With approximately 13,000 magnetic resonance imaging (MRI) instruments installed worldwide, over 35 million MR imaging studies are performed each year. A revolutionary tool, MRI has become the procedure of choice for a diagnosing a wide variety of disorders, including those of the central nervous and musculoskeletal systems. It has been suggested that we are “on the brink of another revolution”1 with MRI, perhaps becoming the diagnostic tool of choice for imaging the cardiovascular system. For example, recent publications have highlighted the growing potential role of MR imaging in coronary artery disease2 with experts editorializing that “the issue should be not if, but when” MR angiography will “become an acceptable alternative to the standard, invasive approach?”3

Over the past several decades, increasingly sophisticated permanently implanted electronic devices have been developed that are used as the standard of care to diagnose and treat important brady and tachyarrhythmic disorders.4 Increasing numbers of these devices are being implanted, with comprehensive data acquired for 1997 showing that over 460,000 pacemaker implants and 38,000 cardioverter defibrillator (ICD) implants were performed worldwide.5–7 Recently expanded indications for pacing in heart failure and the use of ICDs for the primary prevention of sudden cardiac death,4 as well as potentially new device indications,8 highlight the rapidly increasing number of patients who are now and will be treated with device therapy.

Most authorities feel the presence of a pacemaker (PM) and or an ICD is at least a relative if not an absolute contraindication for MRI.9–11 Product literature of all the companies that manufacture pacemakers and ICDs clearly states that MRI is to be avoided. Likewise, the manufacturers of MRI units state that imaging of patients with an PM or ICD is absolutely contraindicated.

Because of expanding indications, the likelihood of being recommended for an MRI procedure during one’s lifetime is increasing. So, too, is the likelihood that an implantable pacemaker or defibrillator will be recommended at some time in a patient’s life. Thus, the concerns of device-device interaction are mounting daily in our ever-increasing use of technology in the care of patients. Will patients with implantable devices simply be denied access to MR imaging? While a contraindication to MRI of these device patients is supported by some previous in vitro, animal and human data, any practice policy should be open to revision as new data is presented that suggests the previous view may be overly restrictive.
It is well known that electromagnetic interference (EMI) can degrade the performance of electronic medical devices.12 Time-varying magnetic forces and radiofrequency (RF) waves are two types of electromagnetic energy. The powerful static and gradient magnetic fields and the radiofrequency energies required for MRI create a hostile environment thought likely to cause severe disruption of pacemaker function. Several concerns including device movement, unexpected programming changes, inhibition of pacemaker output, transient asynchronous pacing, pacemaker reed switch malfunction, rapid cardiac pacing, the induction of ventricular fibrillation, irreversible malfunction of the device after exposure to MRI, and local thermogenic cardiac tissue destruction, have all been cited in support of the view that MRI of patients with pacemakers and ICDs should be avoided.13 While some of these concerns are theoretical, several have been demonstrated during in vitro testing of pacemakers exposed to MRI or in an animal model. Some of the above-noted effects, while often ill characterized, have been seen in humans. Ethical and practical concerns have precluded the deliberate demonstration of these phenomena in humans with implantable rhythm devices. The seriousness of potential adverse effects is highlighted by multiple reports of deaths in pacemaker patients exposed to MRI that have been documented both in the medical literature, FDA data bases, and in the popular press.14–17 However, virtually all serious irreversible adverse events (including all deaths) share at least one fact in common; namely, the patient with the implantable device was somehow placed in the MR environment without the knowledge that the patient had an implantable device.

While adverse events have taken place in device patients who have undergone MRI, and these events should in no way be trivialized, what is true is the following: no clinically meaningful irreversible harm has been demonstrated to have taken place when patients with implantable pacemakers or ICDs were carefully monitored during MRI and the device underwent simple reprogramming prior to the scan. Thus, some have suggested that device patients might safely undergo MRI in selected circumstances.18,19

Approximately 200 patients with pacemakers or ICDs have now been reported in the medical literature to have undergone MRI, with the vast majority having undergone scanning safely when monitoring and reprogramming strategies were applied. Several large series of pacemaker patients safely undergoing MRI have now been reported.18–21 Thus, as has been noted previously, “it may be that the hazards of performing magnetic resonance in patients with pacemakers have been overstated in the past”.22

The reported experience of ICD patients undergoing MRI is substantially smaller.23,24 Like their pacemaker counterparts, when patients with ICDs underwent reprogramming prior to and monitoring during MRI, a safe outcome took place. Inadvertent scanning of ICD patients led to irreversible damage in three reported instances.

Should MRI be considered in a patient with an implantable device, appropriate consultation with a qualified cardiologist or electrophysiologist is strongly advised, so that patients can be adequately screened and correctable physiologic abnormalities can be addressed. When properly monitored and supervised, any abnormal rhythms that might take place during the scanning of a pacemaker patient are likely to immediately terminate once a scan is aborted. If the rhythm does not terminate, appropriately trained individuals should be immediately available with resuscitation equipment.

If MRI is considered, the device should be programmed to OOO if possible in non-pacer dependent patients, bearing in mind that this does not preclude rapid cardiac pacing.25 Pacemaker-dependent patients should be programmed DOO or VOO. The device patient should be monitored with ECG, pulse oximetry and verbal contact during the period in the MR suite. Despite what some have suggested,9 it should be recalled that asynchronous pacing is not always benign, particularly for prolonged periods of time.13 It goes without saying that ICD therapies should be programmed off.

Conclusion.
From a practical perspective, the choice to use a drug, biological product or device involves balancing the benefits to be gained with the potential risks of using a product. “Most injuries and deaths associated with the use of medical products result from their known side effects. Some side effects are unavoidable, but others can be prevented or minimized by careful product choice and use.”26 Thus, it appears with the MRI scanning of pacemaker and defibrillator patients. All clinically significant permanent adverse effects recorded in pacemaker and ICD patients have occurred when the patient entered the MR suite without the staff knowing the patient had an implantable device. Despite the known risks and valid concerns of scanning these patients, MRI is appropriate when the risk-benefit ratio is favorable, and only after informed consent is granted by the patient, cardiovascular consultation is obtained to assist in the implementation of strategies felt to lower the risk during MRI, and proper evaluation and monitoring of the patient takes place before, during and after a scanning event. New technologies may make MRI safer for pacemaker patients.27