An implantable cardiac stimulation device, e.g., a pacemaker or an implantable cardioverter defibrillator (ICD), is provided which prolongs the atrial refractoriness of a heart. The implantable cardiac stimulation device includes a generator that delivers pacing pulses to an atrium of a heart and a detector that detects atrial activations of the heart. An inhibitor is coupled to the detector that inhibits the generator when an atrial activation is detected within an escape interval. A generator control coupled to the generator causes the generator to deliver a primary pacing pulse to the atrium at the end of the escape interval, absent an atrial activation being detected within the escape interval, and causes the generator to deliver a secondary pacing pulse to the atrium a delay time after an atrial activation is detected within the escape interval or the delivery of a primary pacing pulse to the atrium.