Overview

Familiarity with anesthesia equipment, the

∗ Portions of this chapter are from Mason KP, Holzman RS: Anesthesia and sedation for procedures outside the operating room. In Davis PJ, Cladis F, Motoyama EK, editors: Smith’s Anesthesia for Infants and Children , ed 8, Philadelphia, 2011, Elsevier, pp 1041-1057.

In 1986, the American Society of Anesthesiologists (ASA) first established standards for monitoring during anesthesia and subsequently delineated specific standards for the delivery of anesthesia and sedation in sites distant to the operating room (OR). Recognizing that not all care in areas outside the OR require anesthesia, the ASA and other nonanesthesia specialty societies followed with guidelines for sedation and analgesia. In the 1990s, the American Academy of Pediatrics (AAP), American College of Emergency Physicians (ACEP), and American Academy of Pediatric Dentists (AAPD) published sedation guidelines. In 1996, the ASA introduced their guidelines for the administration of sedation and analgesia by nonanesthesiologists; these guidelines were updated in 2010.

The Joint Commission has published guidelines to encourage uniformity and documentation of monitoring at locations throughout the hospital. The delivery of anesthesia and sedation in the MRI environment would fall under the guidelines and regulations not only of the ASA but also of The Joint Commission, other specialty organizations, and the Center for Medicaid and Medicare Services (CMS). In 2009 the CMS published its Revised Hospital Anesthesia Services Interpretive Guidelines—State Operations Manual (SOM); these were subsequently updated in 2010 and 2011. This document defined deep sedation as falling under anesthesia services. The term deep sedation was clearly defined: “a drug-induced depression of consciousness during which patients cannot be easily aroused but respond purposefully following repeated or painful stimulation. The ability to independently maintain ventilatory function may be impaired. Patients may require assistance in maintaining a patent airway, and spontaneous ventilation may be inadequate. Cardiovascular function is usually maintained.”

Accordingly, the CMS guidelines indicate that the administration of deep sedation should be limited to a 1) qualified anesthesiologist; 2) doctor of medicine or osteopathy (other than an anesthesiologist); 3) dentist, oral surgeon, or podiatrist qualified to administer anesthesia under state law; 4) certified registered nurse anesthetist (CRNA), or 5) anesthesiologist’s assistant under the supervision of an anesthesiologist who is immediately available if needed. These guidelines triggered the restructuring of those sedation programs that had been delivering sedation by providers no longer approved by the CMS. One such example is Boston Children’s Hospital, which in November 2010 replaced registered nurses with nurse anesthetists, pediatricians, and anesthesiologists as the providers of deep sedation. Coincident with this restructuring, anesthesiologists have now become more prevalent providers in the MRI environment with a coincident sensitivity to the importance of establishing guidelines for safe practices in the MRI environment.

Many of the challenges encountered with anesthesia delivery and physiologic monitoring in the MRI environment have now been addressed. Technologic advances over the past decade have yielded monitoring, equipment, and devices that are more consistent, reliable, and valuable in the magnetic environment. As anesthesiologists become more prevalent in the MRI environment, they have coincidentally taken a more active role in the design, layout, space allotment, and equipment acquisition of new MRI units.

This chapter outlines some of the basic principles of magnetic fields as well as the challenges and limitations to delivering anesthesia, deep sedation, or monitored anesthesia in the MRI environment. It includes a detailed review of the latest monitors, equipment, and devices.

MRI Physics and Clinical Applications

Imaging by magnetic resonance was first described by Rabi in 1939, the first human images were produced in 1977, and the first commercial scanner was introduced in 1990. The physics behind MR imaging relies on the integrity of the magnetic field. Atoms with an odd number of protons and/or neutrons are capable of acting as magnets. When they are aligned in a static magnetic field, they can be subjected to radiofrequency (RF) energy that alters their original orientation. With removal of the RF pulse, the nuclei rotate back to their original alignment (relaxation), and the energy released can be detected and transformed into an image. Hydrogen is the atom most often used for imaging, as it is present in most tissues as water and long-chain triglycerides.

The application of MRI studies is far reaching, and MRI is used for the evaluation of neoplasms, trauma, skeletal abnormalities, and vascular anatomy. Brain MRIs are frequently performed to evaluate developmental delay, behavioral disorders, seizures, failure to thrive, apnea/cyanosis, hypotonia, and mitochondrial and metabolic disorders. MR angiography (MRA) and MR venography (MRV) are especially helpful in evaluating vascular flow and can sometimes replace invasive catheterization studies for follow-up or initial evaluations of vascular malformations, interventional treatment, or radiotherapy. Functional MRI (fMRI) is an evolving technology that measures the hemodynamic or even metabolic response related to neural activity in the brain or spinal cord, and fMRI is often able to localize sites of brain activation. As a result, it is now dominating brain mapping techniques because of its low invasiveness and lack of radiation exposure.

Some fMRI studies require cognitive facility that demands a conscious and responsive patient. It is unclear how fMRI studies of children unable to respond appropriately, either because of age or cognitive compromise, will evolve in pediatric anesthesiology practice. Recently, dynamic MRI airway studies have used three-dimensional (3D) reconstruction of the images to visualize areas of airway collapse, tracheomalacia, or compromise.

Historically, anesthetic management of children in the MRI suite has been highly dependent on and somewhat limited by the availability of monitors and anesthesia gas machines suitable for use in an MR suite. The American College of Radiology (ACR) established guidelines to minimize the risk of MRI-related mishaps but did not address the needs of the anesthesiologist. These guidelines were written in response to fatalities that occurred when loose, ferromagnetic oxygen cylinders became projectiles when inadvertently brought into the MRI suite with a patient in the bore of the magnet. In 2008 the ASA assembled a task force composed of anesthesiologists and a radiologist with MRI expertise. This Task Force on Anesthetic Care for Magnetic Resonance Imaging created a practice advisory on anesthetic care for magnetic resonance imaging, in which the MRI environment was acknowledged as a hazardous environment that challenges both the delivery of anesthesia and the ability of the anesthesiologist to monitor the patient. This document establishes important recommendations for safe practice and consistency of anesthesia care in the MRI environment. Most important, this practice advisory includes all sedation, monitored anesthesia care, general anesthesia, critical care, and ventilatory support. Conditions of high-risk imaging were defined as imaging in patients with risks related to health, equipment, procedure, or surgery. MRI-guided interventions, cardiac imaging, and airway imaging were all identified as high-risk procedures. The advisory was designed to promote patient and health care provider safety, prevent MRI mishaps, recognize limitations in physiologic monitoring, optimize patient management, and identify potential equipment and health risks.

Anesthetic management depends on the availability of support personnel, equipment, and monitors; the personal style and comfort level of the anesthesiologist; and, of course, the patient’s medical history. Requiring a general anesthetic solely to ensure motionless conditions for a radiologic imaging study is often a frightening concept for parents to embrace. They frequently equate pain and surgical interventions to the need for anesthesia and are reluctant to expose their child to a general anesthetic for an MRI study. Children younger than 5 years most commonly require moderate to deep sedation or an anesthetic to ensure motionless conditions. Pentobarbital, chloral hydrate, propofol, and dexmedetomidine have all been described as successful alternatives to inhalation anesthesia.

In the presence of a magnetic field, anesthesiologists must be aware of many personal items, usually taken for granted, that can be a hazard: clipboards, pens, watches, scissors, clamps, credit cards, eyeglasses, paper clips, and so on. As early as 1985, up to 24% of all MRI centers cited a projectile-related accident. However, projectiles are not the only concern. Conventional electrocardiographic (ECG) monitoring is not possible during MRI because as the lead wires traverse the magnetic field, image degradation occurs; most importantly, however, the ECG leads may inductively heat and cause patient burns. Fiberoptic ECG monitoring is necessary to minimize the risk of patient burns; but even with fiberoptic cables, it is important to recognize that the connections between the ECG pads and the telemetry box are still hard wired, and careful attention must be paid to prevent frays, overlap, exposed wires, and knots in the cables.

To prevent patient injury, care must be used to avoid creating a conductive loop between the patient and a conductor (ECG monitoring/gating leads, plethysmographic gating wire, and fingertip attachment). During the scan, no exposed wires or conductors can touch the patient’s skin, and no imaging coil can be left unconnected to the magnet. Pulse oximeters are also not conventional, rather they are fiberoptic. Failure to remove the conventional pulse oximeter probes and adhesives has resulted in second- and third-degree burns.

Average noise levels of 95 dB have been measured in a 1.5 Tesla (T) MRI machine, comparable to noise levels of very heavy traffic (92 dB) or light road work (90 to 110 dB). Exposure to this level of noise has not been considered hazardous if limited to less than 2 hours per day, although both temporary and permanent hearing loss after an MRI scan have been reported. The 3-T magnet offers the advantage of less image degradation and improved neuroskeletal and musculoskeletal imaging. As the field strength increases, so does the noise. In fact, the peak sound-pressure level of a 3-T magnet exceeds 99 dB, the level approved by the International Electrotechnical Commission. Noise reduction did not differ between earplugs and headphones, although the combination of both was more effective at reducing sound. Earplugs or MRI-compatible headphones should be offered to all pediatric patients and are required for all patients imaged in the 3-T magnet.

Additional challenges in the 3-T environment occur with fMRI: the noise of the magnet can interfere with the acoustic stimulation generated for purposes of obtaining the fMRI. Video goggles compatible with the 1.5-T and 3-T environments (Resonance Technology, Los Angeles) may be worn by the patient during MR imaging to provide a 3D virtual reality system complete with audio integration (see MRI Video Goggles section below). The introduction of this integrated audio-video headset has revolutionized the ability to offer distraction to patients, so that many more patients are now able to tolerate imaging without adjuvant sedation or anesthesia.

Although studies in mice and dogs suggest that exposure to magnetic fields may increase body temperature, it is unlikely that static magnetic fields up to 1.5 T have any effect on core body temperature in adult humans, although this may be of greater concern in infants and small children, such as in cardiac MRI studies that require a long scan time. The specific absorption rate (SAR) is measured in watts per kilogram and is used to follow the effects of RF heating. The Food and Drug Administration (FDA) allows an SAR of 0.4 W/kg averaged over the whole body. Ex vivo exposure of large metal prostheses to fields over six times that experienced in MRI have not revealed any appreciable heating. To date, no conclusive evidence has shown that RF is a significant clinical issue in magnets up to 3 T.

The biologic effects of MRI should be considered when offering parents of pediatric patients the opportunity to be present in the MRI suite during the induction of anesthesia or throughout a deep sedation. Although there are no reports implicating MRI in chromosomal aberrations, pregnant women, particularly during the critical periods of organogenesis, are generally not permitted in the MRI environment except in cases of a need for an emergent study or for fetal imaging. MRI scans during pregnancy are discouraged by the ACR during the first and second trimester, unless fetal imaging is required or the MRI is necessary for emergent medical care. Studies in amphibians demonstrate that exposure to magnetic fields as high as 4 T do not cause any defects in embryologic development, and most hospital MRI machines are 1.5 to 3 T.

Important MRI safety issues include implanted objects (i.e., cardiac pacemakers), ferromagnetic attraction creating “missiles,” noise, biologic effects of the magnetic field, thermal effects, equipment issues, and claustrophobia. Some stainless steel may contain ferritic, austenitic, and martensitic components. Martensitic alloys contain fractions of a crystal phase known as martensite, which has a body-centered cubic structure, is prone to stress corrosion failure, and is ferromagnetic. Austenite is formed in the hardening process of low-carbon and alloyed steels and has ferromagnetic properties. Iron, nickel, and cobalt are also ferromagnetic. For this reason, the components of any implanted device should be carefully researched prior to entering the magnet.

In addition, an external magnetic field may exert translational (attractive) and rotational (torque) forces on stainless steel or surgical stainless objects. Intracranial aneurysm clips, cochlear and stapedial implants, shrapnel, orthodontic devices (braces), intraorbital metallic bodies, and prosthetic limbs may move and potentially dislodge. Special precautions should be taken with cochlear implants in the 3-T environment, because those nonremovable magnets may suffer demagnetization in the scanner. Patients implanted with such devices should only undergo 3-T imaging after special precautions are taken.

It is important to note that some eye makeup and tattoos may contain metallic dyes and may therefore cause ocular, periorbital, and skin irritation. In addition, some tissue expanders used in reconstructive surgery have a magnetic port to help identify the location for intermittent injections of saline. Bivona tracheostomy tubes usually contain ferrous material, although this is not specified in the package insert; these should be replaced with a Shiley tube prior to entering the MRI environment.

In the presence of an external magnetic field, ferromagnetic objects can develop their own magnetic field and become projectiles. The attractive forces created between the intrinsic and extrinsic magnetic fields can propel the ferromagnetic object toward the MRI scanner. Special note should be made of the magnet strength. Over the past few years, 1.5-T magnets have been supplanted by 3-T magnets. The field strength and magnetic force generated by a 3-T scanner is unforgiving to the careless or inadvertent introduction of a ferrous object into the environment. Placing a magnet outside the MRI scanner can be a crude, helpful, and sometimes inaccurate way to test an object. If the object is not attracted to the magnet, this is not an absolute indication that no ferrous material is present. More sophisticated and sensitive detectors of ferrous material have been introduced that include hand-held detectors and walk-through detectors, similar to those at airports. None of these methods of detection should supplant or eliminate the careful, methodical, face-to-face screening that should be performed on everyone and everything prior to entry into the MRI suite. Some unusual objects that have found their way into the MRI suite to become projectiles include a metal fan, pulse oximeter, shrapnel, wheelchair, cigarette lighter, stethoscope, pager, hearing aid, vacuum cleaner, calculator, hair pin, oxygen tank, prosthetic limb, pencil, insulin infusion pump, keys, watches, and steel-tipped and steel-heeled shoes.

Small objects can usually be easily removed from the magnet, but large objects may be subject to so much attractive force from the magnet that it is impossible to remove them by manual force. In these circumstances, the only way to release an object attached to the scanner is by quenching the magnet, which will eliminate the magnetic field over a matter of minutes. This process is not without substantial risk: as helium gas is vented, condensation and considerable noise fills the suite. All personnel are required to vacate the suite during a quench because of a risk of hypoxic conditions, should the helium inadvertently enter the room. The specific indications for quenching a magnet were detailed in 1997 in the ACR’s guidance document for safe MR practices: “Quenching the magnet (for superconducting systems only) is not routinely advised for cardiac or respiratory arrest or other medical emergency, since quenching the magnet and having the magnetic field dissipate could easily take more than a minute. Because of the risks to personnel, equipment, and physical facilities, manual magnet quenches are to be initiated only after careful consideration and preparation.”

Cardiac pacemakers present a special hazard in and around the MRI scanner, especially in patients who are pacemaker dependent. Most pacemakers have a reed relay switch that can be activated when exposed to a magnet of sufficient strength, which could convert the pacemaker to the asynchronous mode. This is an extremely dangerous situation: At least two cases are known of patients with pacemakers who died from cardiac arrest in an MRI scanner. The autopsy of one patient determined that the death was the result of an interruption of the pacemaker in the magnetic environment.

In addition to the risk of pacemaker malfunction, the risk exists for torque on the pacer or pacing leads to create a disconnect or microshock. Recent studies demonstrate that with careful preparation, select patients with permanent pacemakers and implantable cardioverter-defibrillators may safely undergo imaging in the 1.5-T environment without any inhibition or activation of their device. In February 2011, the first and only MRI conditional pacemaker was approved by the FDA. The Revo MRI SureScan Pacing System (Medtronic, Minneapolis, MN) is available for between $5000 and $10,000 and requires specific training of the cardiologist and radiologist prior to use.

The ACR formed a Blue Ribbon Panel on MRI Safety in 2001 to review existing safety practices and issue new guidelines. ACR guidance documents were published in 2002 and updated in 2004 and again in 2007. Pediatric safety concerns were specifically addressed in the 2007 document, with specific emphasis on patient screening, sedation, and monitoring issues. Implanted cardiac pacemakers or implantable cardioverter-defibrillators (ICDs) were considered a relative contraindication to MRI, and patients with such devices should only be scanned in locations staffed with radiologists and cardiologists of appropriate expertise. The recommendation was for radiology and cardiology personnel, along with a fully stocked emergency cart, to be readily available with a programmer to adjust the device if necessary. Following the MRI, a cardiologist should confirm function of the device and recheck it within 1 to 6 weeks. In general, heart valves are not ferromagnetic and are not a contraindication to MRI. It is critical that everyone entering the vicinity of the MRI scanner fill out a screening form that specifically lists every possible implantable device, alerting the MRI staff to any potential hazards.

It is important to note that the magnetic field may affect the ECG. Changes in the T wave are not due to biologic effects of the magnetic field but rather to superimposed induced voltages. This effect of the magnetic field on the T wave is not related to cardiac depolarization because no changes to the P, Q, R, or S waves have ever been observed in patients exposed to fields up to 2 T, and no reports have been made of MRI affecting heart rate, ECG recording, cardiac contractility, or blood pressure. One study, however, found that humans exposed to a 2-T magnet for 10 minutes developed a 17% increase in the cardiac cycle length (CCL), which represented the duration of the R-R interval. The CCL reverted to preexposure length within 10 minutes of removing the patient from the magnetic field. The implications of this finding are unclear, and this change in CCL in patients with normal hearts may be of no consequence; however, the implications of this finding for patients with fragile dysrhythmias or sick sinus syndrome have yet to be determined.

In 2008, The Joint Commission recognized the potential and existent hazards of the MRI environment when they published a Sentinel Event Alert that identified five MRI-related incidents and four MRI-related deaths, one from a projectile and three from cardiac events. The alert specifically identified eight types of possible injury and was designed to prevent accidents and injuries in the MRI suite ( Box 27-1 ).

It is important to recognize the ACR designation of the different zones as they pertain to the proximity of the MRI scanner. In 2002, four zones were defined relative to the magnet that were important in identifying and demarcating the safety precautions that should be taken for each zone ( Table 27-1 ).

TABLE 27-1

Zone Definitions

From Practice advisory on anesthetic care for magnetic resonance imaging: a report by the Society of Anesthesiologists Task Force on Anesthetic Care for Magnetic Resonance Imaging. Anesthesiology 2009; 110:459-479.

Zone I	All areas that are freely accessible to the general public. This area is typically outside the MR environment itself and is the area through which patients, health care personnel, and other employees of the MR site access the MR environment.
Zone II	The interface between the publicly accessible uncontrolled zone I and the strictly controlled zone III. Typically, the patients are greeted in zone II and are not free to move throughout zone II at will but rather are under the supervision of MR personnel. It is in zone II that patient histories, answers to medical insurance questions, and answers to MR imaging screening questions are typically obtained.
Zone III	The region in which free access by unscreened non-MR personnel or ferromagnetic objects or equipment can result in serious injury or death as a result of interactions between individuals or equipment and the MR scanner’s particular environment. These interactions include but are not limited to those with the MR scanner’s static and time-varying magnetic fields. All access to zone III is to be strictly restricted, with access to regions within it, including zone IV (see below), controlled by and entirely under the supervision of MR personnel.
Zone IV	The MR scanner magnet room. By definition, zone IV will always be located within zone III, because it is the MR magnet and its associated magnetic field, which generates the existence of zone III.

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