Using magnetic resonance imaging scans of the heart
wall, researchers at Johns Hopkins have found that people
whose muscle wall thickness contained more than 25 percent
scar tissue were approximately nine times more likely to
test positive for a fast and dangerous heart rhythm known
as ventricular arrhythmia.
Patients at risk of such arrhythmias often have a
heart defibrillator implanted; the small device delivers an
electrical shock to restore cardiac rhythm in case the
heart beats too rapidly to pump enough blood to the rest of
the body. The U.S. Centers for Disease Control and
Prevention estimates that each year more than 400,000
Americans suffer a sudden cardiac death, at least 30
percent of which are due to arrhythmia.
"If further tests confirm that MRI measurements of
scar tissue accurately predict the risk of
arrhythmia-related sudden death, these could become the
gold standard for screening who really needs or does not
need a defibrillator," said the study's senior author,
electrophysiologist Henry Halperin, a professor of
medicine,
radiology and
biomedical engineering at the
School of Medicine and its
Heart Institute. "While tests are widely available to
screen patients with coronary artery disease for risk of
sudden cardiac death, tests are not so effective for
identifying the many who will die suddenly from
arrhythmias."
Indeed, while the U.S. National Center for Health
Statistics estimates that more than 1 million Americans
currently have a defibrillator, national studies published
early this year have shown that only 5 percent of these
devices ever fire to correct a heartbeat.
The latest Johns Hopkins findings, which appear in the
Nov. 1 edition of the journal Circulation, are
believed to be the first to search in the heart's
architecture, rather than its pumping function and
electrical signaling, and so far comprise the only study to
analyze this architecture for clues about arrhythmias in
patients with poor heart function but no arterial
disease.
According to the researchers, defibrillators are
prescribed when tests show abnormalities in the heart's
ejection fraction (ability to squeeze blood to the rest of
the body) and/or its resistance to electrical impulses that
try to stimulate an arrhythmia.
"Our MRI technique has significant advantages over
existing methods because it avoids the risks of infection
that come with surgery, it is noninvasive, there are no
catheters, and it is relatively easy to perform, taking
only 45 minutes," said study co-author and cardiologist
Joao Lima, an associate professor of medicine and
radiology.
Lima said that a patient with an ejection fraction of
60 percent has normal pumping ability, but anything less
than 30 percent for a period of nine months or longer is
considered low and an immediate risk factor for arrhythmia.
If a patient has an ejection fraction that is slightly
above 30 percent, he said, then an electrophysiology test
is used to determine if a patient requires a defibrillator.
In this test, a thin catheter is inserted into the heart to
try to induce an arrhythmia, something that will fail if
the heart is healthy and not at risk. However, if it
happens once, it is known to be two to four times more
likely to happen again, he said.
Twenty-six patients from the Baltimore area
participated in the study, which took place from July 2003
to February 2005. Participants were men and women, with an
average age of 53, referred by community physicians to
Johns Hopkins for cardiac assessment. None had previous
signs of coronary artery disease, another leading cause of
sudden cardiac death, yet they were experiencing other
symptoms of heart disease, such as shortness of breath,
instant fatigue and the inability to walk up stairs.
As part of a baseline MRI, the researchers used a
technique developed at Johns Hopkins to map and gauge the
precise amount and distribution of scar tissue in the
heart's muscle wall. The amount of scar tissue was measured
as a percentage of the thickness of the muscle wall, which
is on average about 1 centimeter. Composed of dense,
fibrous tissue, with little or no blood supply, scar tissue
was clearly visible on the image, the researchers said.
After MRI, each patient underwent a standard
electrophysiological assessment with a catheter.
Statistical analysis showed that the five patients who
tested positive had the characteristic scar pattern,
ranging from 26 percent to 75 percent scar tissue, with
MRI. While MRI did not explain why the scar tissue forms,
such scar patterns have been previously noted on autopsy
studies of patients with heart disease. The researchers
said they believe that previous inflammation, injury or
excess stress on the heart wall may lead to this fibrosis
and scar formation.
"Our study is yet another example of the potential
applications of cardiac MRI in the prevention and treatment
of cardiovascular disease," said the study's lead author,
Saman Nazarian, a cardiac electrophysiology, clinical and
research fellow. "Cardiac MRI is already useful for
assessing the structure and function of the heart and the
extent of structural changes due to coronary artery
disease. MRI can also help identify patients in need of
aggressive medical therapy and can help in the planning of
invasive heart surgery or identification of the best
candidates for bypass surgery."
Nazarian pointed out that these results also offer
promise that cardiac MRI might prove useful in screening
people at moderate risk of sudden cardiac death from
arrhythmias — those without significant coronary
artery disease and ejection fractions between 30 percent
and 50 percent.
Another therapeutic implication, he said, is that
identifying the telltale scar pattern could potentially
improve existing procedures to ablate, or burn off, regions
of the heart muscle that trigger arrhythmia.
Funding for this study was provided by the Donald W.
Reynolds Foundation and the National Institutes of Health.
Halperin is a paid consultant to defibrillator manufacturer
Medtronic, and co-investigator Ronald Berger is a paid
consultant to Guidant Corp., another device manufacturer.
Neither of these companies provided funding for the study,
and the terms of the physicians' arrangements are managed
by Johns Hopkins in accordance with its
conflict-of-interest policies.
Other researchers involved in the study were David
Bluemke, Albert Lardo, Menekhem Zviman, Stanley Watkins,
Timm Dickfield, Glenn Meininger, Ariel Roguin, Hugh
Calkins, Gordon Tomaselli, Robert Weiss and Ronald
Berger.
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