SNOWMASS, COLO. — Magnetic resonance imaging is by far the most problematic medical source of electromagnetic interference with implanted cardiac device function, according to Dr. William H. Spencer III.
Other potential sources of interference include radiotherapy, neurostimulators, electrosurgery, radiofrequency catheter ablation of arrhythmias, and lithotripsy. On the other hand, here are some things device wearers often fret about but needn't: diagnostic x-rays, CT scanning, mammograms, ultrasound, and most forms of laser surgery, Dr. Spencer said at a conference sponsored by the Society for Cardiovascular Angiography and Interventions.
He shed light on sources of interference that may affect implanted cardiac devices:
▸ Radiotherapy. The damage to pacemakers and implantable cardioverter defibrillators (ICDs) by radiotherapy is dose dependent, cumulative, and permanent. “You can fry the system circuitry,” said Dr. Spencer, professor of medicine at the Medical University of South Carolina, Charleston. The radiation oncology center should have protocols for avoiding direct irradiation of the device, creating the greatest possible distance between device and radiation beam, and maximum shielding.
“If you have a woman with breast cancer and positive nodes in the left axilla [who needs] radiotherapy, you may need to move the device over to the other side,” he said.
▸ Neurostimulators. Transcutaneous electric nerve stimulation, and the peripheral and spinal nerve stimulators used to treat neuropathic and orthopedic pain, can often be used safely in patients with modern bipolar pacemakers. But the patient should first undergo testing at the stimulator's maximum output to ensure that it doesn't trigger or deactivate the pacemaker. There has been very little experience to date with ICDs, considered a relative contraindication to neurostimulator therapy.
▸ Electrosurgery. This creates one of the most powerful and dangerous electromagnetic fields found in the medical environment. The best option is to find an alternative form of surgery. Next best is to place the pacemaker in asynchronous mode, disable its antitachycardia and rate-responsive therapies, and employ true bipolar electrosurgery using short, irregular bursts of energy. Afterward, confirm that the device is working properly.
▸ Radiofrequency ablation. This interacts unpredictably with cardiac devices. Turn off rate-responsive and antitachycardia features and program the device to asynchronous mode for the procedure duration. If the goal is to create complete heart block, a temporary pacemaker must be inserted to ensure ventricular capture.
▸ Shock wave lithotripsy. This method of breaking up kidney and other stones is not nearly the problem it once was. Only patients with an abdominally implanted device generator are at high risk, and those are now uncommon. So long as the lithotripsy target and cardiac device are at least 6 cm apart, this therapy appears to be safe.
▸ MRI. The Food and Drug Administration and cardiac device manufacturers list MRI as absolutely contraindicated. Deaths have occurred. It has been estimated that if not for the contraindication, an MRI would be recommended for various indications in up to 75% of U.S. pacemaker and ICD users during the course of the device's service life. Device manufacturers have made development of MRI-safe pacemakers and ICDs a priority, but none exist yet.
Meantime, there are situations in which only the data obtainable from MRI will do. And there are many anecdotal reports of MRI being done safely in pacemaker-dependent as well as nondependent patients. But proper precautions are essential.
The pacemaker should be reprogrammed to VOO or some other uninhibited mode of pacing. Only head and neck or extremity MRI should be done—no chest or abdominal imaging. A cardiologist should be on hand to monitor the ECG and pulse oximeter. And the device must be checked and reprogrammed immediately afterward, Dr. Spencer said.
Manufacturers have prioritized development of MRI-safe pacemakers and ICDs, but none exist yet. DR. SPENCER