Transvenous internal cardioversion of chronic AF using a right atrium (RA) coronary sinus (CS) vector requires more energy than cardioversion of paroxysmal AF. Chronic AF is not terminated in 25% of patients using biphasic shocks up to 10 J. We therefore evaluated efficacy, safety, and tolerability of internal cardioversion using a "unipolar" configuration (RA to skin patch) and biphasic shocks in patients with long-lasting AF and different heart disease. In each patient, biphasic R wave synchronous shocks were delivered between a large defibrillating surface area electrode in the RA and a skin patch in the left prepectoral position. Defibrillation protocol started with a test shock of 0.4 J. Shocks were repeated and increased until termination of AF or a maximum of 34 J. Sedation was used when the patient described the shock as painful. This study included 11 patients with a mean age of 67 +/- 8 years (range 56-83). AF duration was > or = 1 month in all patients with a mean duration of 11 +/- 11 months (range 2-36). Underlying heart disease was present in all patients and the mean left atrial dimension was 43 +/- 9 mm (range 26-57). AF was terminated in 10 of 11 patients (91%) with a mean delivered energy of the successful shocks of 18.7 +/- 8.7 J (median energy 16.9 J; range 7.3-32.5) and a mean leading edge voltage of 564 +/- 129 V. The mean shock impedance at the defibrillation threshold was 71 +/- 13 omega (range 59-103). A total of 131 shocks were delivered without any complication and proarrhythmia episodes. We conclude that low energy "unipolar" internal cardioversion is a simple, safe, and effective technique for termination of chronic AF in patients with heart disease. The procedure is often tolerated under light sedation.