Threshold testing at ICD implantation 'of little clinical value'

11 August 2008

MedWire News: The results of defibrillation threshold (DFT) testing at implantable cardioverter-defibrillator (ICD) implantation have little bearing on clinical outcomes in patients with heart failure (HF), shows a subanalysis of SCD-HeFT.

"Although current literature generally recommends DFT testing, evidence for this practice is minimal," Joseph Blatt (University of Washington, Seattle, USA) and colleagues write in the Journal of the American College of Cardiology.

"The present study represents the only prospective analysis of the relationship between DFT testing and outcomes in patients treated with modern ICDs."

The analysis included 717 patients with New York Heart Association class II or III HF who were assigned to the ICD arm of SCD-HeFT (Sudden Cardiac Death in HF Trial) and had baseline DFT testing data.

The patients' ICDs were programmed with the first shock at 10 J over the DFT, followed by maximum output (30 J) for subsequent shocks. Defibrillation was successful at 30 J or lower in all patients, and at 20 J or lower in 97.8% of the cohort. Most patients (86.6%) therefore had their ICD set to a first shock of 20 J; those set at 30 J were due to elective protocol deviation by study investigators.

Overall, 20.7% of the 547 patients with a DFT ≤10 J died during the minimum 2 years of follow-up, as did 19.4% of the 170 patients with a higher DFT. Mortality rates between these groups did not significantly differ, even after accounting for confounders (p=0.41).

Of the 182 (22.4%) patients who suffered ventricular tachycardia or fibrillation during follow-up, 83.0% had successful defibrillation with the first shock, and the first shock success rate did not differ significantly with DFT. A further 94 patients without DFT testing had a first shock efficacy of 90.4%, which was not significantly different from efficacy in patients with testing (p=0.13).

In an editorial, Anne Curtis (University of South Florida, Tampa, USA) mentioned other reasons to abandon routine DFT testing, including that spontaneous ventricular arrhythmia is often tachycardia, rather than fibrillation as tested, and that testing carries risk for the patient, if only slight.

These points, along with the current findings, "indicate that routine DFT testing in stable patients receiving nonresynchronization ICDs for primary prevention is of little clinical value," she said.

However, Curtis stressed: "A mounting consensus that routine DFT testing may be safely abandoned in selected patients does not mean that it should be discontinued in all patients."

She concluded: "These data justify a prospective trial randomizing patients receiving ICDs for any indication to DFT testing versus no DFT testing, because less testing just might be more than enough."

J Am Coll Cardiol 2008; 52: 551-556, 557-558