Dying Suddenly - Sudden Cardiac Death
Defibrillation is the delivery of an electric shock to the heart to cause the cells of the heart to resynchronize. Defibrillation is depicted frequently on television programs when medical personnel apply paddles to the chest of an unconscious patient and deliver a shock to revive him. In real life, the failure of that shock to be delivered in a timely fashion means death to the unfortunate patient. But defibrillators-devices capable of delivering the shock- are not available everywhere. In fact, up to 80% of victims who suffer a cardiac arrest outside the hospital do not survive to be admitted to the hospital. If they do survive to leave the hospital, the recurrence rate of SCD is up to 40% per yearunless they are fortunate enough to receive an implantable cardioverter-defibrillator (ICD).
Prior to the introduction of ICDs, antiarrhythmic drugs were given to patients with ventricular tachycardia in an attempt to suppress recurrences of the arrhythmia. But there were problems with this approach. First, it was known that abnormalities in the blood chemistry could deactivate the drugs, but it was impossible to detect the chemical changes without blood tests. Second, changes in the heart itself could change the effectiveness of the drugs, but the changes in the heart could be clinically undetectable. Therefore, even if a drug appeared to work well in suppressing arrhythmias in the hospital, one could never be completely confident that the drug would continue to be effective. Third, the drugs used to alter the electrical properties of the heart are not free from risk. Proarrhythmia-the promotion of arrhythmias in cardiac tissue-has been documented to occur in people with heart disease when they are placed on certain types of antiarrhythmic drugs.
The ICD is a device similar to a pacemaker that is implanted in the patient. The ICD monitors the patients rhythm at all times and has the capability of delivering shocks to terminate ventricular tachycardia or ventricular fibrillation automatically. Once an episode of VT or VF begins, the ICD is usually able to recognize it and terminate it with a shock within 8 to 15 seconds. Like pacemakers, ICDs work well within the body over long periods of time, and their effectiveness can be tested. Unlike antiarrhythmic medications, whose purpose is to prevent arrhythmia episodes, ICDs treat the arrhythmias only when they occur. Antiarrhythmic medications may still be used in conjunction with ICDs if arrhythmia episodes are frequent, and the problem of proarrhythmia is eliminated because the device can treat the arrhythmias if and when they occur.
ICDs have been available since the mid-1980s in the United States and have a well-documented success rate in decreasing the rate of death of patients at high risk for SCD. A major trial conducted by the U.S. National Institutes of Health (the Antiarrhythmics Versus Implantable Defibrillator or AVID trial) was actually terminated early because of the striking decrease in death rate observed in patients who received ICDs. The trial compared therapy with the best available antiarrhythmic drugs with ICD therapy for patients with spontaneous ventricular tachycardia or ventricular fibrillation. The overall death rate in the ICD patient group was 39% lower than the death rate of patients treated with antiarrhythmic drugs after only 18 months mean follow-up. Other trials in Canada and Germany have confirmed this finding.
The impact of ICDs seen in the trials noted above may be only the "tip of the iceberg" with regard to the potential impact of ICDs in preventing SCD. Since 80% of people who suffer a cardiac arrest do not even survive to enter the hospital, prevention of SCD in people at risk would appear to be the best strategy. Although 25% of victims of SCD have no antecedent history of heart disease, the remaining 75% of victims do at least have risk factors for SCD. Several groups of people have been identified as being at increased risk for SCD.