A 79-year-old man had replacement of his aortic valve with a bio-prosthesis because of severe calcific aortic stenosis. This ECG tracing is recorded at the end of the operation:

• Sinus rhythm, with Mobitz II second degree AV block
• Right bundle branch block (RBBB) & Left anterior fascicular block (Bifascicular block)
• Left ventricular hypertrophy with repolarization abnormality

Mobitz II (type II) second degree AV block is nearly always at the level of the His-Purkinjie system and caused by bilateral bundle branch block. This is illustrated by this ECG. There is Right Bundle Branch Block and block of the anterior fascicle of the left bundle (the AV conduction is through the posterior fascicle of the left bundle). Then both bundles are blocked, with loss of AV conduction. The PR interval is normal and constant (which confirms that the block distal to the AV node).