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

Abstract

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 strain 3 and fibre shortening, which in turn increase myocardial work 4 and oxygen consumption. 5 The resultant changes in cardiac haemodynamics 6,7 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 failure 11,12 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-bundle 16,17 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 acutely 21-23 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