An implantable triple-chamber cardiac stimulator, comprising - an at least partly electrically conductive housing and - a defibrillation electrode line connector for the connection of a ventricular defibrillation electrode (38, 40) by means of an electrode line, - a defibrillation shock generator that is connected to the defibrillation electrode line connector and that is configured to generate a defibrillation shock and to deliver said shock via the defibrillation electrode line connector, - a right-ventricular sensing unit that is connected or is to be connected via a right-ventricular electrode line (16, 16') to at least one right-ventricular sensing electrode (18, 20) of relatively small surface area compared to the defibrillation electrode (38, 40) and that is configured to receive and to process electric potentials of the ventricular myocardium, - a left-ventricular sensing unit that is connected or is to be connected via a left-ventricular electrode line to at least one left-ventricular sensing electrode (32, 34) and that is configured to receive and to process electric potentials of the ventricular myocardium, - a right-atrial sensing unit that is connected or is to be connected via a right-atrial electrode line to at least one right-atrial sensing electrode (22, 24) and that is configured to receive and to process electric potentials of the atrial myocardium, - a control unit (54; 54') that is connected to the right-ventricular sensing unit and to the defibrillation shock generator and that is configured to detect ventricular fibrillation and to actuate the defibrillation shock generator to deliver a defibrillation shock depending on the detection result, - an additional detection arrangement (280; 460, 470) for perceiving ventricular and atrial events that operates independently of the at least one right-ventricular sensing electrode, wherein the control unit (54; 54') is connected to the additional detection arrangement (280; 460, 470) and is configured to inhibit the delivery of a defibrillation shock in the event that a normal rhythm is reliably detected with the additional detection arrangement (280; 460, 470), characterised in that the evaluation unit is configured to then assess a rhythm as a normal rhythm if, following an intrinsic or stimulated atrial event, a single left-ventricular event is perceived within a predefined time window "AV time expectation" and if, in addition, the interval between successive left-ventricular events corresponds to the interval of the prior atrial events plus a tolerance. - an at least partly electrically conductive housing and - a defibrillation electrode line connector for the connection of a ventricular defibrillation electrode (38, 40) by means of an electrode line, - a defibrillation shock generator that is connected to the defibrillation electrode line connector and that is configured to generate a defibrillation shock and to deliver said shock via the defibrillation electrode line connector, - a right-ventricular sensing unit that is connected or is to be connected via a right-ventricular electrode line (16, 16') to at least one right-ventricular sensing electrode (18, 20) of relatively small surface area compared to the defibrillation electrode (38, 40) and that is configured to receive and to process electric potentials of the ventricular myocardium, - a left-ventricular sensing unit that is connected or is to be connected via a left-ventricular electrode line to at least one left-ventricular sensing electrode (32, 34) and that is configured to receive and to process electric potentials of the ventricular myocardium, - a right-atrial sensing unit that is connected or is to be connected via a right-atrial electrode line to at least one right-atrial sensing electrode (22, 24) and that is configured to receive and to process electric potentials of the atrial myocardium, - a control unit (54; 54') that is connected to the right-ventricular sensing unit and to the defibrillation shock generator and that is configured to detect ventricular fibrillation and to actuate the defibrillation shock generator to deliver a defibrillation shock depending on the detection result, - an additional detection arrangement (280; 460, 470) for perceiving ventricular and atrial events that operates independently of the at least one right-ventricular sensing electrode,