Masquerading Bundle Branch Block

Report:

Atrial fibrillation with ventricular response 80-120/min 1

Fixed-rate ventricular pacemaker rhythm 50/min 2

Fusion beat (third paced complex) 1

R-on-T pacing 1

Right bundle branch block

1

Left anterior hemiblock 1

Standard masquerading bundle branch block 2

Old anteroseptal infarction 1

Comment:

When a high-grade LAHB is combined with RBBB, the appearance in standard Lead 1 may become that of LBBB rather than RBBB. This is called standard masquerading bundle branch block (there is also a precordial form). It signifies diffuse myocardial disease, certainly present in this patient.

This patient also had bilateral bundle branch block (BBBB) demonstrated by both RBBB and LBBB conduction, usually at different times, but sometimes in the same trace (Fig 62a).

The fixed-rate temporary pacemaker should either have its ability to sense restored or be turned off.

Fig 62a. The masquerading BBB in standard leads: RBBB and LBBB beats are almost indistinguishable. V1 strip is indispensable.

Fig 62b. LBBB conduction in sinus rhythm.

Fig 62c. RBBB conduction in AF with RVR.

Fig 63. 85 year old man in CCU for recurrent arrhythmia.