Defibrillators are appropriate for only a minority of patients with HCM. These patients have important risk factors for sudden death. The criteria for placement of ICDs in patients with HCM are evolving.
The most serious arrhythmias that HCM patients can experience are rapid and prolonged venticular arrhythmias. During these sustained, ventricular arrhythmias, there is frequently a fall in blood pressure and even unconsciousness. Unless terminated, some can lead to fatal consequences. These arrhythmias require prompt termination which can be most readily accomplished by the administration of an electrical shock passed across the chest. Outside the hospital, this might be accomplished by an ambulance team who place external paddles on the chest and deliver a shock. This concept is also applied with an implantable device. The premise is that this device, being permanently available to monitor a patient's rhythm, can automatically and in a short period of time, deliver lifesaving electrical energy directly to the heart muscle. Patients who are deemed high risk for the development of these dangerous arrhythmias will often be treated with an implanted device so that they are permanently protected without need for intervention by bystanders or emergency personnel.
These devices are called implantable cardioverter defibrillators (ICD). These are implanted much the way permanents pacemakers are. Using a large vein that passes underneath the collar bone, a wire or lead can be passed intravenously into the right side of the heart. This wire monitors and records the electrical signals from within the heart and tells the device when the heart has gone into a rapid, dangerous arrhythmias. This lead is connected to the device which is then buried under the skin beneath the collar bone. When this device detects a dangerous arrhythmia, it can deliver enough electrical energy through the lead into the heart that the heart will resume its normal electrical activity. The entire process of detection and termination of this potentially fatal arrhythmia can last only a few seconds. Because this period of time is so brief, the patient usually comes to no harm. This device can be highly effective and often life saving in patients who may otherwise succumb to dangerous electrical conditions.
Publications in the New England Journal of Medicine and Journal of the American Medical Association address this strategy. The conclusions of these studies was that "ventricular tachycardia or fibrillation appears to be the principal mechanism for sudden death in patients with HCM. In high-risk patients with HCM, implantable defibrillators are highly effective in terminating such arrhythmias, indicating that these devices have a role in the primary and secondary prevention of sudden death."
A European Registry of HCM patients who have survived cardiac arrest and who were implanted with ICD has been reported. The conclusion was that "ICD therapy offers an effective prophylactic treatment in these patients at high risk of VT or VF recurrences." Both registries highlight that sometimes lethal arrhythmias may occur years after implantation, not necessarily in the first months.
For more information about ventiricular arrhythmias and their treatment, you may wish to read the information provided at the Arrhythmia Service