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Notes: Defibrillation Efficacy. Introduction: Placement of implantable cardioverter defibrillators (ICDs) has been simplified by using the shell of a pectorally implanted ICD as a defibrillation electrode in combination with an endocardial right ventricular defibrillation lead. However, a sufficiently low defibrillation threshold (DFT) cannot be obtained in a few patients. Therefore, alternative approaches were systematically tested in the Active Can versus Array Trial (ACAT). Methods and Results: In the first of two prospective randomized studies, the DFT of a subcutaneous left dorsolateral array anode introduced from a pectoral incision was compared to that of a standard active can anode in 68 patients. Intraoperatively, the DFT was determined twice in each patient using either the active can or, in patients with a subcutaneous array lead, once with all three fingers and once omitting the middle finger of the array. The second prospective randomized study included 40 patients. DFT also was determined twice in each patient using an active can in a left pectoral position as anode alone and combined with a left dorsolateral array electrode with two fingers. In ACAT I, stored energy at DFT decreased from 13.1 ± 7.7 J (active can) to 9.6 ± 6.1 J (three-finger array lead) (P = 0.04), impedance decreased from 53 ± 8 Ω to 40 ± 6 Ω (P 〈 0.0001). Omitting the middle finger of the array lead, stored energy at DFT increased by 0.9 J (P = 0.47) and impedance by 2 Ω (P 〈 0.0001). In ACAT II, DFT and impedance using an active can device were significantly lower when a two-finger array lead was added that decreased stored energy at DFT from 10.1 ± 5.2 J to 6.9 ± 3.9 J (P = 0.001) and impedance from 56 ± 5 Ω to 42 ± 5 Ω (P 〈 0.0001). Conclusion: In combination with a right ventricular defibrillation electrode, a left pectoral subcutaneous array lead improves defibrillation efficacy if used instead of, or in addition to, a left pectoral active can ICD device. Implantation of the array lead can be simplified by using two instead of three fingers, without a significant loss of defibrillation efficacy.