illustrates a fully implantable atrial defibrillator 30 embodying the present invention shown in association with a schematically illustrated human heart 10 in need of atrial fibrillation monitoring and potential cardioversion of the atria the portions of the heart 10 illustrated in fig1 are the right ventricle 12 the left ventricle 14 the right atrium 16 the left atrium 18 the superior vena cava 20 the coronary sinus channel 21 which as used herein denotes the coronary sinus 22 and the great cardiac vein 23 the coronary sinus ostium or opening 24 the left ventricular free wall 26 and the inferior vena cava 27 the atrial defibrillator 30 generally includes an enclosure 32 for hermetically sealing the internal circuit elements of the atrial defibrillator to be described hereinafter an endocardial first lead 34 and a intravascular second lead 36 the enclosure 32 and first and second leads 34 and 36 are arranged to be implanted beneath the skin of a patient so as to render the atrial defibrillator 30 fully implantable enclosure 32 may also be electrically active to act as an electrode during atrial or ventricular defibrillation shocks as a return electrode for pacing or as a sensing electrode the endocardial first lead 34 preferably comprises an endocardial lead having.