VDD single pass lead pacing: sustained pacemaker mediated tachycardias unrelated to retrograde atrial activation.

Pacemaker mediated tachycardias (PMTs) are a well known complication of P synchronous pacing. Although the initiating mechanisms are several, all of them are associated with retrograde atrial activation, which is sensed by the atrial sensing channel, resulting in ventricular pacing. In 19 patients suffering from symptomatic AV conduction disturbances and normal sinus node function, a VDD pacing system connected to a single pass ventricular lead with dual chamber electrodes was implanted. The bipolar atrial electrode, floating in the right atrium, was used to detect endocardial atrial electrograms that were differentially processed within the pacemaker for optimal discrimination and filtering of undesirable signals. The widely programmable atrial sensitivity (amplitude and filtering) allowed stable P synchronized ventricular pacing in all patients, but in five of them, sustained PMTs not related to retrograde atrial activation was documented during the follow-up. The common mechanism for the onset and maintenance of these PMTs was traced to the abnormal sensing of the terminal forces of ventricular activation and/or of the T wave. The possibility of interferences between ventricular and atrial electrodes (crosstalk) was also considered. The reduction of atrial channel sensitivity represented in all cases the only effective procedure to prevent this type of PMT. In conclusion, the bet signal to noise ratio is an important endpoint to assure the proper function of a single lead VDD pacing system. Furthermore, using the differential amplifier built within the pacemaker, consideration should be given to the optimal mode of rejection of the terminal forces of the QRS and T wave.