When high grade AV block occurs in patients with atrial fibrillation of flutter it is usually a manifestation of an underlying conduction system abnormality and/or is caused by drugs which block AV conduction. These include digitalis, the beta adrenergic blocking agents and the calcium channel blocking drugs.
This ECG is from a 92 year old male with a history of 2 prior myocardial infarctions. He was on no medications. The tracing revels coarse atrial fibrillation but the rhythm is regular with RR intervals of 0.94 seconds (a rate of 63 beats per minute), and the QRS complexes are of normal shape and duration. These features suggest that there is high grade AV block and an escape pacemaker located within the AV junction. In this case, the high grade block is presumed due to underlying AV nodal disease since the patient was receiving no drugs that would interfere with AV conduction.