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Entirely subcutaneous implantable cardioverter defibrillators

Dr Andrew Grace
Consultant cardiologist, Papworth Hospital, Cambridge, UK

Each year in the USA, around 340,000 people die from sudden cardiac death (SCD). The proximal cause is a failure in the normal electrical excitation of the muscle of the heart that results in ventricular fibrillation (VF). This breakdown/transition occurs most commonly in those with coronary artery disease.

On occasion, the cause is either a genetic abnormality or structural heart disease. The only effective therapeutic intervention in the protection of patients from SCD is the implantable cardioverter defibrillator (ICD).  

Implantable cardioverter defibrillators can prevent almost all cases of SCD, but a perception has arisen that they can be risky and this view may be reinforced in general cardiologists’ minds when they see patients presenting with the potentially devastating complications that can complicate their use.  

One of the main problems with currently configured ICDs is the lead that attaches the device to the heart. The leads themselves can fail, resulting in inappropriate shocks and, should infection arise, further issues with major cost implications ensue. The good news stories of lives saved by appropriate device activity may not filter back, leading to disillusionment and a disinclination to refer potentially vulnerable patients who could benefit.

Over the last few years, a group of individuals working together internationally have developed an entirely new, essentially disruptive way of providing the positive advantages of ICD therapy while avoiding the classical lead-associated problems. The result is the totally subcutaneous ICD (S-ICD) with no leads in or on the heart. The S-ICD appears to offer a viable alternative to protect against the risks of SCD without having the issues that can arise when a lead is implanted in the vascular system.  

These devices are approved in Europe and have already been used in significant numbers of patients. The purpose of this brief overview is to provide the background to the development of this device, its current utility and its likely future impact on healthcare.  

SCD as a major public health issue
SCD is, after cancer, the second most common cause of death. In the USA, it is estimated to account for around 340,000 deaths per year. In addition, there are 550,000 patients with heart failure and about 5 million with chronic heart problems, all of whom have a finite and potentially measurable increase in the risk of SCD. The problem of SCD is universal, with an estimated 70,000 people dying from this cause each year in the UK.

ICDs have been developed to protect individuals from the consequences of VF that is the cause of SCD. VF arises when the normal heartbeat breaks down. Each heartbeat begins as a wave of electrical excitation at the sino-atrial node and travels in an organised fashion through the atria and then through to the ventricular chambers of the heart. As this electrical wave advances, the muscle contracts and the co-ordinated contraction of the heart as a whole forces the blood out into circulation.

In the diseased heart, however, the flow of the wave of electrical activity that initiates contraction may be disrupted, potentially leading to fibrillation. This compromises the ability to pump blood to the circulation and, if not promptly corrected, results in SCD.

The implantable cardioverter defibrillator
It is over 30 years since the first ICDs were developed. These rudimentary devices had leads attached directly to the external surface of the heart and required open-chest surgery for placement. This procedure entailed significant risk to the patients.  

There have been many technological advances resulting in the current sophisticated but complicated devices comprising two major components. First, is the modest-sized electrically active generator (an active can) placed in a pectoral location just beneath the collarbone that provides detection and efficient defibrillatory shocks to the heart to correct fibrillation. Second, are the leads that are placed in the heart via veins and provide the capacity to detect and deliver therapy.  

In addition to defibrillation, these leads can provide pacemaker functions and the potential, in those who need it, to increase cardiac pumping function through cardiac resynchronisation therapy. The devices have multiple programmable detection algorithms and other functions that can be wirelessly programmed.  

The perceived success of ICD technology has lead to the implantation of around 300,000 devices annually, accounting for an approximately $6 billion market. There are, however, continuing concerns expressed regarding the risks and benefits of currently configured ICDs. In the USA, physicians, the public and politicians have asked questions about their reliability and safety. These have been spurred partially by device recalls due to manufacturing and design concerns. The surrounding negative publicity with breaking new stories, company refutations and subsequent lawsuits has been extensive, protracted and damaging.

Intravascular leads
Clinical studies on device safety have exposed the leads as the Achilles heel of ICD therapy and, in retrospect, one may judge this unsurprising. The vascular environment is hostile with the continuous movement of the heart, the bending of the leads and the mechanical attachment to the heart muscle being especially vulnerable. What is observed in clinical use is that the leads themselves have limited survival rates varying between 85% and 95% at five years; at eight years only 60%–70% of the leads are still functioning. The consequences of lead failure can be severe and potentially life-threatening.  

Extraction of the leads from the vascular system may be required and, on occasion, necessitate open-heart surgery with its attendant risks. Should devices become complicated by sepsis, further problems may emerge with endocarditis and systemic complications. Lead complications are, of course, very costly and one study tracking the costs of ICD therapy calculated that each complication added over $7,000 per patient in hospital costs.

Development of the subcutaneous ICD
The S-ICD has been designed to overcome many of the issues associated with the conventional invasive use of transvenous defibrillation leads. The lead for this new device is placed under the skin across the ribs into a discrete subcutaneous space just to the left of the sternum (Figure 1). All of the hardware is therefore extra-thoracic. This method of employing an ICD lead avoids opening the patient’s vascular system to a foreign body and the procedure to place the device is minimally invasive with a rapid learning curve.

Gust Bardy (Seattle, Washington) and Riccardo Cappato (Milan, Italy) conceived the idea that was turned into an engineering and clinical reality through the enormous efforts of the company they founded (Cameron Health Inc., San Clemente, California). The first round of clinical work was completed mostly in Auckland, New Zealand, and Cambridge, England, between 2001 and 2005. This work established that a device placed under the skin could be engineered in a way that allowed efficient defibrillation at acceptable voltage outputs. Subsequent research resulted in the capability to observe the heartbeat in a sensitive and specific manner.  
A substantial body of research has now been completed indicating that, as a stand-alone system, the S-ICD provides the capabilities needed to correct the heartbeat and protect individuals from SCD.  It is well tolerated, very sensitive to the presence of VF and appears to have a higher specificity to discriminate other rhythms than conventional devices.  

Accordingly, it offers the possibility that the inappropriate shocks – that are one of the most challenging aspects of conventional transvenous ICD systems – can be avoided. Prior to approval in Europe, a regulatory study running from late 2008 to early 2009 was completed. The CE mark was obtained in June 2009 and the first post-approval system was implanted at Papworth Hospital, Cambridge, in July 2009.

Current implantation of subcutaneous ICDs
S-ICDs can be placed without technical difficulty. Although patients often receive general anaesthesia, conscious sedation can also be used. X-rays are not required, with an implant being possible using just anatomical landmarks. The idea of placing the device in hospital locations other than large, specialised units is thereby feasible, with the added flexibility of using a non-imaging suites, thus unburdening the need for additional catheter labs/special procedure rooms.  

The simplicity and predictability of the system also allows for more efficiency in the scheduling of procedures. The system has demonstrated few complications with issues seen with very early device implants – for instance, lead migration easily resolved through greater experience.  

The devices are being extensively used in young adults, as these individuals will require life-long therapy and transvenous-related complications tend to be cumulative, adding to morbidity and mortality. Children have also become a target population, with reports appearing of children as young as eight years at risk of SCD receiving 
S-ICDs with good results.  

Future developments of S-ICDs
The currently configured SICD system is well tolerated, but further decreases in the size of the unit are anticipated, which will make implantation and downstream tolerability even better. Further iterations of the algorithm will further reduce the likelihood of inappropriate shocks. One would anticipate the advantages offered by these devices would result in altered perceptions among physician referrers. Accordingly, thresholds for implantation are likely to fall, leading to greater numbers of referrals, more lives saved and the more efficient use of healthcare resources.

The financial pressure on healthcare costs is going to increase as the population ages, coupled with greater demands for the best available treatments. One hope is that new technologies will solve resource issues, so reducing pressure on scarce materials and limited expert personnel.

Previous trends in ICD therapy have been towards increased complexity requiring greater resources and clinical expertise. The disruptive simplicity of the S-ICD will help solve one of the growing predicaments for hospitals – a greater demand by patients and physicians for the life-saving benefits of efficient, safe ICD technology.     

Competing interests: Cameron Health Inc. (consultant, research support); Medtronic Inc. (consultant, research support).


Grace AA et al. European Society of Cardiology Congress 2005, Stockholm. Website slide resource, session number FP3684
Bardy GH et al. New England Journal of Medicine 2010;363:36-44
Spirito P & Boriani G. Nature Reviews Cardiology 2010;7:543-544