This ECG shown is from a 70 year old female with atrial fibrillation and congestive heart failure who was receiving digitalis. The first 5 QRS complexes on the left side of the strip are irregular with RR intervals that vary from 1.0 to 1.52 seconds. They indicate high grade, but not complete AV block. The ST and T wave changes in leads II and V5 are consistent with the effects of digitalis.
The last 4 QRS are regular with RR intervals of 1.08 seconds. Their shape and duration are abnormal (the QRS duration measures 0.16 seconds). They represent a run of ideoventricular rhythm. The high grade AV block in this patient can be attributed to digitalis and perhaps to underlying AV nodal disease while the ideoventricular rhythm is most likely caused by digitalis and a manifestation of digitalis toxicity.