Biventricular pacemakers
"The device doesn't change the underlying nature of
the problem, but it can be quite successful in treating the disruptive
symptoms of heart failure and is much less risky and invasive than something
like an LVAS or a cardiac transplant."
GREGORY A. EWALD, MD
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ROUGHLY 5 MILLION AMERICANS suffer from congestive
heart failuretheir hearts cannot sufficiently supply the body with
blood. This mechanical malfunction can result for a number of reasons,
but for patients as sick as Keith Norris, the cause is largely irrelevanthe
needed a new heart or a device that would act like a new heart to take
over for his old one. On the other end of the spectrum, patients with
less severe heart failure often can control their conditions using medications.
But there are thousands of patients who are not quite as
sick as Norris who still have trouble breathing in spite of the best possible
drug regimen. According to Mitchell N. Faddis, MD, PhD, assistant professor
of medicine, the key to treating this mechanical malfunction may be to
target a different element of the heartthe electrical system. He
and colleagues at the School of Medicine are part of two nationwide trials
to test a new type of implantable device, the biventricular pacemaker.
The two systems of the heart, mechanical and electrical,
work together to effectively supply the body with blood.
In the electrical system, an internal "pacemaker" called the
sinus node sends an electric signal to the lower chambers of the heart
through a single electrical connection called the atrioventricular node.
This connection immediately divides into three sub-branchesone relays
the signal to the right lower chamber of the heart; the other two diverge
to opposite sides of the left lower chamber. The branches purpose
is to simultaneously deliver the electric message to both sides of the
heart, allowing coordinated contractions (the mechanical function). This
coordinated effort forms the healthy "lubb DUPP" heartbeat.
In some patients, the electrical system is completely askew,
but the mechanical system is relatively healthy. These individuals benefit
from a traditional pacemaker, which serves as a surrogate for the malfunctioning
component of the electrical system, either the sinus node or the atrioventricular
node.
Mitchell N. Faddis, MD, PhD, and Gregory A. Ewald, MD,
display biventricular pacemakers.
In roughly one-third of patients with congestive heart failure,
the sinus and atrioventricular nodes are functioning well, but two of
the three sub-branches are effectively broken. As a result, the electric
signal fails to reach the right and left lower chambers at the same time.
When the left side contracts, the right side is relaxed and vice versa.
Blood then sloshes back and forth, rather than being forced outward by
a two-sided, synchronized effort.
A weakened heart only has a small amount of mechanical energy
left to contract the heart muscles. If the same patient also experiences
a time delay between contraction of the right and left sides of the heart,
the limited mechanical energy left over is used up before enough blood
can be pumped out.
Unlike a left-ventricular assist system, which uses the
hearts natural electric signal to supplement its mechanical pumping
action, the biventricular pacemaker adjusts the electric signal itself.
Traditional pacemakers supply two wires to the heart: one to the upper
right chamber; one to the lower right chamber. The biventricular pacemaker
adds a third wire, which connects to the lower left chamber. With this
third wire, the pumping action of the hearts right and left lower
chambers are re-coordinated.
"The device doesnt change the underlying nature
of the problem, but it can be quite successful in treating the disruptive
symptoms of heart failure and is much less risky and invasive than something
like an LVAS or a cardiac transplant," says Gregory A. Ewald, MD,
assistant professor of medicine.
He, Faddis and Joseph G. Rogers, MD, associate professor
of medicine, are participating in two national trials to test this type
of device. Participants are eligible only if a strict, advanced drug regimen
does not alleviate their symptoms. Patients continue their medical routine
after implantation with a device.
The Miracle InSync ICD trial, sponsored by Medtronic, is
a secondary prevention trial that uses a combination biventricular pacemaker/defibrillator
in heart failure patients who need a defibrillator due to life-threatening
heart rhythm problems. This study will examine whether patients
symptoms and quality of life improve. Medtronics InSync biventricular
pacemaker (without the defibrillator) received FDA approval in August
2001.
In contrast, Guidants Companion trial is a primary
prevention trial with respect to life-threatening heart rhythm abnormalities.
Nearly half of all heart failure-related deaths are sudden, implicating
an anomaly in cardiac rhythm.
"Even if a heart failure patient hasnt had any
heartbeat irregularities, he is at a high risk from dying of an arrhythmia,"
says Faddis.
Thats why the Companion trial is examining whether
biventricular pacing can improve patients quality of life and help
prevent sudden death in heart failure patients who have not experienced
rhythm abnormalities, but who still have trouble breathing, despite medical
treatment.
In this trial, participants who meet the entry criteria
may remain on heart failure medications or be randomly assigned to receive
one of two devices: a biventricular pacemaker or a biventricular pacemaker/
defibrillator. With more than 2,000 projected participants nationwide,
Companion is the largest device study to date, and it is the first to
examine whether either type of device reduces mortality.
"We already have implanted biventricular pacemakers
in about 50 patients. Some have had a dramatically positive response,
many have had an improvement in symptoms," says Ewald. "In some
patients, it may even remove the need for more drastic procedures in the
future, such as heart transplant or implantation of a ventricular assist
device. These clinical trials will allow us to determine which patients
will benefit most so that we can further target the therapy."
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