This 75-year-old woman has undergone replacement of her aortic valve because of severe aortic stenosis. The preoperative ECG has shown left bundle branch block. The following tracing is recorded shortly after transfer from the OR to the cardiac surgical ICU:

• Accelerated junctional rhythm, 72/min
• Left bundle branch block

Comment: Had we not known that this patient’s ECG had previously shown a left bundle branch block, the differential diagnosis would have been either (a) junctional rhythm with left bundle branch block or (b) idioventricular rhythm with retrograde conduction to the atrium.

Second ECG is recorded one day later:

• Ectopic atrial rhythm with short PR
• Left bundle branch block