A 72-year-old man with a history of dilated cardiomyopathy, prosthetic aortic valve, biventricular intracardiac pacemaker, and permanent atrial fibrillation presented with uncontrolled ventricular response from atrial fibrillation despite pharmacological therapy. He underwent radiofrequency (RF) catheter ablation of the atrioventricular (AV) node to achieve optimal biventricular pacing.AV node ablation was performed via the right femoral vein with a 4-mm tip ablation catheter (EPT, BostonScientific) positioned at the AV node just proximal to the His bundle region. RF energy was applied at this site for 60 seconds (55°C) resulting in complete heart block with an escape rhythm of 32 bpm. The next day he was discharged uneventfully. An echocardiogram at 5 months follow-up showed a left ventricular to right atrial (LV-RA) shunt across the membranous septum immediately above the tricuspid valve (online-only Data Supplement Movies I and II and Figure 1A and 1B). This LV-RA shunt (Gerbode-type defect) had a 76-mm Hg gradient across the defect ( Figure 1C). Left ventricular ejection fraction was depressed (25% to 30%) and the right ventricle was mildly to moderately dilated. The mechanical aortic prosthetic valve functioned normally with a mean gradient of 17 mm Hg. Review of the patient's preablation echocardiographic studies did not show any evidence of LV-RA shunt. He remained stable with New York Heart Association class I to II and Ͼ85% biventricular pacing. A repeat echocardiographic study 14 months after the ablation procedure did not show any progression in the size of iatrogenic Gerbode-type defect or shunting.In a second case, a 68-year-old man with a history of severe chronic obstructive pulmonary disease and symptomatic permanent atrial fibrillation underwent RF catheter ablation of the AV node because of ineffective ventricular rate control by medical therapy. Three days after the procedure, the patient developed ventricular fibrillation and could not be resuscitated. Autopsy findings showed lesions at the RF ablation sites. Although he did not develop a frank defect, the ablation lesions extended to the left ventricular side of the membranous septum (Figure 2) at a site similar to the Gerbode-type defect in the first patient.