By Jeanne E Poole
One of the earliest studies to suggest that defibrillation testing did not predict shock efficacy or mortality came from a retrospective review of the SCD-HeFT (Sudden cardiac death in heart failure trial) study, which examined outcomes in 811 patients implanted with a single-lead ICD (Blatt J A et al, J Am Coll Cardiol 2008; 52(7):551-6). Patients underwent a limited 10J safety margin protocol, but regardless of successful shock strength, all patients were implanted with the Medtronic 30J maximum output device used in the trial. Over 45.5 months of follow-up, no difference in first shock efficacy or mortality was observed when patients were stratified by lowest successful implant shock of 10J or less vs. greater than 10J.
Several other studies have found similar findings including the SAFE ICD study, which was a large prospective observational study of 2,120 patients (Brignole M J et al, J Am Coll Cardiol 2012; 60(11):981-987). Significant differences were not noted between defibrillation testing and no-defibrillation testing patients for the outcomes of major complications, first shock efficacy or mortality at two years.
The results of the recently completed SIMPLE study were presented at a Late-breaking trial session during the 35th Heart Rhythm Society Annual Scientific Sessions. The authors reported no difference in first shock efficacy, all-cause mortality or the primary safety composite endpoint between the no-defibrillation testing group and defibrillation testing randomised patients. Therefore, the authors concluded that performing defibrillation testing has no relationship to shock efficacy or long-term mortality and recommended abandoning this practice.
Is there still a role for defibrillation testing?
In light of these data and earlier studies, should we still consider that there is a role for defibrillation testing? To answer this, we need to clarify the reasons why we might want to test the patient and/or the system.
The first potential reason is the belief that we must observe the patient effectively terminated from ventricular fibrillation to assure ourselves that at the time of a spontaneous event, the ICD will in fact work. There are a number of problems with this assumption, beyond the results of SIMPLE. First, the rhythm that is tested in the electrophysiology laboratory is ventricular fibrillation whereas the majority of patients with ICDs experience ventricular tachycardia. Second, the probabilistic nature of defibrillation testing suggests that a failed defibrillation testing may not fail with repeat testing using the exact same configuration, or perhaps on another day. Third, true step-down or step-up defibrillation threshold testing has long been abandoned in lieu of the concept of 10J safety margin testing and finally, the patient clinical milieu in the electrophysiology lab is unlikely to be the situation when a patient has a spontaneous arrhythmia. Triggers such as acute ischaemia, worsened heart failure or metabolic abnormalities may be arrhythmic triggers not present at the time of defibrillation testing. Thus, the efficacy of a “first” shock for a spontaneous arrhythmia may not be predicted by defibrillation testing.
The second potential reason to perform defibrillation testing is to assess the functionality of the ICD generator-lead system. Consider just some of the different components that are involved in this activity: 1) Reliable sensing of a highly variable (in terms of frequency and amplitude) signal source; 2) filtering and processing of the sensed signal; 3) classification of the detection as sustained vs. non-sustained, supraventricular vs. non-supraventricular tachycardia, noise vs. non-noise; 4) acquisition, processing and storage of the electrogram and event information; 5) assuming a sustained ventricular tachyarrhythmia, charging of the capacitors in preparation for delivery of defibrillation therapy, which involves transformation of the energy source to a stored high voltage on the capacitors in a matter of less than 10 seconds; 6) monitoring during charging to ensure the ventricular tachyarrhythmia is ongoing (confirmation); 7) upon completion of charging, identification of appropriate delivery window (synchronisation) if appropriate; 8) discharge of the capacitors through the output circuit, device feed-throughs, lead connections, lead conductors, and electrode/tissue interface, all accommodating peak currents as high as 40 amperes; 9) immediate resumption of signal acquisition from highly-polarised tissue to determine termination/non-termination of a treated episode and 10) re-detection and classification (if necessary) or detection and classification of termination.
The engineering feat required to accomplish these actions with a high level of success has been nothing short of remarkable. But, do we need to place the patient into a life-threatening tachyarrhythmia at the time of implantation in order to feel confident that the device will perform as expected in the event of a spontaneous arrhythmia? That answer appears to be “no”-at least for most patients. The failure rate for pulse generators is low, and when problems have been identified, defibrillation testing at the time of implantation is unlikely to have uncovered the malfunction.
ICD system failure has primarily been confined to the high voltage leads. Patients with apparently functioning advisory/recall leads for whom the physician has decided to retain the lead at the time of pulse generator replacement should undergo system testing. This does not however require the patient to be placed into ventricular fibrillation. A synchronised full output shock could be delivered in normal rhythm and achieve the intended purpose of testing the device-lead system, minimising the risk to the patient. Some might argue that any patient at the time of pulse generator replacement should have such testing performed as the lead, by that point, will be anywhere from on average four to 10 or more years old.
There are other patient factors that might favour defibrillation testing: patients in whom the shock vector is anticipated to be poor (eg. left ventricular mass dominantly posterior in the chest), other patients anticipated to have a high defibrillation threshold; also treatment of the patient with amiodarone, right sided implants (not included in the SIMPLE study) and secondary prevention patients.
While the results of the SIMPLE study are reassuring and can be used to support the increasing practice of implanting physicians to abandon defibrillation testing at the time of de novo ICD implantation, physicians should, nevertheless, consider carefully each patient and device-lead system. As encouraging as the SIMPLE results are that performing defibrillation testing did not appear to harm the patient, the counter of performing defibrillation testing also appeared overall to be a safe procedure.
Jeanne E Poole is with the University of Washington, Seattle, USA