Evolution of extravascular implantable defibrillator 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
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Cited by (3)
Initial Experience With Intercostal Insertion of an Extravascular ICD Lead Compatible With Existing Pulse Generators
2023, Circulation: Arrhythmia and ElectrophysiologyThe defibrillation conundrum: New insights into the mechanisms of shock-related myocardial injury sustained from a life-saving therapy
2021, International Journal of Molecular Sciences
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Disclosures: Dr. Bradley Knight receives speaking and consulting fees from Boston Scientific and Medtronic, manufacturers of implantable defibrillators.