Color Atlas and Synopsis of Electrophysiology, 1st Ed.


Essa Essa, MD, Jose Tores, MD, Raul Weiss, MD


The patient is a 32-year-old white woman, 125 pounds, with a body mass index of 23. She has complex congenital heart disease, has undergone multiple surgical repairs as a child, including repair of the atrial and ventricular septal defects, valvular replacement of the aortic, pulmonic, and tricuspid valves, as well as correction of subvalvular aortic stenosis. Originally she had a transvenous (TV) internal cardioverter defibrillator (ICD) that was implanted for a primary prevention indication. Her TV-ICD became infected after a routine generator replacement, requiring an extraction and replacement of the entire system. Unfortunately, a second infection in the newly implanted system occurred, but at this time it was complicated with prosthetic valve endocarditis. After 6 months of intensive care unit hospitalization and surgical interventions to remove the TV-ICD and replace the prosthetic valves, it was decided not to reimplant her with a TV-ICD. Approximately 1 year after discharged, she had an aborted sudden death episode, and she was referred for a secondary prevention ICD implantation. Given her past medical history a subcutaneous (S-ICD) was implanted (Figure 72-1). Baseline sensing was confirmed, ventricular fibrillation (VF) was induced, and her S-ICD converted VF at 65 J standard polarity two consecutive times.


FIGURE 72-1 PA and lateral view of a chest X-ray of a 32-year-old patient with complex CHD and multiple corrective surgeries.

During follow-up she received appropriate therapy for ventricular tachycardia, but she also experienced inappropriate therapies that were managed by reprogramming the sensing vector of the S-ICD.


Subcutaneous ICDs are commercially available for management of sudden cardiac death. Patients that potentially can benefit from this therapy are those who have accepted indications for ICD implantation,1who do not require antibradycardia pacing, or who have pace-terminable ventricular tachycardia.

Not every patient that has an ICD indication is suitable to be implanted with an S-ICD. Patients have to pass a prescreening 3-lead ECG in the supine and upright position. This prescreening ECG consists of three cutaneous electrodes placed in the proximity where the subcutaneous lead electrodes will be later implanted. This step has to be fulfilled in order to have the device implanted.

The advantage of the S-ICD is that there is no intravascular lead (Figure 72-2). Consequently, the implantation risks related to an intravascular procedure are eliminated, and the long-term risk of endocarditis due to lead infection and the risk associated with lead extraction are also minimal.


FIGURE 72-2 (A) Schematic representation of SICD pulse generator and subcutaneous electrode and sensing vectors. The electrogram obtained by the SICD is similar to an ECG tracing. (B) Chest x-ray of a patient with an SICD and a schematic representation of the vector.

The ability of the S-ICD to sense and defibrillate ventricular arrhythmias has been originally questioned, but the safety and efficacy of the system has recently been confirmed in a landmark study of 330 patients worldwide.2

The system consists of a subcutaneous electrode (SE) and a pulse generator (PG). The SE is a multistrand cable-core design, which is unlike a hollow core-design like TV leads. A multistrand design confers a stronger electrode. The polyurethane insulation was designed to withstand cardiopulmonary resuscitation forces, and the subcutaneous placement avoids intracardiac biomechanical stresses. This SE does not need to be as flexible as a TV lead. The PG is a 69-cc device capable of delivering 80 J, postshock, on-demand pacing at 50 beats per minute for up to 30 seconds and storing electrograms for approximately 48 episodes. The S-ICD can be programmed as a single zone in which the rate cut-off is the only determinant for providing therapy, or as a two-zone in which the second zone offers additional discriminators beyond just rate.


The sensing algorithm used by the S-ICD is a crucial component for appropriate detection and to minimize inappropriate therapies. It differs significantly from the TV ICD in two main ways.

The first difference between the S-ICD and TV-ICD systems is that there is no endocardial sensing lead with an S-ICD. Sensing, therefore, is accomplished via three electrodes. Two of the electrodes are located along the subcutaneous lead (electrode A is at the tip of the lead and electrode B is 15 cm from the tip), and the third electrode is the PG (C electrode) (Figure 72-2). From these three electrodes, three different vectors are generated. The S-ICD automatically chooses the vector with the largest QRS to T-wave ratio, but that vector can be overridden by reprogramming. The QRS to T-wave ratio may vary under different conditions. It can change with body position (patient in the supine versus upright position) or during exercise (Figures 72-3 to 72-5). We have learned that testing sensing during an exercise treadmill test, particularly in young patients, is quite useful in preventing T-wave oversensing (Figure 72-6). In the patient presented in the case story, there were no further inappropriate therapies with reprogramming of the S-ICD sensing vector from secondary to primary vector configuration.


FIGURE 72-3 Baseline and exercise showing the QRS to T-wave ratio. This tracing discarded a single beat because that could not be classified (.).


FIGURE 72-4 Shown is the change in the QRS to T-wave ratio in the alternate vector. At baseline sensing is appropriate, but during exercise the QRS to T ratio decreases, and there is occasional oversensing of the T wave.


FIGURE 72-5 In the primary vector configuration the QRS to T ratio is appropriate at baseline and during peak exercise. Note there is no increase in the amplitude of the T wave as occurred in the other configuration. There is no under- or oversensing of electrical signal.


FIGURE 72-6 Sinus tachycardia during exercise with oversensing of the T wave that led to inappropriate shock due to overcounting. It is important to notice that the algorithm shown in Figure 72-2 does not apply because the device does not have any discrimination. Device is programmed as a single zone, cut off rate 200 in the secondary vector configuration.

The second difference between S-ICD and TV-ICD sensing is that S-ICD utilizes a sophisticated sensing algorithm to help minimize inappropriate shocks. This algorithm evaluates the rhythm, but does not use a beat-to-beat assessment, which is the current methodology for a TV-ICD (Figure 72-7).


FIGURE 72-7 SICD algorithm for detection of ventricular arrhythmias. The programming of a second zone significantly decreases the number of inappropriate shocks.

The S-ICD sensing algorithm has been thoroughly evaluated in the START study2 and in a large clinical trial.3 In the START study, the TV-ICD and the S-ICD sensitivity and specificity were compared. This study was performed by inducing VF (shockable rhythm) and atrial fibrillation (nonshockable rhythm) and then recording the rhythms simultaneously from a TV-ICD lead and from three cutaneous electrodes, which were then evaluated offline. The offline analysis passed the recordings through the TV-ICD sensing algorithms (Boston Scientific, Medtronic, and St Jude Medical) and through the sensing algorithm of the S-ICD (Cameron Health/Boston Scientific) in order to compare the sensitivity of the S-ICD to TV-ICDs. All devices, including the S-ICD, demonstrated 100% sensitivity, meaning that all VF episodes were appropriately detected as shockable rhythm. The specificity (the ability of a system to withhold therapy for nonshockable rhythms) was significantly superior for the S-ICD, 98%. The specificity for a single-chamber TV device was 77%, and for a dual-chamber device it was 68%.

In the largest published study,3 appropriate detection and treatment occurred in 119 spontaneous VT/VF episodes in 21 patients who were treated by the S-ICD (appropriate shocks). In patients with single episodes of VT/VF, the S-ICD appropriately sensed and terminated all episodes (5% of patients required a second shock, and one patient converted while the device was charging to deliver a second shock). Patients with VT/VF storm were also successfully detected and treated by the S-ICD. One of the patients with VF storm is the patient presented in the case story. The ventricular arrhythmias were successfully detected and defibrillated four consecutive times (Figures 72-8 to 72-11). In the study, there was no SCD or death related to the S-ICD.


FIGURE 72-8 Approriate detection with occasional undersensing of Ventricular Fibrillation.


FIGURE 72-9 Appropriate detection and treatment of Ventricular Fibrillation.


FIGURE 72-10 Appropriate detection and treatment of Ventricular Fibrillation. Also this figure shows post shock pacing (P).


FIGURE 72-11 Initial undersensing of Ventricular Fibrillation with successful detection and appropriate the therapy and restoration of normal sinus rhythm.

Due to the nature of the charge (nonprogrammable 80 J), the mean charge time is 14 seconds. This charge time allowed 63% of ventricular episodes to self-terminate and did not result in syncope (Figure 72-12). Similar observations were made in the MADIT-RIT trial.4


FIGURE 72-12 The charge time to 80 Joules in the SICD, although longer than the transvenous system, allows for spontaneous termination of ventricular rhythms. (Data from Weiss et al. Safety and efficacy of a totally subcutaneous implantable-cardioverter defibrillator. Circulation. 2013;Aug 27; 128(9):944-953.)

The incidence of inappropriate shocks over a follow-up of approximately 1 year was 13.1%. Supraventricular tachycardia in the high-rate zone (no discriminators), in which rate alone determines whether a shock is delivered, was the cause in 5.1% of patients (Figure 72-13). These inappropriate shocks were representative of normal sensing behavior by the S-ICD system at rates above the high-rate zone. No patient experienced an inappropriate shock in the lower, conditional zone. The most common cause of inappropriate shocks in this study was T-wave oversensing (see Figure 72-7). For our patient, T-wave oversensing was managed by changing vectors and programming a conditional zone. Programming the device and adding a conditional zone, which offers a detection criteria of rate cut off and discriminators, significantly decreased oversensing by 56% and inappropriate therapies for SVT by 70%. Inappropriate shocks due to T-wave oversensing most often occurs during sinus tachycardia and can be screened by performing exercise treadmill testing and selecting a sensing vector with no T-wave oversensing.


FIGURE 72-13 Reduction of inappropriate shocks by programing a conditional zone with all the rhythm discriminators shown in Figure 72-2. (Modified with permission from Weiss et al. Safety and efficacy of a totally subcutaneous implantable-cardioverter defibrillator. Circulation. 2013;Aug 27;128(9):944-953.)


• S-ICD is a new device that is safe and effective in preventing sudden cardiac death.

• The sensing algorithm utilized by this device appropriately detects ventricular arrhythmias utilizing electrodes positioned subcutaneously and does not require a TV lead.

• The rate of inappropriate shocks with S-ICD is most often due to T-wave oversensing and is not higher than inappropriate shocks with TV-ICDs.

• Programming a second zone (conditional zone) significantly reduces the risk of inappropriate shocks.

• There is no discrimination error reported for atrial fibrillation in the conditional zone.

• Exercise stress testing is clinically helpful to select the best sensing vector to help reduce T-wave oversensing.


1. Epstein AE, DiMarco JP, Ellenbogen KA, et al. 2012 ACCF/AHA/HRS focused update incorporated into the ACCF/AHA/HRS 2008 guidelines for device-based therapy of cardiac rhythm abnormalities: a report of the American College of Cardiology Foundation/American Heart Association Task Force on Practice Guidelines and the Heart Rhythm Society. J Am Coll Cardiol. 2013;61(3):e6-75.

2. 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;23:359-366.

3. Weiss R, Knight BP, Gold MR, et al. Safety and efficacy of a totally subcutaneous implantable-cardioverter defibrillator. Circulation. 2013;128:944-953.

4. Moss AJ, Schuger C, Beck CA, et al; MADIT-RIT Trial Investigators. Reduction in inappropriate therapy and mortality through ICD programming N Engl J Med. 2012;367:2275-2283.