anesthesia protocols for neuormonitoring

8/11/2019 Anesthesia Protocols for Neuormonitoring

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AnesthesiaProtocolsforSurgeryduringNeuromonitoring

Anesthesiology,UniversityofColoradoDenver:[email protected]

April1,2009

Avarietyofanesthesiamethodscanbeusedduringsurgerywhereintraoperativeneurophysiological

monitoringisused.Clearlytheanesthesiamustbetitratedtoeachpatienttoadjustforthevarious

comorbidities,includingthedegreeofneuralcompromisethatmayimpactmonitoring,aswellas

searchingtofindananestheticthatallowsanadequatemonitoringsignalwhilekeepingthepatient

adequatelyanesthetized.Ingeneral,withrespecttomonitoring,thechoiceofanesthesiadependsonthe

particularmonitoringmodalitiesbeingused.Themajorlimitationsarewhentechniquesaresensitiveto

inhalationalagents(IH)andwhentheyaresensitivetoneuromuscularblockingagents(NMB).Some

modalitiesareinsensitivetoboth(e.g.ABR),othersaresensitivetomusclerelaxantsonly(e.g.EMG),or

inhalationalagentsonly(e.g.corticalSSEP),andsomearesensitivetobothinhalationalagentsandmuscle

relaxants(e.g.transcranialmotorevokedpotentials(MEP)).Themostrestrictivetechniquesamongthe

techniquesusedforaspecificsurgerydefinetheoverallanestheticapproachandtheprotocolsbeloware

theprotocolswhichIusuallystartwithforvarioustypesofmonitoring.Ihavealsomentionedsome

alternativestotheapproachIuse.Theseprotocolsareforadults;childrenmayrequiredifferentdosesor

approaches.

MonitoringDuringPosteriorFossaSurgery

WhensurgeryintheposteriorfossainvolvesonlytheAuditoryBrainstemResponse(ABR),thereare

noanestheticconsiderationssincethisisneithersensitivetoinhalationalagentsormusclerelaxants;any

anesthetictechniqueisfinewithrespecttomonitoringandshouldbeguidedtothepatientandsurgery.In

theunlikelyeventthattheEustaceantubeisblockedthenNitrousOxidecouldcauseamiddleeartension

thatwouldmakeitsuseaproblem.

Anesthesia

for

ABR

(techniques

insensitive

to

IH

or

NMB)

Inductionasusual

Maintenanceasusual(IHandNMBasdesired)

ThemostcommonadditiontoABRismonitoringusingEMGofvariouscranialnerves,especiallythe

facialnerve.Assuchthemonitoringthenbecomessensitivetomusclerelaxants.ForsomeEMG

techniqueswherethenervoussystemisstimulated(e.g.MEP,Pediclescrews),partialmusclerelaxationis

oftenacceptable(seebelow),butwhenmonitoringisdesignedtobesensitivetomechanicalstimulation

ofthenerves(asisusuallythecasewithcranialnerves)musclerelaxantsreducetheEMGamplitudeand

makethemonitoringlesssensitivetoimpendingneuralinjury.ForthisreasonItrytoavoidmuscle

relaxationduringthecase.InsurgerieswhereABRiscombinedwithEMG,sincethereisnoinhalational

agentrestriction,

Iusually

use

abalanced

anesthetic

(e.g.

some

opioids

and

inhalational

agents)

and

allow

themusclerelaxantsthatwereusedwithintubationtowearoff.Sincehigherdosesofinhalationalagents

canbeused,thisanestheticworksfine.ThesituationgetsabitmorecomplexwhentheSSEPisusedasin

corticalsurgery.

AnesthesiaforABRwithEMG(insensitivetoIHsensitivetoNMB)

Inductionasusual

Maintenanceasusual(IHasdesired)

LetNMBwearoffafterinduction


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MonitoringtheCerebralCortex

Avarietyofproceduresinvolvemonitoringforpotentialneuralcompromisetothecerebralcortex.A

goodexampleisCarotidEndarterectomy.IftheonlymonitoringmodalityisEEG,theanesthesiaismade

rathereasysinceitisinsensitivetomusclerelaxantsandonlysensitivetohighdosesofinhalational

agents.HencethechoiceofanesthesiaisusuallydesignedtoproducearhythmicEEGinthealpharange

(812Hz)thatisassociatedwithlighttomoderateanesthesiawithinhalationalagents.Higherdoseswill

produceburstsuppressionorelectricalsilencewhichimpairsmonitoringsoinhalationaldosesinthe1

MACorlowerrangeisusuallyfineandcanbetitratedtotheEEG.Thisusuallyproducesexcellent

anesthesiaprovidedthatadditionalopioidsareusedtosupplementtheanalgesia(withtheinhalational

agentsproducingamnesiaandsedation).AprocessedEEGmonitormaybeusedtohelpinsureadequate

sedationinmostpatientsiftheIHdoseislow(lessthanMAC).Theopioidshavetheadditional

advantageofslowingtheheartrateandbluntinghypertensiveepisodeswhichareimportantinreducing

thecardiacriskinthesepatients.Thistechniquealsoallowsmaintenanceofthebloodpressureinthe

patientsusualrangeandisexcellentforTCDmonitoring.ForthisreasonIusuallyloadthesepatientswith

1ug/kgormoreoffentanyl(orasimilardoseofanotheropioid)withinductionandrunabalanced

anestheticwithinhalationalagents.

Anesthesiafor

EEG

(moderately

sensitive

to

IH

insensitive

to

NMB)

Inductionasusual

BalancedMaintenance(6%Des~1MAC)

OpioidsandNMBasneeded

AnesthesiacomesabitmoredifficultifSSEPisusedwiththeEEGforcorticalmonitoringaswouldbe

doneinintracranialaneurysmsurgery.Heretheinhalationalagentmustbekeptlowenoughtokeepthe

corticalSSEPresponsesmonitorable.Ingeneral,corticalSSEPamplitudeswillbeacceptablewith

inhalationalagentconcentrationsbetweenand1MAC,howevertheeffectisnonlinear;thereisusually

aconcentrationthresholdinthatrangeabovewhichthecorticalSSEPresponseismarkedlyreducedin

amplitude.

The

problem

is

that

each

patient

may

have

a

different

threshold

so

the

inhalational

agent

must

betitratedtoeffect.Myapproachistoplanabalancedanestheticwithsomeopioid,musclerelaxantsas

needed,andadjusttheinhalationalagent,observingtheresponse.IuseDesfluraneorSevofluranewhen

possiblebecausetheirinsolubilityallowsrapidincreaseanddecreaseofeffect.Foryoung,healthypatients

withminimalneurologicaldebilityIusuallystartat1MACandtitratedown,andforolderand

neurologicallycompromisedpatientsIstartatMACandtitrateup.Recallthatinhalationaldosesin

excessof1MACmayproducebrainswellingfromincreasedcerebrovasculararterialvolume(aswellas

amplitudedepressionoftheSSEP)soIdontgoabove1MACwithintracranialcases.Ifthedoseof

inhalationalagentsmustbekeptlowtoallowmonitoring,Ioftenwilladdapropofolinfusiontoinsure

adequatesedationandopioidsasneeded.AprocessedEEGmonitorisoftenhelpfulwiththis,providedthe

electrodecontactswiththebrainarenotalteredbythecraniotomyandthebraindoesnotmoveaway

fromthe

frontal

bone.

For

intracranial

surgery

this

additional

infusion

of

propofol

to

prevent

awareness

andsedationisoftennotnecessary(itsomethingaboutoperatingonthebrain),butthereisahigh

possibilityofthisinspinalsurgerywhereSSEPisusedsinceawarenessappearstobemorecommon.

AnesthesiaforCorticalSurgerywithSSEP(sensitivetoIHinsensitivetoNMB)

Inductionasusual,preferablyPropofol

BalancedMaintenance(36%Des1MAC)

OpioidsandNMBasneeded

PropofolinfusionifneededbyEEG


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MonitoringduringSpinalSurgeryusingtheSSEP

WhenIamprovidinganesthesiaforspinalsurgerywhereonlytheSSEPisused(suchasspinal

correctivesurgerybelowL2),Iapproachthechoiceasaboveabalancedanestheticusingopioidsand

musclerelaxationasneededandto1MACinhalationalagent(Des)asacceptabletoacquireacortical

response(titratingasdescribedabove).Asopposedtointracranialsurgery,IfindIusuallyneeda

supplementalinfusionofpropofolandusuallyuseaninfusionofopioid.Thepropofolusuallyrunsat60

120ug/kg/min(oftentitratedwiththehelpofaprocessedEEG).FortheopioidIusuallyusesufentanil

(unlessitsanelderlyfrailpatientwhereIbolusfentanyltoeffect).Sufentanilinfusionsusuallyrun0.150.3

ug/kg/hr,butcanbehigherdependingonthepatientstolerancefrompreoperativeanalgesicuse.Note

thesufentanilinfusionneedstobeturnedoffabout30minutesbeforeending.Notethatfentanyl

(infusion45ug/kg/hr)canbeusedascanremifentanil(0.20.5ug/kg/min).Fortunatelytheinhalational

agentshelpalotwiththeanesthetic.

AnesthesiaforSpinalSurgerywithSSEP(sensitivetoIHinsensitivetoNMB)

Inductionasusual(preferablePropofol)


Opioidssufentanilbolusasneededthan0.150.3ug/kg/hrturnoff30minutesbeforeend

Propofolinfusion

guided

by

EEG

(60

120

ug/kg/min)

NMBasneededifEMGnotmonitored

Analternativeapproachhereistousedexmeditomidineinsteadof,orsupplementarytothe

propofol.SomeindividualsuseDexinfusionsof0.20.5ug/kg/hr.IusuallydontloadtheDex(whichcuts

thecost)ifitsstartedatthebeginningofthecase.TheinfusionofPropofolwillbealowerdoseduetothe

sedationfromtheDex.Becausethemechanismofactionisnotopioidlike(itsacentralalpha2stimulant),

itappearstobehelpfulinopioidtolerantpatients.

AlternateanesthesiaforSpinalSurgerywithSSEP(sensitivetoIHinsensitivetoNMB)

Induction

as

usual

(preferable

Propofol)



Dexmeditomidine(0.20.5ug/kg/hr)

Propofolinfusion(60100ug/kg/min)

NMBasneededifnoEMG

IfEMGisalsomonitoredwiththeSSEP(whichisusuallythecasewithoursurgeries),themuscle

relaxantsmustberestricted.Iprefertoletthemusclerelaxantswearoffafterthebeginningofsurgery.

Afterthebaselinerecordingsaredone,sometimeswewillusesomerelaxationfortheopeningofalarge

spinalsurgerytoreducethemuscleactivityorassistintheexposureofananteriorabdominalcase.

AlthoughIprefer

to

use

no

relaxation

during

the

monitoring

portion

of

the

procedure,

acceptable

EMG

monitoringcanbedonewith2twitchesinatrainoffour,optimallyusingatitratedinfusionofan

intermediateactingdrugsuchasrocuronium(510ug/kg/min)orvecuronium(0.50.8ug/kg/min).Data

suggeststhatadeeperblock(only1twitch),mayartificiallyincreasethepediclescrewthresholdwhich

couldreducetheabilitytosignaltheneedforrepositioningofthescrews.Inaddition,thedetectionof

nerverootcompromisefrommechanicalmeansmightbereducedsimilartofacialnervemonitoring

above,suchthatnorelaxationisdesirable.Ingeneral,sincethesensitivityofmusclegroupstomuscle

relaxantsvaries,wheretheTOFismonitoredisimportant.Sincedistalmusclesaremostsensitive(and

frequentlywheremonitoringisdone),ifwemonitortheTOFusingtheulnarnerveandhandresponseis

probablybestsincemoreproximalmuscles(suchasontheface)mayunderestimatetheeffectinthe


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periphery.ThebestneuromuscularmonitoringofTOFwillbedonebythemonitoringteaminthemuscles

theyaremonitoring(notetheyneedtousethesametechniqueasanesthesiawithaTOFat2Hz).

AnesthesiaforSpinalSurgerywithSSEP&EMG(sensitivetoIH&NMB)




PropofolinfusionguidedbyEEG(50150ug/kg/min)

NMBasneededforinduction,possiblyformuscledissectionthennone

(acceptable2+/4twitchesinTOFinmusclesmonitoredformonitoringnervestimulation)

MonitoringtheSSEPwhenaReductionorEliminationoftheInhalationalAgentsisNeeded

Ingeneral,theabilitytouseinhalationalagentsandpartialmusclerelaxationisveryhelpfulin

anesthetizingthespinesurgerypatients(particularlyiftheyareopioidtolerant).Thesituationbecomes

muchmoredifficultwhentheresponsesaresopoorthattheinhalationalagentmustbereducedor

eliminated.Inthiscasetheanesthesiabecomesatotalintravenousanesthetic(TIVA)withthesedation

beingprovidedbypropofol(75150ug/kg/min,usuallytitratedtoprocessedEEG)withanopioidinfusion

(e.g.sufentanil

0.3

0.5

ug/kg/hr).

If

the

SSEP

remains

too

small

for

monitoring,

an

infusion

of

etomidate

(0.6mg/kg/hr)canbeusedinsteadofthepropofol(asetomidateenhancesthecorticalSSEPatlowdoses).

Alternativelyaketamineinfusion(12mg/kg/hr)canbeusedwiththeopioidinfusion(seebelowforour

approachtoketamine)sinceketaminealsoincreasesthecorticalSSEPresponse.Sinceourspinesurgeries

mostoftenusetranscranialmotorevokedresponseswhenweneedtoeliminatetheinhalationalagents,

wetaketheTIVAapproachdescribedbelowwhenlowdoseofinhalationalagentsarenotacceptablefor

MEP.

MonitoringwhenMotorevokedPotentialsareused

The

most

challenging

anesthetic

is

required

during

monitoring

of

surgery

when

motor

evoked

potentialsarebeingusedbecauseboththeinhalationalagentsandneuromuscularblockingagentsmust

beseverelyrestrictedornotused.WiththesecasesSSEPandEMGarealsousuallybeingmonitored,but

theMEPdefinesthemajorrestrictions.Foramedicallyhealthypatientwhoiswithoutmarked

neurologicalproblems(i.e.usuallypresentswithseverepainthatpromptssurgery),Iusuallystartwitha

TIVAtechniquesupplementedwithMACofinhalationalagent(e.g.3%Des).Somefolksstartwithpure

TIVA,butfrequentlyasmallamountofDesorSevoisacceptableandIbelieveitishelpful,especiallywith

patientswhoareopioidtolerant.Hence,afterastandardinductionwithpropofolandashortor

intermediateactingmusclerelaxant(whichIletwearoff),Iwilluse3%Des,asufentanilinfusion(0.30.5

ug/kg/hr)andapropofolinfusion(75150ug/kg/mintitratedtoprocessedEEG).Notethatsome

individualswouldprefertouse5060%nitrousoxideinsteadoftheDes(butnotbothIHandN2Otogether

atthe

same

time

since

they

are

synergistic

and

the

effect

is

usually

too

much).

This

works

similarly

but

I

prefertonothavemyFiO2restrictedbynitrousoxideandthatwhenturningthenitrousoffinatimeof

concernmaycauseanabruptchangeinanesthesiaandmonitoring.

Thistechniqueusuallyworkswell,butoccasionallytheMEPresponsesaretoosmallwhich

necessitatesturningofftheDesandadjustingthePropofolandsufentanilinfusionsasneeded.Its

importanttonotethatmoderatedosesofbenzodiazepinesandbarbiturateshavebeenreportedtoreduce

theMEPresponseandthatthismaylastalongtime(muchlongerthanthedrugdurationofaction).Itis

notclearhowthispertainstothemodernmultipulsetechnique;however,smalldosesofmidazolam

appearquiteacceptablesuchasthosethatarecustomarilyusedforpreinductionoroccasionallyduring

thecase.


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AnesthesiaforSpinalSurgerywithMEP&EMG(verysensitivetoIH&NMB)


LowdoseIH(3%Des)





MonitoringMEPwithOpioidTolerantPatientsorWhohaveSignificantNeurologicalDisability

Inpatientswhoarenotyoungandhealthyorhavemoderateneuraldisabilityorwhereturningoff

theDesisrequiredintheabovetechnique,IusuallyusepureTIVAusingpropofolandsufentanil.

AnesthesiaforSpinalSurgerywithMEP&EMG(verysensitivetoIH&NMB)


PureTIVAnoIH


Propofolinfusion

guided

by

EEG

(75

175

ug/kg/min)



Ifthisisntsufficienttoallowmonitoring,orinpatientswhoareveryopioidtolerantorwhohave

significantneurologicaldebilitywheretheresponsesarelikelytobepoorIuseTIVAenhancedwith

ketamine.InthiscaseIuseketaminetosupplementtheanalgesia(recallithasNMDAactionthatthe

opioidsdonot).Italsosupplementsthesedationwhichallowsareductioninthepropofolinfusionrate

(andareductioninthedepressanteffectofthepropofol).Thenotablethingaboutketamineisthatitis

metabolizedslowerthanpropofolsothattheinfusionmustbeturneddownearlierthanthepropofol.One

approach

is

to

run

a

separate

infusion

of

ketamine

(1

2

mg/kg/hr),

but

since

we

currently

titrate

the

sedationtotheprocessedEEG,itsmoreconvenienttomixtheketaminewiththepropofol.Assuch,we

mixketamineinthepropofolforaninitialinfusionthathas2mgofketamineineachccofpropofol(e.g.

100mgketamineina50ccsyringeofpropofol).ThisinfusionistitratedtotheEEG(sinceketaminecan

increasethenumericvalueoftheprocessedEEG,ItitratetothehighendoftheacceptableprocessedEEG

range).Thisconcentrationofketamineisreducedwitheachsubsequent50ccsyringeofpropofol.Fora

shortercaseIusuallygo2,then1.5,then1,then0.5mgofketamineperccandusenoketamineinthe

finalsyringes.ForamuchlongercaseItapermoreslowly.NotethattheketaminewillincreasetheSSEP

amplitudesoyoumayseeaslowdeclineinSSEPamplitudeoverthecase(oftento50%)andthisis

expectedandmustbedifferentiatedfromapathologicchange.

Anesthesiafor

Spinal

Surgery

with

MEP

&

EMG

(very

sensitive

to

IH

&

NMB)


PureTIVAnoIH



KetaminemixedinthePropofol(initial2mg/cc)andtaperedtooff




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Themajoralternativetothisistousedexmeditomidineasdescribedabove.Hencesomeindividuals

use


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Usuallytheseprotocolsworkquitewell,althoughIoccasionallyhaveapatientacoupletimesayear

whoIjustcantkeepdown.Myapproachisusuallytoaddinhalationalagentssothatwemaintainthe

SSEPandEMGmonitoring,sacrificingtheMEPratherthanusingNMBandlosingtheEMGandMEP.

anesthesia protocols for neuormonitoring

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