CardioExchange Archive

Last year’s FDA approval of Boston Scientific’s subcutaneous ICD (S-ICD), the first that doesn’t use transvenous leads, was based on a pivotal trial that assessed the device’s safety and effectiveness.  CardioExchange’s John Ryan asks lead investigator Martin C. Burke questions about the trial, recently published in Circulation, and the S-ICD.

Ryan: Should patients ask for the S-ICD preferentially? If it is not available at their center, what options are available for them?

Burke: Patients should be given an even presentation of data and rationale for both the single chamber transvenous ICD and the S-ICD, especially because about 3-5% of patients will fail the pre-implant ECG screening procedure and, and based on the IDE study results, another 2% will fail the acute defibrillation threshold testing part of the procedure. Thus, a small percentage of the patients eligible for the subcutaneous ICD will still require a transvenous ICD system in order to have adequate therapy to prevent sudden cardiac death.

During this pivotal subcutaneous ICD study, patients generally understood the advantages and difference in not having a wire and electrode placed in the heart and took preference with the S-ICD system.  This was especially preferred by the greater than 13% of patients who were implanted after transvenous device infection and recall failure.

The S-ICD system is still only being offered at 31 implant centers in the United States and is widely available across most regions.  Training new implanting electrophysiologists will take place in full swing over the next year with more centers having access by January 2014.  Accessing the Boston Scientific website or talking to local Boston Scientific representatives will direct you to current centers with expertise in the screening and implantation of the S-ICD system.

Ryan: A total of 13% of study patients received inappropriate shocks. This seems high. Do you suspect that this has an effect on outcomes?

Burke: That 13% of patients received inappropriate shocks was an interesting finding in this new subcutaneous implant and sensing platform.  It is certainly too much, but is comparable to pre MADIT RIT programming in the more established transvenous ICD implant platform.  Differences in the mode of inappropriate shock should be clearly delineated.  The S-ICD had a higher rate of inappropriate shock for T wave oversensing rather than rapid SVT, an opposite phenomenon in comparison to the transvenous ICD implant population.  The classification of the rhythm was very accurate using subcutaneous sensing electrodes, while T wave amplitude was more challenging.  Lessons were learned to cut the inappropriate shock rate during the study by more than 50% with reprogramming sensing vectors and rate cutoffs.  Furthermore, the dataset was used to establish more stringent screening criteria for commercial implant populations, making the problem of T wave oversensing more preventable as the experience with the system becomes more wide spread.  It is a goal, and attainable as signal processing exponentially becomes faster, for all ICD systems to have complete freedom from inappropriate therapy and maintain 100% appropriate therapy leading to best clinical outcome. The subcutaneous sensing science is key to successfully realize this goal.

Ryan: Patients with documented spontaneous and frequently recurring VT reliably terminated with anti-tachycardia pacing were excluded from the study —  that would likely be a significant number.  How many patients did this include? And how do you decide a priori if ATP compatible device is preferable for people (bearing in mind that ATP has less effect on morbidity and mortality than ICD firing)? 

Burke: The exclusion criteria for the S-ICD investigational device exemption study was very narrow and did include patients with documented spontaneous and frequently recurring VT reliably terminated with anti-tachycardia pacing.  There were no data prospectively collected as to how many patients were excluded for any of the pre-specified exclusion criteria. However, it should be noted that this number is generally very low in de novo patients indicated for any ICD.  It is only after the implant that the majority of these patients are identified.  Patients with primary and secondary prevention were included in the study cohort at similar rates to the ACC-NCDR database and this clearly indicates that patient enrollment was not overly controlled.

Finally, the subcutaneous device was not removed for any patient due to spontaneous and frequent VT reliably terminated with anti-tachycardia pacing. The interesting correlate to this fact is that 63% of VTs leading to charging of the device did not receive a shock because of the longer, more deliberate sensing algorithm delaying therapy in this S-ICD implant population.  This would correlate to the Group C MADIT-RIT programming methods reported last year.  Anti-tachycardia pacing remains a useful therapy modality but its need is becoming less necessary as large datasets suggest that VT is largely nonsustained and self-terminating.  ICD therapy, either subcutaneous or transvenous, is predominately to reverse fibrillation and prevent sudden death.  A more expansive future role of VT ablation in this population needs more study.