Defibrillators (ICD/AICD) have come a long way. I have implanted approximately 100 per year since 1995. They come with some very special “extras” (multi-programmability) that can be suited to the individual.
I’ll start with an explanation of ventricular arrhythmias and sudden cardiac death followed by a review of the major clinical trials involving AICDs, and end with a brief history on how they came about and recent developments. Finally, I will share a personal insight with you.
Sudden cardiac death is defined as a non-accidental, unexpected death due to cardiac causes within one hour after the onset of acute symptoms. There are approximately 400,000 sudden cardiac deaths in the United States annually with 40 percent to 50 percent of all cardiac death occurring suddenly.
Ischemic heart disease (caused by coronary artery disease leading to myocardial infractions) is the primary cause of sudden cardiac death. The other major type is the non-ischemic cardiomyopathies. Causes of non-ischemic cardiomyopathies include infectious sarcoidosis, hypertrophic amyloidosis, right ventricular dysplasia, prolonged QT syndrome and dilated cardiomyopathy. Most of these cause heart dysfunction.
The risk of sudden cardiac death is proportional to left ventricular dysfunction. One of the terms currently used in the medical community when assessing the degree of left ventricular dysfunction is called the ejection fraction (EF). This is the amount of blood ejected per beat during ventricular contraction. Normal is around 50 percent to 70 percent. Patients with sudden cardiac death usually have an ejection fraction of less than 35 percent, but not always.
As mentioned previously, ventricular arrhythmias are the major causes of sudden cardiac death. There are generally two types of ventricular arrhythmias. Ventricular tachycardia (VT) is a fast, but usually regular rhythm with heart rates up to 200 beats per minute. It may cause palpitations, shortness of breath (dyspnea), loss of consciousness and postural tone (syncope). Ventricular fibrillation (VF) is an irregular rhythm from different parts of the ventricle with heart rates up to 300 beats per minute. It usually causes sudden unconsciousness and cardiac arrest.
These arrhythmias have traditionally been treated with medical therapy, but more recently have been managed with AICDs. The success rate of using anti-arrhythmic drugs is less than 50 percent. There has also been a problem with drug toxicity and side effects. In fact, some of these anti-arrhythmic drugs actually turned out to be pro-arrhythmic in that they caused the same lethal arrhythmias they were trying to prevent.
For patients with coronary artery disease, the role of implantable cardiac defibrillators (ICDs) has been clarified in three major trials. The first is MADIT I (multi-center automatic defibrillator implantation trial). This study was published in 1996, and the trial showed that the ICD improved survival in comparison with anti-arrhythmic medical therapy in high-risk patients.
Patients were classified as high-risk if they had an ejection fraction of less than 35 percent, had asymptomatic non-sustained ventricular tachycardia and a history of a myocardial infraction. They also were required to have inducible ventricular tachycardia during electrophysiological testing not suppressed by the anti-arrhythmic drug procainamide. And they were randomized to drug therapy or assigned to an ICD. At follow-up, those patients who received ICDs had a 54-percent reduction in overall mortality compared to drug therapy.
The MUSTT Trial (multicenter unsustained tachycardia trial) was a similar study that showed the additional benefit of the ICD when used with anti-arrhythmic therapy. The five-year survival rate in patients receiving an ICD was 75 percent.
The MADIT II trial was the first U.S. trial to show that prophylactically implanting an ICD in conjunction with medical therapy improved survival in high risk patients. That means if you had a previous MI and an ejection fraction of less than 35 percent, you should get an ICD.
Dr. Michel Marowski invented the AICD, at one time called AIDS, (automatic implantable defibrillator system, which caused a stir for obvious reasons when his friend died of sudden cardiac death in 1966. The first human implant wasn’t until 1980 at John Hopkins in Baltimore.
There were many challenges at first. Capacitor technology had to be revised to allow small batteries to store and deliver large amounts of energy. The development of transvenous leads would carry energy to the inside of the heart. Circuitry advances would allow the system to sense bad rhythms from good ones and avoid inappropriate shocks.
Finally, radically downsizing the device so it could be implanted in the body was an important step. My first implant was an abdominal implant and the defibrillator was about the size of my fist. Currently the defibrillator is implanted in the pectoral region and is the size of a small beeper. AICD evolution has occurred at a furious rate. The AICDs allow the physician to adjust many aspects of tachycardia therapy. The basic therapies are as follows.
Anti-tachycardia pacing (ATP) uses fast pacing impulses to interrupt the tachycardia and restore normal sinus rhythm. This type of pacing is generally painless and does not use much battery power. Defibrillator therapy uses high-energy shock to terminate the arrhythmia. If you are awake, it feels like a kick in the chest and usually lasts less than a second.
The advantage to the defibrillator is that it requires less energy than the typical pads placed on the outside of the chest and generally delivers the therapy in less than six seconds. No waiting 20 minutes for the paramedics to arrive.
Some of the more recent advances include the ability to pace both ventricles in sync, which allows for improved functional capacity and the ability to shock in both directions, which allows for an increase in defibrillation success.
The improvement in lead technology with the development of true bi-polar leads resulted in improved sensing capability with less inappropriate shocks. We can communicate with defibrillators wirelessly. Today, almost all defibrillators have pacemakers that have rate responsiveness built into them. This allows for an increased pacing rate when the demand for increased activity exists.
Finally, I leave you with a thought for the day. I have been playing golf for a long time. I once had a seven handicap, and for those in the golfing world, I could get up and down from anywhere. My short game was probably the best around, but I was no Phil Mickelson.
Lately, my short game has been pathetic. I have worked on it, but to no avail. I got so frustrated, about two or three weeks ago while playing golf with my friends that I decided to quit. The following week I was talking to a patient about the incident. His friend next to him, whom I did not know, subsequently removed his prosthetic leg and told me I should be thankful for the ability to play.
I did not know what to say to him, but all I can tell you is that I am still hacking the ball around the golf course with my friends these days. Be thankful for what God gives you. Finally, congratulations to Mickelson, who’s a great golfer, but an even better man.
