Imaging in Cardiology: "Impossible" Pacemaker-Mediated Tachycardia during Implantation

A 78-year-old man was referred for pacemaker implantation due to symptomatic 3rd degree atrioventricular (AV) block. Neither the patient's history nor physical examination revealed abnormalities with regard to his cardiovascular status. Tachyarrhythmias such as intermittent atrial fibrillation or ventricular tachycardia had not been documented in recent years, and the patient was not on medication. A 24-hour ambulatory electrocardiogram indicated permanent AV block in the presence of normal sinus rhythm. The atrial and ventricular leads were implanted via the cephalic vein without complications. The tip of the atrial lead (Arox, Biotronik, Germany) was positioned at the right atrial appendage. The acute atrial measurements were: threshold at 0.7 V with a pulse duration of 0.4 ms, a pacing impedance of 480 Ω, and a P-wave of 2.8 mV. The tip of the ventricular lead (Merox, Biotronik) was implanted at the right ventricular apex, and the measurements were: 0.4 V at 0.4 ms for the stimulation threshold, a pacing impedance of 920 Ω, and sensed R-wave amplitudes equal or greater than 12.8 mV. The atrial lead was connected to the Philos DR pacemaker (Biotronik) before the ventricular lead was connected. Immediately after fixation of the connector screw of the ventricular lead, a ventricular rhyhtm was present without pacing artefacts, but the ventricular complex was identical to the paced ventricular complexes observed during the stimulation threshold test (see Figure 1). It has to be stressed that the pacemaker was not inserted into the pocket, and the skin of the patient was covered with an antiseptic foil. Furthermore, the pacemaker was not reprogrammed and, therefore, the default pacemaker program was active: Pacing and sensing in the atrium and ventricle were in the unipolar configurations. When the ventricular pacemaker lead was disconnected, the tachycardia disappeared instantly and did not restart. After reconnecting the lead, a similar ventricular tachycardia was noticed. A pacemaker-mediated tachycardia was suspected, and the atrial lead was hooked up in parallel to the lead used for intracardiac monitoring during implantation in order to visualize detected atrial signal (V6 lead in Figure 2). The V6-lead showed a ventricu-