Author + information
- Claudio Schuger, MD∗ ()
- ↵∗Address for correspondence:
Dr. Claudio Schuger, Henry Ford Health System, Wayne State University, Henry Ford Hospital, 2799 West Grand Boulevard, Room B-1451, Detroit, Michigan 48202-2608.
Since the demonstration of successful clinical defibrillation by electrical shock in 1947 (1), the evolution of defibrillators has focused on the pursuit of defibrillator waveforms that can reliably and reproducibly terminate ventricular fibrillation without incurring myocardial damage. The waveform of an electrical shock is defined as the temporal pattern of its amplitude measured by voltage or current. After a long period of monophasic waveform configurations, a more efficient biphasic waveform was introduced. All current implantable cardioverter-defibrillators (ICD) use biphasic truncated exponential (BTE) waveforms to defibrillate. The use of these exponential decaying morphologies is based on the simplicity and reliability of modern capacitors as the main component of energy delivery, which in turn, makes it possible to generate the current in relatively small implantable devices.
Despite the popularity of descending waveforms, it has been known for years that they are not the most efficient defibrillation waveform. Schuder et al. (2) described the advantage of monophasic ascending ramp (ASC) waveforms compared with descending monophasic waveforms more than 30 years ago. It is by use of the Blair model of the heart as a circuit composed of a capacitor and a resistor in parallel that a passive cell membrane time constant is defined (τm). This membrane time constant is an average value that allow us to predict the passive membrane response to voltage applied within the heart which rapidly rises to a peak and then slowly decays to zero. The time interval in which the peak occurs defines the optimal duration, that is, the shortest duration with minimal applied voltage. Independent of the mechanism behind defibrillation, the goal of a monophasic shock or the first phase of a biphasic shock is to maximize the voltage change across the cell membrane. The goals of the second shock are to bring the membrane potential back to zero, remove the charge of the first phase, and prevent re-fibrillation. Using this model, optimal defibrillation waveforms are those in which the first phase is truncated at the maximum membrane response. Interestingly, this membrane response can be achieved with ascending waveforms at a lower energy and voltage than that of a BTE waveform, resulting in lower defibrillation thresholds (DFT) in terms of energy. Moreover, this reduced peak voltage duration across cell membranes may reduce the cell damage observed with classic defibrillator shocks, believed to be at least partly the result of electrical field-induced electroporation (3,4).
However, the difficulties in achieving ascending ramp waveforms with the technologies available in the past prevented their further development. Moreover, with the introduction of biphasic defibrillation waveforms as described previously, which provided reliable defibrillation in most device recipients, further interest in ascending ramp waveforms was relegated to bioengineering research laboratories.
In this issue of JACC: Clinical Electrophysiology, Huang et al. (5) compared the classic BTE waveform at energies described as DFT plus 10 joules (BTE 25J) to a previously selected ASC first phase in a swine model using conventional defibrillator catheters at DFT plus 10J (ASC 20J) and for comparison at 25J (ASC 25J). The success of ASC waveforms at lower DFT was confirmed, but the focus of the paper was to prove that ASC waveforms result in less myocardial damage, even at comparable energies. For this endeavor, the authors relied on the following surrogates: 1) electrogram evidence of injurious current at the defibrillator delivery site; 2) pathology in the form of histological signs of necrosis; 3) troponin I levels; and 4) contractility was measured in the form of negative dp/dt. Apart from the functional measurement of contractility, all other surrogates showed significantly decreased damage for ASC waveforms. Although the evidence is convincing that local myocardial damage is reduced, the assumption that these findings translate to a clinically significant advantage is lacking, and this is further supported by the investigators’ failure to show meaningful differences in negative dp/dt.
Moreover, in the clinical arena, the winds are blowing in the opposite direction. The potential advantage of ASC waveforms, namely reduced defibrillation energy and more efficient defibrillation, should be measured relative to the increasingly popular practice of not even testing DFT or even defibrillation energy requirements at the time of implantation. The current practice is supported by large clinical trials that have failed to show differences in outcomes between patients who were tested and those who were not at the time of implantation of endocardial devices (6). This fact complicates matters even further for advocating an investment in new, theoretically better defibrillator waveforms, given that not only was defibrillation testing not warranted but also, when it was used, it was safe in a large cohort of patients. The alleged BTE myocardial damage appears then at first glance not to have a significant clinical counterpart, at least when limited use is involved. It is possible that the situation will be different when frequent device interventions are invoked. For instance, both the SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial) and MADIT II (Multicenter Automatic Defibrillator Implantation Trial) trials showed increased mortality associated with appropriate or even inappropriate ICD shocks; however, whether ICD shocks are independent predictors of mortality or mere markers of disease severity remains controversial. As the current practice of programming ICD to maximal output without DFT testing appears to suggest, there is currently no great concern among clinicians about the potential damage induced by conventional ICD shocks.
The renewed interest in waveform investigation is partly related to the development of new amplifier technology capable of delivering ascending waveforms within the confines of modern pectoral ICD volumes. Despite the satisfactory performance of current conventional BTE waveform-based implantable defibrillators, the possibility of implanting an ICD with the ability to deliver multiple and programmable waveforms in response to measured individual variables will provide clinicians with the flexibility to address the occasional problematic patient with high DFTs, which frequently require the addition of further defibrillator hardware. The putative value of waveform programmability in subcutaneous ICD where the margin of defibrillation efficacy appears more limited than with endocardial devices may prompt current manufacturers to pursue this technology further.
The question of clinically significant iatrogenic effects of defibrillation shocks, although still unanswered, is not without merit. For device manufacturers to pursue such a radical change in waveform morphology in implantable ICDs will require a very large clinical trial comparing both technologies to justify their investment.
↵∗ Editorials published in JACC: Clinical Electrophysiology reflect the views of the authors and do not necessarily represent the views of JACC: Clinical Electrophysiology or the American College of Cardiology.
Dr. Schuger has reported that he has no relationships relevant to the contents of this paper to disclose.
The author attests he is in compliance with human studies committees and animal welfare regulations of the authors’ institutions and Food and Drug Administration guidelines, including patient consent where appropriate. For more information, visit the JACC: Clinical Electrophysiology author instructions page.
- 2019 American College of Cardiology Foundation
- Schuder J.C.,
- Rahmoeller G.A.,
- Stoeckle H.
- Huang J.,
- Ruse R.B.,
- Walcott G.P.,
- et al.