Why was a subcutaneous defibrillator removed within one year?

A 50-year-old man presents with palpitations and dizziness at a hospital in Omaha, Nebraska, while being transferred from another facility. He has a history of non-ischemic cardiomyopathy (25% left ventricular ejection fraction) and ventricular tachycardia (VT). Eight months previously, he had undergone the placement of an implantable subcutaneous cardioverting defibrillator (S-ICD).

His records show that 6 months after implantation of S-ICD, he began to deliver several shocks. The clinicians then prescribed amiodarone; however, this created pulmonary toxicity and was discontinued. He began treatment with sotalol and mexiletine, but the recurrent VT and S-ICD shocks continued and he was transferred to hospital for further assessment and management.

On admission, clinicians perform a device interrogation that reveals an occurrence of VT, which they treat upon detection. Additional similar events occur at a heart rate between 150 and 160 beats per minute, which clinicians note to be below the lowest detection zone for S-ICD treatment (170 beats per minute).

Despite the use of a SMART Pass filter, VT is inappropriately detected due to T wave oversensing leading to appropriate S-ICD shocks (Figure 1A). Clinicians note additional events in which the device failed to treat slow VT due to appropriate detection, leading to no or delayed treatment.

They determine that in recurrent slow VT, intermittent T-wave oversensing, and failure of the S-ICD detection zone to appropriately treat all VT episodes, the S-ICD should be removed. .

So, 8 months after S-ICD placement, clinicians place a single-chamber transvenous ICD (TV-ICD) and turn off the S-ICD. They note that the TV-ICD system effectively detects recurrent symptomatic slow TV, which is successfully treated with anti-tachycardia pacing (Figure 1C).

Three months later, the patient received a TV ablation. Clinicians successfully perform substrate modification and ablation of local abnormal ventricular activity in the basal to middle anterolateral wall of the left ventricle (Figure 1D). After surgery, they stop the patient’s mexiletine treatment and continue with sotalol only.

One month later, the patient undergoes a gradual withdrawal of S-ICD (Figure 1B). Unfortunately, 2 months after explanting the S-ICD, a delayed pocket site infection is detected, which requires incision and drainage, as well as a short course of antibiotics.

Figure 1. (A) Interrogation of the implantable subcutaneous cardioverter defibrillator (S-ICD) showing inappropriate T wave oversensing leading to appropriate shock by the device. (B) Chest x-ray before S-ICD removal showing a newly implanted transvenous ICD (TV-ICD). (C) TV-ICD with appropriate detection leading to appropriate therapy. (D) Electroanatomical map of the left ventricle in left lateral view showing ablation points in the basal to middle anterolateral wall.

The patient receives routine follow-up assessments at an arrhythmia clinic. At 15 months after his ablation procedure, he has his final exam, in which no additional delayed or inappropriate TV-ICD therapy is detected.


Clinicians presented this case of a patient whose S-ICD had to be replaced with VT-ICD due to inappropriate shocks resulting from over-sensing events and the inability to process VT below a frequency. heart rate of 170 beats per minute. They noted that implantation and explantation procedures – in addition to incision and drainage of an infection from the site of the pocket – could have been avoided with proper screening and selection of a type of defibrillator. suitable for treating slow TV.

The S-ICD operates in three phases, the case authors noted: a detection phase, a certification phase (to suppress hypersensitive events) and a therapeutic decision phase when the shock is delivered.

Despite some advantages over VT-ICD, the limitations of S-ICD include the inability to treat VT below a heart rate of 170 beats per minute; the potential to produce inappropriate shocks due to oversensing, particularly in poorly screened patients; and a lack of stimulation capacity. The authors cited a study which estimated that hypersensitive events occurred in 16% of S-ICD recipients, resulting in explantation of the device in 5.6% of patients.

They attributed the increasing use of S-ICD systems to efforts to minimize complications associated with the TV-ICD system, such as systemic infections, venous obstruction, and thrombosis.

A meta-analysis (in which the noted case authors did not include any randomized studies) found that although S-ICD systems had fewer lead-related complications than TV-ICD, similar infection rates were noted in both groups. In addition, while the two devices had similar incidences of inappropriate shocks, they were mainly due to the treatment of supraventricular tachycardia in the TV-ICD group and T-wave oversensing in the S-ICD group.

The only random comparison of the S-ICD and TV-ICD systems suggested a tendency for more inappropriate shocks in the S-ICD group, mainly related to over-sensing of T and P waves or extracardiac stimuli, such as myopotentials and noise, case authors. wrote, adding that the trial did not have enough power to assess this outcome.

The group pointed out that inappropriate shocks in TV-ICD patients are usually caused by supraventricular tachycardia and are easily treated by reprogramming the device, while few options exist for treating inappropriate shocks caused by oversensing in S patients. -ICD.

More research is needed to describe the differences in inappropriate shocks between the two devices, the case authors noted. The patient’s young age, low amplitude of QRS complexes, atrial fibrillation, and hypertrophic cardiomyopathy have all been linked to T-wave oversensing. However, selecting the appropriate patients is difficult, given the low specificity of the tools. manual and automated screening, they added.

The occurrence of inappropriate shocks was reduced, but not eliminated, with the SMART Pass filter, the authors said. Although the use of a detection vector may be useful immediately after implantation of the device, the quality of the vector may decline over time. Likewise, inappropriate detection can occur with rate-dependent bundle branch block over time, due to double counting of the notched R wave.

An assessment of factors influencing clinical outcome and cost-effectiveness with the S-ICD System reported in the EFFORTLESS S-ICD Registry identified a complication rate of 2% at 1 year, with an inappropriate shock incidence of 1.5% .

Another large study reported that during a median follow-up of 21 months, 48 ​​of 581 S-ICD patients (71% males, aged 49 ± 18 years) experienced 101 inappropriate shocks (8, 3%), of which nearly three-quarters (73%) caused by over-sensing of the cardiac signal such as over-sensing of the T wave.

Clinicians concluded that given the limited options for managing S-ICD over-sensing issues, additional diligence is required during screening for the pre-implant vector, and that the dynamic nature of this vector must also be taken into account. before implantation of the device.


The authors did not disclose any conflict of interest.

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