Pacing the Right Ventricular Outflow Tract Septum - Are We There Yet?

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DOI
https://doi.org/10.15420/apc.2007:1:1:57

Since the introduction of transvenous cardiac pacing almost 50 years ago,1 the right ventricular (RV) apex has been the preferred site for ventricular lead attachment due to the ease of placement, stability, reliability and lead design. Unfortunately, pacing from the RV apex produces an abnormal late activation of the lateral wall of the left ventricle (LV),2 with ventricular remodelling resuling from neurohormonal and electrophysiological changes. This induces differential muscle strain3 and fibre shortening, which in turn increase myocardial work4 and oxygen consumption.5 The resultant changes in cardiac haemodynamics6,7 cause LV cellular abnormalities at both a gross and ultrastructural level,8 and ultimately lead to ventricular dilatation and impaired lusitropy.

Clinically, there is a higher risk of development of LV dysfunction,9,10 heart failure11,12 and atrial fibrillation.13 These observations have led to an interest in selective non-apical RV pacing sites in order to achieve a less eccentric and more physiological pattern of ventricular activation.12,14,15 To date, the alternative sites for RV pacing have included the low interventricular septum, mid-septum, His-bundle16,17 and RV outflow tract (RVOT). Some of the difficulties encountered with lead placement in these areas relate to the lack of suitable lead technology, the non-standardised nomenclature and difficulty with consistent, accurate and reliable placement of leads in the selected position.

The most studied of these selective sites has been the RVOT, with increasing focus on the septal aspect of this structure. Unil recently, true RVOT septal pacing has been difficult to achieve. We now have a much better understanding of the relationship between the electrocardiograph, fluoroscopy and the anatomy of the RVOT.15,18 This has allowed successful recognition of the lead placement, particularly onto the RVOT septum. It is important to remember that when using standard implantation techniques, the RVOT septum is reached only in 61% of unselected cases, suggesting that new implantation techniques and tools are required.15

The Anatomy of the Right Ventricular Outflow Tract with Relevance to the Pacing Site

The right side of the interventricular septum has been poorly defined in the pacing literature and the term RVOT has been used to describe a variety of pacing sites, including the true outflow tract, the mid-septum and the area above the apex. This confusion persists, despite attempts to standardise the nomenclature of non-apical pacing sites.19,20 The anatomy of the RVOT is complex and includes the septum, the free wall and the anterior wall.

This differentiation is important as activation patterns and wave front propagation will be very different depending on the pacing site from within the RVOT. Improved outcomes, both acutely21–23 and in the medium term,24–26 have been consistently demonstrated with RVOT septal pacing compared with pacing at the RV apex. This is in contrast to the heterogeneous results of studies that did not specify an RVOT pacing site.27–29 Thus, when positioning a lead in the RVOT it is critical that only the septum is considered the target for lead placement.

For purposes of cardiac pacing, the RVOT is bordered by the pulmonic valve superiorly and the superior aspect of the tricuspid apparatus inferiorly. In fact, ‘septal RVOT’ is a misnomer in that it abuts the proximal ascending aorta, and thus the upper part of the RVOT ‘septum’ lies above the aortic valve.18,20 Therefore, only the lower or inferior portion of the RVOT septum can be considered as truly septal. The ‘septal’ component of the conus arteriosus is high and smooth-walled and its position makes it both anatomically and electrophysiologically unsuitable for the attachment of a pacing lead. However, below the level of the supraventricular crest (crista supraventricularis) lies the inferior and truly septal portion of the RVOT, which has a cul-de-sac filled with the septoparietal trabeculations and is ideal for active-fixation pacing lead attachment (see Figure 1). Similar to other areas within the RVOT, this area has characteristic paced electrocardiogram (ECG) and radiographic appearances that allow accurate recognition of lead placement.30

Electrocardiogram Correlates of Lead Position in the Right Ventricular Outflow Tract

RVOT septal pacing is associated with shorter QRS durations than anywhere else in the RV and, in particular, the RVOT free wall.15 This suggests that pacing from the septal RVOT, although not as good as intrinsic conduction, may be the most desirable site for chronic RV pacing, as a narrow QRS duration is associated with an improved LV dynamic.23 Pacing from the RVOT septum typically produces a negative or isoelectric vector in lead I. Conversely, a free wall site is associated with a positive vector in lead I as well as a more prolonged QRS duration and notching of the inferior leads, in particular lead III.15

Radiographic Anatomy of the Right Ventricular Outflow Tract

For recognition of RVOT septal lead placement, three views are essential (see Figure 2). The postero-anterior (PA) and the 40º right anterior oblique (RAO) aspects are the best views for guiding the lead into the low RVOT and excluding inadvertent positioning in the coronary sinus and great cardiac vein. Differentiation between the septal and free wall aspects of the RVOT are best defined by the 40º left anterior oblique (LAO) view. The septal position is characterised by a posterior orientation of the lead tip, while the free wall positioning is seen with the lead tip facing anteriorly. A fourth view, the 90º left lateral (LL), is also very valuable, but can be performed only after the operative procedure. A posterior projection of the lead tip indicates septal placement and is 100% specific.15 In comparison, a lead attached to the free wall passes anteriorly towards the sternum.

Lead Selection and Placement on the Right Ventricular Outflow Tract Septum

In order to position a pacing lead on the septal aspect of the RVOT, an active-fixation lead lying within the RVOT must be guided posteriorly. This can be achieved using a thin (4.1-French) lumenless fixed screw-in lead (Select Secure® Medtronic Inc. Minneapolis, US) passed through a steerable 10-French catheter delivery system (Select Site®, Medtronic Inc.). To date, the only data published are concerned with implant safety and electrical performance31 and, because the procedure requires the use of a workstation and a lead unfamiliar to most implanters, specialised training is required and there is a significant learning curve. In contrast, a novel technique to reliably attach a conventional 7- or 8-French active-fixation lead to the RVOT septum has recently been described.30 The technique uses a specially shaped conventional stylet. Leads with the appropriate shape can be inserted from both the left and right sides using either the cephalic or subclavian venous access, and can be deployed onto the RVOT septum in 100% of cases with a rapid learning curve.

Safety and Reliability of the Right Ventricular Outflow Tract for Chronic Right Ventricular Pacing

In our experience of more than 500 cases of stylet-driven leads attached to the RVOT septum, anterior or free walls, no dislodgements have occurred. The result is similar to other reported series.27,32 Within this group, the last 100 cases have all had the leads positioned on the septum. There have been no pacing complications documented.

In particular, there have been no cases of elevated stimulation thresholds or exit block.30 Due to the anatomical position of the electrode lead perforation, pericarditis, pericardial effusions and diaphragmatic or intercostal stimulation cannot occur. In contrast, the stability and reliability data for the catheter-delivered Select Secure leads revealed a number of complications.31

There were 237 ventricular leads at the RVOT and 79 at the RV apex. Seven acute lead dislodgements were reported for the total group, although the positions were not defined. A number of serious complications were reported, primarily related to injury caused by the catheter delivery system. A modified deployment technique was then instituted and these rates were reduced to acceptable levels.31

Conclusion

Chronic pacing from the right ventricular apex has been shown to result in left ventricular dysfunction. An alternative site, the RVOT septum, may provide more physiological activation of the ventricles and this site is now easily accessible, reliable and stable for lead placement. RVOT septal pacing is associated with shorter ventricular activation times, suggesting that it is the ‘sweet spot’ for chronic ventricular pacing. There exists a need for long-term pacing studies with RVOT septal pacing compared with RVA pacing looking at the incidence of pacing-related LV dysfunction, heart failure, hospitalisation and death.

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