臨床麻醉學(xué)英文版

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