Combined Use of Subcutaneous ICD and Pacemakers: The Beginning of a New Era?
Overall, 2.8% to 3.6% of transvenous ICD recipients suffer some lead‐related adverse event, such as pneumothorax, hemothorax, pericardial effusion and tamponade, or lead dislodgement during hospitalization for implantation.1 Moreover, lead‐related adverse events after hospitalization occur in <0.1 to 6.4% of ICD patients during 2‐ to 70‐month follow‐up; these include central vein thrombosis and occlusion, tricuspid valve insufficiency, systemic infections and endocarditis, and lead malfunction due to insulation defects or lead fractures, with consequent inappropriate/ineffective therapies and recall/withdrawal of the malfunctioning lead from the market.1
The array of devices that defibrillate the heart has expanded in recent years to include the subcutaneous implantable cardioverter‐defibrillator (S‐ICD). Introduced in Europe in 2009, the first‐generation S‐ICD was FDA approved in the United States in September 2012 and the second‐generation (Emblem™, Boston Scientific, Marlborough, MA, USA) in March 2015. The S‐ICD consists of a pulse generator implanted in the left mid‐axillary line at the fifth intercostal space and an extravascular lead tunneled in the subcutaneous space from the lateral pocket medially to the xiphoid process and then cephalad to the sternum‐manubrium just to the left (or right) of the parasternal margin. The subcutaneous lead is equipped with two sensing electrodes separated by an 8‐cm shock coil, the proximal electrode usually being placed at the xiphoid process and the distal one at the sterno‐manubrium level; this lead has no lumen, and is engineered to be stiffer and more robust than a transvenous lead. Because the S‐ICD is not in contact with the myocardium, standard antibradycardia pacing, antitachycardia pacing (ATP), and CRT functions are not provided by this device (with the exception of 30‐second postshock transthoracic backup pacing).
An important feature of the S‐ICD is the sensing and detection function. The device senses subcutaneous signals and detects cardiac rhythm from the two sensing electrodes or from both electrodes and the pulse generator. There are three available sensing vectors (primary: from proximal electrode to pulse generator; secondary: from distal electrode to pulse generator; alternate: from distal to proximal electrode). Arrhythmia detection is performed through the use of one of these three vectors. The system automatically selects the most appropriate vector after implantation, according to the highest R amplitude, the most satisfactory R wave/T wave ratio, and the best noise reduction, to avoid double counting of QRS and T‐wave oversensing.
To verify the adequate sensing of the system, ensure patient eligibility for the S‐ICD, and minimize the risk of inappropriate shocks during follow‐up, preimplantation electrocardiographic screening is recommended. The screening process consists of placing three ECG electrodes on the thorax at the sites where the generator and sensing electrodes of the S‐ICD are to be implanted, thus simulating the three sensing vectors of the S‐ICD. Electrocardiograms from each vector are then recorded at a paper speed of 25 mm/s at gains of 5, 10, and 20 mV for a period of 10 seconds in both supine and upright positions.