Paradoxical atrial undersensing by a dual chamber pacemaker during atrial fibrillation.

This report describes paradoxical atrial undersensing by a dual chamber pacemaker in a patient with paroxysmal atrial fibrillation. Atrial undersensing was present only when the device was programmed to a high sensitivity but sensing normalized when a lower sensitivity was programmed. This unusual response should be differentiated from the recently documented lock-in behavior of pacemakers delivering managed ventricular pacing.