Subcutaneous Implantable Defibrillator: First Experience at Memorial Hermann The Woodlands

Senthil Thambidorai, MD, FHRS, Cardiac Electrophysiologist, Memorial Hermann The Woodlands, and Assistant Professor, University of Texas Health Science at Houston
Senthil Thambidorai, MD, FHRS, Cardiac Electrophysiologist, Memorial Hermann The Woodlands, and Assistant Professor, University of Texas Health Science at Houston


The implantable cardioverter-defibrillator (ICD) prevents sudden cardiac death and is used both in primary and secondary prevention.1 Nevertheless, the transvenous ICD is associated with significant periprocedural and long-term complications such as cardiac perforation, cardiac valve injury, hemothorax, pneumothorax, deep phlebitis, transient ischemic attack, stroke, myocardial infarction, cardiac tamponade, and arterial-venous fistula.2,3 Over time, the incidence of intrinsic lead defects, mainly due to insulation defects, invariably increases with a reported annual failure rate at 10-year-old leads of up to 20%.4

Although the transvenous ICD is highly effective in treating ventricular arrhythmias, there are associated adverse effects mainly related to intravascular lead insertion.4 Therefore, a non-transvenous ICD system is an attractive option that would overcome many of these problems. Recently, a dedicated entirely subcutaneous ICD system (Boston Scientific’s S-ICDSystem) has been developed and recently approved for commercial use in the United States.5,6

Case Study

The patient is a 24-year-old male college athlete who was playing basketball with friends when he suffered a witnessed cardiac arrest. He suffered a syncopal event and fell down face front. As he was in a 24-hour fitness club, they used an AED to shock him back to sinus rhythm, and stabilized him. Rhythm strips confirmed VF arrest and successful defibrillation.

He was admitted to Memorial Hermann The Woodlands Hospital’s ICU for management. His workup included an echocardiogram, which showed no regional wall motion abnormalities or features suggestive of any cardiomyopathy. Coronary angiogram showed no obstructive epicardial coronary artery disease. His electrocardiogram showed prolonged QTc interval. According to his family, he had an episode of near-syncope while playing basketball eight years ago while in high school, but was never worked up after that. After the patient regained full neurological recovery, he noted that two weeks ago, he had experienced another syncopal event for which he did not seek medical attention. In addition, his uncle experienced sudden cardiac death in his 40s. 

Based on the family history, repeated near-syncope, syncope with VF arrest and prolonged QT interval on EKG, long QT syndrome was suspected. The decision was made to implant a primary prophylactic defibrillator. Given his young age and his concerns of intravascular lead placement, we decided to implant a subcutaneous ICD (S-ICD). We performed a screening ECG template to confirm a satisfactory R-wave/T-wave ratio in at least 1 of the 3 available sensing configurations pre-implantation, and the patient qualified for S-ICD implantation.

Device Implantation

The S-ICD System is comprised of a pulse generator, subcutaneous electrode, electrode insertion tool and device programmer. The pulse generator is slightly larger and weighs 145 grams. 

The patient is placed under general anesthesia, as this maximizes patient comfort. Briefly, the generator is placed subcutaneously in a left lateral position over the 5th and 6th intercostal space on the midaxillary line. The pin connector end of the lead is tunneled to the pocket created in the left lateral chest. Using two parasternal incisions, a 3 mm tripolar parasternal electrode (polycarbonate urethane) is positioned parallel to and 1-2 cm to the left of the sternal midline, with the distal sensing electrode localized adjacent to the manubriosternal junction and the proximal sensing electrode positioned adjacent to the xiphoid process. This position is done using the atraumatic tunneling tool. The 8 cm shocking coil is found between the two sensing electrodes.5 The S-ICD automatically selects the most suitable vector for rhythm detection with a satisfactory R-wave/T-wave ratio, in order to minimize the risk for oversensing. During the implantation, ventricular fibrillation is induced, also referred to as induction testing, and a sub-maximal energy shock (65J) is delivered by the S-ICD System to convert the induced tachyarrhythmias and ensure the effectiveness of the therapy in the patient. While a sub-maximal shock is used in induction testing, the system’s 80-Joule maximum energy shock is the only one available outside the hospital.6 It provides high-energy defibrillation shocks through the use of a constant tilt biphasic waveform, and is capable of delivering post-shock bradycardia pacing at 50 impulses per minute using a 200 mA biphasic transthoracic pulse for a period of up to 30 s if >3.5 s of post-shock asystole is detected.6 The chest x-ray shows the typical position of the generator and the leads (Figure 1). 

Detection and Programming Options

The S-ICD System calculates the heart rate as the average of the last four intervals, and performs tachycardia analysis using an 18/24 duration criteria. Tachycardia is reconfirmed after capacitor charging (average time of 14.6 ± 2.9 seconds in IDE trial) but before shock delivery to exclude the presence of non-sustained tachyarrhythmia.5 Apart from a shock zone, the device offers an optional conditional discrimination zone that involves three distinct rhythm analyses to distinguish atrial from ventricular tachyarrhythmia and avoid inappropriate shocks:

  1. Correlation waveform analysis of up to 41 points of each ventricular complex, comparing the current tachycardia beat with the stored template acquired at rest. More than 50% of correlation is considered normal activity and suggests an atrial tachyarrhythmia.
  2. Beat-to-beat analysis that evaluates monomorphic or polymorphic beat relationships. In the case of a polymorphic relationship, ventricular tachyarrhythmia is suspected, and in the case of monomorphic relationship, the algorithm continues.
  3. QRS width analysis, using the baseline template, that indicates ventricular tachycardia (VT) if the QRS complex is wide, and if the beat-to-beat analysis registered a monomorphic relationship. If the QRS complex is narrow, atrial tachyarrhythmia is assumed.6

If ventricular tachyarrhythmia is confirmed, the device is able to deliver up to five shocks of 80 J, with shock polarity reversed if the first shock is unsuccessful. A total of 24 episodes can be stored with a maximum of 120 s of recorded electrograms per event.6


Memorial Hermann The Woodlands was the first community-based hospital to perform the S-ICD in Houston (Figure 2). We have two cardiac catheterization labs equipped with all the equipment to perform these procedures as well as a multidisciplinary team approach to perform the first successful S-ICD outside the Texas Medical Center in Houston (Figure 3).

Advantages of the S-ICD System include that it is easy to implant, it involves no fluoroscopy, and there is no risk of pneumothorax or vascular complications. There are also fewer long-term complications with regards to infection, and it is easy to revise and extract. In the IDE study, the S-ICD System converted 100% of induced VT/VF episodes. The 180-day device- or procedure-related complication-free rate was 92.1%, exceeding the performance goal of 79%. Dual-zone programming reduced the rate of inappropriate shocks for oversensing by 54% and for SVT (programmed rate) by 74%. While there have been over 2,000 patients implanted worldwide, they are still early data in comparison to traditional transvenous ICDs; therefore, we should continue to offer these to patients on a case-by-case basis. Possible limitations of this system may be related to the lack of pacing to terminate arrhythmias and an inability to “painlessly” terminate tachycardia. It also cannot be used if standard pacing indication is present. Table 1 characterizes the more suitable patients for S-ICD.7 

Disclosure: The author has no conflicts of interest to report regarding the article herein. 


  1. Moss AJ, Zareba W, Hall WJ, et al. Prophylactic implantation of a defibrillator in patients with myocardial infarction and reduced ejection fraction. N Engl J Med. 2002;346:877-883.
  2. Curtis JP, Luebbert JJ, Wang Y, et al. Association of physician certification and outcomes among patients receiving an implantable cardioverter-defibrillator. JAMA. 2009;301:1661-1670.
  3. Kleemann T, Becker T, Doenges K, et al. Annual rate of transvenous defibrillation lead defects in implantable cardioverter-defibrillators over a period of >10 years. Circulation. 2007;115(19):2474-2480.
  4. Maisel WH. Transvenous implantable cardioverter defibrillator leads: the weakest link. Circulation. 2007;115(19):2461-2463. 
  5. Bardy G, Cappato R. The totally subcutaneous ICD system (the SICD). Pacing Clin Electrophysiol. 2002;25:578.
  6. Bardy GH, Smith WM, Hood MA, et al. An entirely subcutaneous implantable cardioverter-defibrillator. N Engl J Med. 2010;363:36-44.
  7. Akerström F, Arias MA, Pachón M, Puchol A, Jiménez-López J. Subcutaneous implantable defibrillator: State-of-the art 2013. World J Cardiol. 2013;5(9):347-354.