ECG Challenge: A 78-year-old woman with a history of atrial fibrillation who is taking Coumadin for prevention of embolism and a β-blocker and digoxin for rate control presents to her primary care physician with complaints of fatigue and lightheadedness that have been present for several days.
There are no P waves seen, but there are low amplitude and irregular undulations of the baseline between each RR interval. The underlying rhythm is atrial fibrillation. The rhythm is mostly regular (with a rate of 38 bpm; ↔). However, QRS complexes 4 and 5 (+) are associated with shorter RR intervals that are different from each other (┌┐). As the RR intervals with atrial fibrillation are irregularly irregular, the presence of regular RR intervals (regularized atrial fibrillation) indicates the presence of complete heart block. Complexes 4 and 5 are associated with irregular RR intervals; therefore, these 2 complexes result from conduction through the AV node, indicating that the complete heart block is intermittent (or stated another way that there is intermittent AV conduction associated with the complete heart block).
The morphology of the two conducted QRS complexes (+) is different from the other QRS complexes. These 2 complexes have an increased duration (0.12 sec). Although they resemble a left bundle-branch block, there is an initial R wave in lead V1 (▼), which is a septal force that should not be present with a left bundle-branch block as the septal branch, which innervates the intraventricular septum, originates from the left bundle. In addition there is a prominent terminal S wave in lead V5 (→), indicating terminal impulse directed from left to right. With a left bundle-branch block the entire ventricular impulse is directed from right to left, and there should not be any left to right forces. The prolonged QRS complex duration is therefore an intraventricular conduction delay. The QT/QTc intervals are normal (480/380 ms). The other QRS complexes are wider (duration=0.16 sec) despite a slower rate and have a different morphology. Therefore these QRS complexes are not junctional but originate from the ventricle. Hence this is complete heart block with a ventricular escape rhythm. The block is not within the AV node but is within the His-Purkinje system.
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- © 2014 American Heart Association, Inc.