A 57-year-old woman is being assessed in pre-op clinic before a left breast lumpectomy. 6 years ago she was told that her ECG was   abnormal. She had been active without symptoms and her cardiac investigation, including echocardiogram and exercise test, had been normal:

• Sinus rhythm
• Left posterior fascicular block

(Compared with the previous tracings, there has been no change)

The lumpectomy is carried out without complication.The following is a routine ECG recorded before discharge from the hospital. The patient is asymptomatic:

• Sinus rhythm
• Right bundle branch block (RBBB)
• Left posterior fascicular block(LPFB)

RBBB plus LPFB (bifascicular block)

Comparison of the two tracing shows that the pattern of left posterior fascicular block is clearly present in the first portion of the QRS of ECG 2.
In RBBB the ventricular activation starts in the left ventricle and the first 60 msec of the QRS represent LV depolarization. That’s why the pattern of left posterior fascicular block can still be seen.