Researchers find temporary epicardial leads safe, temporary intracardiac leads unsafe for MRI use*

Although MRI examinations in patients with retained temporary epicardial leads, which consist of electrically conductive material, could theoretically lead to cardiac excitation or thermal injury, such leads are relatively short in length, usually do not form large loops, and are generally not believed to pose a significant risk during MRI procedures.

Hartnell et al reported on 51 patients with retained temporary epicardial pacing wires who underwent clinical MRI procedures. Of those patients examined with electrocardiographic monitoring, no arrhythmias were noted. For all patients, no symptoms suggestive of arrhythmia or other cardiac dysfunction were noted, although the anatomic region examined and the energies used in the examinations were not specifically described.

To date, there is no report of complications associated with performing MRI in a patient with retained epicardial leads.

By comparison, one ex vivo study of temporary intracardiac transvenous pacing leads reported temperature increases of up to 63.1°C. Preliminary results of a recent study confirmed that even unconnected temporary transvenous pacing (as well as permanent pacing) leads can undergo high temperature increases at 1.5T.

In a chronic pacemaker animal model undergoing an MRI examination at 1.5T, temperature increases of up to 20°C were measured, although pathological and histological examination did not demonstrate heat-induced damage of the myocardium. The MRI conditions that generated such elevated lead temperatures included use of the body radiofrequency coil to transmit RF energy over the area of the intracardiac pacing lead (e.g., an MRI examination of the chest/thorax).

To the best of our knowledge (i.e., the Consensus Group: Levine GN, Gomes AS, Arai AE, et al), there are no studies assessing the safety of temporary pacemakers (lead and external pulse generator). Unlike permanent devices, temporary pacemakers use unfixed leads that are more prone to movement, longer leads that may be more prone to induction of lead currents, and a less sophisticated pulse generator, which makes them more susceptible to electromagnetic interference.

Thus, patients with retained temporary epicardial pacing wires are considered able to safely undergo MRI procedures, and patients do not need to be routinely screened for the presence of such wires before scanning. Because of the possible risks involved with temporary pacemaker external pulse generators, however, such generators should not be introduced into the MRI environment.

Although temporary intracardiac transvenous lead heating may be minimized or avoided by scanning anatomic regions above (e.g., head/brain) or below (e.g., lower extremities) the cardiac pacing leads, scanning of patients with temporary intracardiac pacing leads (without the generator) is not recommended. Furthermore, because the harsh electromagnetic environment associated with the MR system can alter the operation of an external pulse generator or damage it, it may not be possible to reliably pace the patient during the MRI examination, which makes the issue of scanning a patient with a temporary intracardiac transvenous lead irrelevant in most cases.

*[Excerpted with permission from Levine GN, Gomes AS, Arai AE, et al. Safety of magnetic resonance imaging in patients with cardiovascular devices: an American Heart Association scientific statement from the Committee on Diagnostic and Interventional Cardiac Catheterization. Circulation 2007;116:2878-2891.]

Dr. Shellock is an adjunct clinical professor of radiology and medicine at the Keck School of Medicine of the University of Southern California in Los Angeles. He is also an adjunct professor of clinical physical therapy in the division of biokinesiology and physical therapy at USC and is president of Shellock R&D Services.BIBLIOGRAPHY

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