Automatic Deffibrillators
Peter Nalos, MD, FACC and FACP
An automatic defibrillator is a device which is implanted in the patient's
upper chest and has an electrode wire which is inserted through the vein into
the chamber of the right ventricle, and this delivers an electrical shock to
restore a patient's heart rhythm during cardiac arrest. Patients who are
candidates for this are people who have had demonstration of serious
life-threatening cardiac arrhythmias (ventricular tachycardia or fibrillation).
Many patients have had previous heart attacks and heart damage and heart
surgery, including bypass and valve replacements. Patients who are candidates
may have had a cardiac arrest previously or have had serious arrhythmias
prompting emergency room admission or periods of fainting and have ventricular
tachycardia inducible at an electrophysiology study. In a number of cases,
patients have already been tried on certain medications to prevent arrhythmias
and these have been deemed not to be effective in protecting them from sudden
cardiac death. The insertion of an automatic defibrillator is done in the
operating room and currently is done under general anesthesia; however, some
cases may be selected for local anesthesia with intravenous sedation. The
surgeon will make an incision below the collarbone on the left side and fashion
a pulse generator pocket. At that time the electrophysiologist will insert an
electrode pacing lead, which has the ability to record signals from the heart,
as well as to deliver electrical shocks between the lead terminals and the can
of the automatic defibrillator pulse generator. During the operation, the
patient will have ventricular fibrillation triggered and the device will be
tested to show that it can safely convert ventricular fibrillation to sinus
rhythm and sense and react appropriately and quickly. During the test, the
patient will have external rescue pads placed on his chest in case an external
shock is necessary to revive heart rhythms. During this study, the patient's
oxygenation, and blood pressure, and vital signs are all measured continuously
to monitor him during the procedure. Following the procedure, the patient will
recover in the hospital, usually for a period of 24 to 48 hours. They will
receive intravenous antibiotics and X-rays and be monitored on the telemetry
floor.
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Specialized Functions of an Automatic Defibrillator -
The first function of the automatic defibrillator will be to provide
life-threatening rescue shocks in the event of very rapid ventricular
tachycardia or fibrillation when the blood pressure ceases. This should react
within a period of 10-12 seconds and restore consciousness and life. The
defibrillator may give up to five electrical shocks to perform this function.
Other functions of the defibrillator are to give cardioversion energy for
ventricular tachycardia and these may be at very low levels (1 joule) or may
require higher levels of energy, but usually less than ventricular
fibrillation. The defibrillator may also provide pacing of the heart for a
period of, perhaps, seven beats to try and terminate ventricular tachycardia
without delivering electrical shocks and in most cases this is effective in
interrupting the abnormal rhythm, and the patient may be completely unaware
that this is happening. All new transvenous defibrillators have a pacemaker
which is programmable of the lower chamber to prevent slowing of the heartbeat.
Once a defibrillator is placed, the patient will need to be monitored
periodically in the office every several months to make sure that the
capacitors work well, that the contact with the heart is good, and that pacing
thresholds are adequate in all of the parameters, such as impedances, voltages,
pulse width, and normal function. As time goes on, we will be able to detect
depletion in the defibrillator's battery and tell when it will require a pulse
generator change. An automatic defibrillator generator change is often a more
simple procedure since the leads are already in place and one only needs to
test them and connect the new generator without inserting new leads; however,
if there is a problem with the old lead system, such as patches or epicardial
screw-in leads which are quite old, it may be wise to replace an abdominal
defibrillator with a new active can transvenous defibrillator which is often
quite smaller. Automatic defibrillators are a very important tool available to
cardiologists and electrophysiologists and have saved numerous lives and
avoided some of the long-term toxicities of drugs.
Peter Nalos, MD, FACC and FACP
- is Board Certified in Internal Medicine, Subspecialty Board of
Cardiolovascular Diseases. Doctor Nalos is a Fellow of the American College of
Cardiology. Dr. Nalos is certified by and a member of NASPE (North American
Society of Physician Electrophysiologists).
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Tetsuo Ishimori, MD
