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Choosing Anesthetic Techniques for Neuro-monitoring Cases

Developing a decision making tree based on the various effects of different anesthetics on neuro-monitoring waveforms

Joel McCreary

on 3 October 2012

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Transcript of Choosing Anesthetic Techniques for Neuro-monitoring Cases

Anesthesia and Neuro-Monitoring Why We Monitor
Patient Safety
Improved Outcomes
Minimize Risk EEG
MEPs Types of Monitors Halogenated Ethers
Nitrous Oxide Monitors of Electrical Function Monitors of Blood Flow Transcranial Doppler Ultrasound
Jugular Venous Oxygen Saturation
Cerebral Oximetry SERs Joel McCreary, DO
Valley Anesthesiology Consultants
Phoenix, AZ BAEPs
SSEPs MERs MEPs Waveforms Factors Affecting
Waveform Morphology Monitoring Locations
Anesthetic Effects
Physiologic Factors
Disease Processes Inhaled Anesthetics IV Anesthetic Agents Sedative Hypnotics
Alpha-2 Agonists
Non-Depolarizing Muscle Relaxants Physiologic Factors Temperature
Tissue Perfusion/O2 delivery
Blood Pressure
Blood Volume
Oxygen Saturation
Blood Glucose
Electrolytes Disease Processes - MEPs Diabetes
Neurologic Symptoms
Nerve Root Compression Considerations Primary Goal?
PATIENT SAFETY Intraoperative Factors Which Waveforms monitored?
Surgical Site
BEST monitoring site
Existing Neurologic Symptoms
Disease Processes
Physiologic Factors "For Best Results" Think PATIENT SAFETY
Take a Team Approach
communicate with surgeon
communicate with neuromonitoring team
Choose a "Stable" Anesthetic History Early development in 1970s
Initially used for Scoliosis Surgeries
Today used for multiple surgical interventions Aneurysm clipping
Intracranial AVM Clipping
Supra- or infratentorial Mass
Trigeminal Neuralgia
Skull Base Surgery
Arnold-Chiari Malformation
ENT and Anterior Neck Surgery Carotid Surgery
Anerior/Posterior Cervical Spine Surgery
Scoliosis Correction
Thoracic Spine Surgery
Lumbar Spine Surgery
Intramedullary Spinal Cord Surgery
Tethered Cord Surgery Monitors of Blood Flow Monitor "Flow" ONLY
NO direct neuro monitoring
Unable to directly relay information regarding neural compromise Monitors of Electrical Function Directly monitor nerve activity/function
INDIRECTLY monitor blood flow
If nerve function changes MAY be an indicator of ischemia (due to hypotension or mechanical compromise from surgical intervention) Definitions Brainstem Auditory Evoked Potentials
(BAEPs) Series of Clicks presented intraoperatively which allows for electrical responses by CN VIII
Electrical Responses recorded as a waveform
Important in surgeries in close anatomical proximity to CN VIII (any surgery in posterior cranial fossa)
VERY resistant to effects of general anesthetic agents
No modification of Anesthetic technique required Visual Evoked Potentials (VEPs) Waveforms recorded over scalp after visual stimulation with light
NOT very useful as a monitor
extremely high variability even W/O surgical injury
intra-op changes UNRELATED to neurologic function may occur in up to 80% of monitored patients
Multiple variables
light shined through closed eyelid
changes in visual gaze
changes in pupil dilation
NOT Commonly employed Somatosensory Evoked Potentials (SSEPs) Recorded nervous system responses to electrical stimulation of peripheral mixed nerves
Monitor waveform responses of nerves stimulated along the Spinothalamic tract (Dorsal Columns)
Dorsal columns have blood supply from TWO posterior spinal arteries from vertebral arteries
Sensitive to effects of Anesthetic agents
Important to tailor Anesthetic technique to ensure good baseline AND consistently reproducible waveforms Motor Evoked Potentials (MEPs) Measured motor responses along motor pathways in anterior portion of spinal cord
Utilizes transcranial electrical/magnetic stimulation of motor cortex to stimulate motor pathways
Responses are measured 2 ways
complex muscle action potential (CMAP)
level of spinal cord in epidural space(D-wave) Why Monitor BOTH SSEPs and MEPs? Monitor different pathways
SSEPs monitor POSTERIOR spinal pathways (spinothalamic tract)
MEPs monitor ANTERIOR spinal pathways
Anatomic considerations Spinal Cord Anatomy Spinal Cord Anatomy Dorsal column Lemniscal Pathway
Spinothalamic tract Anterior Motor Pathways
(Corticospinal Tract) Spinal Cord Anatomy Spinal Cord Anatomy Intra-operative Neuromonitoring and Anesthesiology Indications for MEPs Essentially ANY spine surgery from C1 to sacrum
Specific Spine Surgeries
Spinal deformities with Scoliosis >45 degree rotation
Congenital Spine Anomalies
Resections of Intramedullary and Extramedullary tumors
Extensive anterior and/or posterior decompressions in spinal stenosis with myelopathy
Functional disturbance of the cauda equina and/or individual nerve roots SSEPs Waveforms MEPs Waveforms Latency - time from stimulus to peak of response
Amplitude - measured voltage of the response to the stimulus Why should I care what a waveform looks like? Definitions of significant changes to waveforms
decreased Amplitude of >50%
WITH prolonged Latency of >10%
any waveform change greater assoc. with onset of NEW neurologic post-op if left uncorrected during surgery
Criteria require that UNchanged stimulus parameters produce similar muscle responses (amplitude latency and complexity)
increases in stimulus strength >50 V
increase in # of stimuli required
significant decrease in amplitude (>65-80%) W/O muscle relaxation Monitoring Locations SSEPs
stimulus needs to be distal to surgical site
recording location needs to be proximal
subcortical locations less sensitive to anesthetics Monitoring Locations MEPs - stimulus is transcranial
Spinal Cord Level
surgical placement of electrodes in sterile field (into epidural space)
less sensitive to anesthetic effects
usually utilized for intramedullary spinal cord surgery
do NOT differentiate laterality Complex Motor Action potentials (CMAPs)
measured in distal extremity muscle groups
Upper extremity - thenar eminence (abductor or flexor pollicis brevis)
Lower extremity
Tibialis anterior
Abductor hallucis brevis
Trunk muscles
rectus abdominus
LARGEST and MOST reproducible response is selected and monitored throughout the procedure Monitoring Locations Halogenated Ethers - SSEPs Synaptic Theory Multiple receptor targets for various anesthetics at synapses
gamma-amino-butyric acid (GABA)
n-methyl-D-aspartate (NMDA)
n-Acetylcholine (nACH)
neuromuscular junction (mACH)

For BOTH SSEPs and MEPs, neural pathway crosses multiple synapses prior to recording, allowing anesthetic effects to alter waveforms at multiple sites dose related decrease in amplitude and increase in latency
greater change depending on recording level
cortical>subcortical>spinal>peripheral nerves
Otherwise Healthy individuals with easily obtained baseline waveforms MAY tolerate up to 1 MAC without "significant" changes in waveform morphology
increases options
decreases concerns about anesthesia awareness and patient response to surgical stimuli Nitrous Oxide - SSEPs dose dependent decrease in amplitude and increase in latency (Alone)

SYNERGISTIC decrease in amplitude and increase in latency with halogenated inhalational agents and MOST IV anesthetics Benzodiazepines - SSEPs mild to moderate depressant effects on SSEPs
midazolam 0.2 -0.3 mg/kg (14-21 mg for 70kg person) assoc. with modest to no reduction in amplitude and slight increase in latency
wonderful adjunct as pre-op anxiolitic with minimal effect on waveform morphology Barbiturates - SSEPs dose dependent decrease in amplitude and increase in latency
does not cause significant waveform morphology changes
5 mg/kg induction dose of Sodium thiopental increases latency 10-20% and decreases amplitude 20-30%, but effect lasts <10 mins
at times preferred/utilized in aneurysm clipping to achieve burst supression (surgeon dependent) Propofol - SSEPs dose dependent reduction of amplitude and increase in latency
does NOT usually cause significant waveform morphology changes
better waveform morphology preservation at equipotent doses when compared to Nitrous Oxide and Sevoflurane
Most commonly used for TIVA Propofol Considerations
for patients that require "wake up" test or surgeon desires rapid emergence keep in mind context sensitive half time Etomidate - SSEPs dramatically INCREASES cortical SSEP amplitude
enhancement of amplitude may produce seizures in patients with epilepsy
<10% increase in latency
previously used as infusions
out of favor because of potential for adrenal suppression Ketamine - SSEPs increases cortical SSEP amplitude
NO effect on latency of subcortical waveforms
consider as adjunct for TIVA if propofol requirements are high
don't forget about increased secretions and hallucinations
consider glycopyrollate and a benzodiazepine
Use proper patient selection - sympathomimetic Dexmedetomidine/Clonidine - SSEPs Minimal effects on amplitude and latency
nice adjunct to propofol/opioid TIVA Narcotics - SSEPs minimal effects on latency and amplitude
BEST administered as continuous drip
when given as bolus form, may cause decrease in sympathetic tone -> decreased blood pressure -> decreased perfusion to neural tissue -> decreased amplitude on SSEP waveforms Non-depolarizing Muscle Relaxants NO effects on SSEP amplitude or latency Halogenated Ethers - MEPs Significant Suppression of MEPs with "on/off" phenomenon
loss of acceptable MEP waveforms over NARROW range
SOME patients may maintain acceptable waveforms up to 0.5 MAC
Recording at the Epidural level maintaints well-preserved MEP waveforms Nitrous Oxide - MEPs More profound depression of MEP amplitude than volatile anesthetics
waveforms well-preserved if recorded at epidural level Benzodiazepines - MEPs minimal effects in pre-medication/sedation doses
maintaints MEP waveforms well Propofol - MEPs Minimal effect on MEPs except at Higher doses
IF degradation of signal occurs, consider adding ketamine to decrease propofol dose and maintain MEP waveform amplitude Etomidate - MEPs no effect on MEPs Ketamine - MEPs INCREASES amplitude of muscle and spinal MEPs
useful for patients with increased requirements of propofol that may have signal decrement
keep in mind increase in ICP Dexmedetomidine - MEPs minimal effects on MEPs EXCEPT at higher blood levels
nice supplement to propofol and opioid infusions
decreases overall requirements of other medications Narcotics - MEPs Mild Effects on MEPs
similar to SSEPs, avoid boluses Non-depolarizing Muscle Relaxants QUICKLY obliterate MEP waveforms
Best if avoided with MEP monitoring Blood Pressure Decrease in blood pressure can lead to decrease in Cortical SSEP and MEP amplitude
caused by local ischemia to at risk tissue
IF monitoring signals deteriorate, increasing the systemic blood pressure to patient's preoperative value is MOST effective response Temperature Mild Hypothermia
increases latency, minimal effect on amplitude

Profound Hypothermia
SSEPs disappear

Mild HYPERthermia
decrease latency
no change in amplitude Anemia mild anemia
decrease in SSEP amplitude
increase in SSEP latency (10-15%)
can decrease MEP waveform amplitude

consider transfusion in appropriate patients to increase oxygen carrying capacity SO NOW WHAT? References 1. Koht, A., Sloan, T.B., Toleikis, J.R. Monitoring the Nervous System for Anesthesiologists and Other Health Care Professionals. New York: Springer, 2012.

2. Miller, R.D. Miller’s Anesthesia: Sixth Edition. pp. 1511-1550. Philadelphia: Elsevier, 2005.

3. Banoub M, Tetzlaff J, Schubert A. Pharmacologic and Physiologic Influences affecting Sensory Evoked Potentials: Implications for Perioperative Monitoring. Anesthesiology. 2003; 99:716-37.

4. Pastorelli F, Di Silvestre M, Plasmati R, Michelucci R, Greggi T, Morigi A, Bacchin MR, Bonarelli S, Cioni A, Vommaro F, Fini N, Lolli F, Parisini P. The Prevention of Neural Complications in the Surgical Treatment of Scoliosis: The Role of the Neurophysiological Intraoperative Monitoring. Eur Spine J. 2011. 20 (Suppl 1):S105-S114.

5. Deiner SG, Kwatra SG, Lin H-M, Weisz DJ. Patient Characteristics and Anesthetic Technique Are Additive but Not Synergistic Predictors of Successful Motor Evoked Potential Monitoring. Anesthesia & Analgesia. 2010; 111: 421-25.

6. Reinacher PC, Priebe H-J, Blumrich W, Zentner J, Scheufler K. The Effects of Stimulation Pattern and Sevoflurane Concenration on Intraoperative Motor-Evoked Potentials. Anesthesia & Analgesia. 2006; 102: 888-95.

7. Ku ASW, Hu Y, Irwin MG, Chow B, Gunawardene S, Tan EE, Luk KDK. Effect of Sevoflurane/Nitrous Oxide versus Propofol Anaesthesia on Somatosensory Evoked Potential Monitoring of the Spinal Cord During Surgery to Correct Scoliosis. Br J of Anaesthesia. 2002; 88: 502-7.
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