NS is a common autosomal dominant disease, and the incidence of CHD in those with NS is up to 80%, the most common forms of cardiac disease include PS, atrial septal defect and HCM(6, 7). The severity and prognosis of PS combined with HCM vary greatly. The disease progresses quickly in some patients, and death occurs fairly early. In approximately 70% of infants with NS with HCM, the myocardial hypertrophy can be alleviated spontaneously (7). Restenosis is very likely after balloon dilation for PS. In addition to regular follow-up, HCM can be treated with β-receptor inhibitors, or hypertrophic muscles can be resected by surgery to relieve outflow tract obstruction. For the pediatric patient reported in this study, a modified Konno procedure、PVP surgery was performed to completely relieve the outflow tract obstruction. However, this patient had a grade III AVB after surgery and was indicated for epicardial permanent pacemaker implantation.
The implantation of pacemakers in children can be divided into endocardial implantation and epicardial implantation(5). The advantage of endocardial implantation is that the lead is directly fixed to the endocardium through the venous route. Generally, the pacing threshold is lower, the pacing effect is more stable, and the service life of the pacemaker is longer. The incidence of postoperative complications such as wire fracture and dislocation is low. The disadvantage is that it rarely fits in young infants due to the narrow inner diameter of their blood vessels, thin skin and subcutaneous tissue in the thoracic region, which lead to difficulties in making bags. Moreover, it is not suitable for patients with complex congenital heart diseases whose electrodes cannot be sent to the endocardium by the venous route after the operation. In contrast, epicardial implantation is suitable for children of different ages and weights and is the only option for children with vascular malformations or cardiac structural abnormalities who thus cannot undergo endocardial implantation. Their disadvantage is that their pacing threshold is unstable, and their pacing lead is easily oppressed by bone and muscle, resulting in wear and even fracture.
In this patient, to perform implantation, the chest was opened along the original mid-sternal incision to better expose the atria and ventricles while achieving effective suturing and immobilization of the epicardial pacemaker electrode. As planned, one Medtronic Model 4965 Capsure Epi® steroid-eluting unipolar epicardial pacing lead was placed on the surface of the right atrium and the left ventricle under non-extracorporeal circulation, followed by the placement of a dual-chamber pacemaker. Undoubtedly, the dual-chamber pacemaker can maintain normal sequential atrial and ventricular pacing that mimics a physiological state (8). However, some contingencies occurred during placement of the ventricular electrode. Although the components on the cardiac surface were already sufficiently dissociated and the ventricular electrode was placed repeatedly at different positions on the ventricular surface, the pacing parameters were not satisfactory during testing (threshold 4.0V @ 0.4 ms, 2.6 V @ 0.4 ms, 6.5 V @ 0.4 ms). This may have been related to the fact that epicardial permanent pacemaker implantation was not performed until long after the surgery. For iatrogenic injury after surgery, it is generally believed that the pacemaker should be implanted after 7–14 days(9). Since many Chinese patients are unwilling to receive permanent pacemaker implantation, this interval is usually longer than it should be. Due to the presence of scars and fibrous and hyperplastic tissues after extensive inflammatory responses on the cardiac surface along with the high impedance of the epicardium itself, the capture threshold is generally high. Satisfactory pacing parameters (threshold 1.0 V @ 0.4 ms) were finally obtained after the placement and immobilization of the Medtronic 3830 pacing lead on the surface of the right ventricle near the interventricular septum. The benefits of this procedure can be summarized as follows: (1) There are many sites available for its placement, and there is no requirement for a smooth cardiac surface to ensure the desired pacing threshold; (2) the electrode can be screwed in further away from the phrenic nerve and diaphragm, thus avoiding postoperative stimuli to the diaphragm; (3) a high pacing threshold, large output current and fast battery loss can be prevented, which would otherwise be caused by a loose immobilization of the conventional epicardial pacemaker electrode; (4) the pacemaker electrode is placed into the myocardium and closer to the endocardium, such that the pacing more resembles endocardial pacing.
Conclusion
Cardiovascular surgery sometimes inevitably results in degree III AVB. Surgery is the crucial step to ensure normal epicardial electrode parameters. Proper placement of the epicardial electrode is key its safe and effective operation for better sensing, a lower threshold and longer battery life. Conventional epicardial pacemaker electrodes are usually associated with risks of high thresholds and susceptibility to fracture. Screwing in the epicardial electrode into the myocardium under the naked eye proved to be a feasible approach for implanting ventricular pacing electrodes.