Chapter 57

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 ...

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