Alternative site pacing: accessing normal precordial activation: is it possible?

Pacemaker implantation is necessary in patients with symptomatic bradycardia. The right ventricular (RV) apex is the traditional pacing site because of its ease of access with simple and reliable technology. However, by forcing mechanical desynchronization, this may induce adverse effects, especially in heart failure patients. To avoid these problems, alternative sites for pacing have been explored with the idea of preserving normal ventricular activation. The His bundle is an obvious target, and results are promising, but permanent pacing is fraught with technical difficulties. Assessment of the RV septum and outflow tract has generated inconclusive results. Significant limitations to trials to date are inconsistency of electrode deployment to selected regions (themselves vaguely defined), and the unverified assumption that a single RV region will yield the desired ventricular activation pattern in every patient. Notably, an electrical measure of satisfactory lead positioning has been lacking, despite the fact that QRS abbreviation is associated with improved hemodynamics. Nevertheless, pooled trial results suggest benefits, which tend to accrue with time, especially in patients with LV dysfunction, i.e., the group vulnerable to RV apical pacing. Can results with alternate site pacing be improved? The location of any "sweet spot" may be variable among individuals (or even non-existent), requiring identification by its electrical effect to guide electrode deployment, accurately. The selected alternate sites need to avoid delayed transseptal activation associated with RV apical pacing (similar to LBBB) and result in rapid LV depolarization. However, distinctions may not be binary. For example, apical pacing exerts diverse actions in different individuals, not all of which are deleterious. A relatively underlooked aspect is that both apical and alternate RV site pacing may affect right ventricular activation and thus interventricular dyssynchrony, interfering with ventricular coupling and pump function. Most of these effects are not evident on surface QRS, which is an indirect approximation of electrical action. This may be elaborated with more detailed mapping, e.g., non-invasively with electrocardiographic imaging. In the future, pacing site selection individualized according to detailed biventricular activation effects may enhance outcomes with alternate site pacing.