临床麻醉学-英文版2

整理文档整理文档CHAPTER3ManagingtheAirwayBasictechniques,37Thelaryngealmaskairway,42EmergencyairwaySimpleadjuncts,39Trachealintubation,43techniques,50Maintenanceofapatentairwayisanessentialprerequisiteforthesafeandsuccessfulconductofanesthesia.Inaddition,duringresuscitationpatientsoftenhaveanobstructedairwayeitherasthecauseorresultoftheirlossofconsciousness.Theskillofairwaymaintenanceshouldbeacquiredbyalldoctors,andnotsimplyregardedastheresponsibilityoftheanesthetist.Thedescriptionsofairwaymanagementtechniques,whichfollow,areintendedtosupplementpracticeeitheronamanikinorpreferablyonandanesthetizedpatientunderthedirectionofaskilledanesthetist.BasictechniquesAnesthesiafrequentlyresultsinlossoftheairwayanditismosteasilyrestoredbyacombinationoftheheadtiltalongwithajawthrust(seeChapter2).Thelatterisprovidedbytheanesthetist’sfourthandfifthfingers(ofoneorbothhands)liftingtheangleofthemandible.Theoveralleffectdesiredisthatthepatient’smandibleis‘lifted’intothemaskratherthanthemaskbeingpushedintotheface(Fig.3.1).FACEMASKSThemostcommonlyusedtypeinadultsistheBOCanatomicalfacemask(Fig.3.2)whichisdesignedtofitthecontoursofthefacewiththeminimumofpressure.Leakageofanestheticgasesisminimizedbyanair-filledcuffaroundtheedge.Masksatemadeinavarietyofsizesandthesmallestone,whichprovidesagoodseal,shouldbeused(tominimizetheincreaseindeadspace,whichoccurs).TheAmbumask(Fig.3.2)hasatransparentbody—allowingidentificationofvomit-makingitpoplarforresuscitation.Allmasksmustbedisinfectedbetweeneachpatient.SimpleadjunctsThemostcommonlyandusedaretheoropharyngeal(Guedel)andnasopharyngealairways,insertedaftertheinductionofanesthesiatohelpmaintaintheairwayinconjunctionwiththetechniquesdescribedabove.OROPHARYNGEALAIRWAYThesearecurvedplastictubes,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).NASOPHARYNGEALAIRWAYTheseareround,malleableplastictubes,beveledatthepharyngealendandflangedatthenasalend.Theyaresizedontheirinternaldiameterinmillimeters,withlengthincreasingwithdiameter.Thecommonsizesinadultsare6-8mm,forsmalltolargeadults,respectively.Aguidetothecorrectsizeismadebycomparingthediametertotheexternalnaris.Priortoinsertion,thepatencyofthenostril(usuallytheright)shouldbecheckedandtheairwaylubricated.Theairwayisinsertedalongthefloorofthenose,withthebevelfacingmediallytoavoidcatchingtheturbinates(Fig.3.4).Asafetypinmaybeinsertedthroughtheflangetopreventinhalationoftheairway.Ifobstructionisencountered,forceshouldnotbeusedasseverebleedingmaybeprovoked.Instead,theothernostrilcanbetried.PROBLEMSWITHAIRWAYSThepresenceofsnoring,indrawingofthesupraclavicular,suprasternalandintercastalspaces,useoftheaccessorymusclesorparadoxicalrespiratorymovement(see-sawrespiration)suggestthattheabovemethodsatefailingtomaintainapatentairway.Commonproblemsarisingusingthesetechniquesalongwithafacemaskduringanesthesiaare:inabilitytomaintainagoodsealbetweenthepatient’sfaceandthemask,particularlyinthosewithoutteeth;fatigue,whenholdingthemaskforprolongedperiods;theriskofaspiration,duetothelossofupperairwayreflexes;theanesthetistisnotfreetodealwithanyotherproblems,whichmayarise.Thelaryngealmaskairway(LMA)ortrachealintubationmaybeusedtoovercometheseproblems.ThelaryngealmaskairwayThisdevicewasdesignedforuseinspontaneouslybreathingpatients.Itconsistsofa‘mask’,whichsitsoverthelaryngealopening,attachedtowhichisatube,whichprotrudesfromthemouthandconnectsdirectlytotheanestheticbreathingsystem.Ontheperimeterofthemaskisaninflatablecuff,whichcreatesasealandhelpstostabilizeit.TheLMAisproducedinavarietyofsizessuitableforallpatients,fromneonatestoadults,withsizes3and4beingthemostcommonlyusedinfemaleandmaleadults,respectively.PositivepressureventilationcanbeperformedviatheLMAprovidedthathighinflationpressureisavoided,otherwiseleakageoccurspastthecuff,reducingventilationandcausinggastricinflation.Aversionwithareinforcedtubeisalsoavailable.TheLMAisreusable,providedthatitissterilizedbetweeneachpatient.Theuseofthelaryngealmaskovercomessomeoftheproblemsoftheprevioustechniques:itisnotaffectedbytheshapeofthepatient’sfaceortheabsenceofteeth;theanesthetistisnotrequiredtoholditinposition,avoidingfatigueandallowinganyotherproblemstobedealtwith;itreducestheriskofaspirationofregurgitatedgastriccontents,butdoesnoteliminateit.Itsuseisrelativelycontraindicatedwherethereisanincreasedriskofregurgitation,forexampleinemergencycases,pregnancyandpatientswithahiatushernia.Recently,thelaryngealmaskhasbeenshowntobeusefulintwootherareas:Indifficulttrachealintubationwhereitwilloftenallowmaintenanceoftheairway.Alternatively,asmalldiametertrachealtubeorintroducecanbepassedintothelarynxviatheLMA.Duringcardiopulmonaryresuscitation,ithasbeenshownthatnon-anesthetistsareabletoinsertanLMAmorerapidlyandsuccessfullythanatrachealtubeandachievemoreeffectiveventilationthanusingaself-inflatingbagandfacemask.ItislikelythatinthefuturetheLMAwillfindaroleinairwaymanagementduringresuscitation.TECHNIQUEFORINSERTIONThepatient’sreflexesmustbesuppressedtoalevelsimilartotherequiredfortheinsertionofanoropharyngealairwaytopreventcoughingorlaryngospasm.Thecuffisdeflatedandthemasklightlylubricated(Fig.3.5a).Aheadtiltisperformed,thepatient,smouthopenedfullyandthetipofthemaskinsertedalongthelardpalatewiththeopensidefacingbutnottouchingthetongue(Fig.3.5b).Themaskisthenfurtherinserted,usingtheindexfingertoprovidesupportforthetube(Fig.3.5c).Eventually,resistancewillbefeltatthepointwherethetipofthemaskliesattheupperoesophagealsphincter(Fig.3.5d).Thecuffisnowfullyinflatedusinganair-filledsyringeattachedtothevalveattheendofthepilottube(Fig.3.5e).Thelaryngealmaskissecuredeitherbyalengthofbandageoradhesivestrappingattachedtotheprotrudingtube.TrachealintubationThisisthebestmethodofprovidingandsecuringaclearairwayin-patientsduringanesthesiaandresuscitation,butsuccessrequiresabolitionofthelaryngealreflexes.Duringanesthesia,thisisusuallyachievedbytheadministrationofamusclerelaxant(seeChapter4).Deepinhalationalanesthesiaorlocalanesthesiaofthelarynxcanalsobeused,buttheseareusuallyreservedforuseinthosepatientswheredifficultywithintubationisanticipated,forexampleinthepresenceofairwaytumorsorimmobilityofthecervicalspine.COMMONINDICATIONSFORTRACHEALINTUBATIONWheremusclerelaxantsateusedtofacilitatesurgery(e.g.abdominalandthoracicsurgery)therebynecessitatingtheuseofmechanicalventilation.In-patientswithafullstomach,toprotectagainstaspirationofregurgitatedgastriccontents.Wherethepositionofthepatientwouldotherwisemakemaintenanceoftheairwaydifficult,forexamplethelateralorproneposition.Wherethereiscompetitionbetweensurgeonandanesthetistfortheairway(e.g.operationsontheheadandneck).Inthosepatientsinwhomtheairwaycannotbesatisfactorilymaintainedbyanyothertechnique.Duringcardiopulmonaryresuscitationwhenintubationallows:ventilationwith100%oxygenwithoutleaks;suctionclearanceofinhaleddebris;aroutefortheadministrationofdrugs.EQUIPMENTFORTRACHEALINTUBATIONAvarietyofequipmentexistsandthatchosenwillbedeterminedbythecircumstancesandbythepreferencesoftheindividualanesthetist.Thefollowingisalistofthebasicneedsforadultoralintubation.Laryngoscopewithacurved(Macintosh)bladeandfunctioninglight.Trachealtubesinavarietyofsizesandinwhichthecuffswork..Theinternaldiameterisexpressedinmillimetersandthelengthincentimeters.Theymaybelightlylubricated.Formales:8.0—9.0mminternaldiameter,22—24cmlengths.Forfemales:7.5—8.5mminternaldiameter,20—22cmlengths.syringetoinflatethecuffoncethetubeisinplace.Cathetermountsor‘elbow’toconnectthetubetotheanestheticsystemorventilatortubing.Suction,switchedonandimmediatelytohandincasethepatientvomitsorregurgitates.Extras:asemi-rigidintroducertohelpmouldthetubetoaparticularshape;Magill’sforceps,designedtoreachintothepharynxtoremovedebrisordirectthetipofatube;bandageortapetosecurethetube.TrachealtubesTheseweretraditionallymanufacturedfromredrubberandwerereusable.However,disposableplastic(PVC)onesarenowwidelyusedtoeliminatecross-infectionandarechemicallylessirritanttothelarynx(Fig.3.6).Tubesaresizedanaccordingtotheirinternaldiameterinmillimetersandaremanufacturedinhalf-millimeterintervals.Theyarelongenoughtobeusedorallyornasally.Astandard15-mmconnectorisprovidedtoallowconnectiontothebreathingsystem.Inadultanesthesia,atrachealtubewithaninflatablecuffisusedtopreventleakageofanestheticgasesbackpastthetubewhenpositivepressureventilationisused.Thisalsohelpspreventaspirationofanyforeignmaterialintothelungs.Thecuffisinflatedbyinjectingairfromasyringeviaasmall-diametertube,atthedistalendofwhichisaone-wayvalvetopreventdeflationandasmallpilotballoonwhichindicateswhenthecuffisinflated.Awidevarietyofspecializedtubeshavebeendeveloped,examplesofwhichareshowninFig.3.6.Reinforcedtubesateusedtopreventkinkingandsubsequentobstructionofthetrachealtubeasaresultofthepositioningofthepatient’shead(Fig.3.6c).Preformedtubesareusedduringsurgeryontheheadandneckandaredesignedtotaketheconnectionsawayfromthesurgicalfield(Fig.3.6d).Doublelumentubesareeffectivelytwotubesweldedtogetherside-by-side,withonetubeextendingdistallybeyondtheother.Theyareusedduringthoracicsurgery,andplacedsuchthatthedistaltubelieswithinonemainbronchus(endobronchial).Thisallowstheotherlungtobedeflatedtofacilitateviatheendobronchialportion(Fig.3.6E).Inchildrenunderapproximately10yearsofage,uncuffedtubesareusedasanaturalsealisprovidedbythenarrowinginthesubglotticregion(Fig.3.6F).THETECHNIQUEOFORALINTUBATIONThisrequiresabolitionofthelaryngealreflexesandappropriatemonitoringofthepatient.PositioningThepatientispositionedwiththeneckflexedandtheheadextendedattheatlanto-occipitaljoint.Thisistheso-called‘sniffingthemorningair’position.Thepatient’smouthisfullyopenedusingtheindexfingerandthumboftherighthandinascissorsaction.LaryngoscopyThelaryngoscopeisalwaysheldinthelefthandandthebladeisintroducedintothemouthalongtheright-handsideofthetongue,displacingittotheleft.Thebladeisadvanceduntilthetipliesinthegapbetweenthebaseofthetongueandtheepiglottis,thevallecula.Forceisthenappliedinthedirectioninwhichthehandleofthelaryngoscopeispointing,theeffortcomingfromtheupperarmnotthewrist,toliftthetongueandepiglottistoexposethelarynx.Thisshouldbeseenasatriangularopening,withtheapexanteriorlyandthewhitishcoloredtruecordslaterally(Fig.3.7).IntubationThetrachealtubeisintroducedintotherightsideofthemouth,advancedandseentopassthroughthecordsuntilthecuffliesjustbelowthecords.Thetubeisthenheldfirmlybythefingersoftherighthandandthelaryngoscopeiscarefullyremoved.Thecuffistheninflatedsufficientlytopreventanyleakduringventilation.Finallythepositionofthetubeisconfirmedbylisteningforbreathsoundsinbothaxillaeanditisthensecuredinplace.Fornasotrachealintubation;awell-lubricatedtubeisintroducedusuallyviatherightnostrilalongthefloorofthenosewiththebevelpointingmediallytoavoiddamagetotheturbinates.Itisadvancedintotheoropharynx,whereitisusuallyvisualizedusingalaryngoscopeinthemannerdescribedabove.Itcantheneitherbeadvanceddirectlyintothelarynxbypushingontheproximalend,orthetippickedupwithMagill’sforceps(whicharedesignednottoimpairtheviewofthelarynxanddirectedintothelarynx.Theprocedurethencontinuesasfororalintubation.DIFFICULTINTUBATIONOccasionally,intubationofthetracheaismadedifficultbecauseofaninabilitytovisualizethelarynx.Thismayhavebeenpredictedatthepreoperativeassessmentormaybeunexpected.Avarietyoftechniqueshavebeendescribedtohelpsolvethisproblemandincludethefollowing:manipulationofthethyroidcartilagebydownwardsandupwardspressurebyanassistanttotryandbringthelarynxoritsposterioraspectintoview;atlaryngosopy,agumelasticbougie,60cmlong,isinsertedintothetrachea,overwhichthetrachealtubeis‘railroaded’intoplace;afibreopticbronchoscopeisintroducedintothetracheaviathemouthornoseandisusedasaguideoverwhichatubecanbepassedintothetrachea.Thistechniquehastheadvantagethatitcanbeusedineitheranesthetizedorawakepatients.COMPLICATIONSOFTRACHEALINTUBATIONThefollowingisonemethodsofcategorizingthem,butitisnotanattempttocoveralloccurrences.HypoxiaOesophagealintubation.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).TraumaDirectlyduringlaryngoscogyandinsertionofthetubetolips,teeth,gongue,pharynx,larynx,trachea,andnoseandnasopharynxduringnasalintubation;causingsofttissueswellingorbleeding.Indirectlytothemandible(dislocation),andthecervicalspineandcord,particularlywherethereispre-existingdegenerativediseaseortrauma.ReflexactivityHypertensionanddysrhythmias.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.EmergencyairwaytechniquesThesemustonlybeusedwhenallothertechniqueshavefailedtosecureandmaintainanairwayandoxygenationineitherananesthetizedpatientoroneundergoingresuscitation.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.CHAPTER4DrugsAssociatedWithAnesthesiaIntravenousanestheticNeuromuscularblockingAnalgesicsinanesthesia,71(induction)agents,53drugsandtheir Furtherreading,77Inhalationalinduction,56antagonism,64Inhalationalagentsandintravenousinfusions,57Theanesthetisthastobefamiliarwithawiderangeofdrugstofacilitatethesafeandeffectiveconductofanesthesia.Unlikemostotherbranchesofmedicine,thesearealmostalwaysadministeredpaenterally;eitherintravenouslyorviainhalation.Unfortunately,aswellastheirdesiredeffectonthecentralnervoussystem,thesedrugshaveundesirableactionsonmanyotherbodysystemsofwhichtheanesthetistmustbefullyaware.Intravenousanesthetic(induction)agentsThesesaredrugswhichareusedtostart(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.SystemiceffectsHypotensionoccurssecondarytomyocardialdepressionandvenodilation.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.SystemiceffectsHypotensioniscommon,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.SystemiceffectsCausesaslightfallincardiacoutputandminimalvasodilatation.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.)doseis5-10mg/kganditmaytake8-10minutestoloseconsciousness.Thesubsequentdurationofactionisvariable.Repeatbolusesoraninfusioncanbeadministeredtomaintainanesthesia.Vividhallucinationsarecommonduringrecoveryandcanbeminimizedbytheconcurrentuseofabenzodiazepine(e.g.midazolam).SystemiceffectsHeartrate,bloodpressureandcardiacoutputarewellmaintainedeveninshockedpatients,makingitusefulduringemergencysurgery.Thereisminimaldepressionofventilation.Laryngealreflexesarealsomaintainedbetterthanwithotheragentsandbronchodilatationoccurs.AdvantagesTheabilitytoadministerketaminebyi.m.injectionisusefulwhenvenousaccessisdifficult.Theprofoundanalgesiceffectscanbeobtainedatsubanestheticdoses.Thispropertyisutilizedwhenpatients,oftenchildren,undergorepeatedminorpainfulprocedures,forexampleburnsdressings.Itisoccasionallyusedasthesoleagentinsoleagentinadversecircumstances,forexampleinprehospitalcaretofacilitateextricationofseverelyinjuredvictims.METHOHEXITONE(BRIETAL)Thisisashorteractingbarbituratethanthiopentone.Itwasoriginallypopularforshortproceduresanddaycasesbecauseoftheimprovedrecoverycomparedwiththiopentone.Ithasnowlargelybeensupersededbypropofol.MIDAZOLAMSystemiceffects•Systemiceffects•Causesrelativelylittlecardiovasculardepression.Itisapotentrespiratorydepressant,particularlyintheelderly.Ithasmildanticonvulsantandmusclerelaxantproperties.InhalationalinductionThismaybeusedwheni.v.inductionofanesthesiaisnotpractical,forexampleinanuncooperativechildorapatientwithalackofsuitableveins.Anesthesiaisinducedrelativelyslowlyandrespirationispreserved.Thisisthereforeausefultechniquein-patientswithairwaycompromise,whenani.v.agentmaycauseapnea,andventilationandoxygenationbecomeimpossiblewithcatastrophicresults.Thepatientbreathesanincreasingconcentrationofaninhalationalagentinoxygen(ifthereisairwaycompromise),orinamixtureofoxygenandnitrousoxide.Adequacyofanesthesiaisassessed(andoverdosageavoided)basedonclinicalsignsor‘stagesofanesthesia’.TheoriginaldescriptionofthesestagesbyA.E.Guedelwasbasedonusingether,butthemainfeaturescanstillbeseenusingmodernagents.However,theywillbemodifiedbytheconcurrentadministrationofopiatesoranticholinerfics.FirststageThislastsfromstartingtheinhalationuntilconsciousnessislost.Thepupilswillbenormalinsizeandreactive,muscletoneisnormalandbreathingusesintercostalmusclesandthediaphragm.SecondstageInthisperiodtheremaybebreath-holding,strugglingandcoughing.Itisoftenreferredtoasthestageofexcitation.Thepupilswillbedilatedandthereislossoftheeyelashreflex.ThirdstageThisist