UHF-ECG dyssynchrony during septal pacing
The pacing of the RV septum has been considered to be more physiological than apical pacing [9]. However, its clinical benefit was never shown in a prospective randomized trial [2, 3]. But the right ventricular septum is not a homogenous structure. It consists of an inflow and outflow tract, and also the His bundle and its ramification are localized in its basal part. The pacing in the basal septal area with concomitant myocardial and conduction tissue engagement produced the most physiological UHF-ECG depolarization pattern in our study. In seven patients, proximal RBB capture rather than His bundle capture was seen when this was considered based on the duration of HV intervals. QRSd in these seven patients was significantly shorter but the difference in the LVLWd didn’t reach statistical significance compared to other cSp captures due to the proximal position of the pacing lead within the right Tawara bundle in most of them. An example of how the distance of the lead capturing the septal conduction system from the plane of the tricuspid valve can influence the ventricular depolarization pattern and the QRSd is shown in Figure 7.
An important shortcoming of studies comparing the clinical benefit of RV septal and apical pacing involved inadequate definitions of lead placement in the septum. They were often based on ECG or X-ray criteria, both of which have been shown to be inaccurate [10, 11]. This led to incorrect placement of pacing leads on the anterior wall (e.g., in the anteroseptal grove) in a substantial percentage of patients [2, 3]. To overcome this shortcoming, we used the position of the lead in the para-hisian area displayed in the LAO and RAO projections as a landmark. Its position in the RAO projection was used to approximate the position of the crista supraventricularis [12]. Pacing in the RVOT region above this level has been shown to lead to significantly worse UHF-ECG LV dyssynchrony compared to pacing on or below the level. Interestingly the mean QRSd was the same in RVIT and RVOT captures, which corroborates previous studies showing QRSd to be an imperfect marker of ventricular dyssynchrony during ventricular pacing [13]. The difference in the LVLWd between these two pacing locations is highly likely a result of different distances from the left ventricular subendocardial Purkinje system. Since it is located in the lower third of the LV septum and LV free wall, time to its activation is necessarily shorter during RVIT pacing, which is anatomically directly opposite. Once it is activated, its contribution to left ventricular depolarization is greater during RVIT capture, which was also reflected by a shorter V5-8d compared to RVOT captures.