An implantable pacing system with single, double and triple chamber pacing capabilities, provided individually or in concert on a conditional or continuous basis depending upon ongoing analyses of atrial rhythm status, atrioventricular conduction status and ventricular rate. A mode is selected to reduce the occurrence of any ventricular pacing in favor of intrinsic atrioventricular and ventricular conduction. If excessively long PR intervals are occurring too frequently or atrioventricular conduction is unreliable or absent, the implantable pulse generator is operated in a conditional triple chamber pacing mode that provides atrial-synchronous biventricular pacing in every cardiac cycle for a period of time as necessary to restore and maintain AV synchrony, while minimizing ventricular asynchrony otherwise associated with monochamber RV pacing as in conventional dual chamber pacing systems. Similarly, biventricular pacing is provided in every cardiac cycle when ventricular rates are undesirably slow during atrial fibrillation, where AV synchronization is excluded.La présente invention concerne un système de stimulation implantable doté de capacités de stimulation monochambre, double chambre et triple chambre, pouvant intervenir isolément ou de concert sur une base conditionnelle ou continue en fonction danalyses permanentes du statut du rythme auriculaire, du statut de la conduction auriculo-ventriculaire et de la fréquence ventriculaire. On choisit un mode limitant la mise en œuvre dune quelconque stimulation ventriculaire en faveur dune conduction auriculo-ventriculaire et ventriculaire intrinsèque. Si des intervalles PR excessivement longs surviennent trop fréquemment ou si la conduction auriculo-ventriculaire nest pas fiable ou est carrément absente, le générateur dimpulsions implantable fonctionne selon un mode de stimulation conditionnelle triple chambre qui assure une stimulation biventriculaire synchrone de loreillette au cours de chaque cycle cardiaque pend
