Puzzle 2004 3 123, One to one, one to two, two to one? What is the rhythm? - Answer: Difference between revisions

==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

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.