View source for Puzzle 2004 3 123, One to one, one to two, two to one? What is the rhythm? - Answer

{{NHJ|
|mainauthor= '''A.A.M. Wilde''' and '''H. Tan'''
|edition= 2004:3,123
}}
[[Image:Puzzle 2004 3 123 fig1.jpg|Figure 1|thumb]]
A 18-year-old boy is suffering from palpitations without specific triggers. It started when he was 12 years old. Sometimes he feels dizzy which usually coincides with very fast palpitations. On other occasions, he feels palpitations (less fast) without dizziness. Also, there are transitions between both sensations. On one such occasion, he presents at the hospital. An ECG is taken of which the six precordial leads are shown here. The calibration is as usual (i.e., 10 mm=1 mV and 400 ms, respectively). 

'''What is your diagnosis?'''

==Answer==
The ECG shows a wide-complex tachycardia. The first nine complexes are regular with a cycle length of 200 ms, i.e., 300 beats/min. These complexes are followed by a slower part with similar QRS morphology. This rhythm is also regular with a cycle length of 400 ms
(150 beats/min). This sequence is repeated with nine fast cycle lengths and five 400 ms cycle lengths. The final part is fast again. Thus, the arrhythmia consists of QRS complexes with similar morphology and an alternating rate which doubles from time to time. In
a wide-complex tachycardia, it is always important to look for the P wave. In this particular ECG, a P wave cannot be seen with certainty; one may presume that the small deflections immediately following the QRS complex in both the fast and slower rhythms are P
waves. In that case, during the slower parts, every other P wave is not seen. The rate is typical for atrial flutter (300 beats/min at the atrial level) and the occasional doubling of rate suggests alternating 2:1 and 1:1 conduction to the ventricles.
The QRS width is 160 ms and it should be noticed that the initiating part of the QRS complex is particularly slow. This would be compatible with preexcitation of the ventricles. While a ventricular origin of the rhythm cannot be excluded, the alternating rate with similar QRS morphology renders ventricular tachycardia less likely, as this is only possible in the rare diagnosis of ventricular tachycardia with occasional 2:1 exit block. Similarly, atrial tachycardia with exit block and aberrant conduction is unlikely. Hence, '''atrial flutter with alternating 1:1 and 2:1 conduction to the ventricles over an accessory pathway (i.e., preexcitation) is the most likely diagnosis'''. The bypass is located at the left side of the heart (positive delta wave in lead
V1) but its exact localisation (lateral or posterior) cannot be determined in the absence of limb lead recordings. The differential diagnosis includes atrial flutter with aberrant conduction over the right bundle branch and alternating 1:1 and 2:1 AV-nodal conduction.