An atrial defibrillation pulse generator and lead system particularly adapted to allow for implant via a single incision. The electrode system includes of a coronary sinus electrode and a subcutaneous electrode, which is preferably located on the housing of the implantable defibrillator. The coronary sinus and subcutaneous electrodes are used for atrial defibrillation, or alternatively for both atrial and ventricular defibrillation, with higher energy pulses being delivered during ventricular defibrillation. The defibrillator is preferably planted in the left pectoral region of the body, and includes a pulse generator for applying biphasic defibrillation pulses between the electrodes. Additional electrodes located in the right ventricle and/or superior vena cava may also be employed.