Anesthetic Care for Magnetic Resonance Imaging Reviewed

-Medscape

03/08/2009 - The American Society of Anesthesiologists (ASA) Task Force on Anesthetic Care for Magnetic Resonance Imaging (MRI) has issued a practice advisory in this area and published it in the March issue of Anesthesiology. Anesthetic care for MRI encompasses provision of moderate and deep sedation, monitored anesthesia care, general anesthesia, and ventilatory and critical care support.

"The...MRI suite is a hazardous location because of the presence of a strong static magnetic field, high-frequency electromagnetic (radiofrequency) waves, and a time-varied (pulsed) magnetic field," write co-chair Jan Ehrenwerth, MD, from Madison, Connecticut, and colleagues from the American Society of Anesthesiologists Task Force on Anesthetic Care for Magnetic Resonance Imaging. "Secondary dangers of these energy sources include high-level acoustic noise, systemic and localized heating, and accidental projectiles."

To facilitate decision making in patient care areas, the ASA systematically develops practice advisories based on a review of published studies and other evidence, as well as analysis of expert and practitioner opinion, clinical feasibility data, open requests for comments, and consensus surveys.

These practice advisories are not intended as guidelines or absolute standards because they are not supported by scientific literature to the same extent, given the lack of sufficient numbers of adequately controlled studies. As medical knowledge, technology, and practices advance, the ASA may periodically revise practice advisories.

An expert group of 13 task force members, including 10 anesthesiologists, 1 radiologist, and 2 consulting methodologists, wrote this ASA practice advisory on anesthetic care, which aimed to enhance patient and staff safety, prevent unintentional injuries, address hazards, and identify limits of monitoring.

This practice advisory focuses on level II facilities, in which patients are receiving monitoring or life support, or level III facilities, in which operative procedures are performed. Imaging can be classified as high risk when it involves patients with medical risks, imaging with equipment-related risks, and procedure-related risks.

"Unlike a conventional operating room, the MRI environment frequently requires the anesthesiologist to assume broader responsibility for immediate patient care decisions," the study authors write.

Both static and dynamic magnetic fields, as well as radiofrequency energy emissions, caused by the MRI equipment may pose significant challenges to providing anesthesia and to monitoring the patient. Noise, dim lighting, obstructed line of sight, various distractions, and other characteristics of the MRI environment may hinder direct patient observation.

The MRI suite contains 4 zones. The public has free access to zone I. Zone II is uncontrolled but leads to zone III, which is controlled and restricted because it poses risks for potentially serious injury or death. The MR scanner room itself, with the associated magnetic field, is zone IV.

To ensure safety in anesthetic care for MRI, patient risks regarding medical conditions or implanted devices must be addressed, as well as risks involving equipment and potential emergencies. The anesthesiologist must promote safety through adequate monitoring, anesthetic care, airway management, and prompt and effective response to emergencies.

For patients with high-risk conditions, those who require equipment during imaging, or those who have implantable devices, open communication is essential between the anesthesiologist and the patient, referring clinician and radiologist. The anesthesiologist should evaluate the patient to ascertain if there is a high-risk condition and should control management of high-risk patients during MRI.

Patients at increased risk because of clinical conditions include preterm infants, patients with hemodynamic instability or comorbid illnesses, and those undergoing critical care or changes in level of sedation.

During MRI, neonates may experience greater variability in heart rate, blood pressure, and oxygen saturation. When MRI is performed with gadolinium, the risk for subsequent nephrogenic systemic fibrosis is increased in patients with impaired renal function. Gadolinium should therefore be withheld in patients with acute or severe renal insufficiency.

Patient equipment presenting additional challenges and hazards during MRI include implanted pacemakers, physiologic monitors, and equipment for intubation oxygenation and ventilation equipment. Patients with pacemakers and implantable cardioverter-defibrillators generally should not have MRI because these devices pose an extreme, potentially fatal hazard.

Because of energy transfer to other devices during MRI, these may cause tissue damage, or the device may malfunction or be displaced. Although anesthesiologists should be aware of and document the presence of deep brain stimulators, vagal nerve stimulators and cochlear implants, MRI may be performed on a limited basis for these electronic devices.

The primary risk to safety inherent in MRI is that related to the effect of the magnetic field on ferromagnetic items and implantable devices. These devices should not be permitted in zone III or zone IV of the MRI suite. Ferromagnetic items include aneurysm or surgical clips, prosthetic heart valves, stents, and dental magnetic keepers; these items may become overheated when exposed to the magnetic field.

Eyeliner tattoos may cause image artifacts, burns, swelling, or puffiness, and hemorrhage may result from exposure of iron filings to the magnetic field.

Additional safety precautions that anesthesiologists should follow include use of MRI safe/conditional monitors, remote monitoring, and compliance with ASA standards. During the scan, the anesthesiologist should have access for optimal patient observation. Additional personnel and resources should be available to the anesthesiologist for rapid response if needed in an emergency. These may include medical emergencies, such as cardiopulmonary arrest, and environmental emergencies, such as fire or accidental projectiles. The anesthesiologist should also be able to communicate with the MRI technologist and the facility director.

Evidence to date is insufficient regarding potential long-term hazards of occupational exposure to MRI. However, some data suggest that pregnant and nonpregnant workers are not at increased risk.

"The anesthesiologist should collaborate with the radiologist and other staff in the postanesthetic care of the patient," the study authors conclude. "Patients receiving sedation or anesthesia within the MRI suite should have access to postanesthetic care consistent with that provided in other areas of the institution, including transport to other recovery rooms, dedicated intensive care, or recovery areas within the MRI suite. In all situations, intensive care and recovery areas should include access to vital sign monitors, oxygen, suction, resuscitation equipment, and trained personnel."

The advisory was supported by the ASA and approved by the House of Delegates on October 22, 2008.

Anesthesiology. 2009;110:459-479.

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