The term exotic species is very broad and can cover anything from a skink to an elephant For the purposes of this article it will be tak en to mean those smaller animals encountered in general practice small reptiles small rodents and birds typical ID: 55993
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Practic alconsiderationsinanaesthetisingexoticspecies By KeithSimpson BVScMRCVSAMIIE(Electronics). Thetermexoticspeciesisverybroadandcancoveranythingfroma skink to anelephant.Forthepurposesofthisarticleitwillbetak entomeanthose smalleranimalsencounteredingeneralpractice smallreptiles,small rodentsandbirds ,typicallyunder oraround 1kg . Theprinciplesofgeneralanaesthesiaapplyequallytotheexoticspeciesas theydotosaydogsandcats. Tidalvolu mes,oxygenrequirements,CO 2 elimination , respiratoryrates andbodytemperature mustallbemaintained underanaesthesia. Oneofthemostcommonproblemsindealingwith anaesthesiainthese animals isthesupportandmonitoringofadequate respiration, whererespirationincludeslungventilationaswellastissue respiration. MonitoringSpO2insuchsmallan imalscanbequitechallenging duetothelowpulsevolumesanddifficultyinprobeplacement.Capnography offersaviableandarguablysuperioralte rnativemeansofmonitoringthese animals,althoughthistoohaslimitations aswillbeexplained . Oftenthevolatileanaestheticagentsarepreferred forinduction becauseof therelativeeaseofadministrationandcontrol.Howeverthisroutealso presen tsproblems , inthatduetothesmallsizeofthepatient , relative overdosesarenotuncommonandthedangerofapnoeaorrespiratoryarrest areincreased. Notonlythatbutsomepatientswillsimplydefyyoubynot breathingyouragent forminutesonend !Dependingonthespecies , anaesthesiamayinvolveintravenousagentsfirst , followedbyinhalation : Thereisatendencytoruninthegasat5%untiltheanimalgoestosleepand the nfishitout.Thisisoftenbecausethetimetakentoachieveanaesthesiais otherwisetoolong.Thesolutionis NOT toruninahighergasconcentration, buttochooseasmallerchamber.Otherwisethepatient,sensingthegas, reducesorholdsitsbreath untilitfinallysuccumbsatwhichpointitwilltake several fulllungsof5%agentwhich maythenprovedevastatingintermsof respiratory d epression/cardiovascularoutput.Muchbettertorunina1.5 - 2.0% mixtureforslightlylonger. Onceyourexotic patientisasleepyoumayormaynotchoosetoventilate artificially.OftenIPPVisnotemployedbecauseoftheextraeffort required but therewardsfar out w eigh thateffort. WhyuseIPPV? Oxygenisadequatelydelivered Anaestheticagentisadequatel yandreliablydelivered Wastegases , particularlyCO2 , are reliably eliminated Ifyourelyonspontaneousbreathingthethreeactionsabovemaynot necessarilyoccurasyouwouldlike. ButhowtocontrolthatIPPV? I PPV canbedonemanuallybutextrem ecaremustbetakentoavoid over/under - inflation.I PPV canbedonewithaventilatorbutagaincaremust betakeninchoosinganappropriateventilator.Youarenotgoingtobeableto adaptyourManleyforthistask ! Volumecycledventilatorsshould gen erally be avoidedforthesesmallanimals becauset hemarginoferrorinvolume deliveryisverysmall. I fyoudeterminethatyourbudgierequires 3 mlstidal volumeanditsactualrequirementis 2 50 %over - inflation. Dependingonthecomplianc eofthechest , thatmayormaynotcauseabig riseinlungpressure Thesamewilloccurifyoutemporarilyrestrictthechest (byleaningonitorjustrestingyourhandonit) - t hesamevolumeina restrictedspacewillagainleadtoadramaticrisein pressure. Pressure - cyclingventilatorsare therefore bettersuitedtothesesmallanimals.Choose onethatcanmonitorpressuresdowntoafewcmofwaterpressureandupto atleast25cm. Buthowdoyoudeterminethepressuretoset? The reisnocalcul ationyoucando butnature , thankfullyisfairlyuniform acrossthespecies.Mostanimals requireventilationpressuresofbetween 5 and12cmwaterpressure.Thisappliesacrosstheboard from micetogreat danes. Havingobtainedapressure - cycledven tilatorhowdoyouconnectit toyour, oftentiny,patient ? InordertouseIPPVyourpatientisgoingtohavetobeintubated. Thisoften requiressomeingenuity.Belowafewhundredgrammesyoursmallest2.5mm ETtubeislookingfartoobig.Thenextste pistousevarioussizesofintra - venouscatheters. Youwillneedtochooseoneasclosetothetracheasizeas possible.AsnugfitwillimproveyourIPPV. These catheters comewitha standardluerfittingonthem andyouwillfindthatthismeanstheyw illpush neatlyontoa 2.5mm PortexETconnector.A l so, ifyouchooseoneofthese connectorswithasamplingsideportonit , monitoringend - tidalCO2(sidestream seelater )withminimaldeadspace. Note - ThePorte xcodefora2.5mmRSPsideportconnectoris 10025 - 05S. Picture 1 . PortexETconnectorwithside - port CalculatingtheMinuteVolume a. Weighyourpatient b. CalculatetheTidalVolume.Averybasi cruleofthumbis 10ml/kgtidalvolume c. MinuteVolume=breathsperminutextidalvolume d. Setthefreshgasflowrateto3xMinuteVolume Why3xMinute V olume? theI:Eratiois1:2.Ina spontaneouslybreathinganimalyou mustprovideenough FreshGasFlow ( FGF )tomeetthe transientdemandofinspiration.WithIPPVyouwanttomimic onlygotathirdofthetimetogetthegas in,sotheFGFmustbe 3xtheMinute V olume.IftheI:Eratiowas1:1theFGFcouldbe setto2xMinute V olume. Thefollowingisadescriptionofthestepstofollowwhenusinga p ressure - c ycled v entilator,basedontheVetronicServices,SAV03SmallAnimal Ventilator. Thisventilatorhasbeenusedextensivelyforexoticanaesthesia sinceitsintroductionin1994. P icture 2.System connectionforIPPV withtheVetronic ServicesSAV03 ventilator EstimatetheMinuteVolume.Seeinsetfordetails . SettheExpiratorylengthtominimum. Setthe FGF ra teto3timestheminutevolume(seeinset). Setthetriggerpressureto3or4cmwaterpressure. You initially Ob servetheanimalanda djustthetriggerpressurefornormalchest movements. AdjusttheExpiratorylength togiveanormalrespiratoryrate. Rememberthatintheverysmallanimalsthisdoesnotneedtoequate withtheirconsciousrate.Verysmallanimals expendanawfullotof metabolicenergyjustbreathing , b Therefore,ifyoukeepthemwarm (a mustforanysmallanimal anaesthetic)theiroxygenrequirement(andCO2output)actuallydrops. Oxygendemandcandropbyasm uchasathirdinthissituation. AdjusttheFGFtogiveanormal I nspiratoryduration. Whenanaesthetisingthesepatientsitisoftenaftergasinductionandthey maylightenduringintubation.Thiscancauseaproblemduringtheinitial stagesasthea nimalmakesrapidspontaneousbreathingefforts.Thesolution tothisistoquicklyandadequatelyventilatethelungswithgas/anaesthetic andtoblowofftheirCO2.TodothissettheExpiratorytimetominimumand increasethePressureSettingsothatt heanimalreceivesafullinflationfor everybreath.Thisnormallyovercomestheanimalsowneffortswithinabout 20 - 30seconds. Havingsafelyanaesthetisedandventilatedyourpatient,howcanyouassess itsvitalsignsduringtheanaesthetic? Therear eanumberofparametersyoushouldbecheckingroutinely,justas youwouldinanyanaesthetic. Heartrate Chestmovements/respirationrate Responsetonoxiousstimuli Bodytemperature Justthesealonegiveagoodideaofpatientstatusbutbecauseresp irationis controlled , theonlywarningsignyoumayhaveisheartrate.Thismayleave youfeelingslightlyuneasyasrespirationmay previously havebeenan importantfactorindetermininganaestheticdepth. ADoppler bloodflow monitorcanbeveryreass uringintermsoflisteningtoandfollowingthepulse rateascanastethoscope.Variationsinheartratemayindicate lightening/deepeningofanaesthesia,painresponseetc.Whilstthesearevery important , aventilatedpatientpresentsamonitoringchal lengeastheabove donotgiveareliableguidetotheadequacyorotherwiseoftheventilation. Thereforeadditionalmonitoringtechniquesneedtobeemployed. Sucha dditionalmonitoringaidsare EndtidalCO2 Oxygensaturation Thesetwocanbemonitore dby aCapnograph and aPulse - Oximeter respectively .Pulse - Oximetrymaynotbeasusefulasanticipatedforanumber ofreasons 1) Thepatientsareverysmallandobtainingareadingcanbedifficult 2) Patientsaretypicallybeingmaintainedon100%oxygen so asevere problemmustoccurbeforeanyfallinSpO2isnoticed.Foranimalsthat areeitherbreathingspontaneouslyorarebeingventilatedonroomair aPulse - Oxwouldbeveryusefulaslongasyoucangetareliable reading. IfyouaregoingtouseaPu lse - Oximeterbetterreadingsarelikelytobe obtainedbyareflect ance proberatherthanatrans flectance probe ,although sometimesgoodreadingscanbeobtainedbyplacingtheprobeacrossafoot pad(seeChameleonPhoto)orawingartery .Withsomesmal lreptilesyou cangetareadingbyplacingthereflectiveprobe neartheheart.Reflective probesneedtobetapedintoplace. Picture3. Intubated chameleonwith IPPV,intra - osseousfluids& SpO2 monitoring . PhotoCourtesy ofKevinEatwell, BirchHea th Veterinary Centre. UsingaCapnographcangivegoodinformationontheadequacyofpatient respiration.Thiswillalmostcertainlyneedtobeasidestreamunitifyouwish tomonitorverysmallanimals.Amainstreamunitwouldrequireplacingin line witha15mmconnectorwhichhastwodistinctdisadvantages 1) Thereisafiniteand , dependingontheanimal,alargedeadspace volume. 2) Yourpatient needsto be intubat ed and CO 2 monitoring stop s on extubation. Usingasidestreamdevicewillovercome thesetwoproblems.Asidestream devicedrawsoffasmallamountofgasviaasamplingtube.Thistubecan samplefromtheside - portofanETconnectorordirectlyfromanostrilorfrom insideafacemask.Thelowerlimitofsidestreamdevicesdependsupo nthe samplingrate.TheCapnovet - 10fromVetronicServiceshas aminimum samplingrateof50ml/minute.Thisallowsanimalsdownto75gramstobe reliablymonitored ,althoughyouwillseesomechangestothewaveformin thesesizeanimals.Abigbonustho ughisthatyoucancontinuetomonitor yourpatientafterextubation ormonitoranimalsonafacemask . Therearesomehumanex - hospitalcapnographunitsonthemarketwhichcan oftenbepickedupquitecheaply.Thesewillnormallyhavesamplingratesof around200mls/minutewhichrenderthemunusableforanimalsbelowabout 2 kgsobewareoftheirlimitationsbeforeyoubuy. Therearemanyfactorsthatcomeintoplaywhendeterminingtheend - tidal CO2valueofverysmallanimals.The r eareunavoidableph ysicalfactorsof sizeandanaestheticset - upetcthatwillaffectnotonlytheend - tidalvaluebut alsothewaveformappearance.Thisisequallytrueofmainstreamand sidestreamdevices.ForthisreasonitisasimportanttolookfortrendsinCO2 values andforchangesinthewaveformprofileasitis tolookattheabsolute valueswhenmonitoringthesetinyanimals. Capnographunitsthatonlyhave anend - tidalvalueandnotracearelimitedintheinformationtheycangive. Capnographyisextremelyusef ulinanyanaesthetisedpatientbutespecially soforventilatedanimals.Otherwisehowdoyouknowhoweffectiveyour ventilationis? Whenusingcapnography,lookforthefollowing: Over - ventilation - indicatedbyafalling End - TidalCO2value. Under - ventilation - indicatedbyarisingEnd - TidalCO2value. AimtokeepEnd - Tidalvaluesbetween3.0%and5.0%(23mmHgand38 mmHg) toavoidrespiratoryalkalosisandacidosisrespectively. Lookforarapidriseofthewavefrombaselineonexpirationfollowe dbya slowlyrising (almostflat) plateauphase. An idealwaveformisshowninphoto 4. Picture 4 . Waveformseenwith ventilatedpatient. 20breathsper minute.End - Tidal CO2,4.4% Asrespiratoryrateincreasesthelengthoftheplateau phasewillreduceuntil atrapidbreathingratesthewaveformlooksmorelikeatriangle. Anumberof factorscontributetothisbutthedominantfactoristhedead - spacevolume. Shortrapidbreathsareassociatedwithareducingtidalvolumeuntilatsome pointtheexpiratoryvolumeisapproachingthedeadspacevolume.Theeffect seenthenisdilutiono fexpiredgasinthedeadspace.Thefollowingdiagram showstheeffect: Picture 5 .Dilution effectsoncapnogram appearance Notetheapparen tfallinend - tidalCO2causedbydilution.Asthetidalvolume decreasesthedilutioneffectismoremarked,hencethedecreaseinslopeof phaseII. Theimportantthingiswhetherthedeadspaceispredominantlyphysical ( tubingetc ) ,orphysiological ( justupperairway ) .Ifitispredominantlyphysical thepatientmayhaveanormalEnd - TidalCO2valuebutareducedmeasured End - TidalCO2value.Inthisinstancetryingtoachieveaplateauphase(no furtherdilution,constantCO2elimination)mayprove impossibleandyouwill havetolivewiththefactthatthemeasuredvalueislessthanactual. However,ifyouaremonitoringrightattheendofashortETtubewithasmall samplingrateonasidestreamunit thentheanimalhasatidalvolumenearits p hysiologicaldeadspace. Inthissituation,withnoplateauphase(phaseIII), theefficiencyofCO2eliminationis reducedandthetrueEnd - TidalCO2value maybealittlehigheroralothigherthanthatmeasured.Onlybyachievinga plateauphase(nodi lution)inthissituationcanyoubesureoftheactualEnd - TidalCO2value. Howtoavoidwaveformswithnoplateauphase Usethesmallestfittingsyoucaninthecommon(inspiredandexpired gas)airway. KeeptheETtubelengthasshortaspossible. Sa mpleforyourCapnographasclosetotheendofthetrachealtube aspossible - useaside - portsamplingETtubeconnector. Ifnotventilating,thenconsiderIPPVtocontrol thehyperventilation. If ventilating andthedea dspaceismainlyphysiological, c onsider increasingthetidalvolumeandreducingthebreathingrate. Someusefulstatistics: Waveformchangesassociatedwithincreasedrespiratory rateandfallingtidalvolume II III Item DeadSpace Volume (max) ETConnector 1.8mls 15mmY - connector 6.0mls 15mm - 15mmconnector withoxygenfeed 8.0mls 25mmofNo.3ETTube 0.1mls Luerh ub(female) 0.1mls Photo 5 showsarabbitreceivingIPPVwithcarbondioxidemonitoringviaan ETconnectorwithside - port. Photo 6 . Intubated Rabbit with IPPV, CO 2 andSpO 2 monitoring.PhotoCourtesyof KevinEatwell,BirchHeathVeterinaryC entre. Summary Duetothehugearrayofanimaltypesthisarticlecanonlybeageneralguide toanaesthesiainexoticspecies.Detailsofintravenousdrugsandfluid supportarelefttootherauthors.Thefundamentalsofexoticanaesthesiacan beseent obenodifferentfr omnormalmammaliananaesthesia although , due totheirsize , assistedventilationisdesirableandwiththatcomestheneedto reliablymonitorphysiologicalparameters. Further informationonCapnography ,Pulse - Oximetry andtheMechani csof V entilationcanbefoundonthe website , www.vetronic.co.uk