Delayed restoration of atrioventricular synchrony with beat-to-beat mode switch

This case report describes a patient with complete AV block and a VDD pacem aker who experienced repetitive episodes of symptomatic bradycardia. Episod es occurred due to activation of an automatic beat-to-beat mode switch algo rithm. After mode switch to VDI operation, the pacemaker failed to immediat ely switch back to AV synchronous pacing when regular sinus rhythm (greater than or equal to 100 beats per minute) resumed despite adequate P wave sen sing. Dizziness was felt for up to several minutes of asynchronous pacing a t the lower rate limit until VDD mode was restored. Episodes were completel y eliminated by programming the mode switch function from an automatic beat -to-beat algorithm to a fixed rate algorithm.