This 69-year-old man is in the cardiac ward 6 days after an acute infarction. His condition is stable and he is pain-free:

• Sinus rhythm with Mobitz II second degree AV block
• Inferoposterior myocardial infarction (inferior Q waves with tall RV1)

Comment: Note the long PP interval is twice the “short” PP interval suggesting that the sinus node fires at a rage of 800ms but intermittently there is sinoatrial block, causing a “missing” p-wave and hence twice the “short” PP interval.