People with pacemakers and implanted defibrillators can have MRIs, according to a new study, as long as the pacemaker was manufactured after 1998 or the defibrillator after 1999, and as long as a nurse trained in cardiac life support is standing by.

More than 400 patients at John Hopkins University participated in the study. They all had either a pacemaker or implanted defibrillator and all had at least one MRI, with a cardiac specialist either turning off or turning on the device during the scan. None of the patients were harmed by the scan, though in three cases the device reset to the default settings and had to be reprogrammed.

A report of the study appeared in the Oct. 4 issue of the Annals of Internal Medicine, a leading medical journal.
It is estimated that about 1.8 million Americans have either a pacemaker, which regulates heart rate, or a defibrillator, which can jump start the heart in case of cardiac arrest. About 75 percent of them will eventually have some condition that calls for an MRI.

Experts say that despite the new evidence, many hospitals will not be able to do MRIs on patients with defibrillators or pacemakers, because of the extra precautions that need to be taken and the need to have extra medical personnel assisting.