The use of implantable pacemakers and implantable cardioverter-defibrillators (ICDs) has increased dramatically, due in part to expanded indications for their use and the aging of society. An estimated 75 percent of patients who currently have an implantable cardiac electronic device will need magnetic resonance imaging (MRI) during their lifetimes.
In the past, MRI was contraindicated in all patients with implantable cardiac devices because of concerns that the powerful magnetic and radiofrequency fields generated during imaging might:
- Damage device components
- Inhibit pacemaker function
- Trigger rapid pacing
- Deliver inappropriate shocks
In recent years, several centers have begun offering MRI to patients with cardiac pacemakers. The current guidelines from the American Heart Association and the Food and Drug Administration (FDA), however, do not support MRI in patients with pacemakers, nor do any of the device manufacturers' guidelines (except for new MRI-conditional devices).
"The American College of Radiology recognizes that MRI in patients with pacemakers is never routine and should be conducted only when the case is properly triaged and deemed medically necessary and when alternative radiologic methods have not been diagnostic," according to Robert E. Watson Jr., M.D., Ph.D., a radiologist at Mayo Clinic in Rochester, Minn. "Also, it is stipulated that there is cardiology pacemaker support and careful pacemaker and physiologic monitoring during the MRI, as well as MRI physicist support during imaging."
The first MRI-conditional pacemaker received FDA approval for use in the United States in February 2011. This first-generation device has important limitations:
- It requires a special lead system, so the generator cannot be simply replaced and connected to in situ intracardiac leads.
- Cardiac MRI is excluded because of potential overheating of the new lead system (second-generation devices currently available in Europe use a lead system that is compatible with cardiac MRI).
- Additionally, the first-generation devices are limited to 1.5-tesla scanners.
"While it is likely that, in the next decade, MRI-conditional pacemakers and possibly ICDs will become standard, there is a large population of patients who in the interim may require MRI scanning," says Win-Kuang Shen, M.D., a cardiac electrophysiologist and chair of the Division of Cardiovascular Diseases at Mayo Clinic in Arizona. Recent studies have suggested that MRI can be done safely in many patients with standard cardiac pacemakers.
Physicians from the Department of Radiology and the electrophysiology group in the Division of Cardiovascular Diseases at Mayo Clinic in Arizona and Minnesota devised a pilot protocol for patients with standard pacemakers in whom MRI was the preferred imaging modality:
- After device interrogation, prospective patients met with an electrophysiologist to determine whether the patient was pacemaker dependent and the relative need for pacing under baseline conditions.
- Only nondependent patients with mature lead systems (longer than 90 days) were considered.
- Patients with devices that demonstrated inadequate function (such as high capture threshold, high pacing impedance or depleted battery voltage) were excluded.
- Although initially only head MRI was performed, later in the protocol body scanning was allowed.
According to the protocol, the pacemaker is programmed in an asynchronous mode at the intrinsic heart rate plus 20 beats per minute prior to scanning. Patients are supervised by a cardiologist or pacemaker nurse through the procedure. Pulse oximetry and ECG are monitored. The device is then reprogrammed to original settings after the scan is complete.
The MRI pulse sequences are determined by the radiologist and the physicist. "All the exams take place on a 1.5-tesla magnet, and the specific absorption rate is limited to 1.5 W/kg for a maximum of 30 minutes," according to Joel P. Felmlee, Ph.D., a radiation physicist at Mayo Clinic in Minnesota. "If pre-scan sequences demonstrate heart rate synchronization to the transmission-receive interval of the radiofrequency coil, the scan is discontinued."
No clinically adverse events have been noted. Power-on resetting (POR) and magnet mode pacing have been observed in some patients and seem to be device specific. Premature ventricular contractions have been observed, but they have been clinically insignificant. Lead model has not been predictive of abnormal pacing function during MRI studies, nor has region of the body scanned.
All pacing abnormalities appear to have been transient and reversible. No effects on generator voltage or lead function have been observed either immediately after scanning or at one-month follow-up.
During POR, battery voltage declines to less than a critical preset level (the trip level) at which point operation of the device is unpredictable. After recovery of battery voltage, devices typically reset to the manufacturer's nominal settings. "Because of the potential for POR and the unpredictability of pacemaker function during MRI scanning, patients with pacemakers should not undergo MR imaging," says Dr. Shen.
Magnet mode pacing occurs as a result of reed-switch activation by the magnetic field generated during MRI. Theoretically, programming the device to an asynchronous mode should prevent reversion; however, magnet-mode pacing has been seen during scanning despite asynchronous programming and could initiate arrhythmias.
The Centers for Medicare and Medicaid Services has approved reimbursement for MRI in patients with the new MRI-conditional pacing system. However, it continues not to reimburse for MRIs performed in patients with other pacemaker systems.
The Mayo Clinic Department of Radiology has been providing this service to patients in need of MRI regardless of reimbursement issues.
Points to remember
- MRI in patients with non-MRI-conditional devices should be considered only if the patient is not pacemaker dependent.
- MRI should not be performed if there is evidence of generator or lead malfunction.
- MRI in selected patients with ICDs is currently under investigation.