Heart & Vascular Institute Physician eNewsletter - Winter 2013
Subcutaneous ICDs could be a real game-changer in implant technology.
Subcutaneous implantable cardioverter defibrillators (ICDs) are a less invasive and potentially better alternative than transvenous ICDs for some patients with life-threatening ventricular tachyarrhythmias, particularly those on dialysis, with infections or venous access issues.
“Implant technology has changed a lot over the years, and so has the data supporting it,” says Bruce Wilkoff, MD, Director of Pacemakers and ICDs at the Heart & Vascular Institute.
Because a subcutaneous ICD lead does not enter the vascular space and is not in direct contact with the heart, arrhythmia detection is accomplished differently than with transvenous ICDs.
Transvenous vs. Subcutaneous ICDs
Transvenous ICD leads fed directly into the vascular space detect heart rate only from a specific location dictated by the lead’s placement. This method provides limited insight about electrical activity outside of the target area. Although it allows for excellent sensitivity for the detection of ventricular tachyarrhythmias, the trade-off can be diminished specificity and frequent inappropriate shocks. Performance is highly dependent on the programming of the device.
“With the subcutaneous ICD, you are looking at a larger view of the heart because the device does not actually touch the heart and the performance is less dependent on the programming,” says Dr. Wilkoff.
By using a series of sensing electrodes to record vectors of cardiac electrical conduction and application of discrimination algorithms, subcutaneous ICDs “wait” until there is certainty that a tachyarrhythmia is detected before shocking a patient back into rhythm.
“This is a brand new way of identifying arrhythmias,” Dr. Wilkoff says. “This algorithmic approach to identifying arrhythmias may very well be a superior way of going about it.”
Subcutaneous placement obviates the possibility of leads dislodging from the heart. In addition implanting and explanting electrodes is much easier with a subcutaneous ICD.
Shocking News: The MADIT-RIT Trial
A recent study showed that patients with tachyarrhythmias who have implantable ICDs fare better when the devices are programmed to wait out irregular heartbeats that aren’t sustained.
A randomized trial known as MADIT-RIT (Multicenter Automatic Defibrillator Implantation Trial—Reduce Inappropriate Therapy) showed that ICD therapy results in fewer inappropriate shocks and a reduced rate of all-cause mortality when programmed to detect ventricular tachycardia or fibrillation with two strategies 1) deliver therapy at a heart rate of 200 bpm or higher or 2) with a prolonged delay in therapy at 170 bpm or higher. Both of these strategies were compared with conventional programming.
During 1.4 years of follow-up, the high rate and delayed therapy programming strategies reduced the risk of a first occurrence of an inappropriate shock by about 80 percent and reduced the rate of all-cause mortality by 55 percent.
The study was conducted using transvenous ICDs, but the encouraging results are consistent with the approaches used by subcutaneous ICD technology. Essentially, fewer shocks reduce risk for patients, and the subcutaneous ICD is more reliable at arrhythmia discrimination than conventionally programmed transvenous ICDs. However, the transvenous ICD technology can further reduce shocks by using antitachycardia pacing, which causes no pain, if the ICD is appropriately programmed.
“Recent data collected through the MADIT-RIT trial indicates that it is probably smarter if a device is programmed not to ‘rush’ to shock the heart back into rhythm, and to be sure there is a sustained tachyarrhythmia before treating the rhythm,” Dr. Wilkoff says. “Appropriately programmed transvenous ICDs and subcutaneous ICDs seem to reduce a lot of the mechanisms for inappropriate detection.”
About half of the patients who need an ICD could be candidates for subcutaneous devices, Dr. Wilkoff estimates. The other half requires a pacing device, which is only possible with the transvenous ICD. That includes patients with cardiac dyssynchrony and congestive heart failure, who require cardiac resynchronization in which placement of three leads into the heart is necessary.
Patients with venous access problems and those on dialysis in whom additional leads in their veins should be avoided are strong candidates for subcutaneous ICDs.
Subcutaneous ICDs could be a game changer, believes Dr. Wilkoff, although a final conclusion awaits head-to-head comparison between the subcutaneous and transvenous devices. Improvements to first-generation subcutaneous ICDs are expected, including devices that are less bulky.
“We don’t know which is the best approach, but we do know there is a group of patients who are not suitable for venous leads, and those patients clearly will benefit from these developments in implant technology; on the other hand, some patients need protection from bradycardia or tachycardia with pacing and traditional ICDs with intelligent programming will be the best solution for these patients,” Dr. Wilkoff says.