This 59-year-old man with a two year history of angina is admitted to the cardiac surgery ward to undergo coronary artery bypass surgery:

• Sinus rhythm with bigeminal sequence
• Consider: Type I sino-atrial block or premature atrial complexes (originating in an area close to the sinus node) – the computer’s diagnosis was: Mobitz I sino-atrial block
• Inferior infarction, age undetermined

The P waves are positive in I and aVF, indicating their origin from the sinus node, and they all have the same morphology. The PR interval is prolonged at 240 msec and constant. There is a bigeminal rhythm, and the longer PP interval is not a multiple of the short one. These findings suggest the possibility of 3:2 type I sino-atrial block. A possible alternative diagnosis is premature atrial complexes with a pattern of bigeminy, originating near the sinus node. Because this possibility cannot be excluded, the diagnosis of 3:2 sino-atrial block is usually open to question, unless other conduction/block ratios are also documented.

In type I sino-atrial block, calculation of the sinus rhythm is obtained by assuming that the sinus node discharges regularly and the sino-atrial conduction time increases until a sinus discharge is blocked. In this example of 3:2 type I block, the interval between the P waves at the end of 2 successive long pauses corresponds to 3 sinus discharges.