This 50-year-old man presents to the Emergency Department with severe central anterior chest pain for the past hour:

• Sinus rhythm with Mobitz I second degree AV block
• Acute inferior infarction

The taller T wave in V2 in a patient with an inferior myocardial infarction suggests the possibility of posterior wall ischemia.

The change of the QRS pattern in V1 after the long pause is due to aberrant conduction. While aberrant conduction more often occurs after shorter RR intervals, it may occasionally be seen after a longer pause. In an abnormal cell during phase 4 there may be a gradual reduction of the negative action potential. If the RR interval is prolonged, the activation of the cell from a less negative potential results in impared conduction (“phase 4 aberration”).