Preventive Measures Ensure Safety
Peter Zimetbaum, MD
, and Matthew Reynolds, MD, recently wrote an editorial that appeared
Annals of Internal Medicine
regarding the use of magnetic resonance image (MRI) scans for people
with pacemakers or implantable cardioverter-defibrillators (ICDs). We
talked with Dr. Zimetbaum about this issue.
Can you explain why it's been considered unsafe for patients with
implantable cardiac devices to undergo an MRI scan?
Both pacemakers and ICDs are devices that contain wires with electrodes
that are connected to one or more of your heart's chambers. These are
designed to restore normal rhythm to the heart by sending electrical pulses
through the wires.
Magnetic resonance imaging works by generating a strong magnetic field. The
fear has been that the powerful magnetic fields and radio waves that are
part of MRI scans could cause the ICD wires to overheat, potentially
damaging heart tissue. There has also been concern that MRIs could induce
unwanted currents that would either make the heart beat wildly, or in the
case of ICDs, cause an unnecessary shock.
Your editorial in the Annals of Internal Medicine provided
commentary on a large study. Can you describe the study findings?
The study was conducted over a seven-year period beginning in 2003. A total
of 555 MRI scans were performed on 438 people, 237 of whom had a pacemaker
and 201 of whom had an ICD. Forty percent of the MRI scans were of the
brain, 22 percent were of the spine and 16 percent were of the heart. The
findings concluded that the vast majority of patients had no problems.
It appears that one reason why the MRIs proved to be safe was because, in
all cases, the pacemakers or ICDs were reprogrammed in advance of the
patient entering the MRI scanner.
How are the devices reprogrammed?
It is actually a fairly straightforward process that can be done
noninvasively. In the case of the pacemakers, for patients who are
dependent on pacing from the device — the device is put into a full-time
pacing mode (asynchronous) to avoid it being inhibited (not pacing) when it
should as a result of the MRI. For ICDs, the device is programmed to not
detect any fast rhythms during the scan (to avoid inappropriate sensing of
the magnetic field-related generated noise and potential shock). In the
case of the ICDs (which sense tachycardia, a racing irregular heartbeat),
this function was temporarily disabled.
Throughout the course of the scans, the patients were carefully monitored
by a nurse trained in programming the cardiac devices.
What advice would you give to patients?
Patients should not undergo MRI scans until the CRMD (cardiac rhythm
management devices) have been in place for at least six weeks and careful
considerations are made regarding the type of MRI scanner and the medical
personnel who are on hand.
However, this new data, as well as other recent findings, suggest that
patients with implantable cardiac devices should be assessed on an
individual basis to determine whether they should undergo an MRI. This is
not to say the scans are risk-free, but it does appear that risks are
Above content provided by the CardioVascular Institute at Beth Israel
Deaconess Medical Center. For advice about your medical care, consult