Leadless pacemaker (2018)

Introduction Every year millions of patients receive a pacemaker for the prevention or treatment of symptomatic bradyarrhythmias or atrioventricular blocks, and among them a significant proportion is treated with system utilizing an active anchoring to the myocardium. The concept of leadless pacing is an attractive concept in interventional electrophysiology. The traditional transvenous intracardiac pacing system has its risks (acute, subacute and chronic), and a certain number of them incur in the necessity for wires or generator removal.1–4 The need for improvement has brought forward the leadless pacing technology.5–7 Technological ingenuity provided various modalities able to eliminate the need for intracardiac wires for cardiac sensing and pacing; one interesting solution consists in a wireless transmission of ultrasound from a subcutaneous generator to an intracardiac transductor, located in the left intraventricular region. Currently, this technology (EBR Systems Inc., Sunnyvale, California, USA) is under clinical experimentation and employs an hybrid system with ultrasound technology as well as traditional transvenous biventricular pacing in heart failure.5 Another example of new technology is represented by two very innovative models of cylindrical miniaturized pacemaker with embedded technology for all the required pacing functions. The first device uses an active anchoring to the right ventricular (RV) apex after catheter delivery; the second, instead, has a passive ‘barb’ anchoring system. LEADLESS II6 and MICRA7 studies evaluated the safety and efficacy of these innovative miniaturized cylindrical pacemakers. For the first study, primary end-points were an acceptable pacing (≤2.0 V at 0.4 ms) and sensing (R wave ≥5.0 mV, or a value similar or higher than implant threshold) threshold during the first 6 months of monitoring. For the second study, the primary end-points were the percentage of patients with a low capturing threshold which remains stable at 6-month follow-up (≤2.0 V width 0.24 ms with an increment of ≤1.5 V from implant). The two studies reported the mid-term results for the safety and efficacy of the two pacemaker systems delivered in the RV and capable of single-chamber sensing and pacing (Nanostim LEADLESS Pace-maker; St. Jude Medical and MICRA Transcatheter Pace-maker System Medtronic; Abbott, Saint Paul , MN, USA). This technology is truly remarkable because it contains in the implantable microcylindrical device, released inside the heart, all the hardware normally housed in the traditional generator implanted in subcutaneous position. The LEADLESS release and anchoring technology for the RV wall is very attractive as it potentially prevents many of the complications common to the traditional technology.1–4 During the immediate postimplant phase, these complications may affect one in eight patients and consist of hematoma and infection of the subcutaneous pocket, problems with access for the lead such as pneumothorax or hemothorax, or lead placement itself such as dislocations or malfunctions, systemic infection-like septicemia or endocarditis, vascular obstruction which negate reimplants or upgrading to defibrillator or resynchronization devices. The patients selection process as well as the single-chamber pacing mode followed similarly to the criteria commonly used for traditional pacemakers and was similar for the two intracardiac pacemakers investigated (Table 1). Somewhat different were the criteria which defined the successful implant, with capturing and width threshold for the LEADLESS study, and threshold stability for the MICRA study. The success rate for the implants was high for both the devices (95.8% for the LEADLESS and 99.2% for the MICRA). Likewise the safety (93.3% for the LEADLESS and 96.0% for the MICRA) and efficacy levels (90.0% for the LEADLESS and 98.3% for the MICRA) were high. In both studies, the safety and efficacy levels were higher than the predetermined levels set for use in clinical practice.Table 1: Demographic data, indications, efficacy criteria and clinical outcomes in the LEADLESS II and MICRA studiesThese results are encouraging for the future clinical introduction of these two systems, particularly considering this was the very first clinical experience with them. Major complications occurred in 6.5% of the patients in the LEADLESS study and 4% in the MICRA study. There was one death for metabolic acidosis (not otherwise specified) in the MICRA study, and 1.3% incidence of early cardiac perforation in the LEADLESS study. These data suggest that the leadless systems are not free from risks, at times severe, and their occurrence should be considered in view of the possible advantages for the individual patients as they compare with the traditional technology. Overall, it can be said that the LEADLESS and the MICRA studies suggest that the preliminary results for the new systems display high standard of safety and efficacy, similar to the traditional systems, at least during the first few months of follow-up. These results wait for confirmation with larger studies with longer follow-up. There are aspects of these study, though, deserving further attention. Intuitively, the ‘barb’ anchoring system of the MICRA system appears to be less invasive than the ‘screw-in’ of the Nanumed system. The ‘screw-in’ anchoring system, when faced with the contractile RV wall, particularly the free wall, is more prone to not only early perforation (1.3% in the EFFORTLESS study), but also late, when monitoring and diagnosis are more difficult in the ambulatory setting and the consequences potentially ominous. For both systems, the anchoring of the cylinder to the contracting ventricular wall is such that antegrade (pulmonary) or retrograde (caval) embolization is possible. Albeit infrequent, these complications pertain exclusively to these new pacemaker systems. The precise incidence of these complication still remains uncertain as uncertain are their clinical consequences (in the European trial of Nanumed system three deaths occurred, the causes of which are not reported). Accordingly more data are necessary, and until that time, it would be prudent to recommend these new systems for carefully selected few patients and only if they are part of a study or registry. Acknowledgements Conflicts of interest There are no conflicts of interest.