There are several issues here.
First, those responding that the narrow QRS complex is a result of a pause occurring in a rate-dependent (or acceleration-dependent) bundle branch block may be absolutely correct. I certainly cannot say that they aren't.
But I don't think it is what occurred at all.
I hinted that the key to this rhythm was in the ninth "beat." Please note that I did not say in the ninth QRS.
Recognizing right ventricular ectopy in an atrial fibrillation with a left bundle branch block cannot be based on the morphology of the QRS (same QRS morphology) or on the prematurity of the beat ("prematurity" has no meaning in atrial fibrillation). It will depend on the appearance of repolarization - the ST-T portion. Depolarization is altered in ventricular ectopy so repolarization will be altered also. [PEARL: You can often recognize junctional ectopy by the difference in repolarization even when the QRS complexes are seemingly identical to the sinus conducted beats.]
The ST-T in the ninth beat is very different than the other beats, so this represents a ventricular ectopic beat (again, we really can't call it a "PVC" because this is, after all, atrial fibrillation). So we know that ectopy is occurring in this tracing.
The other thing to remember is that while a rate-related bundle branch block may manifest in the left bundle, it is fairly rare for this to happen. Why? The right bundle branch has an inherently longer refractory period and is more likely to experience a delay or block at a lower critical rate than the left bundle. The causes for a rate-related RBBB are mostly physiological and not really pathological. The cause of a LBBB is almost always pathological. While that doesn't rule out a rate-related LBBB in this case (after all, it does occur after the longest pause), it doesn't make it the most plausible explanation.
What this actually represents is a fusion beat caused by a ventricular extrasystole that occurred in the left ventricle simultaneously with the depolarization of the right ventricle. Because the depolarization of the ventricles occurred at the same time, a normal QRS appeared. This is called normalizing fusion.
The repolarization looks more normal also, but since we don't have any normally-conducted sinus beats to compare it to, we really don't know exactly what normal repolarization should look like on this tracing.
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