The subject of cardiac implantable electronic devices (known as CIEDs) is vastly complicated, and falls under the specialty of the heart rhythm physician or Cardiac Electrophysiologist. Devices used to manage heart rhythm problems are also referred to as CRM (cardiac rhythm management) devices, and include pacemakers and implantable cardioverter-defibrillators (ICDs). You should read a few basic articles on this website to understand some of the heart arrhythmias discussed here. The purpose of this article is to discuss the indications and function of an ICD and present how an ICD can benefit certain cardiac illnesses.
The Impact of Heart Disease
Heart disease is the leading cause of death among adults in the United States. Heart disease is responsible for one in four deaths and kills over 600,000 people every year. Cardiovascular disease includes heart attacks, stroke, high blood pressure, cardiac arrhythmias and congenital heart disease (structural cardiac disease that a person was born with).
The medical costs associated with heart disease are staggering. About one in six dollars spent on healthcare is directly related to heart disease, amounting to about $1 billion per year. The most serious type of heart problem is coronary artery disease. Heart attacks affect over 700,000 people a year. Fortunately, technology has progressed at an amazing rate and patients with a heart attack can now be treated rapidly with interventional procedures, including coronary stenting, which has revolutionized our approach to a heart attack and has saved countless people from having major heart damage.
Unfortunately, a heart attack can still result in significant damage to the heart, which leaves a scar in the heart muscle. Other conditions can adversely affect the heart muscle, including hypertension, valvular heart disease, and even genetic conditions that affect the muscle directly. Any time the muscle of the lower chambers (ventricles) becomes diseased, the electrical signals that control the heart rhythm can become disturbed. This can give rise to extremely rapid heartbeats in the bottom chambers, known as ventricular tachyarrhythmias. Ventricular tachycardia and ventricular fibrillation are two such arrhythmias that could be life-threatening.
As the heart works, it fills up with blood coming in from the veins. The ventricles squeeze and eject the blood into the arteries of the body, including the aorta, which is the main artery coming off of the left ventricle. After squeezing, the ventricle relaxes again, allowing the chamber to fill up once more. If a ventricular tachyarrhythmia occurs, the ventricles will beat so rapidly that they don’t have enough time in between beats to fill. With less blood in the heart, the cardiac output drops dramatically. This gives rise to a precipitous drop in blood pressure, which can lead to sudden loss of consciousness. If the heart rate is fast enough, the heart cannot pump enough blood to the vital organs (including itself), and “cardiac arrest” occurs. Note that the word “arrest” suggests that the heart stopped, but it’s much more common for someone to die suddenly because their heart went too fast. Of course, once the heart itself is deprived of oxygen long enough, it will stop beating altogether and death will occur.
The rapid electrical disturbance that gives rise to ventricular tachycardia or ventricular fibrillation can be stopped in its tracks by applying a brief high-energy shock to the heart. Such a device is known as a “Defibrillator,” which can shock a person out of a lethal arrhythmia and save their life. The “AED” or automatic external defibrillator has become a universally accepted life-saving device that can be found at most major businesses like shopping malls, corporations, banks, and even schools. Defibrillators are standard equipment on all ambulances and rescue vehicles, and anyone trained in ACLS (advanced cardiac life support) should be familiar with the use of this device. Unfortunately, few people are successfully resuscitated from a cardiac arrest, so the survival rate is extremely low unless they happened to be at the right place and at the right time.
What Is An ICD?
An Implantable Cardioverter-Defibrillator or ICD is a relatively small battery-operated device that is about the size of a small deck of cards. It is surgically implanted under the skin, generally in the upper chest area. It is connected to the heart through thin flexible wires (called leads) that are inserted into a vein and advanced to the inside of the heart chambers. The leads are able to sense the heart rate and also relay information about the rhythm of the heart to the device. If a life-threatening arrhythmia occurs, the device analyzes the signal and then decides to treat the arrhythmia according to the way it is programmed (it is indeed a sophisticated mini-computer). It can treat a rapid ventricular arrhythmia immediately with a painless series of tiny stimuli to interrupt the electrical disturbance. If necessary, the ICD is also capable of delivering an strong electrical shock to stop a lethal arrhythmia and restore a normal heart rhythm.
ICDs are mainly designed to prevent sudden cardiac death. Only patients who are at risk of sudden death qualify to receive an ICD. Anyone who is fortunate enough to have survived a cardiac arrest would be a candidate for an ICD. Most patients who receive an ICD to prevent cardiac arrest have never had one but are considered at risk for life-threatening arrhythmias. In that case the device is recommended for “Primary Prevention.” ICDs are prescribed by cardiac electrophysiologists. They also are the doctor who will implant the device in a relatively simple procedure that generally takes less than an hour. Most patients can go home the next day, and although the device can be felt as a lump under the skin, it is for the most part comfortable and should not affect a person’s lifestyle to a major degree. Depending on the severity of the heart condition, patients may be routinely monitored every 1 to 3 months. Most patients find that they can return to a normal level of activity once the device is in place.
ICDs are very sophisticated devices that store important data about a patient’s heartbeat. Many models can also detect if a patient starts to accumulate too much fluid, which is a sign of impending heart failure. This data is used by doctors and medical practitioners to monitor the heart’s condition, its performance, and help guide other treatment such as medical therapy for heart failure.
ICDs are similar to pacemakers in that they help monitor and regulate heart the heartbeat, but pacemakers typically are used to correct an excessively slow heart rate, whereas an implantable cardioverter defibrillator is more commonly used in patients who are susceptible to rapid life threatening arrhythmias. However, all defibrillators have a built-in pacemaker and can support the heart rate if the patient needs. Different ICD models provide varying degrees of pacing capabilities, and it is up to the electrophysiologist to decide which model would be best for each particular patient. The best models can provide state-of-the-art pacing capabilities as well as treat ventricular tachyarrhythmias.
Aside from preventing sudden death, one particular ICD model may actually help improve heart muscle weakness. Some patients with a weak heart muscle also have underlying electrical problems. The branch of the electrical system that leads to the left ventricle may not be working properly, which is a condition known as “left bundle branch block” or LBBB. This can adversely affect the heart’s ability to contract in a synchronous manner. In patients with congestive heart failure due to weakness of the left ventricle who also have a LBBB, a pacing system known as “Cardiac Resynchronization Therapy” or CRT can be used. CRT is designed to stimulate the left ventricle at the same time as the right ventricle, which can improve the heart function in the majority of patients with this condition. An ICD with CRT capabilities is commonly referred to as a “Bi-Ventricular ICD” and can be prescribed by the electrophysiologist in patients that qualify for this particular therapy. These systems are more complicated than conventional ICDs because they require that three leads are implanted. Because of this they are more expensive and carry a higher risk of complications. Approximately 65-70% of patients who qualify for CRT feel better with that system because their heart function improves, but some patients do not respond to this strategy.
Why Has My Doctor Recommended an ICD?
If your doctor has discussed an implantable cardiac defibrillator with you, it is likely that your heart has some type of damage and you could be a significant risk of sudden cardiac death. In some cases where the heart is weak and the electrical system is damaged (LBBB), a Bi-Ventricular ICD might help the heart to beat more effectively. However, most ICD recipients don’t need this advanced type of pacing therapy and are better of with a simpler model.
You will need to have a consultation with a cardiac electrophysiologist regarding the type of device best suited to your condition. Be sure to discuss the risk, benefits, and alternatives with the doctor, as this is a life-long therapy that needs to be undertaken with care and much consideration. Keep in mind that the battery in the ICD should last about 7-10 years, at which point the device will need to be replaced with a new one.