Current Value of Defibrillation Threshold Testing
Abstract & Commentary
By John P. DiMarco, MD, PhD
Source: Blatt JA, et al. No benefit from defibrillation threshold testing in the SCD-HeFT (Sudden Cardiac Death in Heart Failure Trial). J Am Coll Cardiol. 2008;52:551-556.
The sudden cardiac death in heart failure trial (SCD-HeFT) was a study involving patients with ischemic and non-ischemic cardiomyopathy and a left ventricular ejection fraction < 35% and class II or III heart failure that tested the value of ICD therapy for the primary prevention of sudden cardiac death. In SCD-HeFT, patients were randomized to receive a single lead ICD, amiodarone, or placebo. This report focuses on the value of defibrillation threshold testing in the ICD cohort. Patients randomized to receive ICD underwent limited defibrillation threshold (DFT) testing using a standard protocol. Ventricular fibrillation was induced, and an initial shock energy of 20 joules was tested. If the 20 joule shock was successful, 10 joules was tested. If the 20 joule shock was unsuccessful, a shock at 30 joules, the maximum output of the device, was tested. If satisfactory defibrillation was not achieved, the protocol discouraged the use of alternative lead systems or devices and the device was implanted anyway. Follow-up data were routinely downloaded at three month follow-up visits and after known ICD therapy. In this paper, the success of the first appropriate shock experienced by patients in SCD-HeFT was analyzed and correlated to the DFT results.
In SCD-HeFT, 829 patients were randomized to the ICD arm. The results of baseline DFT testing were available from 717 patients. Of these, 97.8% had successful defibrillation at ≤ 20 joules, and all 717 were defibrillated at ≤ 30 joules. Patients were divided into a low DFT group (DFT < 10 joules-86.8%) or a high DFT group (DFT > 20%-13.2%). There was no difference in overall mortality between the low DFT group (20.7%) and the high DFT group (19.4%). First shock efficacy was also not predicted by the baseline DFT results. First shock efficacy was 82.6% in those with a DFT < 10 joules, 81% for those with a DFT < 20 joules, and 100% in those with a DFT > 20 joules. There were 31 patients who had an unsuccessful first shock and, in 28 of these, 31 subsequent shocks were successful. In three patients, all subsequent shocks failed, and these patients died on the day of that arrhythmia.
Blatt et al conclude that there is a poor correlation between baseline DFT testing and either shock efficacy or survival. They suggest that the practice of DFT testing in patients receiving an ICD for primary prevention may be abandoned.
Commentary
Defibrillation testing has been a part of ICD implantation since the devices were first introduced. Early epicardial lead systems and monophasic shock transvenous systems often had poor defibrillation thresholds, and modifications to the systems were frequently required. As ICD technology has advanced, the efficacy of the delivered shocks has improved, and charge times, even to maximum energy, have been greatly reduced. The latter development means that there is now relatively little advantage to programming output to less than the maximum capability of the device. As a result, the large majority of patients have a very adequate safety margin. I certainly agree with Blatt et al that full DFT testing protocol to determine the actual DFT are rarely required during routine clinical practice. However, there still are reasons why I believe limited defibrillation testing may be valuable. In SCD-HeFT, Blatt et al were allowed to reposition the RV lead if the initial shock at 20 joules was unsuccessful. The initial test also provided data on the integrity of the system, sensing, lead connections, as well as the defibrillation safety margin. However, only the final results were reported on the SCD-HeFT report forms and could be analyzed in this paper. ICD implanters have all had examples where they thought they had a satisfactory lead position and the initial DFT shock was unsuccessful. However, if they could reposition the lead in a different position, a satisfactory DFT could be achieved even without using additional lead systems. I would suspect that this happened during SCD-HeFT, but the data went unreported.
Other studies have shown that there is some small risk associated with DFT testing. In a large review from Canada, Bernie et al reported three deaths and five strokes after defibrillation testing in a group of over 19,000 patients (Heart Rhythm. 2008; 5:391-392). However, if patients are carefully selected, and those who are hemodynamically unstable or have associated high risk conditions (for example, recent stroke or MI, hypotension, atrial fibrillation without anticoagulation, etc.), defibrillation threshold testing should be safe.
It is clear that defibrillation testing is now much less important than it was when ICDs were first introduced. In certain patients, the risks of defibrillation testing may outweigh the benefits. At this time, however, I still prefer to perform simple DFT tests in most of my patients.
The sudden cardiac death in heart failure trial (SCD-HeFT) was a study involving patients with ischemic and non-ischemic cardiomyopathy and a left ventricular ejection fraction < 35% and class II or III heart failure that tested the value of ICD therapy for the primary prevention of sudden cardiac death.You have reached your article limit for the month. Subscribe now to access this article plus other member-only content.
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