Extracardiac Implantable Cardioverter Defibrillators in Infants and Children: Is Routine Defibrillation Threshold Testing Appropriate?

Objectives: In pediatric patients with extracardiac implantable cardioverter defibrillators (EC-ICD) proper identification and termination of ventricular arrhythmias is of paramount importance. Patients´ growth may result in a shift of the shock vector and an increased defibrillation threshold (DFT) which may occur in failure of the EC-ICD. This potentially life-threatening event can be prevented by repeated DFT testing. Risk and benefit of routine DFT testing has not systematically been established.

Patients and Methods: Data from all pediatric patients with an EC-ICD implanted at our institution between 07/2004–09/2017 were reviewed. EC-ICD had been implanted using 3 different techniques. T1: abdominal device/subcutaneous shock coil along third to sixth rib. T2: abdominal device/pleural shock coil along third to sixth rib. T3: subcardiac device/intrathoracically shock coil. Sufficient DFT was considered intraoperatively ≤20 Joule (J) allowing double safety margin to maximal output (35–40 J); during follow up a DFT of ≤ 10 J below maximum output was considered sufficient. Ventricular fibrillation (VF) was induced by T-wave shock or ventricular burst pacing. First shock during testing was programmed at 15 J with a subsequent increase in 5 J-steps. DFT testing schedule: intraoperatively, before hospital discharge, three months after implantation and subsequently every 12 months.

Results: A total of 349 DFT tests were performed in 43 patients. Indication for EC-ICD implantation was primary prevention in 15 (35%) and secondary prevention in 28 (65%) subjects. EC-ICD implantation technique T1/T2/T3 was used in 7/5/31 individuals. Median age at implantation was 5.8 (IQR 1.8–8.3) years, median body weight 20 (IQR 10.3–25.6) kg. Median follow-up was 5.1 (IQR 1.5–7.3) years. A relevant DFT increase indicating need for surgical revision was detected by routine DFT testing in 15/349 (4%) tests in 10 patients. (T1: 5/7, T2: ⅕, T3: 4/31 patients; p = 0.006). External defibrillation due to failure of the EC-ICD was required in 6/349 (1.7%) tests. None of the patients with significant DFT increase or need for external cardioversion had evidence for lead problems during routine ICD interrogation. No major complications related to DFT testing were noted.

Conclusion: Routine DFT testing in growing patients with EC-ICD was safe and revealed a remarkable number of ICD systems with insufficient safety margin. Therefore, regular DFT testing should be performed in growing children with EC-ICD.