The limitless possibilities of ICDs

The biggest trends in ICD technology have been a movement to smaller devices, simplified implantation, and reducing or eliminating leads implanted in the veins or the heart.

Since the first pacing system implanted in 1958 by Lillehei (University of Minnesota), cardiac pacing has emerged as a field in constant evolution, from its conception to its methods of insertion. Cardiac defibrillation also underwent some great technological advances in a relatively short period of time, since the first implantable cardioverter defibrillator (ICD) implantation in 1980, and the first biventricular pacing report in 1994.

ICDs are a fast-growing market, primarily driven by the increasing prevalence of arrhythmia as well as advances in technology by combining the features of a traditional pacemaker into the devices. GlobalData estimates that the global ICD market is expected to reach approximately USD 4.3 billion by 2028, growing at a compound annual growth rate (CAGR) of 4.1 percent, with traditional single- and dual-chamber ICDs growing at an average CAGR of 5 percent.

The biggest trends in ICD technology have been a movement to smaller devices, simplified implantation, and reducing or eliminating leads implanted in the veins or the heart. The US Food and Drug Administration (FDA) also has certified numerous ICDs as magnetic resonance imaging (MRI)-safe with MR-conditional labeling, to allow patients with ICDs to undergo MRI scans. New software is helping increase battery life, reduce unnecessary shocks, and better monitor heart-failure patients to prevent acute hospitalizations. Use of antibacterial envelopes was also recently shown to reduce ICD infection risk.

ICD controversies include heightened FDA concerns about ICD cybersecurity, recent data showing ICD implantation disparities based on race and gender, the safety of ICD patients being scanned at airport security checkpoints, a high percentage of patients who meet ICD guidelines but are not receiving them, and patients who do not qualify for ICDs but are receiving them. Some of these issues and new technologies include:

Advancements in subcutaneous technology. One issue with ICDs is the need to implant transvenous leads, which come with their own set of complications including infection risks, more invasive procedures, and potential obstruction of the vessels as the leads become encased in scar tissue. This last point also makes the leads difficult to remove, if and when they need to be replaced. To overcome these difficulties, an entirely subcutaneous ICD (S-ICD) has been designed. These devices have the advantages to avoid any risk of device-related endocarditis, and all the problems inherent to the presence of an endovenous lead. The main disadvantage is the current impossibility to deliver anti-tachycardia-pacing (ATP), and long-term conventional pacing. Nevertheless, these systems will be coupled with leadless pacemakers in the near future to overcome this specific, but important, issue. The question will arise if the S-ICD will then become the device of first choice. Challenges will be the device-to-device communication ensuring a reliable anti-bradycardia and painless anti-tachycardia therapy without the risk interfere with required shock therapies. The S-ICD market is currently the fastest-growing segment, where GlobalData expects the market to grow at a CAGR of 11 percent over the next 10 years.

ICD for primary prevention. Current guidelines give a class-I recommendation for ICD implantation in patients with either ischemic or non-ischemic cardiomyopathy with severely depressed systolic function. At five years from implantation, about 37 percent of primary prevention patients will have an appropriate device intervention, against 51 percent of patients implanted for secondary prevention. Frequent episodes of ICD-appropriate therapy warrant a clinical reassessment, as it has been demonstrated that ICD shocks correlate with increased risk of heart-failure hospitalization and mortality. The clinical cardiologist should, therefore, identify potentially reversible causes of ventricular arrhythmias and treat them, whenever possible. Nowadays, the sole parameter to decide on ICD implantation for primary prevention in heart failure is LVEF, even if the incidence of ICD-appropriate shocks is relatively low. It is also true that only a minority of people who experience SCD have an LVEF ≤35 percent. In the past, attempts to stratify patients better according to their arrhythmic risk have been performed, especially in ischemic patients (e.g., heart-rate variability analysis, programmed ventricular stimulation (PVS) to induce arrhythmias in the electrophysiology laboratory), but none of these methods has translated into effective clinical use. Recently, there has been an increase of interest in cardiac magnetic resonance (CMR) for the prediction of ventricular arrhythmias. It is well known that the presence of late gadolinium enhancement (LGE) indicates myocardial scar, which is a potential arrhythmogenic substrate, and has a negative prognostic value. The presence and extent of LGE has been found to correlate with the incidence of ventricular arrhythmias and appropriate ICD shocks in both ischemic and non-ischemic dilated cardiomyopathy patients. Further studies will clarify the potential role of CMR imaging in stratifying the arrhythmic risk of patients with heart failure or with normal/near normal cardiac function.

Device infections. Microbial colonizations involving the leads pose a high risk for endocarditis and sepsis. The majority of these episodes are encountered in patients with comorbidities and who are immunocompromised. These conditions increase the risk for life-threatening infections, which occur in about 1.2 percent of implant patients, and require long-term antibiotic therapy. Device infections can occur even years after implantation, as a result of sepsis and colonization by microbial agents. In recent years, subcutaneous ICDs (S-ICDs), which do not require transvenous leads, have been associated with a better safety profile than conventional ICDs in terms of sepsis and endocarditis. In future, randomized clinical trials, evaluating S-ICDs in patients at high risk of infection or with a previous conventional ICD explanted because of infection, will clarify the utility of this new generation of devices in this subset of patients. The use of an absorbable antibacterial envelope during implantation of conventional ICDs and pacemakers has shown promise in reducing the rate of electronic device infections. However, the envelope does not provide protection for transvenous leads, which are the most dangerous sites of microbial colonization, and costs, which are high, limit its use to patients at high risk. For these reasons, device infections remain an issue.

Outlook
Technological advances are considerable in the field of cardiac pacing and defibrillation, from the latest venous accesses for implantation to recent locations for permanent pacing, or non-endovenous ICD. Imagination of researchers and device companies seems to be limitless.