Masquerading Bundle Branch Block: Positive Concordant Precordial Pattern

Report:

Atrial fibrillation with rapid ventricular response

Left anterior hemiblock

Right bundle branch block

Masquerading (standard + precordial) bundle branch block

Anterior infarction, probably old

Comment:

This is a case of RBBB with high-grade LAHB that has come to resemble LBBB in both the frontal and left precordial leads. As always, lead V1 provides the correct diagnosis of basic RBBB: all the delay is in the right ventricular (positive) territory of that lead. The masquerade can be confined to only one set of leads, usually the limb leads (standard form); in this case it is present in both. In either case, it is almost invariably a sign of severe diffuse myocardial disease, not unlike the obverse defect of LBBB with right axis deviation7.

This is also one of the causes of positive concordant precordial pattern. It is easy to see than an SVT with this morphology would “pass” for a VT on morphological grounds. In AF only the relatively slow ventricular rate distinguishes it from WPW ‘A’ conduction. In the latter, too, one would expect to see some non-anomalous complexes as well.

Fig 29a is a trace in sinus rhythm, showing mild first degree A-V block and biatrial abnormality. The P waves and the cycle lengths show discrete but definite alternation in what may be atrial or sinoatrial bigeminy.