View source for Puzzle 2005 6 244 - Answer

{{NHJ|
|mainauthor= '''A.A.M. Wilde, R.B.A. van den Brink'''
|edition= 2005:6,244
}}
[[Image:Puzzle_2005_6_244_fig1.jpg|Figure 1|thumb]]
[[Image:Puzzle_2005_6_244_fig2.jpg|Figure 2|thumb]]

A20-year-old male is having palpitations. They
occur without a specific trigger, although episodes
are sometimes related to emotion or exercise. Duration
is between two minutes and one hour. He does not
feel well during an attack, but has never fainted.
Physical examination reveals no abnormalities nor does
laboratory investigation or echocardiography. His
baseline ECG is normal (not shown). He was asked to
come to the emergency room if an episode lasted long
enough, which he did (figure 1).

Upon presentation during an attack the ECG
recorded a narrow-complex tachycardia with an RR
interval of 280 msec (214 beats/min). There is a slight
rightward deviation of the electrical axis. ST morphology
is normal and no P wave can be identified. The
differential diagnosis is:
# atrioventricular nodal reentry tachycardia (AVNRT), 
# orthodromic tachycardia with a concealed bypass (AVRT) and 
# atrial tachy-cardia. Adenosine was administered and the ECG presented in figure 2 was recorded.

'''What is your diagnosis and what would your further
treatment be?'''

==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>