Alternative pacing sites at the atrial level
1 Thoraxcentre, Erasmus MC, Rotterdam, The Netherlands
2 St Mary's Hospital, London, UK
* Corresponding author. 5 Devonshire Place, London, W1G 6HL, UK. Tel: +44 2079351011; fax: +44 2079356718. E-mail address: r.sutton{at}imperial.ac.uk
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Abstract |
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 Top Abstract IntroductionGeneral considerations: specific...Pacing of the interatrial...Multi-site pacing techniques:...References |
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Pacing the atria at alternative sites may have value in preventing atrial fibrillation but the available data so far fails to prove this point. This may be explained by inadequate lead positioning. Improvement in lead placement can now be achieved by use of advanced imaging techniques such as intracardiac echocardiography. Further studies are required in order to demonstrate the potential benefit that, for example, Bachmann's bundle stimulation should offer in atrial arrhythmia prevention.
Key Words: Atrial fibrillation • Interatrial sepum • Bachmann's bundle • Atrial pacing • Alternative pacing sites • Intracardiac echocardiography • Atrial fibrillation prevention
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Introduction |
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TopAbstractIntroductionGeneral considerations: specific...Pacing of the interatrial...Multi-site pacing techniques:...References |
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The issue of the optimal atrial pacing site or sites has received much attention particularly with respect to suppression of atrial fibrillation. Several sites have been suggested for stimulation of the atria tissue with the aim of reducing atrial activation time.
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General considerations: specific site pacing techniques |
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TopAbstractIntroductionGeneral considerations: specific...Pacing of the interatrial...Multi-site pacing techniques:...References |
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Recently, novel ways of pacing have been proposed for the treatment of patients with various types of arrhythmia.1–6 There is growing evidence to suggest that pacing at the atrial level in the region of Bachmann's bundle, the interatrial septum (IAS) or even multiple atrial sites may have advantages in patients with atrial fibrillation.6–10 However, these specific site pacing techniques require extremely accurate lead positioning compared with those used for conventional bradycardia indications.11 This may explain that adequate results have not yet been obtained with these new pacing approaches in a considerable number of patients.12 One possibility may be that anatomical variations may adversely affect efficient lead positioning.13,14 Improved imaging has a potential role to facilitate these pacing techniques as discussed below.14
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Pacing of the interatrial septum |
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TopAbstractIntroductionGeneral considerations: specific...Pacing of the interatrial...Multi-site pacing techniques:...References |
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Pacing the IAS has been reported to be effective to attenuate the progression of AF, but the optimal atrial pacing site remains controversial.7–9
Anatomical and pathological considerations
Anatomically, the most prominent part of the atria is the IAS with its very characteristic appearance. It is defined as the thin wall separating the two atria, running obliquely from the anterior aspect, posteriorly and to the right. When it is seen from the right atrium, the most definable feature is the fossa ovalis surrounded by a muscular rim. When seen from the left atrium, the crater-like appearance on the right side is absent. The membrane in the fossa itself is predominantly composed of fibrous tissue with relatively few myocytes.15 Its form can be explained by its embryonic development, which is outside the scope of this review. An important electrophysiological aspect of the IAS is related to the natural conduction pathways between the two atria. These special fibres are located both anteriorly above the fossa ovalis and posteriorly near the orifice of the coronary sinus. Recent data suggest that interatrial conduction delay may play an important role in the onset mechanism of atrial fibrillation.16 Thus, stimulation of these regions may, conversely, play a role in prevention of atrial fibrillation.
Electrical activation of the atria
The electrical conduction from the right to the left atrium occurs in regions with fast right to left conduction. This is mainly determined by the conduction properties and orientation of atrial muscular structures. These structures are very complex and developed during the early embryonic phase. Within the anterior superior atrial wall parallel bands of atrial muscle fibres extend from right to left in the interatrial groove. This serves as the preferential conduction route from the right to the left atrium, called Bachmann's bundle. This is located above the fossa, while another potential fast conducting pathway between the atria are the muscular margins of the fossa ovalis located inferiorly to its membrane. The remaining pathway is located in the septum primum.16
Rationale for septal pacing and reduction of P-wave duration
Bradycardia, disturbances of the autonomic nervous system, atrial dilatation, atrial stretch, and the presence of triggers (including atrial premature beats) are factors, which predispose to paroxysmal AF.2,17–19 These factors cause loss of homogeneity of refractoriness and conduction. Pacing can abolish several of these factors. The conventional site for atrial pacing is the right atrial appendage, but stimulation at this site delays both intraatrial and interatrial conduction. This was confirmed in recent trials, which failed to show any benefit from atrial appendage pacing for patients with paroxysmal atrial fibrillation. Therefore, alternative pacing sites within the right atrium have been investigated. The antifibrillatory effect of Bachmann's bundle (BB) pacing was demonstrated in both acute and chronic settings including a randomized multi-centre study.7 Other alternative pacing sites have also been tested such as the posteroseptal region just above the ostium of the coronary sinus (CS).9 Reduction in P-wave duration varied in a wide range from 9 to 36 ms in early reports of septal pacing. Interestingly, in the study of Bailin et al.7 pacing of the region of BB resulted in less reduction in P-wave duration than in the earlier study of Padeletti, where the septum was paced near to the ostium of the CS.9 A possible explanation for these controversial findings is related to methods of measurement of P-wave duration. Simultaneous recording of at least three standard leads and obtaining the total P-wave duration is better for identifying patients at risk rather than just taking the duration in the longest lead (e.g. lead II).
A more likely explanation is the accuracy of the pacemaker lead positioning, which was not optimized in the above-mentioned studies. Using 3D echocardiography guided lead placement, a remarkable 43 ms mean reduction of P-wave duration was achieved, which is more than any of the previous studies.14 By using this direct method for identification of the optimal site less variability in P-wave duration and, moreover, the final lead position could be verified (Figure 1). These data point to the importance of guided IAS pacing with accurate lead placement.
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Imaging guided pacing
Echocardiography has a potential role in guiding pacemaker lead implantation for special indications. The feasibility of transoesophageal echocardiography (TOE) and 12-lead ECG guided septal pacing was demonstarted.1,20 However, in their study, Szili-Torok et al.21 clearly demonstrated that the 12-lead ECG can be misleading. A reliable trend could not be observed in P-wave axis changes, especially from the suprafossal lead position group. TOE seems to be a feasible tool but has some disadvantages. It is uncomfortable for patients during long lasting procedures, has its own complications and the visualization of intracardiac structures is less accurate. Intracardiac echocardiography (ICE) allows visualization of important anatomical landmarks with excellent accuracy and has been shown to be a feasible tool to guide interventions in clinical electrophysiology. With 3D reconstruction ICE becomes a tool which provides an outstanding opportunity for on-line assessment of the anatomy.13,14,21,22 Using 3D echocardiography, the most relevant right atrial structures are readily recognized. In addition, after implantation the atrial lead can be visualized without technical problems despite the small amount of metal content of the pacing lead. The results of intracardiac measurements demonstrate that significant variations exist in right atrial anatomy including distances between structures.21 The location of the fossa ovalis is quite unpredictable,21 and simply cannot be adequately judged using standard fluoroscopy imaging (Figure 2). In some patients, it is situated low in the right atrium and in others it is placed much higher. Therefore, visualization of this landmark has advantages during the implantation procedure.
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Multi-site pacing techniques: atrial level |
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TopAbstractIntroductionGeneral considerations: specific...Pacing of the interatrial...Multi-site pacing techniques:...References |
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It was demonstrated that maximal interatrial conduction delay occurred at the coronary sinus os and IAS in response to right atrial ectopy. Multi-site pacing can be either dual-site right atrial or bi-atrial (left and right) pacing.5 Dual-site right atrial pacing is performed with one atrial lead in the high right atrium (HRA) and one close to the coronary sinus ostium (CS os).5 Bi-atrial pacing requires one coronary sinus lead (proximal or distal) for left atrial activation and one HRA lead.5 In addition to the potential benefits of single-site right atrial pacing previously discussed, multi-site atrial pacing been shown to reduce intra-atrial conduction delay and reduce the dispersion of atrial refractoriness, both known to be substrates of atrial fibrillation.23 Studies with these pacing techniques were designed in patients with a bradycardia indication for prevention of atrial fibrillation. Results consistently showed that these pacing modalities were associated with significant decrease in atrial activation time.23 Unfortunately, this obvious pathophysiological advantage was not associated with a positive clinical impact in the only available multi-centre randomized trial.12 Interestingly enough, several small studies have shown that dual-site RA pacing decreases the incidence of atrial tachyarrhythmias when compared with no pacing or single-site RA pacing. Similarly, with the bi-atrial stimulation, a multi-centre randomized trial of dual-site atrial pacing vs. high RA or support (DDI or VDI) pacing did not reveal a statistically significant benefit of dual-site atrial pacing.21
In conclusion it is questionable whether pacing will ever be a major therapeutic option for the prevention of atrial fibrillation. However, the results obtained from these alternative pacing sites studies have had a significant impact on our current pacing practice. Pacemaker leads and devices have improved significantly in the past two decades allowing stable, safe, and more physiological pacing positions in the right atrium. More importantly, studies of these pacing sites have definitely added to our understanding of atrial arrhythmia mechanisms.
Conflict of interest: none declared.
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References |
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