Dr. Stefano Cappelli
Dr. Alina Olaru
Dr. Elia De Maria
If a given patient necessitating implantation of a cardioverter defibrillator has no indications for cardiac resynchronisation, bradycardia support or antitachycardia pacing and provided this patient's condition or previous history would make the absence of leads within the heart and the preservation of central venous circulation preferable, than the newly developed subcutaneous cardioverter defibrillator is a first line indication. See here specifically who stands to benefit from the subcutaneous defibrillator in this review of the performance, approval, limitations and advantages of the newly available device.
In the battle against sudden cardiac death (SCD), an implantable cardioverter-defibrillator, with the treatment of coronary artery disease, risk stratification and pharmacological interventions is a therapeutic cornerstone for the management of ischemic and nonischemic cardiomyopathy or impaired ventricular function in the prevention of potentially lethal ventricular tachycardia. Today's devices are miniaturised pectoral systems capable of defibrillation and pacing with leads inserted into venous circulation through cephalic, axillary or subclavian veins whereas in 1980, surgically-applied pericardial patches required a large abdominally-placed generator (1,2). Great progress in this area has been made.Several large randomised trials have demonstrated reduction in total mortality in primary and secondary prevention with the implantable cardioverter-defibrillator (3). However, insertion of the electrode into the central venous circulation and inside cardiac chambers can cause vascular obstruction, thrombosis, infection and cardiac perforation (4,5). Moreover, lead failure has been estimated at 0.58%/year and up to 20% at 10 years; failures may require the leads to be extracted, however this is a highly challenging procedure: major complications rates are about 1% and mortality is at 0.3%, even in experienced centers (6,7,8,9). Some leads have even necessitated recall due to increased mortality. The development of the subcutaneous defibrillator was initially motivated by cases of children with congenital heart diseases or patients with who were unsuitable for transvenous ICD due to impaired venous access. The result has been the development of a device with no leads within the heart and preservation of the central venous circulation.
1 - Description
The subcutaneous implantable cardioverter defibrillator consists of a 3-mm tripolar parasternal lead (12 French, 45 cm) connected to an electrically active pulse generator. The lead is vertically positioned in the subcutaneous tissue of the chest, parallel to and 1-2 cm to the left sternal midline followed by a horizontal segment, at the level of the 6th rib, until it reaches the left anterior axillary line. The lead has a 8-cm shock coil, flanked by two sensing electrodes - the distal one positioned adjacent to the manubriosternal junction and the proximal one adjacent to the xiphoid process. Compared to a traditional ICD, the pulse generator is double in size. It measures 78x65x15 mm with a volume of 69 cc and a mass of 145 grams. Its estimated longevity, defined as 3 full-energy capacitor charges per year, is of nearly 5 years (which is half that of the traditional device).The generator is positioned in the subcutaneous tissue of the chest over the 6th rib between the left midaxillary and the left anterior axillary lines (Fig.1).Fig. 1: External appearance of an implanted device.The sensing and detection function identifies subcutaneous signals (“subcutaneous ECG”) and cardiac rhythm from the two sensing electrodes or from electrodes and a pulse generator. There are three available sensing vectors: primary - from proximal electrode ring to can -, secondary - from distal electrode ring to can- and alternate - from distal to proximal electrode. (Fig.2).
Fig. 2: The three available sensing vectors of S-ICD.The system automatically selects the optimal vector for detection to prevent inappropriate therapy due to myopotentials, noise or multiple counting (in particular double QRS and T-wave oversensing). A dedicated algorithm or template matching is used for the discrimination of supraventricular arrhythmias. Its system is conceptually similar to that used by transvenous devices, but the S-ICD evaluates a greater number of points (up to 41) of ventricular complex arrhythmias to improve signal resolution.
2 - Performance
In the START study, a recent head-to-head comparison between the S-ICD and the ICD showed that the S-ICD was superior to the transvenous ICD with regard to specificity for supraventricular arrhythmia discrimination (10). In this context, compared to late stage morphology discriminators in transvenous ICD, earlier use of morphology discriminators may represent an advantage. Moreover, the ultra far field signal recorded by S-ICD lead mimics the surface ECG more closely, theoretically improving diagnostic capability. A shock zone, at 240 beats per minute and a conditional one between 170 and 240 beats per minute can be set to deliver therapy. These zones are to distinguish supraventricular from ventricular tachyarrhythmias. Testing during implantation is performed with a 65-J shock on induced VF, however after implantation the device delivers only up to five consecutive biphasic 80-J shocks per episode. It does not deliver anti-tachycardia pacing however it can automatically reverse shock polarity if initial shock is unsuccessful. Charge time is approximately 15” to maximum output, which is longer than transvenous systems. Demand transthoracic pacing (200-mA biphasic pulse) at a fixed rate of 50 beats per minute is available for 30” after a shock, activated only after >3500 msec of post-shock asystole. Programming is simple as all device settings are automated apart from shock therapy (on/off), pacing after a shock (on/off), conditional discrimination of supraventricular tachyarrhythmias (on/off) and upper-rate cut off for the conditional shock zone (170 to 240 beats per minute). Implanting the system is relatively simple and guided only by anatomical landmarks and with no fluoroscopy required; however a learning curve, consisting of at least 3-4 cases, is necessary for an operator to become familiar with the procedure (11,12).
3 - Approval
A number of pivotal studies initiated more than 15 years ago demonstrated the ability of the subcutaneous system to terminate ventricular arrhythmias using different shock vectors and with variable energy requirements. The first human study provided information regarding optimal lead configuration, lead and generator placement and shock efficacy on both induced and spontaneous arrhythmias (13). After obtaining CE marking in 2009, several single-center and multicenter experiences across Europe demonstrated safety and efficacy of the S-ICD in a “real world” scenario (14,15,16,17). Finally the first prospective, single-arm, non-randomised, multicenter trial was completed in the U.S. It showed that both effectiveness and safety endpoints were met compared with traditional devices; ventricular fibrillation (VF) was detected without delay in >99% of cases and all spontaneous VT/VF episodes were terminated by the device with a first shock efficacy >92% which is comparable to the efficacy of transvenous ICD. These results led to the US Food and Drug Administration approval in late 2012.
4 - Advantages
The advantages of the S-ICD are:
5 - Limitations
Described further are the limitations of the device:
Fig. 3: Two examples of oversensing during premature ventricular contractions. Top: double QRS counting. Bottom: T wave oversensing. S indicates a sensed event.6. Patient selection
In as much as there are patients needing an ICD but in whom a transvenous ICD is not feasible, the S-ICD may be the only option.
Fig. 4: VF episode in Brugada syndrome treated by the subcutaneous defibrillator. S-ICD electrogram of spontaneous ventricular polymorphic tachycardia degenerating into ventricular fibrillation and treated with a shock in a patient with Brugada Syndrome. C= capacitor charging; S= sensing of an event not classified as tachycardia; T= sensing of an event classified as tachycardia. A dot indicates sensing of an unclassifiable event that is discarded. Black lightning symbol indicates a shock. Red arrows indicate post shock transthoracic pacing (reproduced with permission from reference 24).Contraindications
Contraindications of the ICD are the following:
A summary of proposed recommendations for patient selection is as follows: S-ICD as a first choice:
S-ICD as a reasonable choice:
When to avoid the S-ICD:
8 - Future perspectives
Approximately 55% of patients in routine clinical practice needing an ICD are potential candidates for a subcutaneous device despite current limitations. Nevertheless, to maximise clinical outcome and cost/benefit ratio, it is fundamental to choose candidates that can benefit the most. Next generation devices might address many of the current limitations by offering improvement of battery technology, a downsized generator; improved algorithm design, remote monitoring and pacing capability with perhaps leadless ultrasound-based cardiac stimulation.Currently there are still no clinical data that demonstrate S-ICD can prevent sudden death or improve survival in the same way as traditional ICD. The possible use of S-ICD as a first-line strategy or as an alternative approach for specific populations will require the confirmation of large clinical trials and registries that are currently ongoing (26,27).
1 - Current evidence base for use of the implantable cardioverter-defibrillator. Hohnloser SH, Israel CW. Circulation. 2013 Jul 9;128(2):172-83. 2 - Ventricular arrhythmias and sudden cardiac death. John RM, Tedrow UB, Koplan BA, Albert CM, Epstein LM, Sweeney MO, Miller AL, Michaud GF, Stevenson WG. Lancet. 2012 Oct 27;380(9852):1520-9.3 - Sudden cardiac death and implantable cardioverter defibrillators: two modern epidemics?. Katritsis DG, Josephson ME. Europace. 2012 Jun;14(6):787-94. 4 - Implantable cardioverter defibrillator harm?. Gasparini M, Nisam S. Europace. 2012 Aug;14(8):1087-93.5 - Implantable cardioverter defibrillators: risks accompany the life-saving benefits. Atwater BD, Daubert JP. Heart. 2012 May;98(10):764-72.6 - Transvenous implantable cardioverter-defibrillator leads: the weakest link. Maisel WH. Circulation. 2007 May 15;115(19):2461-3.7 - Implantable cardioverter-defibrillator lead performance. Maisel WH, Kramer DB. Circulation. 2008 May 27;117(21):2721-3. 8 - Management of the patient with implantable cardioverter-defibrillator lead failure. Kalahasty G, Ellenbogen KA. Circulation. 2011 Mar 29;123(12):1352-4.9 - The subcutaneous implantable cardioverter-defibrillator. Grace A. Curr Opin Cardiol. 2014 Jan;29(1):10-9.10 - Head-to-head comparison of arrhythmia discrimination performance of subcutaneous and transvenous ICD arrhythmia detection algorithms: the START study. Gold MR, Theuns DA, Knight BP, Sturdivant JL, Sanghera R, Ellenbogen KA, Wood MA, Burke MC. J Cardiovasc Electrophysiol. 2012 Apr;23(4):359-66.11 - Implantation of a completely subcutaneous ICD system: case report of a patient with Brugada syndrome and state of the art. De Maria E, Bonetti L, Patrizi G, Scrivener J, Andraghetti A, Di Gregorio F, Montin A, Zuccon G, Cappelli S. J Interv Card Electrophysiol. 2012 Jun;34(1):105-13.12 - Subcutaneous implantable cardioverter defibrillator. Rowley CP, Gold MR. 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The Subcutaneous Implantable Cardioverter Defibrillator (ICD) Should Be Considered in all ICD Patients Who Do Not Require Pacing. Poole JE, Gold MR. Circ Arrhythm Electrophysiol. 2013 Dec 1;6(6):1236-45.20 - Evaluation of acute cardiac and chest wall damage after shocks with a subcutaneous implantable cardioverter defibrillator in Swine. Killingsworth CR, Melnick SB, Litovsky SH, Ideker RE, Walcott GP. Pacing Clin Electrophysiol. 2013 Oct;36(10):1265-72.21 - Which Patients Are Not Suitable for a Subcutaneous ICD: Incidence and Predictors of Failed QRS-T-Wave Morphology Screening. Olde Nordkamp LR, Warnaars JL, Kooiman KM, de Groot JR, Rosenmöller BR, Wilde AA, Knops RE. J Cardiovasc Electrophysiol. 2013 Dec 9. doi: 10.1111/jce.12343. [Epub ahead of print].22 - Who should receive the subcutaneous implanted defibrillator? Timing is not right to replace the transvenous implantable cardioverter defibrillator. Acha MR, Milan D. 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Elia De Maria, EP Cath Lab, Cardiology Unit, Ramazzini Hospital, Carpi (Modena), Italy.Alina Olaru, Department of Cardiovascular Medicine, University of Modena ad Reggio Emilia, Modena, Italy.Stefano Cappelli, Cardiology Unit, Ramazzini Hospital, Carpi (Modena), Italy.Address for correspondence:Elia De Maria, EP Cath Lab, Cardiology Unit, Ramazzini Hospital, Via Molinari, Carpi (Modena), Zip Code 41012. Tel +39059659320. E-mail: email@example.comAuthors' disclosures: None declared.
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