Bilateral Bundle Branch Block

A patient c/o of dizziness.

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The rhythm is sinus at a rate of about 100 bpm. The first 8 QRS complexes has a left bundle branch block configuration with PRI of 0.20 sec. The next QRS complexes has a right bundle branch (RBBB) configuration with QR configuration in V1 and wide S in I with PRI of 0.32 sec.

Is this dual AV node physiology + rate-related aberrancy?

It is possible to have normal PRI and a long PRI in one strip because some individuals have 2 pathways in the AV node where a supraventricular impulse can pass. The fast pathway will create a short PRI and a slow pathway will create a long PRI on the surface ECG.

It is also possible that RBBB and LBBB would appear depending on the heart rate.

In most "funky" arrhythmia, there is a unifying phenomenon

In this case, there is alternating QRS morphology (LBBB and RBBB) and changing PRI interval. So, is there is unifying phenomenon to explain this?

"AV conduction delay or block can be assumed to be caused by bilateral bundle branch block (BBB) only in the presence of alternating or intermittent RBBB and LBBB with a changing PR interval. - Fisch and Knoebel 2000."

In the first 8 QRS complexes, the sinus impulse is blocked in the left bundle and transmitted in the right bundle. After that the sinus impulse was blocked in the right bundle but delayed in the left bundle.

The unifying explanation for this strip is Bilateral bundle branch block. The bradycardia noted during admission could have been the AV block (block on both branches) expected among these groups of patients.

In this case, the patient came in due to dizziness and heart rate dropped to the 30's. This patient eventually had a pacemaker.