Case builds against routine ICD defibrillation-threshold testing

March 07, 2008

London, ON - Defibrillation-threshold (DFT) testing performed on a routine basis at ICD implantation is unlikely to significantly affect one- or five-year mortality, according to a statistical analysis derived from outcomes and other data from the literature[1].

Such testing has long been performed to fine-tune the devices' ability to deliver appropriate and effective therapy in individual patients, but it may not be needed in the modern era of ICDs with sophisticated programming and multiple-shock capability, write the authors, led by Dr Lorne J Gula (University of Western Ontario, London).

Based on their analysis, "it is clear that the mortality benefit attributable to DFT testing, if it exists, would be small for the modern ICD recipient," the group writes in a Journal of Cardiovascular Electrophysiology report published online February 21, 2008.

Perhaps the proper question is not ' to test or not to test,' but 'for which patients is testing important?'

However, Gula et al are careful not to recommend that such testing necessarily be abandoned. "Given the small number of patients who require therapy for VF, the propensity of the device to work successfully, and according to one's perception of the risks," they write, "a case can certainly be made for continuing, modifying, or even abandoning defibrillation-threshold testing in the modern era."

According to the authors and an accompanying editorial[2], there is enough clinical equipoise to justify a prospective investigation of whether defibrillation testing improves the survival of ICD recipients. "Perhaps the proper question is not 'to test or not to test,' but 'for which patients is testing important?' " proposes editorialist Dr Steven M Markowitz (Cornell University Medical Center, New York, NY).

"Perhaps a selective strategy is called for, in which patients are tested only if they are likely to have a high DFT based on known clinical criteria, such as lower ejection fraction, nonischemic cardiomyopathy, and amiodarone therapy," he writes. "A rigorous reassessment is indicated before this practice is abandoned."

The statistical model assumed a 95% rate of success at terminating potentially lethal arrhythmias for ICDs set with a shock-strength safety margin of >10 J (ie, set at >10 J higher than the defibrillation threshold) and a 75% success rate for devices with a <10-J safety margin; the annual risk of such arrhythmias was assumed to be 5%.

On that basis, one-year survivals with and without defibrillation testing were 92.2% and 92.1%, respectively, and survivals at five years were 59.7% and 59.4%, respectively.

Markowitz observes that there are few data on complications associated with defibrillation testing, and while their risk is probably small, it must be considered.

We stopped performing defibrillation threshold testing during most ICD implantations a long time ago.

In one of the rare large studies of the risk of such complications, published in the March 2008 issue of Heart Rhythm, Dr David Birnie (University of Ottawa, ON) and associates retrospectively noted a defibrillation-testing-related mortality of 0.016% and stroke/TIA risk of 0.026% in the six-year composite experience of 21 Canadian centers[3]. Prolonged resuscitation was required in 0.14% of cases. The authors concede their numbers may underestimate those in clinical practice.

An editorial accompanying the Birnie et al report calls their data "the best available" on the risks of DFT testing and notes that, in addition, such testing can raise troponin levels and depress LV function, both signs of myocardial damage[4]. The writers, Dr Sami Viskin (Tel-Aviv Sourasky Medical Center, Israel) and Dr Raphael Rosso (Tel-Aviv University, Israel), include such risks among their "top 10 reasons to avoid defibrillation threshold testing."

Their other reasons center on uncertainties about testing's effectiveness based on differences between induced VF and spontaneous VF, variations in type of VF, and whether testing can reflect the likelihood of spontaneous-VF termination, risk of sudden death, or long-term survival.

As a result, Viskin and Rosso write, "We stopped performing defibrillation threshold testing during most ICD implantations a long time ago."

Of Gula ' s coauthors, Dr Andrew D Krahn (University of Western Ontario) reports being a consultant to Boston Scientific and Drs Raymond Yee and George J Klein (University of Western Ontario) report being consultants to Medtronic. Markowitz reports receiving a research grant from Medtronic. Birnie et al detail potential conflicts of interest in their report; Viskin and Rosso report none.




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