HHE Cardiovascular supplement 2018 - Page 13

The leadless pacemaker was developed to avoid many of the potential complications encountered with traditional transvenous pacemakers. In December 2012, the world’s first leadless pacemaker was implanted in a series of 11 patients at Homolka Hospital in Prague, Czech Republic, heralding the start of a new era in cardiac pacing. 7 These small, capsule-shaped, self-contained devices are implanted directly into the right ventricle, and are entirely intracardiac, eliminating the need for a lead or subcutaneous pocket, and thereby avoiding many of the attendant complications. Their major limitation is the restriction to single-chamber ventricular pacing, and thus to a comparatively small subgroup of patients who require pacing – predominantly those with concomitant atrial arrhythmia, those in whom there is an expectation of only infrequent pacing, or the elderly. There are two kinds of leadless pacing system: the Nanostim™ Leadless Cardiac Pacemaker (LCP) by St Jude Medical (now Abbott), and the Micra™ Transcatheter Pacing System (TPS) by Medtronic. Currently only the Micra™ has Food and Drug Administration (FDA) approval for use in the U nited States. Some of their characteristics are summarised in Table 1, along with those of a standard transvenous single chamber pacemaker. 8,9 Micra™ Transcatheter Pacing System (TPS) The Micra™ TPS is a 25.9mm x 6.7mm device weighing 2g, with a volume of 0.8 cm 3 . Figure 2 illustrates the difference in size between it and Table 2 Electrical parameters of the Micra™ TPS and the Nanostim™ LCP at implantation and six months Parameter Micra™ TPS Mean capture threshold Mean R wave amplitude Mean impedance Nanostim™ LCP Mean capture threshold Mean R wave amplitude Mean impedance Figure 3 Four flexible nitinol tines affix the Micra™ to the right ventricular myocardium Reproduced with permission of Medtronic, Inc. 13 HHE 2018 | hospitalhealthcare.com Implantation Six months 0.63V 11.2mV 724 Ω 0.54V 15.3mV 627 Ω 0.82V 7.8mV 700 Ω 0.53V 9.5mV 486 Ω a standard single chamber pacemaker. At the distal end of the device, four flexible nitinol (nickel titanium) tines affix the Micra™ to the myocardium. At the proximal end is a docking button by which it attaches to the delivery catheter during the implantation procedure, and from which it can later be snared for retrieval if required. Implantation of the Micra™ takes place via the femoral vein through a large introducer sheath. The steerable delivery catheter enables the device to be positioned in the right ventricle (RV). As the device is deployed, the fixation tines emerge from the delivery system and engage with the myocardium (see Figure 3). Device stability and electrical parameters are then tested, and, if needed, the Micra™ can be retrieved and positioned in a different location, multiple times if necessary. When a satisfactory position is found with good electrical parameters, a tether is cut, the delivery catheter and sheath are withdrawn, and haemostasis achieved at the access site. 10 The largest study of the Micra™ TPS, an international, multi-centre, single arm study, included 725 patients who underwent attempted implantation. Of these, 719 (99.2%) were successfully carried out. At six months, 96.0% of patients remained free from major complications. 11 The remaining 4.0% included 11 cardiac injuries, five puncture site complications, two cases of thromboembolism, two pacing issues and eight other events. One death from metabolic acidosis in a dialysis-dependent patient was felt to have been contributed to by a prolonged procedure. There was no incidence of radiologically apparent device dislodgement, no systemic infections and no failures in telemetry. Comparison was made in this study with a historical control cohort of 2667 patients who had undergone transvenous pacemaker implantation. The 4.0% major complication rate in the Micra™ group compares with 7.4% in the transvenous pacemaker group (hazard ratio 0.49; 95% CI 0.33–0.75, p=0.001). The rates of hospitalisation (2.3% vs 3.9%) and system revision due to complications were also lower in the Micra™ group (0.4% vs 3.5%). There was no significant difference between the two cohorts in rates of cardiac injury, access site issues or problems with pacing. 11 In 98.3% of patients capture thresholds were low and stable, and paired electrical data from implantation and six-month follow up indicate a tendency toward improvement over time. Electrical parameters are shown in Table 2. 11