|
Pacemakers and MRIs don't mix
2006-10-09
Medical experts are laboring to resolve a high-technology era conflict that confounds doctors: sophisticated scans needed to detect cancer, stroke and other ailments cannot now be permitted for patients with lifesaving implants like pacemakers. More than 2 million Americans depend on pacemakers or defibrillators to keep their hearts beating right. But those lifesaving implants come with a price: They're not allowed in MRI machines, leaving patients out of luck if they later need scans to detect cancer, stroke or myriad other ailments. That soon will change. Doctors at a handful of U.S. hospitals are beginning to give MRIs to certain patients despite the implants, careful experiments to find ways to shield the heart devices from potentially deadly meltdowns or misfires. The first human study of a pacemaker specially designed to withstand MRIs is expected to begin by year's end. The attempts come none too soon. Use of the heart implants is growing rapidly, and numbers of recipients estimated in the hundreds of thousands are turned away every year from MRIs that their doctors sought to help diagnose or manage other diseases. "It's a critical issue," said Owen Faris, a heart device specialist at the Food and Drug Administration, which has long urged manufacturers to create MRI-compatible implants. The irony is that for most people, an MRI is supersafe. The scanner itself is a powerful magnet. Most modern implants are made with materials that are not very magnetic, meaning an MRI will not move them around once they've healed in the body. But MRIs are off-limits for a handful of implants -- mostly pacemakers, heart-shocking defibrillators and some brain devices -- because the scans can heat them and burn surrounding tissue. Also, MRIs emit radio frequency waves that could confuse electronic implants, leading them either to quit working or fire when they should not. Johns Hopkins University Hospital in Baltimore, Maryland, a leader in the fledgling MRI trend, is getting two or three requests a day to scan pacemaker or defibrillator recipients, after scientists there reported safety steps that have allowed MRIs for more than 100 of the risky patients so far. "Even with all these precautions, we can't guarantee that nothing adverse would happen," warns Dr. Saman Nazarian, a Hopkins cardiac electrophysiologist who monitors patients' hearts while they are inside the scanner, ready to intervene at signs of trouble. "Our hope ... is the devices that will come out in the future will be built from the ground up to be safe." Scientists are trying three approaches: _Medtronic Inc. is awaiting permission from the Food and Drug Administration to begin the first patient tests of a pacemaker designed to be compatible with MRI scans. Medtronic added filters to prevent the pacemaker's heart-penetrating wires from picking up MRI signals, so that they should not heat or misfire, explains Vice President Warren Watson. _Recall how dentists cover patients' bodies with a lead apron before X-raying teeth? University of Pittsburgh researchers are preparing to test a similar shield approach, covering pacemaker recipients' chests with material that blocks MRI energy while scanning other body parts. Called an "MRI isolation blanket," it is made by a company that provides radar blockers to the military. In laboratory tests using mannequins, the blanket kept MRIs from dangerously heating pacemaker wires, says Dr. Emanuel Kanal, the university's MRI chief and head of the American College of Radiology's MRI safety committee. _Then there is Hopkins' method. Lead researcher Dr. Henry Halperin first exposed pacemakers and defibrillators to extra-high-dose MRIs to winnow out 24 modern brands that withstood strong magnetic fields without becoming too hot. Animal testing identified more possible safety problems. If a pacemaker wire, called a lead, should go bad, doctors sometimes just unhook it from the device's battery and let it lie quietly in the heart. It turns out that unattached leads get far hotter than attached ones; so do even attached leads if they're placed on the outside of the heart instead of inside. So Nazarian turns away patients with those two lead problems. For other candidates, he temporarily reprograms their pacemakers or defibrillators to reduce electronic interference, and runs the MRI scanner at half-strength. So far, no one has been harmed, the team reported in last month's journal Circulation. The MRIs have uncovered cancers and strokes, helped doctors plan artery-opening treatments. In one woman the procedure spotted a brain disorder that other tests had missed, which allowed surgery to end the patient's seizures. For now, pacemaker patients should be wary if a doctor orders an MRI, Kanal warns. Very few medical centers have on hand the specialized equipment and specially trained electrophysiologists and radiologists needed to monitor their safety during the scan. "It can be done," Kanal said, "but it's not at all the state of the art." ___ Lauran Neergaard covers health and medical issues for The Associated Press in Washington. Muzi.com News
| 
