Evolution of extravascular implantable defibrillator technologies

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Abstract

The implantable cardioverter-defibrillator (ICD) has been successfully treating patients with lethal ventricular arrhythmias for decades. The main acute and chronic complications of this therapy modality are related to the use of a transvenous lead. An entirely extravascular ICD concept was developed over the last 20 years, with emergence of the subcutaneous ICD (S-ICD). This device was approved for clinical use seven years ago, and accumulating real-life experience confirms its safety and efficacy. The main limitations related to this system include the lack of pacing capabilities for bradycardia, tachycardia or resynchronization therapy, a large size, and relatively high energy requirements for effective defibrillation. This review article summarizes current knowledge and potential future developments of the extravascular ICD technologies.

Section snippets

S-ICD: development, current applications and limitations

The S-ICD was developed to provide reliable detection and treatment of life-threatening ventricular arrhythmias while preventing complications related to TV leads. Although animal models were used early during the development of the ICD with extrathoracic electrodes,12 animal models cannot sufficiently replicate the human conditions needed to test various cardiac sensing and defibrillation vectors during the development of an extravascular defibrillator. The first human feasibility trial was

Improved sensing algorithms

To refine detection of arrhythmias and minimize oversensing, several algorithms were developed over time. First, as described in the IDE trial, a conditional zone was added at rates below the shock zone to help discriminate SVT from VT.20 When the heart rate falls within the conditional zone, the morphology of the QRS complex and T wave are compared to a template obtained while in normal rhythm. If it matches the template, it is sensed as supraventricular in origin. If the tachycardia does not

Refinement in implantation

Currently, defibrillation testing is recommended for all patients receiving a S-ICD. In the EFFORTLESS registry, 17 patients required repositioning of either the generator or electrode for defibrillation failure.15 A computer modeling study investigated the possible causes for high DFTs.24 The model was based on MRI imaging and was used to test different generator and coil positions. The generator was virtually moved from the standard position (at the level of the 6th rib in the midaxillary

Substernal lead placement

Chan et al. first reported the use of the substernal space for lead placement instead of the left parasternal location used for the S-ICD system.28 The rationale was that closer proximity to the heart would allow effective defibrillation with a lower shock energy, as well as painless pacing at a lower output than with the conventional S-ICD. Sixteen subjects who were scheduled to undergo a sternotomy or ICD implantation were enrolled in the feasibility study. Prior to their scheduled procedure,

The modular cardiac rhythm management system

Tjong et al. proposed another approach to circumvent the pacing limitations of the S-ICD: by combining existing S-ICD technology with a leadless pacemaker, pacing capabilities could be added to the subcutaneous system in a modular fashion.32 Both devices have shown clinical efficacy and safety.11,33., 34., 35., 36. The authors propose the use of the currently available EMBLEM™ S-ICD platform (Boston Scientific Corporation) that would require only a firmware update for unidirectional

Conclusions

The S-ICD has reduced the complications related to traditional ICDs, largely by the elimination of the TV lead, while providing effective primary and secondary prevention of SCD. While the rate of significant complications has remained low, the current S-ICD technology is limited by an inability to provide antibradycardia pacing or ATP, and by a fairly high rate of inappropriate shocks and high DFTs. Recent improvements in algorithms have minimized the rate of inappropriate shocks. Two

References (36)

Disclosures: Dr. Bradley Knight receives speaking and consulting fees from Boston Scientific and Medtronic, manufacturers of implantable defibrillators.

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