MRI perilous for pacemaker patients:IMAGING STUDY

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Exposure to an MRI magnetic field can cause unintended cardiac stimulation and considerably alter pulse in patients implanted with pacemakers–conditions that can have potentially devastating consequences–according to research published Dec. 15 in BioMedical Engineering Online.

Howard I. Bassen, a researcher with the FDA in Rockville, Md., and Gonzalo G. Mendoza, a biomedical engineer at Catholic University of America in Washington, D.C., measured the electric fields (E-fields) induced near the tips of pacemakers by a simulated MRI gradient system to assess the risks involved in patients with a cardiac device who undergo MRI.

According to the researchers, “patients are generally not allowed (by present practices) to undergo MRI procedures if they have implanted cardiac and neurological stimulations devices,” however, some clinicians “condone scanning patients” implanted with cardiac pacemakers. In addition, Bassen and Gonzalo noted that there has been a push to develop medical implants that are MRI compatible.

The investigators configured data by mapping a magnetically induced E-field with a 0.1 mm resolution as close as 1 mm from the devices lead tip. Two-electrode probes, .5 mm and .2 mm diameter American Wire Gauges were utilized.

To generate a magnetic flux density, researchers placed a 24 cm diameter cylindrical plastic tank inside a Helmholtz coil and filled it with saline to produce conductivity.

Two cardiac pacemaker pulse generators—the Medtronic KDR401 and the Guidant Insignia pacemaker—were connected to the tined tip lead to compare the values of the E-field in the saline solution at the distal tip to the magnetically induced E-field at the same location. Utilizing a SPEAG DASY5 robotic Dosimetric Assessment System, the researchers took 15,000 voltage readings.

Three leads, a simulated, tined tip and an active fixation tip were configured inside the circumference of the saline tank for evaluation.

Researchers found that when comparing measurements of the active fixation lead and the tined tip lead that “the E-field strengths for these leads were significantly higher than the E-field strength induced at the blunt cut tip of a simulated lead made of an insulated wire.”

“The intended stimulation waveform delivered by an implantable pacemaker pulse generator to a distal tip unipolar electrode can be significantly altered (decreased or increased),” the authors found. This, the researchers said, is due to the combination of the magnetically-induced E-field from a time-coincident MR gradient pulse and the injected pulse from the pacemaker.

The authors found that MRI systems normally operate with a 30 to 100 T/s gradient field rate of change. When a gradient pulse is coincident (within 30 ms) with an implantable pacemaker pulse the researchers found that changes in pacemaker stimulation voltages can occur.

In addition, findings showed that “unintended stimulation” can be induced by leads if proximal ends are not “capped” with insulation by surgeons. In addition, the researchers noted that due to the magnetically-induced E-field from a time-coincident MR gradient pulse, pacemaker pulses can be “drastically altered.”

Bassen and Mendoza suggested that “a simple way to reduce the possibility of unintended stimulation of a patient is to turn off their pacemaker during an MRI imaging session, provided they are not pacemaker dependent.” However, they noted that this “opens them to the risk of no available pacemaker therapy if they were to need it during the duration of the session.”

The authors suggest that future research be obtained by computational modeling of the “experimental measurement system” in order to compare measured data.

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