br Conflict of interest br Introduction Far field sensing

Conflict of interest
Introduction Far-field sensing of the atrial electrogram by a ventricular inhibited (VVI) pacemaker is a rare but potentially catastrophic phenomenon if there is no escape rhythm [1–3]. In this setting, the tip of the pacemaker lead is usually located in close proximity to the tricuspid annulus [1,2]. Of the several options available, the triggered mode (VVT) might be most effective in select cases [1–3]. In this report, we describe a patient in whom far-field atrial sensing by a VVI pacemaker led to ventricular asystole that was successfully treated by changing the programming to the VVT mode.
Case report A 61-year-old man with a history of ischemic cardiomyopathy and congestive buy Ubenimex failure had undergone implantation of a single-chamber pacemaker at an outside hospital for intermittent complete heart block 1 year before his presentation at our institution. The patient was hospitalized after he experienced an episode of syncope. He developed complete heart block with no ventricular escape rhythm (Fig. 1). Notably, there were no pacemaker spikes and a pacemaker malfunction was suspected. A 12-lead electrogram showed sinus rhythm with a right bundle branch block and an extreme left axis deviation, suggestive of a bifascicular block. A radiograph of the chest indicated that the tip of the pacemaker lead was located near the tricuspid annulus (Fig. 2). Interrogation of the pacemaker revealed a Medtronic single-chamber pacemaker (model number 8340) programmed to a bipolar VVI mode at a rate of 60 beats per minute with a sensitivity of 1.25mV and a ventricular refractory period of 325ms. Lead impedance (Medtronic bipolar passive-fixation lead: model number 4024) was measured at 509Ω. The intracardiac marker channel indicated that the ventricular pacing lead had detected atrial activity. The measured P wave amplitude was 2.5mV. There were no R waves. Since the patient was unstable with worsening congestive heart failure, the pacemaker was reprogrammed to the VVT mode with a rate of 60 beats per minute and a sensitivity of 1.25mV. With this setting, the pacemaker showed ventricular pacing after each of the sensed P waves; it increased the ventricular rate and atrioventricular (AV) synchrony (Fig. 3A and B). VVT pacing with intermittent failure of atrial sensing and adequate ventricular sensing is shown in Fig. 3C. This resulted in a pacing spike with either the P or R wave. The sensitivity was set at 2.5mV. After adjusting the ventricular sensitivity to 1.25mV, no further sensing problems occurred.
Discussion Far-field sensing of the atrial electrogram by the ventricular lead is a rare but potentially serious complication of a VVI pacemaker in patients with complete heart block [1–3]. This phenomenon has usually been noted in a pacemaker where the tip of the pacing lead is located near the tricuspid annulus, as was seen in our case (Fig. 2). It has also been noted in bipolar as well as unipolar lead pacemakers. Treatment options include applying a magnet over the pacemaker generator, changing the programming to the asynchronous or triggered mode, and decreasing the sensitivity [4]. However, only a few case reports of the effectiveness of the VVT mode in this setting are available [1–3]. The VVT mode is similar to the VVI mode in that it delivers a stimulus at a lower rate interval if no spontaneous ventricular activity is detected. When there is a spontaneous ventricular complex, the pacing spike comes with the QRS. There is an upper rate interval beyond which sensing is lost. VVT pacemakers are seldom used, primarily because of battery drainage. They were initially developed to solve the issue of external interference in patients with no intrinsic rhythm. In contrast to the VVI or VOO mode, the VVT mode can offer rate responsiveness and possible AV synchrony in the case of atrial oversensing. The AV interval depends on the timing of atrial sensing by the VVI pacemaker [1–3].