Puzzle 2009 08 Answer

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The ECG (precordial leads) shows an irregular rhythm without clearly discernable P waves. Atrial fibrillation is the most likely supraventricular rhythm. QRS complexes show a left bundle branch block pattern (LBBB) with the exception of the last but 5th QRS complex. This complex is narrow and has a normal morphology. There are two potential explanations for this QRS complex. The first and probably most likely is that aberrant conduction occurs in the right bundle (concomitant conduction slowing in the contralateral bundle, the right bundle is contralateral to the left bundle that shows conduction delay in the other com- plexes) (figure 2). Indeed, aberrant conduction is most prevalent after the longest RR intervals during atrial fibrillation, as in this case. This relates to the longer refractory period of the bundles after a long preceding RR interval. The second possibility is ventricular ectopy in the ipsilateral bundle at the time that conduction passes through the right bundle. This possibility cannot be excluded. Ectopy should occur at the time that the supraventricular input reaches the site of the block. In both cases the QRS complex that ensues will mimic a normally conducted QRS complex.