A year old female patient was referred
A 55-year-old female patient was referred to us for fbpase of an implantable cardioverter-defibrillator (ICD) after cardiac arrest due to idiopathic ventricular fibrillation. As she was young and had no indications for cardiac resynchronization therapy or bradycardia support, she was screened for a subcutaneous ICD (S-ICD). We recorded the surface eletrocardiogram from the anticipated location of the device׳s three sensing electrodes in the supine and standing positions and during a limited treadmill exercise test (using the Modified Bruce protocol as she was still recovering from cardiac arrest and had limited mobility). When compared with the manufacturer׳s template, the patient passed the screening in all three vectors and an S-ICD (SQ-RX® pulse generator model 1010 and Q-TRAK® lead; Boston Scientific, Marlborough, MA, USA) was implanted with a successful defibrillation test. The device automatically selected the primary sensing vector and was programmed with a conditional zone over 200 beats per minute and a shock zone over 240 beats per minute.
One month later, while the patient was swimming, the device delivered two shocks and the patient collapsed, followed by a third shock and recovery of consciousness. The device was interrogated and revealed inappropriate detection of ventricular tachycardia and ICD discharge due to T wave oversensing (TWOS). The second shock induced ventricular fibrillation, which was successfully terminated by a third, appropriate shock (Fig. 1). When reviewing the stored electrograms immediately prior to the first shock, we observed that the QRS and T waves morphology changed, with decreased R wave amplitude and increased T wave amplitude (resulting in decreased R/T wave ratio), leading to TWOS and double counting. The patient underwent repeat treadmill exercise testing, this time through stage 3 of the full Bruce protocol. On this occasion, we observed a rate-dependent right bundle branch block (RBBB), which reproduced the altered R/T wave ratio observed in the primary sensing vector prior to the inappropriate shock (Fig. 2). The other sensing vectors were also screened during the exercise test for altered QRS-T wave morphology with rate-dependent RBBB; the secondary sensing vector was unaffected (Fig. 2), allowing for successful sensing reprogramming. The patient was able to continue her usual exercise activities and has had no further shocks during 12 months of follow-up.
Discussion The S-ICD is a recently introduced technology for the prevention of sudden cardiac death with clinical trial-supported efficacy . The advantage of the S-ICD is that energy is entirely subcutaneous, avoiding the need for transvenous leads and their associated complications. As the leads are extracardiac, sensing and programming differ compared to those of the transvenous ICD (T-ICD), posing new challenges. Currently, this device should be considered for patients with an indication for ICD when pacing therapy for bradycardia support, cardiac resynchronization, or antitachycardia pacing is not necessary. The S-ICD could be especially considered for young patients, those at high risk for bacteremia (due to indwelling catheters/hardware or immunocompromised states), and those with difficult venous access . The 360-day rate of inappropriate shocks observed in the EFFORTLESS S-ICD Registry was 7% . With T-ICDs, supraventricular tachycardias account for the majority of inappropriate shocks , whereas with the S-ICD, oversensing (especially TWOS) is the primary cause of inappropriate shocks . These findings align with those of a simulation study comparing single- and dual-chamber ICD algorithms to the S-ICD conditional zone discriminator algorithms based on morphology and R/T ratio, which were more specific for supraventricular arrhythmia discrimination than the manufacturers’ T-ICD algorithms . However, because the S-ICD relies on “far-field” electrograms (resembling a surface electrocardiogram), it may be more sensitive to QRS-T wave morphology changes compared to the local “near-field” ventricular electrograms from the T-ICD, rendering the S-ICD more prone to TWOS. Further, the S-ICD has a fixed sensing algorithm that can only be adjusted by changing the therapy zone frequency or by changing the sensing vector and stored template. TWOS incidence in T-ICDs has decreased over time due to the development of several algorithms for filtering and rectification, minimum sensing threshold, automatic sensitivity adjustment, and T wave rejection . Even so, TWOS still induces inappropriate shocks in 1–3.8% of T-ICD recipients, depending on underlying etiology [6,7].