A 78-year-old woman with a history of diabetes and a three year history of mild stable angina presents for routine follow-up. Complete occlusion of the LAD has previously been demonstrated, with good collateral circulation. She also has moderately severe left ventricular dysfunction, with evidence of a previous anterior infarct. She denies any presyncope:

• Sinus rhythm possibly with 3:2 Mobitz I SA block (Sinoatrial Exit Block)
• Left bundle branch block

In this case, the differential diagnosis includes sinus rhythm with premature atrial complexes, and sinus rhythm with 3:2 SA block. The fact that all the P waves show the same morphology favors (but does not prove) a sinoatrial exit block. In 3:2 sinoatrial block, 2 P waves correspond to 3 sinus discharges. The presumed sinus cycle is therefore calculated by dividing the interval between the P waves at the end of 2 successive long pauses by 3.