Chapter 57. Respiratory Arrest in the Magnetic Resonance Imaging Suite

Mr. S is a 52-year-old entrepreneur in the waste management industry. He weighs 262 lb (approximately 119 kg), is 5 ft 9 in (approximately 175 cm) tall, and is being investigated for dizzy spells that appear to be panic attacks. His medical problem list includes obesity, untreated hypertension, and possible obstructive sleep apnea (OSA) (based on his wife's nocturnal observation that "sometimes he just stops breathing"). When questioned, he admits to extreme claustrophobia, possibly the result of a protracted period of time spent in a car trunk as a child. A previous attempt at a magnetic resonance imaging (MRI) scan was unsuccessful because Mr. S, startled by the onset of the loud noises made by the MRI machine, panicked and tried to get out of the MRI scanner, pulling out his IV in the process.

On this occasion, the MRI team decides that Mr. S might be more cooperative with pharmacologic assistance and to this end has given him 5 mg of IV midazolam (Versed®). Unknown to the clinical team, just before entering the MRI suite, Mr. S had also taken 6 mg of sublingual lorazepam (Ativan®) to help reduce his considerable anxiety. For the scan, a pulse oximeter and nasal capnograph are used to monitor respiration. Oxygen is administered by nasal prongs at 3 L·min−1.

About 10 minutes into the scan, the pulse oximeter alarm activates, drawing attention to an oxygen saturation reading of 83%. The pulse oximeter waveform quality appears to be good. However, no waveform can be obtained from the capnograph. Since Mr. S is deep inside the MRI machine, it is hard to see how well he is actually breathing. You are summoned to the MRI suite to help manage this patient.

57.2.1 What Is MRI and Why Is It Done?

MRI has steadily increased in popularity as a noninvasive, painless diagnostic imaging procedure. MRI images are produced using a strong (typically, 1.5 tesla [15,000 gauss]) magnetic field into which radiofrequency (RF) pulses are injected. MRI is the imaging method of choice for examinations in which water content differences make it possible to differentiate tissue types.1 It offers distinct advantages over computed tomography, both in terms of the quality of the obtained images for certain types of tissue (like brain) and the lack of exposure to ionizing radiation. MRI scans are frequently ordered by neurologists and neurosurgeons for patients of all ages with neurological disorders. In addition to intra-axial pathology, orthopedic problems such as osteomyelitis, soft tissue muscle tumors, and damaged knee menisci can be assessed using MRI techniques.1

57.2.2 What Is Unique About the MRI Suite in Terms of Caring for a Patient?

The extreme strength of the magnetic field in an MRI scanner can be hazardous. For example, patients with implanted ferromagnetic objects like aneurysm clips have had these fatally pulled out of position by the magnetic field.2 Similarly, some authorities have expressed concerns about carrying out MRI scans in patients with pacemakers.3 Likewise, ferromagnetic objects like wrenches, scissors, IV poles, pens, stethoscopes, and even hair barrettes can become accidental projectiles. In one case, a pillow containing metal springs, not detectable using a handheld magnet, flew into the magnet during positioning of a patient, fortunately without causing injury,4 However, projectile oxygen and nitrous oxide tank cylinder accidents causing injuries and even death have been reported during the last decade.5-7

Zimmer et al8 relate the following interesting cautionary tale. A 2-year-old boy underwent abdominal MRI scanning under general anesthesia. During the procedure, an anesthesia practitioner carried a portable sevoflurane vaporizer into the MRI suite. When the vaporizer was placed on an examination table, it was vigorously attracted toward the scanner, and it was only by the strength of two people that the vaporizer was directed to strike against the gantry, instead of flying directly into the magnet where it might have hit the child. Quenching the magnet, that is, emergency release of liquid helium from the scanner to collapse the magnetic field was initially considered, but the vaporizer could be removed with the help of a third individual. Of interest, immediately after the mishap the portable vaporizer was tested for magnetism with a strong handheld magnet, and no attraction was apparent. A review of the event revealed that the vaporizer contained ferromagnetic material in the temperature compensation module.

In addition to the attractive forces of the magnetic field on ferromagnetic objects, the strong magnetic field and associated RF pulses can interfere with the operation of ordinary anesthesia machines, as well as with patient-monitoring equipment, sometimes resulting in patient injury.9

It should be emphasized that some anesthesia machines and patient monitors that are alleged to be MRI compatible may still contain ferromagnetic components and may pose risks when safety precautions (often described in fine print in the user's manual) are violated.

57.2.3 Why Might MRI Require Moderate or Deep Sedation, or General Anesthesia?

Patients must remain motionless during MRI scans. However, the long duration (up to 20 minutes or more) of some MRI scans and the loud noise of the MRI machine may eventually lead to significant discomfort for many patients. In addition to fidgeting, many MRI patients are fearful or claustrophobic. Moderate to deep sedation, and sometimes general anesthesia, may be required to immobilize these patients sufficiently to obtain good quality scan images, particularly in children and mentally challenged patients.

57.2.4 What Special Precautions Must Clinicians Take When Responding to or Working in the MRI Suite?

Clinicians caring for patients in MRI suites must be careful to rid themselves of all objects with possible ferromagnetic components, such as pagers, mobile phones, keys, pens, and stethoscopes. In addition, credit cards and ID badges may be demagnetized by the magnetic field.

There are serious concerns regarding the clinical monitoring modalities available in an MRI unit. In order to monitor the patients undergoing general anesthesia properly, it is necessary to have MRI-compatible systems that support automatic noninvasive blood pressure monitoring, electrocardiography, pulse oximetry, capnography, and even multichannel invasive pressure monitoring. To avoid burns10 and fires,11 electrocardiogram electrodes must be applied at a distance from the imaging area, or (in special cases) should be replaced with special MRI-compatible carbon electrodes.

57.2.5 Where Can One Obtain MRI-Compatible Anesthesia Equipment?

The list of MRI-compatible anesthesia equipment needed in an MRI suite can be extensive and includes, but is not limited to, anesthesia machines, patient monitors, oxygen cylinders, and laryngoscopes. Hospital purchasing departments should be able to provide useful information on the availability of this equipment. The author recommends getting information from the web site www.magmedix.com. Additional resources appear in Appendix 57.1.

57.3.1 What Airway Equipment May or May Not Be Used in an MRI Unit?

The answer to the question regarding which airway management devices are safe to use in an MRI unit can be both simple and complex. The simple answer is that devices, such as conventional laryngoscopes that contain substantial amounts of ferromagnetic materials, can easily become dangerous projectiles, while items completely free of ferromagnetic materials are completely safe. The complex answer is that most airway instruments and devices that have not been specifically designed for use in an MRI environment are likely to have at least some ferromagnetic components. Even apparently benign products, like endotracheal tubes and laryngeal mask airways (LMAs), may have small amounts of ferromagnetic materials, such as metallic springs in the cuff inflation valve. While such small ferromagnetic objects do not generally present a projectile safety hazard, they can interfere with the image quality, possibly introducing an information hole if located near the imaging area.

57.3.2 What Are Some Other Airway Management Issues in the MRI Environment?

Since most airway devices are not specifically designed for the MRI environment, equipment like flexible bronchoscopes, rigid fiberoptic laryngoscopes (such as the Bullard laryngoscope), or video laryngoscopes (such as the GlideScope®) must be specifically tested for suitability in the MRI environment by experienced MRI personnel. The same applies to the LMA-Fastrach™ and LMA-CTrach™, though the LMA-Fastrach™ single-use device is metal free. In addition, there is a theoretical concern that armored endotracheal tubes and other airway devices with wire-reinforced elements may either interfere with image quality, or undergo self-heating from absorbed electromagnetic radiation.

It should also be pointed out that airway equipment containing electronic circuits could possibly be affected by strong magnetic fields, for example, by the mechanism of closing a normally open switch containing ferromagnetic elements. This has been alleged to sometimes occur with the Trachlight™ intubating lightwand.

57.4.1 What Are Immediate Management Options for This Patient?

Before we proceed with the management of the patient, for patient's safety, all health care providers, including the airway practitioners, should remove all objects containing ferromagnetic substance, before entering the MRI unit. Mr. S, who is now turning blue is likely over-sedated and has an obstructed airway. Based on the history from his wife that he sometimes "just stops breathing" while asleep, it is likely that OSA is involved. The management options available include the following:

1. If the patient is accessible, a simple jaw thrust or head tilt often suffices to restore respiration. Most of the time, however, the scan will have to be temporarily suspended to permit this to occur. (This does not, however, mean that the magnet needs to be turned off). Some practitioners have tried taping the patient's jaw to the MRI head frame to keep the airway open.

2. The patient may also benefit from a nasopharyngeal airway. An oropharyngeal airway may also be considered but these tend to be less well tolerated.

3. If a simple jaw thrust, head tilt, or artificial airway does not promptly restore effective ventilation and oxygenation, positive pressure ventilation with 100% oxygen, using bag-mask-ventilation (BMV), will be needed to restore oxygenation. Certainly, when the patient is severely hypoxemic, this should be the first step undertaken.

4. Pharmacologic reversal of the lorazepam/midazolam may be helpful to reduce the level of sedation and restore the airway. Intravenous flumazenil, administered in 100 μg increments, is a benzodiazepine antagonist that antagonizes the sedation produced by all benzodiazepines. In the case of opioids, pharmacologic reversal can be achieved using intravenous naloxone (Narcan), also using 100 μg increments. It should be emphasized that since both naloxone and flumazenil have relatively short durations of action, resedation can occur. As Mr. S is a chronic user of benzodiazepines, he may be physically dependent on them. As such, it is possible that flumazenil, administered in traditionally recommended doses, may induce seizures.12

5. Insertion of an LMA, or even tracheal intubation, may be necessary in the absence of ventilation. Although LMAs may compromise the MRI image, this is not a constant finding. As mentioned, ferromagnetic laryngoscopes cannot be used. Commercially available MRI-compatible laryngoscopes along with MRI-compatible batteries should be readily available in the vicinity of the MRI unit.

Given that Mr. S has developed respiratory difficulties, it is decided to remove him from the scanner to allow for positive pressure ventilation, or other intervention. With the commotion of being moved about, Mr. S becomes aroused, and spontaneous respirations resume. Instead of positive pressure ventilation with a bag-mask-ventilation (BMV) unit, a non-rebreathing face mask is used and the pulse oximeter is soon providing reassuring oxygen saturation. Mr. S remains drowsy when left undisturbed and he continues to have intermittent obstruction of his airway. The radiologist is eager to have the scan completed, emphasizing that Mr. S's neurologist has called repeatedly about the results (being concerned about ruling out a brain tumor) and also pointing out that this is the second time an MRI has been attempted on Mr. S.

Your examination shows that Mr. S has a Mallampati Class II view, with good mouth opening and good jaw protrusion. However, his considerable obesity and his apparent history of OSA raise concerns regarding possible difficult BMV, difficulty with direct laryngoscopic intubation, and difficulty establishing an emergency surgical airway. In addition, there are issues regarding cooperation, possible substance abuse, and an increased potential for rapid desaturation (because of a small functional residual capacity).

57.5.1 How Should We Proceed (If We Did Proceed) with Managing the Airway?

With the concerns listed earlier, and the urgency to proceed, a variety of means to secure the airway can be considered. Provided that BMV is not anticipated to be difficult, the first option, Plan A, is to induce anesthesia using propofol and achieve muscle relaxation using succinylcholine. Assuming that the patient is not prone to aspiration, and expert help is readily available, induction of anesthesia could either be done in the induction area some distance from the MRI scanner or in a regular operating room. Ramped positioning should be employed. An appropriate variety of airway devices and adjuncts, like an intubating stylet, an Eschmann tracheal introducer (bougie), and an LMA must be immediately available.

If the view at laryngoscopy proves to be unsatisfactory, even with external laryngeal manipulation, and especially if the use of an Eschmann tracheal introducer is unlikely to be successful (eg, Grade 4 airway), this author recommends the use of Plan B with a GlideScope®. This instrument has proven to be especially valuable.13 Failing that, Plan C would be to use bronchoscopic intubation, either awake (having awakened the patient) or under general anesthesia.

There are several possible options for the use of the flexible bronchoscope in the anesthetized patient. These include tracheal intubation using the flexible bronchoscope through the LMA-Classic™ (or other extraglottic device) with the aid of an Aintree catheter14 or using the GlideScope® to facilitate bronchoscopic intubation.15

If one's overall clinical impression is that inducing general anesthesia is not a prudent course, a more cautious approach would likely be awake intubation under topical anesthesia, using a flexible bronchoscope or using the GlideScope®.15

57.5.2 Discuss Postintubation Management

At the end of the MRI scanning procedure, a decision should be made whether it is safe to perform tracheal extubation in the MRI suite or in the post-anesthetic care unit after a period of elective ventilation (as might be appropriate if intubation was difficult and it is suspected that the airway structures are edematous). Consideration will, at times, be given to extubation over a tube exchanger.

57.6.1 How Do You Manage a Patient with a History of Difficult Airway in the MRI Suite?

A review of the patient's previous medical records (especially anesthesia records) can be valuable to determine why previous intubation attempts may have been difficult. Depending on what information is found in the medical records, and the results of the airway examination, options will range from maintaining spontaneous ventilation in the patient in a setting of minimal sedation, to full general anesthesia preceded by awake endotracheal intubation. In particular, patients with severe reflux may require rapid-sequence intubation, or awake intubation methods, to minimize the risk of aspiration. As most of the airway devices and intubating equipment may not be compatible in the MRI unit, if the patient requires tracheal intubation for the MRI scanning, it would perhaps be wise to secure the airway in the operating room, prior to going to the MRI suite.

57.6.2 Does the American Society of Anesthesiologists Have Any Advice on Working in the MRI Suite?

In March 2009, the American Society of Anesthesiologists published a comprehensive document that should be very helpful to anesthesiologists.16 The following are some highlights from the report:

• Anesthesiologists should work with their institutions to properly identify and label anesthesia-related equipment according to convention (safe, unsafe, or conditional) for each MRI scanner.
• For every case, the anesthesiologist should communicate with the patient, referring physician, and radiologist to determine whether the patient presents with a high-risk medical condition, requires equipment, has implanted devices (eg, pacemakers, cardioverter–defibrillators, nerve stimulators), has been screened for the presence of implanted ferromagnetic items, and imbedded foreign bodies.
• For every case, the anesthesiologist should have a plan for providing optimal anesthetic care within the MRI suite. In addition to addressing the medical needs of the patient, features of the plan should include: (1) requirements of the scan and personnel needs; (2) positioning of equipment; (3) special requirements or unique issues of the patient or imaging study; (4) positioning of the anesthesiologist and the patient; and (5) planning for emergencies.
• Unique features of airway management during an MRI scan include: (1) the limited accessibility of the patient's airway; and (2) the difficulty of conducting visual and auditory assessments of the patient. Airway providers should have a pre-formulated plan in place to deal with instrumentation of the airway and common airway problems when patients are in an MRI environment.
• If a patient presents with a high-risk medical condition, the anesthesiologist should collaborate with all participants to determine how the patient will be managed during the MRI procedure.
• For patients with acute or severe renal insufficiency, the anesthesiologist should not administer gadolinium because of the increased risk of nephrogenic systemic fibrosis.
• Cardiac pacemakers and implantable cardioverter–defibrillators generally contraindicate MRI scanning.
• Anesthesiologists should have a clear line of sight of the patient and physiologic monitors, whether by direct observation or by video camera.
• Anesthesiologists should prepare a plan for rapidly summoning additional personnel in the event of an emergency.

The key message from this chapter is that the MRI suite is a potentially hostile environment for patients with a difficult airway and numerous special precautions must be taken to prevent airway-related problems in these patients. Such precautions include having an additional airway practitioner readily available, as well as having primary and secondary backup plans for airway management. It is also essential to consider in advance what equipment can or cannot be used near the MRI scanner. The unique role of the recently introduced single-use, metal-free LMA Fastrach™ is yet to be determined, although it seems uniquely suited to the MRI environment, particularly for patients with a potentially difficult airway.

Because it can be difficult to directly observe whether a patient is breathing adequately when they are deep inside the MRI scanner, other means of respiratory monitoring (such as capnography) are especially important. In some situations, tracheal intubation is necessary because of the high likelihood of airway obstruction with sedation; in such cases, consideration must be given to the possibility that tracheal intubation may be difficult, without special equipment and/or special techniques.

Where can I get more information on MRI safety issues?

A good place to start is on the web at www.mrisafety.com. Unfortunately, one must register to use this site. Another valuable resource is from the American College of Radiology Blue Ribbon Panel on MR Safety. This site offers a number of useful safety guidelines and clinical protocols and can be accessed at http://www.acr.org/SecondaryMainMenuCategories/quality_safety/MRSafety/safe_mr07.aspx.

An interesting report from the Institute for Safe Medication Practices (http://www.ismp.org/newsletters/acutecare/articles/20040408.asp) explains that patient burns can occur when medication patches employing an aluminized backing are used (like many of those in common use containing nicotine, nitroglycerine, scopolamine, etc). What happens is that the RF pulses heat up the metal involved, even if the metal is not ferromagnetic. Of interest, this problem can also occur when patients have tattoos containing metal pigments.

A comprehensive list of MRI-forbidden objects is available at www.newmri.com/html/mr_safety.asp.