Several clinical studies have shown enormous benefits in terms of survival after ICD implantation, in patients at risk of developing malignant ventricular arrhythmias. A so significant efficacy that overshadowed the role of antiarrhythmic drugs.
Nevertheless, the use of a conventional ICD is not free from risks and complications. In particular, during implantation there is a risk of causing pneumothorax, hemothorax, cardiac perforation, with or without tamponade, and damage to the tricuspid valve. In addition, over time a displacement or an infection of the catheter may occur.
Finally, if for any reason it is necessary to remove the catheter placed inside the heart, this surgery is still complicated and not risks-free.
To overcome these drawbacks, in 2012 a newly developed implantable defibrillator was approved by the FDA: the subcutaneous defibrillator.
However, the subcutaneous defibrillator also has its limitations. The main one is that being positioned outside the heart, it can perform a defibrillation, but not pacing. For this reason, important functions, such as antitachycardia pacing, bradycardia pacing, or cardiac resynchronization are completely impossible.
To date, the recommendations of the North American guidelines, dated 2017, suggest the implantation of the subcutaneous defibrillator, in Class I, in patients who have inadequate vascular access or are at high risk for infection, and in whom pacing for bradycardia or ventricular tachycardia termination, or as part of resynchronization therapy is neither needed nor anticipated.
Similarly, European guidelines, dated 2015, recommend the use of a subcutaneous defibrillator, as an alternative to transvenous defibrillators when pacing therapy for bradycardia support, cardiac resynchronization or antitachycardia pacing is not needed.
Researchers from the multicenter study PRETORIAN, recently published in the New England Journal of Medicine, conducted a non-inferiority analysis, in which patients with an indication for an ICD implant, but no indication for a stimulation, were assigned to receive a subcutaneous ICD or a transvenous ICD.
The primary endpoint of the study was a composite of device-related complications and inappropriate shocks. Secondary endpoints included death and appropriate shock.
After a median follow-up of 49.1 months, the primary endpoint occurred in 68 patients in the subcutaneous ICD group and in the same number of patients in the transvenous ICD group, achieving the goal of non-inferiority.
Regarding device-related complications, these occurred in 31 patients in the subcutaneous ICD group and in 44 in the transvenous ICD group.
This study was designed to gathering more information on the clinical efficacy and safety of the subcutaneous defibrillator, by means of a direct comparison with the conventional transvenous implant.
Even if the statistical analysis has achieved the goal of non-inferiority, thanks to the definition of a combined endpoint, it is evident that results highlights a more complex picture.
The authors point out that these shocks were more frequently induced by oversensing phenomena of cardiac signals, mainly T waves, and non-cardiac signals. They precise that the use of a special filter, while the study was ongoing, has led to a reduction of inappropriate shocks.
In this sense, the results are obviously all to be verified, and testify that the subcutaneous defibrillation systems still have room for improvement, from a technical point of view.
Subcutaneous ICD can of course potentially offer great advantages for specific patients. On the other hand, the limitations are not few. Not having antitachycardia pacing is a major limitation. This alternative therapy can often be effective in suppressing ventricular tachycardias, while preserving the device’s battery. Furthermore, it is often difficult to exclude, a priori, that a patient will not need bradycardia pacing in the future. Finally, what about the importance of bradycardia pacing for post-shock asystole?
Other studies will soon arrive in this field, but to understand the true impact of this new device it will be interesting to observe if the guidelines will extend in the future the indications for the subcutaneous defibrillator.
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