This tracing is from a different patient with atrial fibrillation. It reveals regular narrow QRS complexes with RR intervals of 1.20 seconds (rate 47), indicating high grade AV block and an AV nodal escape rhythm.
This patient was receiving digoxin and the high grade AV block and nodal escape rhythm are manifestations of digitalis excess. Although digitalis is now employed much less frequently in the management of patients with congestive heart failure and/or atrial fibrillation than it was 10 years ago, its potential to cause a wide variety of cardiac arrhythmias should be remembered.