What is the rhythm shown in the strip
A. AF with pre-existing RBBB
This ECG shows a regular broad complex tachycardia. It has several features of VT with an unusual morphology for RBBB, an axis of –90°, and positivity in aVR. The 12th beat is a fusion beat which clinches the diagnosis. The fact that this is ‘RBBB-like’ suggests that it arises from the left ventricle and then crosses over to the right ventricle in a way that is analogous to conduction in RBBB.