A patient with PMH of HTN and DM presents to the ED with chest pain. What is the interpretation of the ECG?

Answer: Sinus tachycardia at 100 BPM. Left axis deviation. Bifascicular block (RBBB+LAFB). ST elevation in V3-V5.

Discussion:

A bifascicular block is the combination of a RBBB plus either a left anterior fascicular block (LAFB) or left posterior fascicular block (LPFB). Conduction to the ventricles occurs down only a single fascicle. This is a sign of extensive conduction disease. 

The EKG will show a RBBB plus either a left axis deviation (if LAFB) or right axis deviation (if LPFB). A RBBB alone does not cause an axis deviation. 

A LAFB has an rS pattern in the inferior leads (II, III, aVF), and a qR pattern in the lateral leads (I, aVL), causing a left axis deviation. 

A LPFB has an rS pattern in the lateral leads (I, aVL), and a qR pattern in the inferior leads (II, III, aVF) causing a right axis deviation.

A bifascicular block should raise suspicion for anterior MI, especially if it is new. It can also be seen in progressive CAD, HTN, AS, and hyperkalemia.