The incidence, risk factors, and management of very high defibrillation thresholds (DFTs) during present-day implantable cardioverter defibrillator (ICD) testing are not well known.

The purpose of this study was to assess (1) the incidence of very high DFTs and (2) the efficacy/safety of routinely adding a subcutaneous (SQ) array for these patients.

The study evaluated patients undergoing first-time ICD implantation at Southlake Regional Healthcare Centre from January 2006 to December 2007. All implanted ICDs had a maximal output of 35 J. Patients with DFTs greater than a 10-J safety margin from maximum output were considered to have very high readings and underwent SQ array insertion after other attempts at lowering DFT (group I). These patients were compared with the rest of the patients who had acceptable DFTs (group II) using both univariate and multivariate logistic regression analysis. Outcomes of array insertion were also assessed.

A total of 313 patients underwent first-time ICD implantation during the analysis period. Of those, 16 (5.1%) had very high DFTs (group I). By univariate analysis, advanced New York Heart Association class (3 or 4), congestive heart failure hospitalization, non-ischemic cardiomyopathy, amiodarone use, implant of a biventricular device, and highest quartile of left ventricular (LV) chamber enlargement were all significant predictors of very high DFTs (p < 0.05). By multivariate analysis, only amiodarone use [odds ratio (OR) = 10.3, 95% confidence interval (95% CI) = 3.7–32.6] and being in the highest quartile for LV diastolic diameter [OR = 5.4, 95% CI = 1.4–20.8] predicted very high DFT. In all 16 cases, other methods to lower DFT prior to array insertion were attempted but failed for all patients: reversing shock polarity (n = 15), removing the superior vena cava coil (n = 14), reprogramming shock waveform (n = 9), and repositioning right ventricular lead (n = 9). Addition of the array successfully decreased DFT to within safety margin for all patients (33 ± 2 vs 21 ± 5 J, p = 0.02). Complication due to array insertion occurred in one patient (pneumothorax).

Very high DFTs occur in about 5% of patients undergoing ICD implantation and may be predicted by LV dilation and amiodarone use. SQ array insertion reliably corrects this problem over other interventions with a low rate of procedural complication.