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.