Washington, DC - A retrospective look at SCD-HeFT has added to a good deal of observational evidence questioning the routine need for defibrillation threshold (DFT) testing in most patients receiving implantable cardioverter defibrillators (ICDs) for primary prevention of sudden death [1].

Advances in ICD technology and experience "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," writes Dr Anne B Curtis (University of South Florida, Tampa) in an editorial [2] accompanying the SCD-HeFT analysis in the August 12, 2008 Journal of the American College of Cardiology.

The new SCD-HeFT analysis suggests that DFT testing had little bearing on the efficacy of delivered shocks or long-term mortality. It supports other observational studies suggesting that it offers little clinical value in most stable patients receiving primary-prevention ICDs who don't have an indication for resynchronization therapy but who were on "standard-of-care background medical therapy for heart failure," according to Curtis, a recent president of the Heart Rhythm Society.

"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," she concludes.

The current analysis doesn't, on its own, prove that DFT testing shouldn't be performed in primary-prevention ICD recipients, "but when we look carefully at the data, we don't really see that it provides any particular benefit," Dr Joseph A Blatt (University of Washington, Seattle), lead author of the analysis, told heartwire.

As we get better at device implantation, we have to think about whether putting the patient under further risk is really worth it.

"As we get better at device implantation, we have to think about whether putting the patient under further risk is really worth it. We know that there is a chance of sudden death after DFT testing, although it's low, in the range of 0.1% or less," Blatt said.

Embolic strokes have also been described as a complication, he observed, and there is increasing evidence that the shocks themselves can somehow damage the myocardium. "Future studies are needed to really tease this out and to see whether or not [DFT testing] is beneficial or possibly harmful."

SCD-HeFT randomized more than 2500 patients with NYHA class 2-3 heart failure and LVEF <35% but no history of sustained VT or cardiac arrest to receive ICDs, amiodarone, or placebo on top of then-standard therapies. Those getting devices received single-lead Medtronic ICDs programmed to deliver only shock therapy with a maximum output of 30 J. Importantly, antitachycardia pacing wasn't a treatment option.

The findings of a 23% reduction in mortality (p=0.007) over five years, regardless of HF etiology, and no significant effect in the amiodarone group were publicized in March 2004 and published the following year [3] and have been extensively covered by heartwire.

The analysis from Blatt et al included the trial's 811 patients who had been randomized and agreed to receive ICDs; baseline DFT thresholds were known for 717. The average follow-up was almost four years. SCD-HeFT had excluded patients with an indication for dual-chamber pacing and forbade the use of biventricular devices.

The success rate for the first appropriate shock ranged nonsignificantly from 81.0% to 100% among patients with implant DFTs of <10 J, 11-20 J, and 21-30 J and those with unknown baseline DFTs. The implantation DFT was <20 J in all but three, or 97.8%, of the 717 for whom it was known.

Mortality was 20.7% among the 547 patients with DFTs <10 J and 19.4% for those with higher DFTs, for an adjusted hazard ratio of 1.19 (p=0.41) for the latter group; the results were similar whether the 94 patients without baseline DFT data were included in the lower-DFT or higher-DFT group.

Blatt said the SCD-HeFT outcomes are entirely relevant to device therapy today. Despite the trial's exclusive use of single-lead defibrillators limited to therapy with shocks, the devices were "essentially modern" in terms of implantation technique, shock waveforms, and levels of delivered energy. Concomitant HF drug therapy was also consistent with today's standards, he said.