In this exploratory study with an intermediate follow-up duration, STE still adjudicated the presence of subtle LV dysfunction in patients with CCAVB despite targeted lead placement strategies in childhood that obviated RV apical pacing. Their concurrent assessment by conventional echocardiographic parameters was ostensibly normal. Furthermore, STE revealed subtle differences in the degree of LV dysfunction among this smaller population with LV epicardial vs. RV septal pacing.
As enunciated below, our findings should be contextualized and contrasted to the well-established framework of detrimental effects of RV apical pacing and usual assessment of the consequent LV dysfunction using conventional echocardiographic surrogates such as SF and LVEF. Children with CCAVB and a permanent pacemaker accrue a chronic, obligatory high ventricular pacing burden. The choice of ventricular pacing site is shown to affect long term hemodynamic and ventricular function [6, 17, 18]. Standard echocardiographic assessment of LV function is simpler to perform, but is limited in its ability to accurately evaluate mechanical contractility or detect subtle segmental dyssynchrony. STE is a load independent technique for accurately assessing regional/global cardiac contractility. It confers additional advantages over LVEF/SF and has emerged as a robust predictor of cardiac morbidity/mortality in a various cardiac conditions [19].
With the aforementioned paradigm, the intention of pursuing an alternative site (non-RV apical) of pacing in a vulnerable population of children is then to attenuate any LV electro-mechanical dyssynchrony and preserve its function. This is deemed to be better assessed by STE. Thus, our unique study population and methodology employed here represents a suitable set up to test the validity of this approach. Our preliminary results suggest that an alternative pacing site to RV apex (LVEp and RVSp) is still associated with detectable abnormalities on STE. There are only limited and methodologically different studies to reference when evaluating the use of STE in patients with CCAVB. Kovanda et al., used STE to evaluate systemic LV function in LV paced patients with complete AV block in the presence (n = 14) or absence (n = 22) of repaired congenital heart disease vs. age-matched normal controls (n = 25) [18]. They reported preserved global LV function with no significant difference between the groups with regards to global longitudinal strain or EF.
An erudite discussion regarding the results of our study suggesting that RVSp may yield less LV dysfunction than LVEp as detected by STE is precluded by small numbers. If this is indeed portended, then a hypothesis based on leveraging the cardiac conduction system by RV septal pacing to more optimally synchronize the LV becomes tenable [20]. However, this merits further validation in larger study in this specific population. In this context, Karpawich et al. from our own center have previously advocated for RV lead implantation closer to the normal conduction system in children [21].
There is no antecedent to our study specifically comparing LVEp to RVEp using STE in children with CCAVB. Only limited studies exist on the long-term effect of RV endocardial pacing in this population. Most studies only acknowledge the superiority of LV compared to RV pacing when utilizing an epicardial site based on preservation of LV function overtime. In a retrospective multicenter study involving 178 children with AV block and a normal heart that required chronic pacing, Janoušek et al. compared LV function utilizing conventional and STE echocardiographic parameters with respect different sites of pacing. This included epicardial (n = 97) and RV endocardial (81) locations including RV septum (n = 29). Overall LV pacing was deemed to be superior to RV pacing (endocardial or epicardial), but there was an unequal distribution of patients in each subgroup. Nevertheless, it was suggested that selective RV septal lead placement might bear promising results [22].
The inability of conventional echocardiographic assessment to pick up LV dysfunction in our own patients may be related to its inherent technical limitations and our smaller sample size. Avoidance of RV apical pacing can also be speculated to have attenuated the effect of LV dysfunction to a subtler degree that fell below the threshold of detection by using LVEF/SF. In contrast, a larger study may be more suitable to capture the effect of site of pacing predicated by conventional assessment of LV function. In such a multicenter study of 297 children with CCAVB, Van Geldorp et al. noted abnormal (SF < 28%) or depressed (SF < 25%) LV function in 14% and 10% patients respectively with RV pacing (endocardial or epicardial). They also reported better LV function (SF) in patients who were paced from the LV epicardium [6]. Similarly, Guerra et al. evaluated the prevalence of LV dysfunction in 50 patients with CCAVB [median age 20 (5 months to 62 years) years, 68% female] using different echocardiographic techniques such as conventional 2D echocardiography, real time 3D echocardiography (RT3E) and Tissue Doppler imaging (TDI). LV systolic dysfunction was noted more frequently using RT3E (40%) than 2D echocardiography (32%). Intraventricular dyssynchrony was reported to be similar using TDI (28%) and RT3E (24%) [23].