Pacemaker mediated reentrant tachycardias have been seen frequently during use of dual chamber sensing pacemakers and are dependent on the presence of intact retrograde (ventriculo-atrial) conduction. The status of a patient's retrograde (VA) conduction cannot be determined from the surface electrocardiogram. At the time of pacemaker implant the state of antegrade conduction should be determined at incremental atrial pacing rates, the exact timing measured from the intracardiac electrograms, i.e., from the intrinsic deflection of the intrinsic deflection of the ventricle. With this information obtained in 53 patients, it was possible to determine which patients could safely receive dual chamber sensing pacemaker devices, the appropriate pacemaker atrial refractory period setting to avoid pacemaker mediated tachycardias in those patients with intact retrograde conduction, and the optimal timing for programming the pacemaker's AV delay. Patients who have been studied in this manner and who were felt to be suitable for a dual chamber sensing device have not displayed pacemaker mediated reentrant tachycardias. Forty-seven percent of all patients who require pacemaker implant have VA 1:1 conduction; 67% of those with sinus node dysfunction and 14% of those with complete antegrade block have VA conduction at a mean interval of 235 +/- 50 ms (range 110-380 ms).