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| [[Puzzle_2006_08_268 Answer|Answer]] | | [[Puzzle_2006_08_268 Answer|Answer]] |
| Figure 1 shows a narrow-complex tachycardia (rate
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| 180 beats/min). The QRS complex is 84 msec wide,
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| and has an intermediate axis. A P wave is not easy to
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| discern, but could be present in the mid part of the T
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| wave (most clearly seen in leads II and aVF). The Pwave
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| axis cannot be determined. The differential
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| diagnosis of this tachycardia is orthodromic circus
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| movement tachycardia, AV-nodal re-entry tachycardia
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| or atrial tachycardia. The position of the P wave at
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| some distance from the QRS complex is in favour of
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| an orthodromic tachycardia (or an atrial tachycardia
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| with first-degree atrioventricular (AV) block).
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|
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| Isoptin terminates the tachycardia and that is usually
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| indicative of involvement of the AV node. Hence, an
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| orthodromic tachycardia is the most likely diagnosis.
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| The ECG after termination of the tachycardia (figure
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| 2) showed sinus rhythm with a normal PQ interval
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| (140 msec), followed by a normal QRS complex in
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| the first six beats. In the rhythm strip at the bottom of
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| the panel, the next three QRS complexes are preceded
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| by the same configuration P wave but with a shorter
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| PQ interval (90 msec). These QRS complexes start
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| with a Q wave. In the three precordial leads above the
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| rhythm strip a discrete Δ-wave is visible (electroneutral
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| in V1 and positive in leads V2 and V3). In the left
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| precordial leads (last complex) the Δ-wave changes
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| from positive (V4) to neutral-negative (V6). Hence,
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| this ECG shows evidence of intermittent pre-excitation
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| with a Kent bundle in the inferior wall, presumably
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| just left of the septum. For the exact location of the Kent
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| bundle pre-excitation should be present in the
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| extremity leads.
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| '''In conclusion, in the presence of (intermittent) preexcitation the tachycardia is most likely an orthodromic re-entrant tachycardia.'''
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