Early in the development of the transvenous implant- able cardioverter defibrillator (ICD), defibrillation threshold (DFT) testing was performed by connecting the transvenous lead to an external cardioverter defibril- lator (ECD) using high-voltage cables.The device itself was only implanted when external testing was successful. Over time, device-based testing could be performed.This has been standard practice for many years.
There was also a time when defibrillation testing was routinely performed using a step-down approach to accu- rately define the defibrillation energy requirement (DER).The rationale was that the output could be programmed as low as possible when the DER was defined, which would reduce the time to therapy.As battery technology, sensing algorithms, and charge times have improved, there has become less of an incentive to precisely define the defibrillation threshold or to test rede- tection after a failed shock. However, limited DFT testing is still usually performed to be sure that there is an adequate safety margin. Studies such as the Low Energy Safety Study (LESS)1 demonstrated that a single effective shock at 14 Joules predicts an adequate safety margin when using a 31-Joule maximum output device.
Defibrillation technology has improved even more over the past decade with better biphasic shock waveforms and higher energy devices, to the point now where it is not clear that even limited DFT testing is necessary at the time of implantation. In addition, there is evidence that the efficacy of modern ICDs for conversion of spontaneous rhythms is very high, and that spontaneous rhythms may require less energy to convert compared to fine ventricular fibrillation (VF) that is induced in the EP lab.
Whether or not DFT testing should be performed at implant was addressed in a study presented during Heart Rhythm 2014, the Heart Rhythm Society’s Annual Scientific Sessions in San Francisco last month. Dr. Jeff Healy presented the “Shockless IMPLant Evaluation (SIMPLE):A Randomized Trial of Routine Defibrillation Testing, Compared to No Testing, at Time of Defibrillator Implantation.” The international group of investigators randomized 2,500 patients who were undergoing initial ICD implantation to have DFT done at the time of implant or not. For those who underwent testing, the protocol required at least one successful termination ofVF at 17J or two successes at 21J. Otherwise, the patient was required to undergo system revision.The first shock energy in all zones was programmed to 31J in all patients.
In SIMPLE, DFT testing was concluded to be safe based on identical 30-day safety outcomes in each group. The primary outcome, which was a composite of failed appropriate shock or arrhythmic death, was seen in 8.3% of patients who underwent testing but only 7.2% in those who did not.The odds ratio for failed shock or arrhythmic death was 0.86 in the non-testing arm, and the p-value for non-inferiority of not testing during implantation versus testing was <0.001.This study also demonstrated comparable efficacy in the secondary endpoint of first shock efficacy.As an aside, although the study was unrelated to and did not include patients with the subcutaneous ICD, it is interesting that the 91% first shock efficacy rate in the no-DFT group was comparable to that in the recently reported S-ICDTM System EFFORTLESS Registry (88 percent).
So, a simple interpretation of SIMPLE is that DFT testing is safe, but does not appear to have an impact on post-implant outcomes. But what is the definition of safe? Clearly there are risks to DFT testing.Although adverse events are not common, patients can have complications such as airway issues, cardiogenic shock after a shock, and rarely, failure to defibrillate with both the device and rescue shocks resulting in death.The use of a 30-day safety endpoint to compare DFT testing to no-DFT testing inherently results in dilution of the major adverse events and takes away from the fact that there are complications associated with testing that do not occur when the device is not tested.
With regard to efficacy of the device, it was surprising to find that DFTs at implant were high in about 10% of patients. If DFT testing was not found to improve long-term outcomes, then can one assume it is because spontaneous rhythms are easier to terminate? Perhaps that is the explanation. However, another possible explanation might be that subsequent shocks are more effective than previously thought. Based on SIMPLE, it appears that when a maximum shock does not defibrillate a patient for spontaneous rhythms, there is still a good chance that subsequent shocks of the same energy will be effective. Interestingly, this issue has never been systematically tested in the EP lab.
The key finding of SIMPLE is that DFT testing at the time of ICD implantation has no impact on post-implant outcomes and first shock efficacy.This study does not apply to patients undergoing implantation of the subcutaneous ICD, or to patients who might have a potential problem with their ICD system post implant that might warrant defibrillation testing. Based on this data, however, it is difficult to justify DFT testing at the time of implantation of a transvenous ICD in most patients.
Bradley P. Knight, MD, FACC, FHRS Editor-in-Chief, EP Lab Digest®
- Higgins S, Mann D, Calkins H, et al. One conversion of ventricular fibrillation is adequate for implantable cardioverter-defibrillator implant: an analysis from the Low Energy Safety Study (LESS). Heart Rhythm. 2005;2(2):117-122.