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CHAPTER3
ManagingtheAirway
Basictechniques,37Thelaryngealmaskairway,42Emergencyairway
Simpleadjuncts,39Trachealintubation,43techniques,50
Maintenanceofapatentairwayisanessentialprerequisiteforthesafeandsuccessfulconductofanesthesia.Inaddition,duringresuscitationpatientsoftenhaveanobstructedairwayeitherasthecauseorresultoftheirlossofconsciousness.Theskillofairwaymaintenanceshouldbeacquiredbyalldoctors,andnotsimplyregardedastheresponsibilityoftheanesthetist.Thedescriptionsofairwaymanagementtechniques,whichfollow,areintendedtosupplementpracticeeitheronamanikinorpreferablyonandanesthetizedpatientunderthedirectionofaskilledanesthetist.
Basictechniques
Anesthesiafrequentlyresultsinlossoftheairwayanditismosteasilyrestoredbyacombinationoftheheadtiltalongwithajawthrust(seeChapter2).Thelatterisprovidedbytheanesthetist’sfourthandfifthfingers(ofoneorbothhands)liftingtheangleofthemandible.Theoveralleffectdesiredisthatthepatient’smandibleis‘lifted’intothemaskratherthanthemaskbeingpushedintotheface(Fig.3.1).
FACEMASKS
ThemostcommonlyusedtypeinadultsistheBOCanatomicalfacemask(Fig.3.2)whichisdesignedtofitthecontoursofthefacewiththe
minimumofpressure.
Leakageofanestheticgasesisminimizedbyanair-filledcuffaroundtheedge.
Masksatemadeinavarietyofsizesandthesmallestone,whichprovidesagoodseal,shouldbeused(tominimizetheincreaseindeadspace,whichoccurs).
TheAmbumask(Fig.3.2)hasatransparentbody—allowingidentificationofvomit-makingitpoplarforresuscitation.
Allmasksmustbedisinfectedbetweeneachpatient.
Simpleadjuncts
Themostcommonlyandusedaretheoropharyngeal(Guedel)andnasopharyngealairways,insertedaftertheinductionofanesthesiatohelpmaintaintheairwayinconjunctionwiththetechniquesdescribedabove.
OROPHARYNGEALAIRWAY
Thesearecurvedplastictubes,flattenedincross-sectionandflangedattheoralend,whichlieoverthetongue,preventingitfromfallingbackintothepharynx.
Theyareavailableinavarietyofsizesfromneonatestolargeadults.Thecommonestsizesare2-4,forsmalltolargeadults,respectively.
Aguidetothecorrectsizeisdeterminedbycomparingtheairwaylengthtotheverticaldistancefromthecornerofthepatient’smouthtotheangleofthemandible.
Itisinitiallyinserted'upsidedown'asfarasthebackofthehardpalate(Fig.3.3a),rotated180(Fig.3.3b)andfullyinsertedutiltheflangeliesinfrontoftheteethorgumsinanedentulouspatient(Fig.3.3c).
NASOPHARYNGEALAIRWAY
Theseareround,malleableplastictubes,beveledatthepharyngealendandflangedatthenasalend.
Theyaresizedontheirinternaldiameterinmillimeters,withlengthincreasingwithdiameter.Thecommonsizesinadultsare6-8mm,forsmalltolargeadults,respectively.
Aguidetothecorrectsizeismadebycomparingthediametertotheexternalnaris.
Priortoinsertion,thepatencyofthenostril(usuallytheright)shouldbecheckedandtheairwaylubricated.
Theairwayisinsertedalongthefloorofthenose,withthebevelfacingmediallytoavoidcatchingtheturbinates(Fig.3.4).
Asafetypinmaybeinsertedthroughtheflangetopreventinhalationoftheairway.
Ifobstructionisencountered,forceshouldnotbeusedasseverebleedingmaybeprovoked.Instead,theothernostrilcanbetried.
PROBLEMSWITHAIRWAYS
Thepresenceofsnoring,indrawingofthesupraclavicular,suprasternalandintercastalspaces,useoftheaccessorymusclesorparadoxicalrespiratorymovement(see-sawrespiration)suggestthattheabovemethodsatefailingtomaintainapatentairway.Commonproblemsarisingusingthesetechniquesalongwithafacemaskduringanesthesiaare:
inabilitytomaintainagoodsealbetweenthepatient’sfaceandthemask,particularlyinthosewithoutteeth;
fatigue,whenholdingthemaskforprolongedperiods;
theriskofaspiration,duetothelossofupperairwayreflexes;
theanesthetistisnotfreetodealwithanyotherproblems,whichmayarise.
Thelaryngealmaskairway(LMA)ortrachealintubationmaybeusedtoovercometheseproblems.
Thelaryngealmaskairway
Thisdevicewasdesignedforuseinspontaneouslybreathingpatients.Itconsistsofa‘mask’,whichsitsoverthelaryngealopening,attachedtowhichisatube,whichprotrudesfromthemouthandconnectsdirectlytotheanestheticbreathingsystem.Ontheperimeterofthemaskisaninflatablecuff,whichcreatesasealandhelpstostabilizeit.TheLMAisproducedinavarietyofsizessuitableforallpatients,fromneonatestoadults,withsizes3and4beingthemostcommonlyusedinfemaleandmaleadults,respectively.PositivepressureventilationcanbeperformedviatheLMAprovidedthathighinflationpressureisavoided,otherwiseleakageoccurspastthecuff,reducingventilationandcausinggastric
inflation.Aversionwithareinforcedtubeisalsoavailable.TheLMAisreusable,providedthatitissterilizedbetweeneachpatient.
Theuseofthelaryngealmaskovercomessomeoftheproblemsoftheprevioustechniques:
itisnotaffectedbytheshapeofthepatientsfaceortheabsenceofteeth;
theanesthetistisnotrequiredtoholditinposition,avoidingfatigueandallowinganyotherproblemstobedealtwith;
itreducestheriskofaspirationofregurgitatedgastriccontents,butdoesnoteliminateit.
Itsuseisrelativelycontraindicatedwherethereisanincreasedriskofregurgitation,forexampleinemergencycases,pregnancyandpatientswithahiatushernia.
Recently,thelaryngealmaskhasbeenshowntobeusefulintwootherareas:
Indifficulttrachealintubationwhereitwilloftenallowmaintenanceoftheairway.Alternatively,asmalldiametertrachealtubeorintroducecanbepassedintothelarynxviatheLMA.
Duringcardiopulmonaryresuscitation,ithasbeenshownthatnon-anesthetistsareabletoinsertanLMAmorerapidlyandsuccessfullythanatrachealtubeandachievemoreeffectiveventilationthanusingaself-inflatingbagandfacemask.ItislikelythatinthefuturetheLMAwillfindaroleinairwaymanagementduringresuscitation.
TECHNIQUEFORINSERTION
Thepatient’sreflexesmustbesuppressedtoalevelsimilartotherequiredfortheinsertionofanoropharyngealairwaytopreventcoughingorlaryngospasm.
Thecuffisdeflatedandthemasklightlylubricated(Fig.3.5a).
Aheadtiltisperformed,thepatient'smouthopenedfullyandthe
tipofthemaskinsertedalongthelardpalatewiththeopensidefacingbutnottouchingthetongue(Fig.3.5b).
Themaskisthenfurtherinserted,usingtheindexfingertoprovide
supportforthetube(Fig.3.5c).Eventually,resistancewillbefeltatthepointwherethetipofthemaskliesattheupperoesophagealsphincter(Fig.3.5d).
Thecuffisnowfullyinflatedusinganair-filledsyringeattachedto
thevalveattheendofthepilottube(Fig.3.5e).
Thelaryngealmaskissecuredeitherbyalengthofbandageor
adhesivestrappingattachedtotheprotrudingtube.
Trachealintubation
Thisisthebestmethodofprovidingandsecuringaclearairwayin-patientsduringanesthesiaandresuscitation,butsuccessrequiresabolitionofthelaryngealreflexes.Duringanesthesia,thisisusuallyachievedbytheadministrationofamusclerelaxant(seeChapter4).Deepinhalationalanesthesiaorlocalanesthesiaofthelarynxcanalsobeused,buttheseareusuallyreservedforuseinthosepatientswheredifficultywithintubationisanticipated,forexampleinthepresenceofairwaytumorsorimmobilityofthecervicalspine.
COMMONINDICATIONSFORTRACHEALINTUBATION
•Wheremusclerelaxantsateusedtofacilitatesurgery(e.g.abdominalandthoracicsurgery)therebynecessitatingtheuseofmechanicalventilation.
In-patientswithafullstomach,toprotectagainstaspirationof
regurgitatedgastriccontents.
Wherethepositionofthepatientwouldotherwisemake
maintenanceoftheairwaydifficult,forexamplethelateralorproneposition.
Wherethereiscompetitionbetweensurgeonandanesthetistfor
theairway(e.g.operationsontheheadandneck).
Inthosepatientsinwhomtheairwaycannotbesatisfactorily
maintainedbyanyothertechnique.
Duringcardiopulmonaryresuscitationwhenintubationallows:
ventilationwith100%oxygenwithoutleaks;
suctionclearanceofinhaleddebris;
aroutefortheadministrationofdrugs.
EQUIPMENTFORTRACHEALINTUBATION
Avarietyofequipmentexistsandthatchosenwillbedeterminedbythecircumstancesandbythepreferencesoftheindividualanesthetist.Thefollowingisalistofthebasicneedsforadultoralintubation.
Laryngoscopewithacurved(Macintosh)bladeandfunctioning
light.
Trachealtubesinavarietyofsizesandinwhichthecuffswork..The
internaldiameterisexpressedinmillimetersandthelengthincentimeters.Theymaybelightlylubricated.
Formales:8.0—9.0mminternaldiameter,22-24cmlengths.
Forfemales:7.5—8.5mminternaldiameter,20-22cm
lengths.
syringetoinflatethecuffoncethetubeisinplace.
Cathetermountsor‘elbow’toconnectthetubetotheanesthetic
systemorventilatortubing.
Suction,switchedonandimmediatelytohandincasethepatient
vomitsorregurgitates.
Extras:asemi-rigidintroducertohelpmouldthetubetoaparticular
shape;Magill’sforceps,designedtoreachintothepharynxtoremovedebrisordirectthetipofatube;bandageortapetosecurethetube.
Trachealtubes
•Theseweretraditionallymanufacturedfromredrubberandwerereusable.However,disposableplastic(PVC)onesarenowwidelyusedtoeliminatecross-infectionandarechemicallylessirritanttothelarynx
(Fig.3.6).
Tubesaresizedanaccordingtotheirinternaldiameterin
millimetersandaremanufacturedinhalf-millimeterintervals.Theyarelongenoughtobeusedorallyornasally.
Astandard15-mmconnectorisprovidedtoallowconnectiontothe
breathingsystem.
Inadultanesthesia,atrachealtubewithaninflatablecuffisusedto
preventleakageofanestheticgasesbackpastthetubewhenpositivepressureventilationisused.Thisalsohelpspreventaspirationofanyforeignmaterialintothelungs.
Thecuffisinflatedbyinjectingairfromasyringeviaa
small-diametertube,atthedistalendofwhichisaone-wayvalvetopreventdeflationandasmallpilotballoonwhichindicateswhenthecuffisinflated.
Awidevarietyofspecializedtubeshavebeendeveloped,examplesofwhichareshowninFig.3.6.
•Reinforcedtubesateusedtopreventkinkingandsubsequentobstructionofthetrachealtubeasaresultofthepositioningofthepatient’shead(Fig.3.6c).
Preformedtubesareusedduringsurgeryontheheadandneckand
aredesignedtotaketheconnectionsawayfromthesurgicalfield(Fig.3.6d).
Doublelumentubesareeffectivelytwotubesweldedtogetherside-by-side,withonetubeextendingdistallybeyondtheother.Theyareusedduringthoracicsurgery,andplacedsuchthatthedistaltubelieswithinonemainbronchus(endobronchial).Thisallowstheotherlungtobedeflatedtofacilitateviatheendobronchialportion(Fig.3.6E).
Inchildrenunderapproximately10yearsofage,uncuffedtubesare
usedasanaturalsealisprovidedbythenarrowinginthesubglotticregion(Fig.3.6F).
THETECHNIQUEOFORALINTUBATION
Thisrequiresabolitionofthelaryngealreflexesandappropriatemonitoringofthepatient.
Positioning
Thepatientispositionedwiththeneckflexedandtheheadextendedattheatlanto-occipitaljoint.Thisistheso-called‘sniffingthemorningair’position.Thepatient’smouthisfullyopenedusingtheindexfingerandthumboftherighthandinascissorsaction.
Laryngoscopy
Thelaryngoscopeisalwaysheldinthelefthandandthebladeisintroducedintothemouthalongtheright-handsideofthetongue,displacingittotheleft.Thebladeisadvanceduntilthetipliesinthegapbetweenthebaseofthetongueandtheepiglottis,thevallecula.Forceisthenappliedinthedirectioninwhichthehandleofthelaryngoscopeispointing,theeffortcomingfromtheupperarmnotthewrist,toliftthetongueandepiglottistoexposethelarynx.Thisshouldbeseenasa
triangularopening,withtheapexanteriorlyandthewhitishcoloredtruecordslaterally(Fig.3.7).
Intubation
Thetrachealtubeisintroducedintotherightsideofthemouth,advancedandseentopassthroughthecordsuntilthecuffliesjustbelowthecords.Thetubeisthenheldfirmlybythefingersoftherighthandandthelaryngoscopeiscarefullyremoved.Thecuffistheninflatedsufficientlytopreventanyleakduringventilation.Finallythepositionofthetubeisconfirmedbylisteningforbreathsoundsinbothaxillaeanditisthensecuredinplace.
Fornasotrachealintubation;awell-lubricatedtubeisintroducedusuallyviatherightnostrilalongthefloorofthenosewiththebevelpointingmediallytoavoiddamagetotheturbinates.Itisadvancedintotheoropharynx,whereitisusuallyvisualizedusingalaryngoscopeinthemannerdescribedabove.Itcantheneitherbeadvanceddirectlyintothelarynxbypushingontheproximalend,orthetippickedupwithMagill’sforceps(whicharedesignednottoimpairtheviewofthelarynxanddirectedintothelarynx.Theprocedurethencontinuesasfororalintubation.
DIFFICULTINTUBATION
Occasionally,intubationofthetracheaismadedifficultbecauseofaninabilitytovisualizethelarynx.Thismayhavebeenpredictedatthepreoperativeassessmentormaybeunexpected.Avarietyoftechniqueshavebeendescribedtohelpsolvethisproblemandincludethefollowing:
manipulationofthethyroidcartilagebydownwardsandupwardspressurebyanassistanttotryandbringthelarynxoritsposterioraspectintoview;
atlaryngosopy,agumelasticbougie,60cmlong,isinsertedintothetrachea,overwhichthetrachealtubeis‘railroaded’intoplace;
afibreopticbronchoscopeisintroducedintothetracheaviathemouthornoseandisusedasaguideoverwhichatubecanbepassedintothetrachea.Thistechniquehastheadvantagethatitcanbeusedineitheranesthetizedorawakepatients.
COMPLICATIONSOFTRACHEALINTUBATION
Thefollowingisonemethodsofcategorizingthem,butitisnotanattempttocoveralloccurrences.
Hypoxia
Oesophagealintubation.Thisisbestdetectedbymeasuringthecarbondioxideinexpiredgas;lessthan0.2%indicatesoesophagealintubation.Analternativeistoattacha50ml‘bladder’syringetothetrachealtubeandwithdrawtheplungerrapidly(Wee’soesophagealdetector).Ifthetrachealtubeisintheesophagus,resistanceisfeltandaircannotbeaspirated;ifitisinthetrachea,airiseasilyaspirated.Lessreliablesignsate‘burping’soundsasgasescapes,diminishedbreathsoundsonauscultation,anddecreasedchestmovementonventilationandgurglingsoundsovertheepigastrium.Pulseoximetryonlychangeslate,particularlyifthepatienthasbeenpreoxygenated.
NB.Ifthereisanydoubtaboutthepositionofthetubethenitshouldberemovedandthepatientventilatedviaafacemask..
Failedintubationandinabilitytoventilatethepatients.Thisisusuallyaresultofabnormalanatomyorairwaypathology.Manycasesarepredictableatthepreoperativeassessment(seepage6).
Failedventilationafterintubation.Possiblecausesincludethetubebecomingkinked,disconnected,orinsertedtoofarandpassingintoonemainbronchus,severebronchospasmandtensionpneumothorax.
Aspiration.Regurgitatedgastriccontentscancauseblockageoftheairwaysdirectlyorsecondarytolaryngealspasmandbronchospasm.Cricoidpressurecanbeusedtoreducetheriskofregurgitationpriortointubation(seebelow).
Trauma
Directlyduringlaryngoscogyandinsertionofthetubetolips,teeth,gongue,pharynx,larynx,trachea,andnoseandnasopharynxduringnasalintubation;causingsofttissueswellingorbleeding.
Indirectlytothemandible(dislocation),andthecervicalspineandcord,particularlywherethereispre-existingdegenerativediseaseortrauma.
Reflexactivity
Hypertensionanddysrhythmias.Thisoccursinresponsetointubationandmayjeopardizepatientswithcoronaryarterydisease,aorticorintracranialaneurysms,In-patientsatrisk,specificactionistakentoattenuatetheresponse,forexamplepretreatmentwithB-blockers.Potentanalgesics(fentanyl.Alfentanil)orintravenouslignocaine.
Vomiting.Thismaybestimulatedwhenlaryngoscopyisattemptedin-patientswhoareinadequatelyanesthetized.Itismorefrequentwhenthereismaterialinthestomach;forexampleinemergencieswhenthepatientisnotstarved,inpatientswithintestinalobstruction,orwhengastricemptyingisdelayed,asafteropiateanalgesicsorfollowingtrauma.
Laryngealspasm.Reflexadductionofthevocalcordsasaresultofstimulationoftheepiglottisorlarynx.
CRICOIDPRESSURE(SELLICK’SMANOEUVRE)
Thecricoidcartilageistheonlycompleteringifcartilageinthelarynx.Pressureexerteduponitanteroposteiorlyforcesthewholeringposteriorly,compressingtheesophagusagainstthebodyofthesixthcervicalvertebra,therebypreventingpassiveregurgitation.Itisperformedbyanassistantusingthethumbandfirsttwofingerstoapplythepressurewhilsttheotherhandisplacedbehindthepatient’snecktostabilizeit(Fig.3.8).Pressureisappliedasthepatientlosesconsciousnessandmaintaineduntilthetubehasbeensuccessfullyinserted,thecuffinflatedandtubespositionconfirmed.Ifthepatientstartstoactivelyvomit,pressureshouldbereleasedduetotheriskoftheesophagusrupturing,andthepatientshouldbeturnedontotheirsidetominimizeaspiration.
Emergencyairwaytechniques
Thesemustonlybeusedwhenallothertechniqueshavefailedtosecureandmaintainanairwayandoxygenationineitherananesthetizedpatientoroneundergoingresuscitation.
Needlecricothyroidotomy.Thecricothyroidmembraneisidentifiedandpuncturedusingalargeborecannula(12-14gauge)attachedtoasyringe.Aspirationofairconfirmsthatthecannulalieswithinthetrachea.Thecannulaisthenangledtoabout45caudallyandadvancedofftheneedleintothetrachea(Fig.3.9).Ahigh-flowoxygensupplyisthenattachedtothecannulaandinsufflatedfor1secondfollowedbya4-secondrest.Expirationoccursviatheupperairwayasnormal.Thistechniqueonlyoxygenatesthepatientanddoesnoteliminatecarbondioxide.Itisthereforelimitedtoabout30minutesusewhileadefinitiveairwayiscreated.
Surgicalcricothyroidotomy.Thisinvolvesmakinganincisionthroughthecricothyroidmembranetoallowtheintroductionofa5.0-6.0mmdiametertracheostomytubeortrachealtube(Fig.3.10).Itismoredifficulttoperform.Andresultsinsignificantlymorebleeding,thantheabove.Theadvantagesarethatonceatubeofthisdiameterhasbeeninserted,thepatientcanbeadequatelyventilated,ensuringoxygenation,eliminationofcarbondioxideandtheairwaysuctionedtoremoveanybloodordebris.
CHAPTER4
DrugsAssociatedWithAnesthesia
Intravenousanesthetic Neuromuscularblocking Analgesicsin
anesthesia,71
(induction)agents,53drugsandtheir Furtherreading,77
Inhalationalinduction,56antagonism,64
Inhalationalagentsandintravenousinfusions,57
Theanesthetisthastobefamiliarwithawiderangeofdrugstofacilitatethesafeandeffectiveconductofanesthesia.Unlikemostotherbranchesofmedicine,thesearealmostalwaysadministeredpaenterally;eitherintravenouslyorviainhalation.Unfortunately,aswellastheirdesiredeffectonthecentralnervoussystem,thesedrugshaveundesirableactionsonmanyotherbodysystemsofwhichtheanesthetistmustbefullyaware.
Intravenousanesthetic(induction)agents
Thesesaredrugswhichareusedtostart(induce)anesthesia.Afterintravenous(i.v.)administration,consciousnessislostinlittlemorethanthetimeittakesforthesedrugstogetfromthesiteofadministrationtothebrain(onearm-braincirculationtime).Generally,consciousnessisregainedbyredistributionfromthebraintoothertissues.Currently,onlypropofolisalsousedtomaintainanesthesia.Anesthesiacanalsobeinducedbytheinhalationofanincreasingconcentrationofavolatileagent.
Becauseoftheadverseeffectsofthei.v.agentsonthecardiovascularsystem,muchlowerdosesshouldbeadministeredinelderly,frailorshockedpatients.
SODIUMTHIOPENTONE(INTRAVAL)
Thisisshortactingbarbiturate,thedoserequiredforinductionisusuallybetween2and7mg/kg.Inductionofanesthesiaisrapidandsmooth,takingabout15-20seconds,exceptinthosewhosecirculationisdelayed,forexamplepatientswhoareelderly,hypovolemic,orhavecardiacdisease.Patientsmaycommentonbeingabletotasteonionsorgarlicduringadministration.Consciousnessusuallyreturnsafter4-10minutesastheresultofredistribution,followedbyaprolongedperiod(=24hours)ofmetabolismbytheliverandexcretionbythekidneys.Repeateddosesareassociatedwithaccumulationanddelayedrecovery.
Systemiceffects
Hypotensionoccurssecondarytomyocardialdepressionandvenodilation.Thisisexaggeratedinthosewhoarehypovolemicorhavealimitedcardiovascularreserve.
Ashortperiodofbreathholdingoccursfollowedbydepressionofventilation.
Sodiumthiopentoneisapotentanticonvulsant.Cerebralmetabolism,bloodflowandintracranialpressurearereduced.
Aswithallbarbiturates,administrationmayexacerbateporphyria.PROPOFOL(DIPRIVAN)
Propofolispreparedasanemulsion.Thedoserequiredtoinduceanesthesiaisusuallybetween1.5and2.5mg/kg.Thirtytofortypercentofpatientscomplainofpainorburningoninjection.Inductionofanesthesiaisrapidbutlessdefinitethanwiththiopentone.Involuntarymovementsaresometimesseen.After4-7minutes,thereisarapid,fullrecoveryofconsciousness,apropertythathasmadepropofolpopularforday-casesurgery.Postoperativenauseaandvomitingareuncommon.Repeatdoses(oraconstantinfusion)canbeusedtomaintainanesthesia.
Systemiceffects
Hypotensioniscommon,secondarytovasodilatation.
Apneaiscommonafteraninductiondoseandmaylastfor40-50seconds.
Ventilationandtheresponsetocarbondioxidearedepressed.laryngealreflexesappeartobedepressed,laryngealspasmuncommon.
Cerebralmetabolismandbloodflowarereducedalongwithintracranialpressure.
Musclerelaxationinmorepronouncedafterpropofol,particularlywhenusedasaconstantinfusiontomaintainanesthesia.
ETOMIDATE(HYPNOMIDATE)
Thedoseforinductionofanesthesiais0.2-0.3mg/kg.Painoninjectioniscommon.Inductionisrapid,butisfrequentlyassociatedwithmuscletwitchingandinvoluntarymovement.Recoveryoccursafter4-8minutes,followingredistributionfromthebrain.Etomidateisnon-cumulative,evenafterseveraldoses.
Systemiceffects
Causesaslightfallincardiacoutputandminimalvasodilatation.Bloodpressureisbettermaintainedthanwithotheragents,makingitpopularforuseinsickpatients.
Causesdose-dependentdepressionofventilationandadecreasedventilatoryresponsetocarbondioxide.
Reducescerebralmetabolism,bloodflowandintracranialpressure,anditisanticonvulsant.
Itdoesnotcausehistaminereleaseandallergicreactionsareextremelyrare.
Despitebeingnon-cumulative,prolongedusesuppressesadrenocorticalfunction,impairingrecoveryincriticallyillpatients.
KETAMINE(KETALAR)
Thei.v.doseis1-2mg/kg,followingwhichconsciousnessislostover1minute,lastingfor10-20minutes.Ketaminecanalsobeadministeredintramuscularlytoinduceanesthesia.Theintramuscular(i.m.)doseis5-10
mg/kganditmaytake8-10minutestoloseconsciousness.Thesubsequentdurationofactionisvariable.Repeatbolusesoraninfusioncanbeadministeredtomaintainanesthesia.Vividhallucinationsarecommonduringrecoveryandcanbeminimizedbytheconcurrentuseofabenzodiazepine(e.g.midazolam).
Systemiceffects
Heartrate,bloodpressureandcardiacoutputarewellmaintainedeveninshockedpatients,makingitusefulduringemergencysurgery.
Thereisminimaldepressionofventilation.
Laryngealreflexesarealsomaintainedbetterthanwithotheragentsandbronchodilatationoccurs.
Advantages
Theabilitytoadministerketaminebyi.m.injectionisusefulwhenvenousaccessisdifficult.Theprofoundanalgesiceffectscanbeobtainedatsubanestheticdoses.Thispropertyisutilizedwhenpatients,oftenchildren,undergorepeatedminorpainfulprocedures,forexampleburnsdressings.Itisoccasionallyusedasthesoleagentinsoleagentinadversecircumstances,forexampleinprehospitalcaretofacilitateextricationofseverelyinjuredvictims.
METHOHEXITONE(BRIETAL)
Thisisashorteractingbarbituratethanthiopentone.Itwasoriginallypopularforshortproceduresanddaycasesbecauseoftheimprovedrecoverycomparedwiththiopentone.Ithasnowlargelybeensupersededbypropofol.
MIDAZOLAM
Thisisashort-acting,water-solublebenzodiazepine.Itisnotatrueinductionagentasitdoesnotproducearapidlossofconsciousness.Thedoserequiredtoinduceanesthesiaisbetween0.1and0.3mg/kg.Itmaytake30-45secondsbeforeconsciousnessislostwiththeend-pointbeingveryindistinct.Recoveryoccursafter10-15minutesandisassociatedwithalongerperiodofamnesia.
Systemiceffects
Causesrelativelylittlecardiovasculardepression.
Itisapotentrespiratorydepressant,particularlyintheelderly.
Ithasmildanticonvulsantandmusclerelaxantproperties.
Inhalationalinduction
Thismaybeusedwheni.v.inductionofanesthesiaisnotpractical,forexampleinanuncooperativechildorapatientwithalackofsuitableveins.Anesthesiaisinducedrelativelyslowlyandrespirationispreserved.Thisisthereforeausefultechniquein-patientswithairwaycompromise,whenani.v.agentmaycauseapnea,andventilationandoxygenationbecomeimpossiblewithcatastrophicresults.
Thepatientbreathesanincreasingconcentrationofaninhalationalagentinoxygen(ifthereisairwaycompromise),orinamixtureofoxygenandnitrousoxide.Adequacyofanesthesiaisassessed(andoverdosageavoided)basedonclinicalsignsor‘stagesofanesthesia’.TheoriginaldescriptionofthesestagesbyA.E.Guedelwasbasedonusingether,butthemainfeaturescanstillbeseen
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