DISCUSSION
This prospective, randomized study assessed the feasibility of right ventriculography in assisting deployment of MicraTMleadless pacemaker in the RV mid-septum and has several important findings. First, this technique yielded a high accuracy (93.8%) of the MicraTM pacemaker deployment in the target location compared to the accuracy (48.1%) of the standard procedure without right ventriculography. Second, with the assistance of right ventriculography the fluoroscopy exposure time was significantly reduced while the appropriate pacing parameters were preserved, and no implantation-related adverse events were observed. Finally, septal pacing, especially in the mid-septum, provides a relatively narrow ECG QRS duration (123 ms in average).
The MicraTMCoverage with Evidence Development Study (the Micra CED) in 5746 patients implanted with MicraTM leadless pacemakers found that the incidence of complications following implantation was 3.3% in a 6-month follow-up period, in which myocardial perforation was the most common complication.11, 12 Other complications included pericardial tamponade and anterior descending artery injury.7, 11, 13 To reduce these implantation-related complications, the right ventricular septum has been a target location for leadless pacemakers.6, 14The presence of more myocardial trabeculae and columnae carneae in the right ventricular septum, particularly prominent in the middle of the septum, than in the right ventricular apex and right ventricular outflow tract makes the RV septum an ideal location for MicraTM leadless pacemakers with a less likely occurrence of perforation. In the Micra CED study, 52.1% of MicraTM leadless pacemaker locations were in the RV septum, 39.3% in the RV apex, 1.9% in the RV outflow tract, and 6.3% in junctions (including apex-septum and inferior septum).11 Sharma P et al.(2020)6reported that 51.4% of leadless pacemaker implants were located in the ventricular septum, 28.6% in the right ventricular apex, and 20% in the RV outflow tract.
Currently, conventional fluoroscopic method is used in leadless pacemaker implantation. In the study by Hai JJ et al.(2019)15 in which multi-angle fluoroscopy was used to guide leadless pacemaker implantation in the mid-septum, 90% of the pacemaker implants were intended in the interventricular septum and 5% were in the right ventricular outflow tract under the RAO view. However, when left-sided fluoroscopy was performed, the actual implantation site was observed at the anterior free wall in 17.6% of patients with intended septal implantation. Therefore, developing an accurate and effective method for implanting leadless pacemakers in the mid-septum remains a challenge.
The present study observed a higher accuracy of the MicraTM leadless pacemaker deployment at the target location with assistance of right ventriculography than that without right ventriculography. Furthermore, the present study demonstrated not only the safety of the technique but also a significant reduction in X-ray exposure dose and time. The less fluoroscopic use time in the radiography group was likely because the operator could directly maneuver the distal part of the delivery catheter against the mid-septum according to the right ventriculography while in the non-radiography group the operator used more time and fluoroscopy repositioning to figure out the target site. The implantation site in the inferior septum is close to the RV apex, which may have a high risk of cardiac perforation while the implantation site in the superior site has fewer myocardial trabecula, which can cause difficult in the leadless pacemaker deployment. In consideration of the effects of right ventricular anatomic variants and cardiac transposition on the leadless pacemaker implantation process, right ventriculography provides more detailed information on the right ventricular contours and anatomic landmarks in the fluoroscopic LAO and RAO positions. It may also be used to identify the septal tangent and position of the tricuspid valve. Additionally, right ventriculography can show the specific anatomic IVC and heart structural variants and allow easier access to the right ventricle with guidewires and sheaths. Right ventriculography has previously been used to identify the target site in the RV mid-septum for the lead deployment for left bundle branch pacing and His bundle pacing.16 Therefore, right ventriculography can be a useful technique to facilitate the MicraTM leadless pacemaker as well as transvenous pacing lead deployment at an intended location.
Conventional RV apical pacing causes a significant wide ECG QRS duration and interventricular dyssynchrony that are associated with an increased risk of cardiac dysfunction in patients with frequent pacing. Pursuit of more physiological pacing method is continuing. Previously, septal pacing has been proposed as an alternative to RV apical pacing though there has been no conclusive clinical evidence. The true location of the pacing lead tip using fluoroscopy is often inaccurate, leading to conflicting results of septal pacing in acute and chronic studies. Recently, the study by Sharma et al demonstrated that a mid-septal pacing by MicraTM leadless pacemaker resembles physiological pacing and tends to achieve a relatively narrow paced QRS interval.6 Our study also found that pacing by MicraTM leadless pacemaker at the mid-septal location generated a relatively narrow QRS duration and left ventricular activation time, suggesting a more physiological pacing modality (Supplemental Material Figure S2, a comparison in 12-lead ECG between RV septal pacing vs. apical pacing). Currently, left bundle branch pacing has been considered as a physiological pacing modality.17-19 To achieve left bundle branch pacing the pacing lead tip is screwed into the left side of the septum from the right side of the RV mid-septum.20 In the future, MicraTM leadless pacemaker with a modified long pacing electrode and RV ventriculography for the target entry site in the RV septum can also achieve left bundle branch pacing.
Study limitations. The study was performed in a single center with relatively small patient numbers and in a non-blinded fashion that might introduce potential bias. The study could not address the potential difference in safety between the ventriculography group and the non-ventriculography group because the operator made the effort to place the leadless pacemaker in the RV septum in both groups. The generalization of the technique of right ventriculography in routine clinical practice needs further validation in a multi-center study with a large population and diverse pacing indications. Furthermore, long-term clinical outcomes with this approach and pacing in the mid-septum remain unknown though the present study found a relatively narrow QRS duration and a recent report showed a possibility of leadless pacemaker in prevention of pacing-induced cardiomyopathy.21 Thus, clinical studies are needed to evaluate the long-term clinical effects of MicraTMleadless pacemaker implanted in the mid-septum in comparison with the pacing site in the RV apical septum or apex.
Conclusion. In the present study, use of right ventriculography yielded a high success and accuracy rate to deploy a MicraTM leadless pacemaker in the mid-septum with reduced time in X-ray exposure without compromising safety. Furthermore, the study found that pacing at the mid-septum had a relatively narrow ECG QRS duration. Septal deployment of the MicraTMleadless pacemaker has a potential to reduce complications such as perforation, effusion and pericardial tamponade when compared to the implantation site at the RV apex. Thus, right ventriculography to guide MicraTM leadless pacemaker implantation in the mid-septum can be a useful method, especially for beginners, less experienced centres, and patients with specific cardiac anatomical variations in a routine clinical practice.
Funding
National Natural Science Foundation of China (Grant number:81873487) awarded to Yaodong Li.
Conflict of interest:
All authors report no conflict of interest.