Arrhythmia Induction During Atrial Pacing With Atrial «Floating» Electrodes

INTRODUCTION

The use of single-lead VDD pacemakers enables atrioventricular synchrony to be maintained in patients who have impaired atrioventricular conduction with preserved sinus node function. The principal advantages of these pacemakers are the simplified implantation procedure and reduced overload of the venous system.¹

For a number of years, there has been an interest in developing single-lead DDD pacing.^2,3 During 1997 and 1998, at the Hospital Clínico in Zaragoza, Spain, we implanted a number of VDD pacemakers that were also able to provide atrial stimulation by means of overlapping biphasic impulses (OLBIs) from "floating" electrodes. This mode of stimulation involves the generation of two broad large-amplitude unipolar impulses of opposite polarity.⁴

CASE REPORT

Our patient was an 85-year-old ex-smoker with a history of trifascicular block and episodic syncope of cardiogenic origin. In 1997, a permanent VDD pacemaker (Eikos SLD, Biotronik, with an Intermedics UniPass electrode) was implanted. Use of a Holter monitor prior to pacemaker implantation confirmed the existence of severe paroxysmal atrioventricular block but there was no evidence of concomitant sinus node dysfunction.

The pacemaker was programmed to function in a VDD mode just after implantation and performed without problems. During the follow-up period, and in accordance with the standard procedure for evaluating this type of pacemaker, atrial stimulation using OLBIs was started.

During a subsequent routine check-up, intermittent failure of atrial capture was observed during atrial stimulation using AAI pacing at a near-threshold voltage. This resulted in reproducibly induced sustained episodes of intranodal tachycardia (Figures 1 and 2).

Fig. 1. AAI pacing using a floating electrode (external recording). Intermittent sensing and stimulation failures induced intranodal tachycardia. P indicates spontaneous P wave; E, capture failure; A, stimulated P wave; P', P wave during tachycardia.

Fig. 2. a: external lead; b: atrial electrocardiogram; c: ventricular electrocardiogram during intranodal tachycardia. P' indicates P wave; A, atrial activity; V, ventricular activity.

DISCUSSION

The electrode sensors designed for use with single-lead VDD pacing comprise two poles that "float" within the atrium. These enable the atrial depolarization signal to be detected without the need for contact with the atrial wall.

Initially at least, this "floating" state means that the relative position and orientation of the two poles will change in response to numerous factors, including postural alterations, breathing, physical exercise and heart failure.⁵

During VDD pacing, these variations in position and orientation can modify the amplitude of the atrial signal detected such that, in some cases, there is an intermittent loss of atrioventricular synchronization, which is usually of no clinical significance. However, pacemaker syndrome or pacemaker-mediated tachycardia can occur.

During single-lead DDD pacing, failures in signal detection or atrial capture can lead to asynchronous stimulation, which can induce arrhythmias under certain conditions, as were observed in the patient described above. Such arrhythmias can result in episodes of sustained intranodal tachycardia despite the underlying deterioration in atrioventricular conduction. Asynchronous stimulation can also be produced by other types of pacing.⁶

Our experience has shown that the variable nature of this type of pacing can even cause a different chamber to be stimulated by the bipolar electrode in the atrium (Figure 3).

Fig. 3. Variations observed during single-lead DDD pacing (external leads). A indicates effective atrial impulse; V, effective ventricular impulse; a, atrial impulse that triggered ventricular capture; v, ineffective ventricular impulse; P, spontaneous atrial activity.

The possibility that asynchronous stimulation, whether atrial or ventricular, can occur is one of the main obstacles to the development of single-lead DDD pacing. Pacemakers should not be programmed to operate in a DDD mode, at least in ambulatory patients, in view of the possibility that potentially fatal ventricular arrhythmias can be induced.⁴

Full English text available at: www.revespcardiol.org

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Correspondence: Dr. R. Izquierdo González.
Servicio de Cardiología, Hospital Xeral-Calde.
Severo Ochoa s/n. 27004 Lugo. España.
E-mail: [e-mail]