Evoked Potential Monitoring - University of Washington

Evoked Potential Monitoring

Anesthesia Protocol
University of Washington Medical Center
Department of Anesthesiology

Volatile Inhalational Anesthetics

SSEP (somatosensory evoked response)

„ All agents produce a dose-dependent increase in latency and reduction in amplitude of
cortically recorded SSEP responses.

„ Smaller effects are seen on responses recorded over cervical spine, and minimal effects are
seen in epidural or peripherally recorded responses.

„ Potency of these effects is probably N2O> isoflurane = sevoflurane = desflurane > enflurane >
halothane.

„ Subcortical effects such as at Erb’s point and the cervical spine are minimal (0-9%) and are
not dose related.

„ Most of the dose-dependent effect occurs at lower concentrations (such as 0.7-1.3 MAC).
There is a diminishing dose-dependent effect at higher concentrations.

„ Combining N2O with other inhalational anesthetics produces unpredictable effects, suggesting
drug interaction.

„ Usually the effect of adding N2O to other inhalational anesthetics is more than additive.

BAER (brainstem auditory evoked response); Cortical AER

„ BAER little affected by inhalational anesthetics.
„ Latency change in wave V, less in III, and minimal in I.
„ Amplitude changes are minimal. AER are affected like cortical SSEP.

VER (visual evoked response)

„ Dose related effect; increase in latency and decrease in amplitude.

MEP (motor evoked response)

„ Dose related effect.
„ Unfortunately, major effect occurs at very low concentrations (such as less than

0.2 to 0.5 percent isoflurane).
„ Magnetically induced MEP may be more sensitive than electrically induced MEP.

WRITTEN BY DR. JOHN LECKY, 9/30/94
REVIEWED BY DR. ALAN ARTRU, 9/03/04
APPROVED BY DR. ALAN ARTRU, 10/20/04
FOR MODIFICATIONS AND SUGGESTIONS, PLEASE WRITE TO [email protected]

EVOKED POTENTIAL MONITORING

Sensitivity

„ For latency change: SSEP (Erb’s) = SSEP (spinal cord) = BAER I < BAER III < BAER V <
SSEP (cortex) < VEP (cortex) < cortical AEP < tcMMEP = tcEMEP.

„ For amplitude change: SSEP (Erb’s) = SSEP (spinal) = BAER I < BAER III < BAER V <
cortical AEP < SSEP (cortex) < VEP (cortex) < tcMMEP = tcEMEP.

„ For SSEP, lower extremity responses appear to be more sensitive than upper extremity
responses.

Intravenous Anesthetics

Opioids

„ Effects are mild; less when opioid given as infusion as compared to intermittent bolus.

Ketamine

„ Minimal effects; may even increase amplitude of cortical SSEP.

Barbiturates

„ After bolus dose used to induce anesthesia, increased latency of cortical SSEP with increasing
effects on longer latency waves and minimal effects on brainstem responses.

„ BAER virtually unaffected. MEP more sensitive than SSEP.

Benzodiazepines

„ Initial mild increase in latency and decrease in amplitude gradually resolving over time.
„ Interestingly, for median nerve SSEP mild decrease amplitude for early peaks (N18, P22),

moderate decrease amplitude for late peaks (N60), loss of very late peaks (200 to 400 msec),
but increase amplitude for middle latency peaks (N35 to N60).
„ MEP more sensitive than SSEP.

Etomidate

„ Induction doses cause little change in BAER, cortical AEP and VEP.
„ Increase amplitude of cortical components of SSEP.
„ Continuous infusion of etomidate has enhanced cortical responses to permit monitoring in

cases where it would have been otherwise impossible.
„ Minimal amplitude depression and no latency changes in MEP.

Propofol

„ Induction dose depresses amplitude of cortical AEP and SSEP, with rapid recovery after dose
no longer being given (bolus or infusion).

„ When used without any other anesthetics, may increase SSEP amplitude.
„ Increase BAER latency without amplitude decrease.
„ No effect on epidurally recorded SSEP. Decrease amplitude of MEP.

UNIVERSITY OF WASHINGTON MEDICAL CENTER, DEPARTMENT OF ANESTHESIOLOGY

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EVOKED POTENTIAL MONITORING

Recommendations

For subcortical responses, any anesthetic technique (except regional anesthesia) is usually
acceptable.
For cortical responses, 0.5 MAC inhalational anesthetic combined with opioid (currently,
remifentanil is used most often). If responses cannot be monitored with this anesthetic, discontinue
inhalational anesthetic and replace with another class of anesthetic (etomidate or ketamine appear
best based on the above information; propofol has also been used successfully; dexmedetomide is
another option).
For motor evoked responses, don’t even try inhalational anesthetics. Most commonly used
anesthetics are opioids and ketamine.

References

1) Banoub M, Tetzlaff JE, Schubert A: Pharamologic and physiologic influences affecting sensory
evoke potentials, Anesthesiology, 99: 716-737, 2003.

2) Black S, Mahla ME, Cucchiara RF: Neurologic Monitoring, Anesthesia, 6th edition. Edited by
Miller RD. Philadelphia: Churchill Livingstone, 2000.

3) Levy WJ: Neurophysiologic Brain Monitoring: Electroencephalography, Anesthesia and
Neurosurgery, 4th edition. Edited by Cottrell JE, Smith DS. St. Louis: Mosby, 2001.

4) Mahla, ME: Neurologic Monitoring, Clinical Neuroanesthesia, 2nd edition. Edited by Cucchiara
RF, Black S, Michenfelder JD. New York: Churchill Livingstone, 1998.

5) Sloan TB: Evoked Potentials, Anesthesia and Neurosurgery, 4th edition. Edited by Cottrell JE,
Smith DS. St. Louis: Mosby, 2001.

6) Sloan TB: Evoked Potentials, Textbook of Neuroanesthesia. Edited by Albin MS. New York:
McGraw-Hill, Inc. 1997.

UNIVERSITY OF WASHINGTON MEDICAL CENTER, DEPARTMENT OF ANESTHESIOLOGY

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