Author + information
- Anne B. Curtis, MD, FACC⁎ ()
- ↵⁎Reprint requests and correspondence:
Dr. Anne B. Curtis, University of South Florida, 12901 Bruce B. Downs Boulevard, MDC 87, Tampa, Florida 33612.
Since implantable cardioverter-defibrillators (ICDs) were first introduced in the early 1980s, defibrillation threshold testing (DFT) at the time of implantation has been considered mandatory because the results have been used to predict the likelihood that these devices would successfully terminate sustained ventricular tachyarrhythmias when they occurred clinically. From the initial days of epicardial patch placements through the early generations of transvenous devices with monophasic shocks, failure to defibrillate, even with high-output shocks, was not uncommon. In addition to intraoperative testing, the performance of DFT testing 1 or 2 days after implantation but before hospital discharge became routine practice. The data acquired occasionally led to the need for system revision. With the advent of biphasic shocks in ICDs, mean defibrillation thresholds were lower, and it became much less common for system revision to be necessary. With the ability to reverse polarity, adjust the tilt of the defibrillation waveform, or change the vector for defibrillation, the need for the addition of a subcutaneous patch or electrode array to the system is very rare in clinical practice today.
The established reliability of current ICD systems has led most electrophysiologists to abandon the practice of routine DFT testing before hospital discharge. Even annual DFT testing, once a common clinical practice, is performed infrequently today. In fact, DFT testing after implantation is usually reserved now for specific situations, such as a concern about lead status or a change in drug therapy (e.g., the addition of amiodarone) in the setting of a previously marginal DFT.
Although practice patterns in the surveillance of patients with ICDs have evolved remarkably, there has been little change to date in the approach to the patient at the time of implantation. Defibrillation is a probabilistic phenomenon in which the higher the shock strength, the higher the likelihood of successful defibrillation. The most common approach in clinical practice to approximating the DFT is to induce ventricular fibrillation at least twice and defibrillate the patient with an energy setting at least 10 J less than the maximum output of the ICD. Given the shape of the defibrillation curve, this method should ensure an adequate safety margin in the overwhelming majority of patients, although it does not technically determine the actual DFT (1). The rare need for system revision and the proven reliability of ICDs in terminating spontaneous, sustained ventricular tachyarrhythmias have called into question whether DFT testing at the time of implantation is still required. Yet, physicians have been reluctant to abandon it, despite mounting evidence that it may no longer be necessary (2–6).
In this context we have new data acquired from patients enrolled in the parent SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial) (7). As reported previously, SCD-HeFT was a primary prevention ICD trial that established the superiority of single-chamber ICDs compared with amiodarone or placebo in improving survival in patients with symptomatic heart failure and left ventricular ejection fractions ≤35% (8). In a SCD-HeFT substudy by Blatt et al. (7), reported in this issue of the Journal, baseline DFT data were available for 717 patients implanted with ICDs. The DFT protocol in the study was straightforward: ventricular fibrillation was induced and 20-J shocks were delivered. If defibrillation was achieved, ventricular fibrillation was reinduced and the effects of a 10-J shock were then determined. If 20 J failed the first time, 30 J was delivered during the second induction. Regardless of the outcome of DFT testing, all patients were programmed with the first shock set at 10 J above the DFT (unless the DFT was 30 J, in which case 30 J was programmed), with the maximum output for subsequent shocks and reverse polarity for the last 2 shocks. Defibrillation was achieved in all patients with the use of shock strengths ≤30 J. In addition, 97.8% of the patients had a DFT ≤20 J. The main findings of the study were that there was no difference in survival between patients who had a DFT ≤10 J and those who had a DFT >10 J. First-shock efficacy for spontaneous events was 83.0%, with no significant difference according to baseline DFT. Of the 31 patients who had failed first shocks in follow-up, all survived the event, with 3 of the patients subsequently dying in the hospital from progressive heart failure. In most of these patients who survived a failed first shock, the arrhythmia terminated spontaneously or subsequent shocks successfully terminated the arrhythmia.
Of the 16 patients with a DFT between 21 and 30 J, only 3 patients had an appropriate shock in follow-up, all of which were successful. Granted, this is a small number of patients and events. However, there were over 700 patients in this substudy, with a median follow-up of 45.5 months. The 2.2% risk of an elevated DFT is comparable to what has been seen in other studies. With that low level of risk for a high DFT, and a markedly lower risk of an adverse outcome in follow-up, a much larger study with a prolonged follow-up would need to be done to detect a difference in outcome, if indeed one exists. Whether such a slight difference in outcome would be clinically meaningful is also questionable.
It should be noted that defibrillation using shock energies up to 30 J was successful in all patients in this substudy. It is thus unknown what kind of outcome might be expected in patients who failed defibrillation with the maximum output of an ICD at the time of implantation. However, it is clear that in patients similar to those tested in SCD-HeFT, that finding would be exceedingly rare.
It is important to stress that the results of this study can be applied only to similar patients who are receiving single-chamber ICDs for primary prevention and are receiving standard-of-care background medical therapy for heart failure. The results should not be extrapolated to patients receiving cardiac resynchronization therapy, who often have more advanced disease. Whether the results could be applied to patients receiving dual-chamber ICDs is debatable because the reason for the atrial lead may be a history of atrial arrhythmias or a rare need for pacing that may not have an impact on defibrillation. The results should also not be applied to patients who are receiving ICDs for secondary prevention, in whom life-threatening ventricular arrhythmias have already occurred.
A recent decision analysis and Markov model found that 5-year survival was nearly identical with or without DFT testing (9). Only if the annual risk of a lethal arrhythmia was at least 5% was there a slight advantage to DFT testing, but the incremental benefit was minimal. Another factor that must be considered in recommendations regarding DFT testing is the risk associated with this practice. Birnie et al. (10) recently reviewed the experience in Canada over a 6-year period and found that among 19,067 patients with ICD implants, 3 deaths, 5 strokes, and 27 episodes of prolonged resuscitation, some of which were associated with serious clinical sequelae, were directly attributable to DFT testing.
The current study is not the first to challenge the conventional wisdom of routine DFT testing (2–6). Viskin and Rosso (5) recently enumerated elegantly a myriad of reasons for avoiding DFT testing, including the lack of correlation between induced and spontaneous ventricular fibrillation; the fact that many patients will never have spontaneous ventricular fibrillation in follow-up; the reality that successful DFT testing at implant is so likely that little information is gained as a result of the procedure in most patients; the observation that spontaneous arrhythmias are often ventricular tachycardia, which is easier to cardiovert; and the fact that DFT testing is not without risk, among others.
These observations coupled with the results of the present study indicate that routine DFT testing in stable patients receiving nonresynchronization ICDs for primary prevention is of little clinical value. Driven by these results, it would be reasonable to consider whether practice guidelines should be revised to reflect these findings, with 2 important caveats. First, a mounting consensus that routine DFT testing may be safely abandoned in selected patients does not mean that it should be discontinued in all patients. We do not have information on patients receiving cardiac resynchronization devices and other specific situations as mentioned previously to make recommendations at this time. However, proof-of-concept has now been established that routine DFT testing is unlikely to provide sufficient information that impacts ICD programming in the overwhelming majority of patients, and it carries some slight risk. Collectively, these data justify a prospective trial randomizing patients receiving ICDs for any indication to DFT testing versus no DFT testing, because less testing just might be more than enough.
Dr. Curtis is a consultant for and has received honoraria and grant support from Medtronic, Inc.; has received honoraria from Boston Scientific; and is a consultant for and has received honoraria from St. Jude Medical.
↵⁎ Editorials published in the Journal of the American College of Cardiology reflect the views of the authors and do not necessarily represent the views of JACC or the American College of Cardiology.
- American College of Cardiology Foundation
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- Klein G.J.
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- SCD-HeFT Investigators