Possible AV dissociation after cardioversion.

Dr. Hancock is Professor of (Cardiovascular) Medicine Emeritus, Stanford University School of Medicine, Stanford, Calif. A 62-year-old man was hospitalized for worsening congestive heart failure. His history included diabetes of four years' duration, episodic atrial arrhythmias, recurrent cellulitis of the lower extremities, and alcoholism. At the time of admission, he had moderate right and left-sided congestive heart failure, atrial flutter with a ventricular rate of 90 to 100/min, and a loud murmur of mitral regurgitation. His medications included warfarin, glyburide, furosemide, arnlodipine, and folic acid. The patient improved slowly, but pleural effusion and hypoxemia continued despite salt restriction and higher doses of furosemide. Cardioversion did not convert atrial flutter to a stable sinus rhythm with a 1:1 conduction; instead, an irregular rhythm developed, as well as what appeared to be AV dissociation. The patient was not taking digoxin at this time. The ECG is shown. What is the rhythm mechanism? What is the underlying physiologic abnormality? What further therapy is indicated?