It's Not What You Think it Is: Difference between revisions

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It's Not What You Think it Is (view source)

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[[Answer]]
[[Puzzle 2005_6_244 - Answer|Answer]]
 
Figure 2 shows a narrow complex rhythm, with an RR
interval of 560 msec, i.e. a rate exactly half of that
before adenosine. P waves are now clearly discernable
with an axis compatible with sinus rhythm. This suggests
either sinus rhythm, or a supraventricular
tachycardia with 2:1 block in the AV node caused by
adenosine and comparable P wave morphology. AVRT
can be excluded as a 1:1 relation between atrium and
ventricle is required. AVNRT with 2:1 block to the
ventricle is also highly unlikely because of the
morphology of the P waves, which in that case would
be negative in the inferior leads due to retrograde
activation of the atrium. The only remaining alternative
is atrial tachycardia from an area in the vicinity of the
sinus node. Indeed, on closer examination there is
evidence of a second P wave partly hidden in the
terminal part of the T wave in lead V1.
Shortly after this ECG was obtained the tachycardia
(figure 1) resumed. A higher dose of adenosine
terminated the tachycardia and sinus rhythm (60
beats/min) appeared. Atrial tachycardias occasionally
respond to adenosine. In those cases the underlying
electrophysiological mechanism is triggered activity
based on delayed afterdepolarisations.<cite>Markowitz</cite><cite>Lerman</cite> These tachycardias
usually respond well to β-blockade or
verapamil, and are generally amenable to catheter
ablation.
 
==References==
<biblio>
#Markowitz pmid=10355690
#Lerman pmid=8743762
</biblio>
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