DISCUSSION
This study compared patients with an increased pacing threshold shortly
after TPS implantation with those who maintained a stable pacing
threshold. Our results suggest that impedance and threshold at the time
of implantation may serve as predictive factors for an increased pacing
threshold (AUC: 0.737–0.943 and 0.586–0.926, respectively). All
patients requiring a higher pacing threshold were in the IPT group,
suggesting that if a threshold increase can be avoided within the first
month after TPS implantation, a stable threshold may be obtained in the
medium-to-long term.
Impedance is the sum of resistance in the electrode lead and the
myocardium owing to contact between the two. Therefore, the difference
in impedance is a measure of whether the TPS is sufficiently anchored to
the myocardium. To anchor the device, four nitinol tines at the tip of
the TPS are hooked to the myocardium. It has been shown that at least
two tines must be applied to minimize dislodgement of the device, which
may occur due to the cardiac rhythm.10 However, there
are conflicting reports where, despite two tines being anchored to the
myocardium, the TPS tail was reported to shift, leading to an increase
in the threshold with a changing bodily position.11,12To achieve sufficient fixation to the myocardium with two tines, the
tines must be attached perpendicular to the myocardial
wall.10 If the tines are leaning against the
myocardial wall, they do not provide enough force to secure the device;
thus, the device may shift with an increase in the cardiac rhythm during
exercise conditions. Therefore, a slight movement between the heart
muscle and the device could result in micro-dislodgement. In this study,
at least two or more tines were anchored by the pull-and-hold method in
all patients. However, the low impedance at the time of implantation in
the IPT group meant that the tines were not sufficiently anchored to the
myocardial wall, resulting in micro-dislodgement. We observed that the
pacing threshold increases shortly after implantation in this group,
suggesting that additional investigations are required for the
evaluation of adequate anchoring of the TPS to the myocardium other than
the number of tine connections. While it was expected that impedance at
the time of implantation would function as a predictive factor for the
altered threshold status, it was also observed that the pacing threshold
at the time of implantation served as a predictor in that it tended to
be low if the device was well-anchored. This finding is similar to a
report by Tolosana et al., who showed that impedance and threshold at
the time of implantation play important roles in achieving a pacing
threshold of ≤2.0 V/0.24 msec.7
We observed that the pacing threshold improved in the majority of cases
in the IPT group, while a higher threshold was maintained in a few
cases. This may be due to fixation and adhesion of the dislodged device.
However, we do not have sufficient evidence to support this claim, and
further studies are needed.
We experienced two cases of pacing failure in the IPT group. This may be
because the pacing threshold was higher than the initial pacing
threshold (+ 1.5 V) set at implantation. Our results suggest that if the
electrode impedance is low and the pacing threshold level is high
immediately after TPS implantation, maintenance of a sufficient margin
to the established output with periodic monitoring will help lower the
risk of pacing failure. The results of this study indicate that if the
electrode impedance is low or the pacing threshold is high at the time
of TPS implantation, it is better to maintain a sufficient margin of the
set output.
In addition, pericardial effusion was observed in one case in the SPT
group. In this case, the Micra-TPS was attached to the inferior wall.
When positioning the Micra-TPS, the thickness of the myocardium should
be considered to ensure safety of the right ventricular
septum.13 To achieve sufficient compression fitting of
the Micra-TPS and myocardium, it is necessary to press the Micra-TPS
with adequate force onto the myocardium. It is, therefore, recommended
that the Micra-TPS be positioned on the right ventricular septum, if
possible.