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| isoprenaline, the induction of an AV-nodal reentrant
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| tachycardia. Slow pathway ablation successfully
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| modified the arrhythmogenic substrate.
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