This section describes efficacy outcomes from the published literature that the committee considered as part of the evidence about this procedure. For more detailed information on the evidence, see the interventional procedure overview.
4.1 In a matched-controlled study of 138 patients comparing 69 patients with subcutaneous implantable cardioverter defibrillators (ICD) and 69 patients with transvenous ICDs, the conversion rates of induced ventricular fibrillation at implantation were similar (p=0.81): 90% (60/67) for 65 J of energy (15‑J safety margin) in the subcutaneous ICD group and 91% (59/65) for a device-dependent 10‑J safety margin in the transvenous ICD group. In a systematic review of 5,380 patients from 16 studies, the defibrillator threshold test was successful on the first attempt in 89% of patients (range 70 to 100%) and in 96% after reprogramming.
4.2 In a retrospective propensity-matched cohort study of 280 patients (140 with subcutaneous ICDs and 140 with transvenous ICDs), appropriate ICD intervention rates (shocks and anti-tachycardia pacing) were lower in the subcutaneous ICD group, at 17% (95% confidence intervals [CI] 6 to 26%) compared with 31% (95% CI 23 to 40%) in the transvenous ICD group (hazard ratio [HR] 2.42; p=0.01). However, the incidence of appropriate shocks was similar in both groups (HR 1.46; p=0.36). In a case series of 889 patients, which combined patients from the IDE study and from an international registry (Effortless), 111 episodes of spontaneous ventricular arrhythmias were treated in 59 patients within a mean 22‑month follow-up; 90% (100/111) of these events were stopped with 1 shock and 98% (109/111) were stopped within the 5 available shocks. In the systematic review of 5,380 patients, the range of the first shock efficacy rate was 58 to 90% and the overall shock efficacy rate was 96% or more.
4.3 In the prospective case series of 321 patients, the mean time to therapy (defined as the interval starting 2,000 milliseconds after the last induction artefact and ending at the onset of the shock deflection on a standard ECG) was 14.6 seconds (range 9.6 to 29.7 seconds). A time to therapy of greater than 18.0 seconds was noted in 13% of episodes. In an international registry of 985 patients, there was a statistically significant difference between the mean (± standard deviation) time to therapy for induced episodes and for spontaneous episodes (15.1±3.5 seconds compared with 18.4±4.3 seconds, p<0.001).
4.4 In the retrospective propensity-matched cohort study of 280 patients comparing 140 patients with subcutaneous ICDs and 140 patients with transvenous ICDs, 5‑year patient survival was similar in both groups (96% and 95% respectively, p=0.42).
4.5 In a propensity-matched case-control study of 334 patients comparing 167 patients from the Effortless registry with 167 patients with transvenous ICDs from the Midas prospective observational study cohort, there were no statistically significant differences between groups on physical (p=0.8157) and mental quality-of-life scores measured using the SF‑12 questionnaire (p=0.9080) at baseline, and 3 months and 6 months after implantation in adjusted analyses. The evolution in physical (p=0.0503) and mental scores (p=0.3772) during 6‑month follow-up was similar for both cohorts. Both patients with subcutaneous ICDs and patients with transvenous ICDs experienced statistically significant improvements in physical and mental quality of life between implantation and 3‑month follow-up (p<0.0001) and 6‑month follow-up (p<0.0001). However, the difference between 3- and 6‑month follow-up was not statistically significant.
4.6 In the systematic review of 5,380 patients, the median device longevity was 5.0 years (range 4.4 to 5.6 years).
4.7 The specialist advisers listed the following key efficacy outcomes: successful detection of ventricular arrhythmias, successful delivery of shock to restore normal rhythm, prevention of sudden death and low rate of inappropriate shocks.
4.8 Seven commentaries from patients who had experience of this procedure were received, which were discussed by the committee.