Editorials
Implantation Site Does Matter for Defibrillation Threshold
2016 Volume 80 Issue 4 Pages 816-817
Details
2016 Volume 80 Issue 4 Pages 816-817
With the advent of optimal biphasic waveform, higher-shock energy devices, inadequate safety margins for defibrillation have become uncommon. Nevertheless, post-implant defibrillation testing occasionally reveals a high defibrillation threshold (DFT) at which the reliability of defibrillation might be questioned. Multiple factors influence the DFT, including the size of the heart, location of the right ventricular electrode, system impedance, heterogeneity of conduction, the number of propagating wavelets, etc.1 In such cases, an intravenous coil lead, subcutaneous small electrode patch, array, or single coil has been added as a safety measure if the implanted device system’s configuration adjustment fails to resolve it. However, this invasive approach may increase implant complexity and morbidity much more. Collins et al were the first to report implantable cardioverter-defibrillator (ICD) implantation in the left axilla of a 10-year-old boy without any problems.2 They also showed that ICD implantation in the left axilla decreased the DFT because it may improve the direction of shock current propagation through the myocardium. This finding implied that the implantation site does matter for the DFT.
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Axillary implantation of a cardiac device is considered if the patient is thin, emaciated, a small child or young female who needs excellent cosmetic results and comfort.2–4 Shefer et al described a retropectoral transaxillary pacemaker implantation with a single axillary incision and direct axillary vein puncture (Figure 1).3 The retropectoral space exists between the clavipectoral fascia anteriorly and the thoracic cage posteriorly, serving as a natural pocket for the pacemaker generator (Figure 2).5 They placed the pacemaker generator behind the pectoralis major muscle literally. Noro et al adopted this method but placed the pacemaker generator more laterally in the deep left axilla, under the sweat glands and fat tissue. From the left axillar incision, they separated the abundant fat tissue up to the fascia of the serratus anterior muscle, and the leads and generator were anchored to the fascia and tendinous cord. They reported that left axillary pacemaker implantation may be performed safely if functional or cosmetic outcomes are required by the patient.4
Direct puncture of the left axillary vein. (A) Contrast-enhanced image in the anterior view. (B) The needle is advanced vertically to puncture the axillary vein directly. The patient’s left arm is elevated and bent. After the puncture, an axillary pocket is created by extending the incision from the needle puncture site by separation up to the fascia of the anterior serratus muscle. (Reprinted with permission from Noro M, et al.4)
Left retropectoral space (axillary pocket). Dotted line depicts the left retropectoral space3 that exists between the clavipectoral fascia anteriorly and the thoracic cage posteriorly. The left axillary vein is not shown in this schematic diagram. (Reprinted and modified with permission from Gray’s anatomy for students, 3rd edn.5)
In this issue of the Journal, Noro et al6 expand their experience with left axillary implantation to ICD and cardiac resynchronization therapy with defibrillator (CRT-D). They confirm that left axillary ICD/CRT-D implantation was acceptable to patients (note that the left ventricular lead for CRT-D is inserted from the subclavian vein, not by direct puncture of the axillary vein). Moreover, and importantly, they show that ICD/CRT-D implantation in the left axilla had a lower DFT compared with conventional left anterior chest implantation, both theoretically and clinically. Although the details of the computer simulation they used6,7 are not easy to comprehend quickly, the strikingly finding was that 86.7% of the patients with left axillary implantation had DFT <5 J, but only 27.5% of the patients with left anterior chest implantation in their human clinical experience. So far, the defibrillation shock’s electrical conduction, efficiency or damage to the myocardium has not been evaluated scientifically for axillary ICD implantation. The authors are to be congratulated on their original and impressive work. Recently, the need for ICD/CRT-Ds in pediatric patients who have congenital heart disease has been increasing.8 For these young patients with small physique, axillary implantation would benefit them not only aesthetically, but also prevent excess myocardial damage with low DFT, ensuring enough energy safety margin for defibrillation, and to save battery life, which may decrease the frequency of generator exchange. In the near future the ICD/CRT-D placement site may be tailored for each patient based on computer simulation before device implantation.
