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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.
Some of the most serious arrhythmias that HCM patients can experience
are the rapid and prolonged arrhythmias that come from the pumping
chambers of the heart. During these 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 is accomplished by an ambulance team
who places paddles on the chest and delivers the 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 can record the electrical
signals from within the heart and tell 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.
A recent publication in the New England Journal of Medicine
by Dr. Barry Maron and his collegues addresses this question.
The St. Luke's-Roosevelt HCM program contributed patients
to this registry. The paper is entitled "Efficacy of Implantable
Cardioverter-Defibrillators for the Prevention of Sudden Death
in Patients with HCM." NEJM 2000;342:365-73. In this paper
128 patients who received ICDs were followed for mean of 3.1
years. The rate of appropriate, life saving activation of
the devices was 7% per year. In the patients implanted for
secondary prevention, after malignant arrhythymias, the rate
was 11% per year. In the patients implanted for prophylactic
reasons, because of high risk features, but before a malignant
event, activation occurred in 5% per year. The conclusion
of this study 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 by
Dr. Martin Borrgreefe and Dr. William McKenna at the American
Heart Association meeting in 1999. Results were similar to
Maron's findings. With a follow up of 30 months, 29% of patients
implanted had appropriate activation of the device. 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 arrhythmias and their treatment, you
may wish to read the information provided at the Arrhythmia Service. |
