Christopher P. Rowley, MD, Peter H. Belott, MD, Michael R. Gold, MD, PHD
A 35-year-old woman with hypertrophic cardiomyopathy, a family history of sudden cardiac death (SCD), and a 3.2-cm interventricular septal diameter presents for consideration of a primary prevention implantable cardioverter defibrillator (ICD). She has no other relevant past medical history. She has no history of atrial or ventricular dysrhythmias. A 12-lead electrocardiogram demonstrates sinus rhythm and LVH with normal PR interval and QRS duration of 121 ms.
This patient has a history of hypertrophic cardiomyopathy (HCM) with high-risk features for SCD and therefore has an indication for an ICD.1 Until recently, traditional ICDs required either transvenous access to place leads endocardially, or less commonly, patches could be surgically placed on the epicardium. Whereas ICDs provide excellent protection from sudden death, long-term lead durability is imperfect, and lead failure often requires extraction of a chronic lead or placement of a new lead. Such procedures can be complicated by cardiac perforation, tamponade, or systemic infection due to the requirement for venous access. This is of particular importance in young patients whose life expectancy is anticipated to be far greater than battery longevity or lead durability. As such, a subcutaneous approach that does not require leads to be placed on or within the heart may be especially advantageous in younger populations that do not require pacing. In this regard, the subcutaneous defibrillator is not capable of sustained pacing for bradyarrhythmias or cardiac resynchronization therapy. This patient is young and has no current or anticipated indication for pacing and therefore may be offered a subcutaneous ICD (S-ICD) for SCD prevention.
The S-ICD (Cameron Health/Boston Scientific) comprises a pulse generator and a single coil defibrillator lead placed to the left of and parallel with the sternum (Figure 71-1). The lead has a distal sensing electrode placed next to the manubriosternal junction and a proximal sensing electrode placed next to the xiphoid process. These electrodes flank an 8-cm shocking coil. The two sensing electrodes and pulse generator provide three sensing vectors for rhythm detection.
FIGURE 71-1 Posteroanterior (PA) chest X-ray demonstrating the left lateral position of the pulse generator as well as the parasternal lead location.
The S-ICD provides the following features:2
• The optimal sensing vector is automatically selected by the device and applies three algorithms to prevent double counting and T-wave oversensing.
• One- or two-zone configurations are available for arrhythmia detection with a minimal rate of 170 bpm. Typically, a 2-zone configuration is programmed, and in a young patient with HCM, the l conditional zone, which utilizes discrimination criteria, would typically be programmed about 200 bpm with the shock zone (ie, no discrimination algorithm active) at about 230 bpm. The number of intervals to detect an episode is nonprogrammable, and the algorithm is designed as a rhythm rather than beat detector, which tends to prolong detection.
• Arrhythmia discrimination analysis is performed either to deliver therapy appropriately for lethal ventricular arrhythmias or withhold therapy for supraventricular arrhythmias, myopotentials, or noise.
• The S-ICD has excellent arrhythmia detection and discrimination when compared to transvenous systems using either single- or dual-lead devices.3
• Defibrillation is performed with 80 J shocks. If initially unsuccessful, the S-ICD automatically reverses shock polarity. Due to the higher energy requirement for subcutaneous defibrillation, the current generation pulse generator is larger at 69.9 cc compared to transvenous pulse generators which are typically 40% to 50% smaller (Figure 71-2). Though larger, the laterally placed pulse generator is well tolerated and has been successfully implanted without regard to body habitus.
FIGURE 71-2 The S-ICD pulse generator (right) is larger than a typical transvenous pulse generator (left).
• Up to 30 seconds of transcutaneous pacing is available for postshock bradycardia.
• In addition to an established safety profile, the S-ICD has been demonstrated to identify and terminate invoked as well as spontaneous ventricular arrhythmias effectively.4
• Young patients at risk for SCD may be particularly well suited for an S-ICD given the likely need for multiple pulse generator replacements and concerns regarding lead durability. This device is ideal for patients with channelopathies such as long QT syndrome and Brugada syndrome.
• Patients with congenital heart disease (CHD) may have anatomy that limits transvenous lead placement. The S-ICD can be placed without being limited by such anatomy.
• Immunocompromised patients may be at higher risk for systemic infection when transvenous access is required.
• The presence of indwelling catheters as well as active fistulas or grafts for hemodialysis may preclude standard transvenous approaches. Additionally, venous obstruction, as often found in this population, requires additional procedures such as venoplasty to place transvenous leads.
• Patients with previous transvenous device infections requiring lead extraction and pulse generator removal may have a lower risk of systemic infection by implanting an S-ICD. This can potentially avoid more costly options such as providing a wearable defibrillator vest while reimplantation is postponed for prolonged antibiotic treatment.
• Other than 30 seconds of pacing for postshock asystole, the S-ICD is not able to provide sustained bradyarrhythmia therapies. Therefore, patients with any pacing indication, including cardiac resynchronization therapy, are not candidates for the S-ICD.
• Monomorphic VT can frequently be terminated by using nonshock therapy such as antitachycardia pacing (ATP), thereby avoiding high-energy shocks. Since the S-ICD is not capable of nonshock therapy, it should be avoided in patients for whom ATP is likely to be needed.
The S-ICD can be implanted in any standard sterile procedure or operating room without requiring fluoroscopy. Though fluoroscopy is not required, our practice has used a single fluoroscopic image to mark the ideal location of the pulse generator on the mid axillary line at the level of the apex. The S-ICD is then implanted in the following manner6:
• The left lateral wall and left chest are cleaned and draped in sterile fashion. Perioperative antibiotics are administered according to institutional policy. After the patient is sedated and local analgesic applied to the skin, an approximate 4-inch incision is made along the left mid axillary line and a subcutaneous pocket fashioned by standard technique utilizing electrocautery for hemostasis.
• Two incisions, inferior and superior, are made 1 to 2 cm to the left of the sternal border at the xiphoid process and manubriosternal junction (Figure 71-3). Of note, some implanters are avoiding the superior incision at the manubrium for cosmetic reasons.
FIGURE 71-3 Two incisions are made 1 to 2 cm to the left of the sternum.
• A tunneling tool is then used to guide the defibrillator lead in the subcutaneous space from the device pocket to the inferior incision and then from the inferior to superior incision. Suture sleeves are applied to the lead and anchored to the fascia to stabilize the lead (Figure 71-4).
FIGURE 71-4 A tunneling tool is attached via suture to the distal end of the defibrillator lead and tunneled from the pocket to the inferior parasternal incision and then to the superior incision. The lead is secured by suture sleeves at both parasternal incision sites.
• The pulse generator is then connected to the lead and placed within the pocket. The pocket and parasternal incisions are closed with suture in standard fashion (Figure 71-5).
FIGURE 71-5 After the lead is secured, it is connected to the pulse generator, which is placed within the pocket. The pocket and parasternal incisions are closed in standard fashion.
• Defibrillation testing is typically performed at 65 J to establish an adequate safety margin, though the device is programmed to only deliver 80 J biphasic shocks.
An initially unsuccessful attempt at defibrillation warrants reevaluation of the pulse generator and lead location. Minor adjustment of these elements can significantly alter the shock vector and improve defibrillation success. For this reason our practice has been to place the pulse generator laterally with a preference for slight posterior rather than anterior placement.
Management of patients after S-ICD implantation is similar to management for transvenous ICDs. Device interrogation is typically performed within 24 hours of implant to ensure adequate sensing. Postoperative evaluation of incisions is performed in accord with institutional practice for transvenous systems. In contrast to transvenous ICDs, which are typically capable of remote monitoring, the S-ICD is not currently equipped with this feature and therefore requires in-person visits for follow-up interrogations. However, there are minimal parameters to measure, so it only takes 1 to 2 minutes to perform device interrogations and assessment of function.
1. Epstein AE, Dimarco JP, Ellenbogen KA, et al. ACC/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities. Heart Rhythm. 2008;5(6):e1-62.
2. Bardy GH, Smith WM, Hood MA, et al. An entirely subcutaneous implantable cardioverter-defibrillator. N Engl J Med. 2010;363(1):36-44.
3. Gold MR, Theuns DA, Knight BP, et al. Head-to-head comparison of arrhythmia discrimination performance of subcutaneous and transvenous ICD arrhythmia detection algorithms: The START study. J Cardiovasc Electrophysiol. 2012;23(4): 359-366.
4. Weiss R, Knight BP, Gold MR, et al. The safety and efficacy of a totally subcutaneous implantable-defibrillator. Circulation. 2013;128(9):944-953.
5. Rowley CP, Gold MR. Subcutaneous implantable cardioverter defibrillator. Circ Arrhythm Electrophysiol. 2012;5(3):587-593.
6. Lobodzinski SS. Subcutaneous implantable cardioverter-defibrillator (S-ICD). Cardiol J. 2011;18(3):326-331.