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.