https://orcid.org/0000-0002-4499-0806
Overview of developments in implantable cardiac pacemaker technology, looking speculatively at futuristic advances in the realm of symbiotic nanogenerators capable of extracting energy from myocardial motion.
This editorial refers to ‘Leadless pacemakers at 5-year follow-up: the Micra transcatheter pacing system post-approval registry’, by M.F. El-Chami et al., https://doi.org/10.1093/eurheartj/ehae101.
Around 30 years ago, when our paper describing a leadless miniature pacemaker implanted endocardially was published in a decent quality but limited recognition scientific journal,1 the response did not suggest that our peers were very excited.
At major cardiology conferences, I was often called upon to explain the advantages of a ‘bullet pacemaker’, as the participants called our experimental device, compared with traditional transvenous pacing. However, we remained optimistic that the future of heart rhythm devices would lie in the concepts of miniaturization, leadless and scarless pacemakers, and—indeed why not—devices fuelled with energy by the myocardium itself.
This issue of the European Heart Journal includes an article by El-Chami et al.,2 based on the Micra post-approved registry (Micra PAR) study, which reports the findings from 1804 patients who underwent placement of a Micra pacing system, in 179 centres in 23 countries, over a median follow-up of 51.1 months. Micra VR PAR is a non-randomized, prospective registry study that was designed to assess the performance of the Micra VR system when used in real-world clinical practice throughout the device’s life cycle.3
The objective of this current study, the latest in a series of publications from the experienced international group of M. El-Chami, was to record any major complications, note any cases requiring system revision, and compute all-cause mortality during the study period. ‘Major’ complications were defined as system- or procedure-related adverse events that resulted in death, permanent loss of device function, or prolonged hospitalization (≥48 h). The study found the Micra leadless pacemaker to have high reliability, with low rates of complications and system revisions (both <5%) maintained over the 5-year follow-up.
These findings gain even more credibility when taken together with those of the ‘Micra coverage with evidence development’ study (Micra CED), a continuously evolving, observational cohort study of leadless VVI pacemakers in the US Medicare fee-for-service population.4 In the latter study, which included a population of 6219, a real-world comparative evaluation of leadless VVI vs. transvenous VVI de novo pacemaker implants demonstrated that leadless pacemakers were associated with fewer hospitalizations and infections than transvenous devices over a 3-year follow-up.5
All these benefits accruing to the innovative concept of leadless pacing should fill us with satisfaction, because our bold predictions have been vindicated. Any initial reservations as to the longevity of the device, the stability of its implantation within the ventricular myocardium, its behaviour, mainly in the long term, as regards sensing and pacing thresholds, and, of course, the possibility of thrombogenesis, seem to have been largely dismissed by technological developments.
Nonetheless, we must remember that, despite the enormous technological advances exemplified by the success of the Micra miniature pacemaker, we still have a long way to travel along the path we envisaged 30 years ago.
Today’s realities still highlight leadless pacing as a secondary option in the therapeutic armamentarium of pacing specialists,6 not only because of the high cost of the Micra compared with traditional VVI/VVIR devices, but also because, despite all the advances during the 10 years since the first clinical presentation of leadless VVI, a great deal of improvement will still be required if this radically new concept is to find wider acceptance. In particular, the following should be noted. (i) There is a need for well-documented data regarding the actual longevity of the leadless devices. At present, it has been estimated by the manufacturer, as well as by individual scientific peers, that the projected life of the Micra VR can reach 12.6 years for patients with a pacing burden >90%. Also, based on recent Food and Drug Administration- (FDA) approved devices Micra AV2 and Micra VRAx, the median projected battery life is expected to be 15.6 years and 16.7 years, respectively. This optimistic assessment needs to be confirmed by carefully organized retrospective analyses, which are understandably not yet available. (ii) The strategic therapeutic approach to replacing devices whose batteries have been depleted is also a matter for further consideration, as the cost to right ventricular functionality from hosting two or more leadless devices is not known. (iii) Finally, we must keep in mind that atrial pacing, preservation of the atrioventricular sequence, and ventricular synchronization continue to be vital targets, all of which are currently met by traditional pacing systems.
On the other hand, we should not neglect the fact that, during this decade, the industry has achieved great progress in leadless pacing development, including such landmarks as accelerometer atrioventricular synchrony, the Micra AV model, as well as the Aveir DR concept, a dual-chamber leadless pacemaker system.7,8,9
It is reasonable to expect that these innovative developments should be backed by reliable trials to establish their clinical utility and overall performance, as well as their safety and longevity, along with an array of other important parameters. However, the striking developments in technology leave little room for doubt about the rapid advances in leadless cardiac stimulation, while there is still plenty of scope for further innovations. It has become increasingly clear that leadless pacing can aspire to new levels of excellence and clinical utility, provided that today’s miniature devices become even smaller and can be supplied by a fundamentally new model of energy support. A number of recent publications prompt optimism with respect to significant future developments, perhaps in the form of symbiotic nanogenerators, typically very small up to the microscale range, that are capable of implementing the principle of harvesting energy from myocardial motion.10,11
In this regard, we have seen the advent of Triboelectric nanogenerators for implantable medical devices, which claim to provide excellent output performance, high power density, and good durability.12 According to a recent relevant publication, the energy harvested from each cardiac cycle is 0.495 mJ, which exceeds the threshold of 0.377 mJ required for endocardial pacing.13
To close this brief overview, when we look back over the last 50 years.1,14 to when our experimental models of leadless pacing envisaged devices with 10–12 years’ longevity, up to 25 mm in length and up to 7 mm in diameter, non-thrombogenic, and capable of stable placement in the myocardium, I think we should be delighted to see these impressive technological developments that have partially realized that dream.
However, the vision of leadless pacing should not be limited to today’s conventional, small devices. True leadless pacing requires tiny devices, capable of responding optimally to any need for myocardial pacing or synchronization. I am confident that radically new technologies for energy harvesting will open the door to fundamentally new concepts of leadless stimulation.
Declarations
Disclosure of Interest
The author declares no disclosure of interest for this contribution.
References
Author notes
The opinions expressed in this article are not necessarily those of the Editors of the European Heart Journal or of the European Society of Cardiology.
