Implantable Cardioverter-Defibrillators (ICDs):
Implantable Cardioverter-Defibrillators or ICDs perform all of the features of a permanent pacemaker (such as treating slow heart rhythms) but they also have the capacity to treat dangerously rapid heart rates with rapid pacing or with shocks. They are implanted in the same way as a permanent pacemaker (PPM) with leads traveling down to the heart through a vein but the device itself is larger than a PPM and the lead that is capable of giving shocks is also a little larger (thicker) than a normal pacemaker lead. The device needs to be larger because shocks (or high voltage therapies) require more energy and more battery capacity than low voltage therapies like pacing the heart.
A number of studies have shown that ICDs can save lives in patients who are survivors of a cardiac arrest or in patients who have a cardiomyopathy (heart muscle weakness) where the left ventricular ejection fraction is less than 35% (the normal ejection fraction or amount of blood the heart pumps with each beat is ≥50-55%) (1, 2). If an ICD is being implanted purely to treat a possible future cardiac arrest (and not to perform any pacing function) then it will not change your heart failure symptoms but is purely a treatment aimed at extending a patients life. Treatments aimed at directly targeting rapid heart rates to improve symptoms are often needed in combination with an ICD (these include medications or ablation procedures). Potential complications with ICD implantation include a 1:300 risk of bleeding around the heart that may require drainage, a 1:100 risk of injury to the lung as the leads are passed through the vein at the top of the lung and a 1-2% risk of bleeding or infection. If any complications do occur they will generally be identified and treated at the time of the procedure. All current ICDs are compatible with ‘home monitoring’ so that if a rapid heart rhythm does occur this can be transmitted back to your cardiologist through a device like a modem from your home. Generally ICDs should be checked with your cardiologist every 6-12 months to make sure you are well and that no changes to the programming need to be made.
A newer ICD called a subcutaneous ICD can also treat patients for dangerously rapid heart beats or a cardiac arrest without having to pass leads through a vein to reach the heart. Instead a lead is passed in front of the sternum (breastbone) to deliver shocks. This device is easier to implant and it may be more suitable for young patients as the 'lead' part of the device is more durable over many years. These subcutaneous ICDs don't have all of the features of a conventional ICD, however, so choosing the right ICD is an important discussion with your cardiologist.
Cardiac Resynchronisation Therapy (CRT):
Cardiac Resynchronisation Therapy (CRT) is similar to PPM or ICD implantation but it involves an additional lead that is passed through a vein travelling behind the heart to pace the heart from the left hand side. It is also referred to as biventricular pacing because it results in pacing from the left and right ventricles at the same time. CRT is indicated in patients with at least moderate to severe impairment in their left ventricular ejection fraction (heart muscle weakness) and who also have an impairment in the hearts normal electrical system known as left bundle branch block or LBBB for short. This LBBB causes the heart to beat inefficiently as the walls of the heart do not contract at the same time and instead it gives the heart a ‘wobbling motion’ as it contracts. By placing another lead on the opposite side of the heart we can stimulate both sides of the heart to contract in time again and the heart beats more efficiently.
This therapy has the potential to both improve symptoms from heart failure and improve prognosis in patients who respond favourably (3). There is usually a 50-90% chance of responding depending on a number of factors including whether or not there is a large amount of scarring in the heart from previous heart attacks. The risks of CRT implantation are similar to the risks for implanting a PPM or ICD, however, the procedure usually takes longer (1.5-3 hours on average) and because the left sided lead is more complicated to position, it can move from its original location and need repositioning in up to 3-4% of cases.
1. Moss AJ, Zareba W, Hall WJ, Klein H, Wilber DJ, Cannom DS, Daubert JP, Higgins SL, Brown MW, Andrews ML, Multicenter Automatic Defibrillator Implantation Trial III. Prophylactic implantation of a defibrillator in patients with myocardial infarction and reduced ejection fraction. The New England journal of medicine Mar 21 2002;346:877-883.
2. Connolly SJ, Hallstrom AP, Cappato R, Schron EB, Kuck KH, Zipes DP, Greene HL, Boczor S, Domanski M, Follmann D, Gent M, Roberts RS. Meta-analysis of the implantable cardioverter defibrillator secondary prevention trials. AVID, CASH and CIDS studies. Antiarrhythmics vs Implantable Defibrillator study. Cardiac Arrest Study Hamburg . Canadian Implantable Defibrillator Study. Eur Heart J Dec 2000;21:2071-2078.
3. Tang AS, Wells GA, Talajic M, et al. Cardiac-resynchronization therapy for mild-to-moderate heart failure. The New England journal of medicine Dec 16 2010;363:2385-2395.
The picture above is an X-ray of the chest following implantation of a CRT-D or a cardiac resynchronisation therapy defibrillator. Three leads are passed down through the veins and into the heart. The atrial lead is placed into the right atrial appendage, the defibrillation lead is placed at the tip of the right ventricle (the larger diameter shock coil can be seen) and then the left ventricular lead is passed through the coronary sinus (vein behind the heart) and into a branch to sit on the left hand side of the heart. Pacing from the right and left ventricular leads at the same time causes the opposite walls of the heart to contract simultaneously and the heart to beat more efficiently. Wires can be seen in the sternum of this patient as they have had previous open-heart surgery.
The picture above shows a typical example of different implantable cardiac devices. On the left (the largest device) is a subcutaneous defibrillator. This device uses a lead that travels in front of the sternum and not into the heart. It might be used in younger patients who would otherwise have leads inside their hearts for many years. The next largest device is an implantable defibrillator that uses leads implanted through a vein and into the heart. It can perform pacing functions for slow and rapid heart beats and shocks in the event of a cardiac arrest. The next largest device is a dual chamber permanent pacemaker and the smallest device is an implantable loop recorder that can be used for diagnosing cardiac arrhythmias in patients with recurrent syncope (fainting). It continuously records the hearts rhythm for up to three years and can transmit alerts over a mobile network for monitoring from home. A 50-cent coin is shown for comparison.