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The aim of this study was to determine the relationship between improved ejection fraction (EF) and occurrence of arrhythmias in patients with cardiac resynchronization therapy devices with defibrillator function (CRT-D). The hypothesis was that patients who experienced a marked improvement in EF also had fewer appropriate defibrillator interventions.We analysed data of 270 patients from2 prospective registries with follow-up of ≥12 months and echocardiography performed ≥8 months after CRT-D implantation. The discriminator was whether left ventricular ejection fraction (LVEF) improved to >35% [cut-off for primary prevention implantable cardioverter-defibrillator (ICD) implantation]. Mean age was 61 ± 11 years, LVEF 22 ± 5%, and follow-up 40 ± 22 months. Ischaemic cardiomyopathy was present in 48%, and secondary prevention indication was present in 25%. Implantable cardioverter-defibrillator interventions were delivered to 35% of patients. Echocardiography (20 ± 15 months after implantation) showed an improvement in LVEF from 22% (SD 5.4%) to 30% (SD 9.8%). Improvement to >35% was seen in 21% of patients. Those who improved to >35% had fewer ICD interventions than those who did not (23 vs.38%; P-value 0.03). Analysing only patients with a primary prevention indication and stratifying again in patients with and without improvement of LVEF to >35%, the latter had highly significant more ICD-therapies (6 vs. 31%; P-value 0.0008).Patients with CRT-D for primary prevention, whose LVEF improved to >35% during mid-term follow-up, are at low risk of first ICD therapies beyond year 1. If similar findings are reported in other patient cohorts, this might impact on decision-making at the time of battery depletion.