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December 1, 2013 Volume 77, Number 12
ACE Question


A 65-year-old woman who develops the abrupt onset of lower-extremity weakness due to spinal cord compression is taken to the O.R. for emergency T5 to T8 decompressive laminectomy. Monitoring of motor-evoked potentials (MEPs) is planned. Which of the following is likely to provide the best conditions for monitoring MEPs in this situation?

 

(A) Administer a volatile anesthetic only.

(B) Use controlled hypotension.

(C) Use a nondepolarizing muscle relaxant.

(D) Maintain hemoglobin above 10 mg/dL.

 

Whenever the spinal cord is at risk of injury, in all procedures above L1, both the motor and sensory function are commonly monitored due to their segregation to anterior or posterior spinal cord locations. Thus, MEPs will also be performed. Unlike somatosensory evoked potentials, where the response to a peripheral nerve stimulus is measured by waveform response in the cerebral cortex using scalp electrodes, the MEP response, created by an electrical stimulus to the motor cortex, is most often measured by an electromyographic response in the hand, leg, and foot muscles.

 

Total intravenous anesthesia is advocated in all patients who require MEP testing. MEPs are very sensitive to volatile anes-thetics, but recent research suggests that up to 0.25 minimum alveolar concentration of volatile anesthetic can be used to supplement an intravenous anesthetic technique. In patients with preexisting motor neurologic changes, volatile anesthetic often cannot be used. Most intraoperative neuromonitoring groups do not use nondepolarizing neuromuscular blocking agents during the procedure when MEPs are being performed. It has proved difficult to maintain a constant level of neuromuscular blockade. Thus, changes in MEP amplitude lose significance and the quality of monitoring decreases. Train-of-four suppression to one or two twitches would likely make it impossible to perform MEP monitoring.

 

The spinal cord is an extension of the cerebral cortex; all maneuvers used to maintain cerebral perfusion apply to the spinal cord. MEPs have more synapses in the spinal cord and are more sensitive to changes in perfusion and oxygen availability. Blood pressure (BP) is used as a surrogate for adequacy of neuronal perfusion, so controlled hypotension is not advocated because of the potential to increase injury in already compromised neurologic tissue. Mean arterial pressure (MAP) of at least 70 mm Hg or, when known, the patient’s usual MAP provides the best perfusion. Since this patient has spinal cord compression, any decrease in BP will lead to increases in hypoperfusion and greater spinal cord injury. Hypotension should be avoided and certainly not used as a therapeutic modality.

 

The red blood cell mass that provides the best oxygen delivery remains somewhat controversial. Best practice anesthetic management would include maintaining a normal BP and maintaining the hemoglobin no lower than 10 mg/dL, by transfusion if necessary. Since IOM is a functional test of neurologic integrity, conditions that maintain an optimal neurologic function improve the ability to detect neurologic changes associated with surgical activities.

 

Answer: D





Bibliography:

  • Miller RD, ed. Miller’s Anesthesia. 7th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2010:1477–1514, 2906–2907.
  • Mendiratta A, Emerson RG. Neurophysiologic monitoring of scoliosis surgery. J Clin Neurophysiol. 2009;26(2):62–69.
  • Jameson LC, Sloan TB. Monitoring of the brain and spinal cord. Anesthesiol Clin. 2006;24(4):777–791.
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