For all patients requiring general anesthesia or monitored anesthesia care (MAC) in locations remote from the operating room, the same principles and requirements for anesthesia equipment, monitoring standards, patient preparation, and postanesthetic care outlined in Chapters 1, 9, 10, and 36, respectively, should be met.

1. The anesthetist must determine that all standards are met before initiation of an anesthetic.

2. A central supply of oxygen and suction is a minimum requirement: two independent supplies of oxygen and suction (patient use and waste gas scavenging) are required at an anesthetizing location. In addition, a full reserve tank of oxygen must be available for each case. For locations that do not have a central supply of nitrous oxide, a reserve tank is available on the machine. Adequate lighting and electrical power outlets connected to the emergency power supply are required. A source of medical grade compressed air is desirable but not mandatory.

3. Functioning anesthesia machine appropriate for the anesthetic. Extralong gas supply hoses may be necessary; anesthesia breathing circuit tubing may require extensions to reach the patient.

4. Anesthesia supply cart should be readily available, containing appropriate supplies and drugs necessary for provision of anesthesia.

5. Resuscitation equipment must be immediately available: defibrillator, medications, and self-inflating hand resuscitator bag for transport.

1. Adequate space for the anesthesia machine, equipment, and access to the patient is required.

2. Areas outside the operating room where anesthesia is performed regularly should be designated as “approved anesthetizing locations” by the hospital.

3. A direct means of communication is necessary in case of emergency.

4. Monitoring must be adapted if the anesthetist cannot remain in the room (e.g., during irradiation); viewing the patient by window or closed circuit television may be necessary. Anticipate and provide necessary monitoring during patient transport.

5. Patient positioning may be difficult in confined spaces of magnetic resonance imaging (MRI) and computed tomography (CT) scanners. Additional padding may be necessary to prevent injury from compression of soft tissues during prolonged procedures.

6. Imaging often requires that the patient repeatedly moves a significant distance during the scan. Adequate lengths of ventilation, intravenous (IV), and monitoring cables are necessary; a “test move” of full excursion of patient movement is helpful before the procedure begins.

7. Anesthetists should take appropriate precautions to minimize radiation exposure to themselves during procedures.

1. Specially trained nurses provide sedation for most patients requiring invasive procedures outside of the operating room. Procedural sedation is defined as a medically controlled state of depressed consciousness that allows protective reflexes to be maintained and retains the patient’s ability to maintain a patent airway and to respond appropriately to physical and verbal stimulation. American Society of Anesthesiologists (ASA), Joint Commission on Accreditation of Healthcare Organizations, and state agencies (licensing boards) have set guidelines for provision of conscious sedation by nonanesthesiologists and nonphysicians.

2. MAC is a physician-directed service that may include the provision of sedatives and analgesics. It requires constant assessment and management of the patient’s medical problems and physiologic derangements with the ability to convert to general anesthesia if needed. When available, capnography should be used as an aid to detect apnea, especially during pediatric cases. Postprocedural patient management is required until adequate recovery from the procedure and sedation/anesthesia. An anesthesiologist is required to provide sedation for any patient in which airway management is considered complex (i.e., mask ventilation anticipated as difficult or impossible or potentially difficult intubation) or if significant comorbid pathology exists (i.e., ASA physical status classes III and IV) that require medical management by a physician.

A. Contrast Media. Ionic and nonionic contrast media are administered intravenously and intra-arterially to supplement imaging; complexed gadolinium may be administered for both MRI and x-ray-based imaging. Hypo- and isoosmolar iodinated media are used instead of hyperosmolar because they are considered less nephrotoxic. Contrast material can produce a brisk diuresis, and bladder catheterization may be necessary.

B. Acute Contrast Media Reactions. Serious or fatal reactions are rare but unpredictable and are not dose related. They are considered anaphylactoid because they possess features of anaphylaxis but are not IgE mediated.

1. Risk factors include history of previous adverse reaction, asthma, hay fever allergy requiring medical therapy, and concurrent use of β-blockers or interleukin-2.

2. Symptoms develop within 5 to 30 minutes of exposure and present as generalized skin reactions, airway obstruction, angioedema, or cardiovascular collapse.

3. Treatment of acute reactions is supportive. Generalized anaphylactoid reactions should be treated with immediate administration of corticosteroids and H₁ and H₂ blockers. Oxygen, epinephrine, β₂-agonists, and intubation may be necessary to treat bronchospasm and laryngeal edema; circulation is supported with IV fluids and vasopressors.

4. Routine prophylaxis of all patients is not supported by all authors. Strategies commonly used include prednisone 50 mg PO given 13, 7, and 1 hour prior to imaging with 50 mg diphenhydramine 1 hour prior to contrast administration. Hydrocortisone 200 mg IV can be substituted for oral prednisone in patients who cannot take oral medications. Alternatively, methylprednisolone 32 mg is given orally 12 and 2 hours prior to contrast administration with or without antihistamine. For emergency procedures, treatment with 50 mg of diphenhydramine IV and 200 mg of hydrocortisone IV or 40 mg of methylprednisolone every 4 hours until the procedure is completed has been used successfully.
Patients with methylprednisolone, aspirin, or NSAID allergies, especially if they have a coexisting diagnosis of asthma, should be pretreated with 7.5 mg of dexamethasone or 6 mg of betamethasone every 4 hours until the study has been completed.

1. N-acetylcysteine is often administered before and after the procedures to reduce the incidence of contrast-induced nephropathy. Data supporting this practice continue to be controversial. Periprocedural hydration is generally recommended with crystalloid, though no optimal dosing regimen is supported. Data to support hydration with sodium bicarbonate rather than sodium chloride continue to be equivocal.

2. Gadolinium. Nephrogenic systemic fibrosis is a sclerosing skin condition that occurs in patients with advanced or end-stage renal disease, with or without dialysis. This typically occurs days or months following a gadolinium exposure.

A. CT scans are usually performed without general anesthesia. Children and uncooperative adults (e.g., head-injured patients) may require sedation or general anesthesia to minimize motion artifacts; if so, standard monitors as outlined in Chapter 10 are required. Capnography is useful to provide evidence of ventilation during sedation; fitting a sidestream sampling tube to nasal cannula or oxygen facemask provides qualitative assessment of ventilation.

B. Adults. Small IV doses of benzodiazepines, narcotics, or short-acting hypnotics (e.g., propofol or dexmedetomidine) are useful for sedation; continuous infusions should be titrated to effect.

C. Infants and children less than 3 months of age may not need sedation; most children, however, will require some level of sedation or general anesthesia. Children provide specific challenges to the anesthetist as outlined in Chapter 31.

a. Small IV doses of benzodiazepines, narcotics, or short-acting hypnotics (e.g., propofol) may be used for sedation as either bolus or continuous infusions titrated to effect.

b. Chloral hydrate (30 to 50 mg/kg orally or per rectum [PR] administered 30 to 60 minutes before the procedure) is an adequate mild sedative for children.

c. Methohexital, PR (25 to 30 mg/kg), has more rapid onset (5 to 10 minutes) than does chloral hydrate and lasts approximately 30 minutes. It is useful for the induction of general anesthesia; however, effects may vary because absorption is unpredictable. Because deep sedation or general anesthesia may occur, only an anesthetist should administer methohexital with appropriate monitoring and provisions to secure the airway. The drug is not appropriate in patients at risk of reflux of gastric contents.

2. General anesthesia maintained with either IV or inhalational agents may be required. The airway may be maintained naturally, with laryngeal mask airway (LMA) or endotracheal intubation as necessary.

A. The physical environment of the MRI suite presents several challenges for anesthetizing patients.

1. The long, narrow bore of the magnet in which the patient reclines does not allow ready access to or viewing of the patient during imaging.
Scanners are located in shielded rooms that contain the magnetic field and shield against radiofrequency noise that would produce image artifact.

2. The static magnetic field is present at all times and exerts a force on all ferromagnetic materials (e.g., steel gas tanks, batteries, and standard stethoscopes). Ferromagnetic objects brought near the magnetic field can be forcibly pulled toward the magnet, potentially injuring people or equipment in their path. The static field and magnetic gradients generated during scanning can interfere with mechanical components (solenoids) in automated noninvasive blood pressure monitors, ventilators, and infusion pumps; specialized compatible equipment is necessary. Only plastic stethoscopes and commercially available magnet-compatible laryngoscopes should be used in the magnet area. Credit cards, watches, and pagers must be left outside the scanning room.

3. Radiofrequency signals and shifting magnetic fields generated during scanning may lead artifacts on the ECG and pulse oximeter.

4. Metallic implants (e.g., joint prostheses, aneurysm clips, and cochlear implants) or implanted devices (e.g., pacemakers, implantable cardioverter defibrillators [ICDs], insulin infusion pumps, intrathecal pumps, or spinal cord stimulators) may potentially be dislodged, dysfunction, or suffer permanent damage by the magnetic field, and scanning or heating may occur from radiofrequency signals generated during scanning. Some specific pacemakers, ICDs, or pulmonary artery catheters are considered “MRI safe,” and patients with these devices may undergo MRI scanning. Imaging of patients with these devices should not be considered routine, and individual cases should be carefully reviewed for medical necessity. Imaging may require that the pacemaker be turned off while the patient is in the magnetic field; the device is turned on after scanning. Cerebral aneurysm clips are not considered an absolute contraindication to MRI; it is important, however, to identify the type of clip present to determine MR compatibility. Not all clips are compatible. Each MRI site carries a list of medical devices designated MRI compatible by the U.S. Food and Drug Administration (FDA). Because medical devices may be upgraded or altered by a manufacturer without notifying the FDA, MRI centers should, in addition, contact the manufacturer if questions arise about specific devices.

B. Monitors must be safe for the patient, function within the magnetic field, and have a minimal effect on imaging. Specialized compatible monitoring equipment is available that can remain in the magnetic field and communicate to a “slave” monitor outside the shielded magnet area.

1. The standard ECG is subject to interference during scanning.

2. Scanning interferes with standard pulse oximeters, which may also interfere with image acquisition. Specialized “MRI-compatible” monitoring systems utilizing fiberoptic cables are available.

3. A large amount of noise can be generated during imaging.

4. Temperature probes are not used because of the potential for thermal burns.

5. Visualization of patients during scanning is imperative and may occur via a shielded window or closed circuit video.

6. Electrical currents induced in coiled cables during scanning can burn patients; cables should be kept as straight as possible to minimize this risk.

C. General Issues. The duration of an MRI scan varies. Immobility is required only during the actual scanning, 3 to 12 minutes at a time. General anesthesia is necessary for most infants and children, usually utilizing an LMA or endotracheal tube. General anesthesia can be induced in the magnet area. Alternatively, induction can be performed in an area out of the static magnetic field and the anesthetized patient moved into the scanner. Maintenance of anesthesia is provided with specially modified anesthesia machines that contain only nonferrous metals. The patient must be removed from the magnetic field if cardiopulmonary resuscitation is required.