Spinal anesthesia.docx

Spinal anesthesia.docx

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Spinalanesthesia

Spinalanesthesia

Spinalanesthesiainvolvestheadministrationoflocalanestheticintothesubarachnoidspace.

A.Anatomy

1.Thespinalcanalextendsfromtheforamenmagnumtothesacralhiatus.Theboundariesofthebonycanalarethevertebralbodyanteriorly,thepedicleslaterally,andthespinousprocessesandlaminaeposteriorly

Vertebralanatomy.

2.Threeinterlaminarligamentsbindthevertebralprocessestogether:

a.Superficially,thesupraspinousligamentconnectstheapicesofthespinousprocesses.

b.Theinterspinousligamentconnectsthespinousprocessesontheirhorizontalsurface.

c.Theligamentumflavumconnectsthecaudaledgeofthevertebraeabovetothecephaladedgeofthelaminabelow.Thisligamentiscomposedofelasticfibersandisusuallyrecognizedbyitsincreasedresistancetopassageofaneedle.

3.Thespinalcordextendsthelengthofthevertebralcanalduringfetallife,endsataboutL-3atbirth,andmovesprogressivelycephaladtoreachtheadultpositionnearL-1by2yearsofage.Theconusmedullaris,lumbar,sacral,andcoccygealnerverootsbranchoutdistallytoformthecaudaequina.Itisinthisareaofthecanal（belowL-2）thatspinalneedlesareplaced,becausethemobilityofthenervesreducesthedangeroftraumafromtheneedle.

4.Thespinalcordisinvestedinthreemeninges:

a.Thepiamater.

b.Theduramater,whichisatoughfibroussheathrunninglongitudinallytheentirelengthofthespinalcordandistetheredcaudallyatS-2.

c.Thearachnoid,whichliesbetweenthepiaandduramater.

5.ThesubarachnoidspaceliesbetweenthepiamaterandthearachnoidandextendsfromtheattachmentoftheduraatS-2tothecerebralventriclesabove.Thespacecontainsthespinalcord,nerves,cerebrospinalfluid（CSF）,andbloodvesselsthatsupplythecord.

6.CSFisaclearcolorlessfluidthatfillsthesubarachnoidspace.ThetotalvolumeofCSFis100to150mL,whereasthevolumeinthespinalsubarachnoidspaceis25to35mL.CSFiscontinuouslyformedatarateof450mL/daybysecretionorultrafiltrationofplasmafromthechoroidarterialplexuseslocatedinthelateral,third,andfourthventricles.CSFisreabsorbedintothebloodstreamthroughthearachnoidvilliandgranulationsthatprotrudethroughduratolieincontactwiththeendotheliumofthecerebralvenoussinuses.

B.Physiology

1.Neuralblockade.SmallerCfibersconveyingautonomicimpulsesaremoreeasilyblockedthanthelargersensoryandmotorfibers.Asaresult,thelevelofautonomicblockadeextendsabovethelevelofthesensoryblockadebytwotothreesegments.Thisistermeddifferentialblockade.Similarly,fibersconveyingsensationaremoreeasilyblockedthanthelargermotorfiberssothatsensoryblockadewillextendabovethelevelofmotorblockade.

2.Cardiovascular.Hypotensionisdirectlyproportionaltothedegreeofsympatheticblockadeproduced.Sympatheticblockaderesultsindilatationofarteriesandvenouscapacitancevessels,leadingtodecreasedsystemicvascularresistanceanddecreasedvenousreturn.IftheblockisbelowT-4,increasedbaroreceptoractivityproducesanincreaseinactivitytothecardiacsympatheticfibersandvasoconstrictionoftheupperextremities.BlockadeaboveT-4interruptscardiacsympatheticfibers,leadingtobradycardia,decreasedcardiacoutput,andafurtherdecreaseinbloodpressure.Thesechangesaremoremarkedinpatientswhoarehypovolemic,elderly,orhaveobstructiontovenousreturn（e.g.,pregnancy）.Theseeffectscanbeminimizedwithprehydration,vasopressors,andanticholinergics.

3.Respiratory.Lowspinalanesthesiahasnoeffectonventilation.Withascendingheightoftheblockintothethoracicarea,thereisaprogressiveascendingintercostalmuscleparalysis.Thishaslittleeffectonventilationinthesupinesurgicalpatientwhostillhasdiaphragmaticfunctionmediatedbythephrenicnerve.Ventilationinpatientswithpoorrespiratoryreserve,suchasthemorbidlyobese,however,maybeprofoundlyimpaired.Bothintercostalandabdominalmuscleparalysisdecreasetheefficiencyofcoughing,whichmaybeimportantinpatientswithchronicobstructivepulmonarydisease.Epiduralanalgesiawithopioidsandlowdoselocalanesthetics,whichproducesminimalmotorblockade,ishelpfulinthepostoperativecareofthoracicsurgicalpatients.

4.Visceraleffects

a.Bladder.Sacralblockade（S2-S4）resultsinanatonicbladderthatisabletoretainlargevolumesofurine.Blockadeofsympatheticefferents（T5-L1）resultsinanincreaseinsphinctertone,producingretention.Aurinarycathetershouldbeplacedifanesthesiaoranalgesiaismaintainedforaprolongedperiod.

b.Intestine.Sympatheticblockade（T5-L1）producedbyspinalanesthesiahasapromotilityeffectonthegutbecauseofpredominanceofparasympathetictone.

5.Renalbloodflowismaintained,becauseofautoregulationbylocaltissuefactors,exceptwithseverehypotension.Urineproductionisusuallyunaffected.

6.Neuroendocrine.PeriduralblocktoT-5inhibitspartoftheneuralcomponentofthestressresponse,throughitsblockadeofsympatheticafferentstotheadrenalmedullaandblockadeofsympatheticandsomaticpathwaysmediatingpain.Othercomponentsofthestressresponseandcentralreleaseofhumoralfactorsareunaffected.Vagalafferentfibersfromupperabdominalvisceraarenotblockedandcanstimulatereleaseofhypothalamicandpituitaryhormones,suchasantidiuretichormoneandadrenocorticotropichormone.Glucosetoleranceandinsulinreleasearenormal.

7.Thermoregulation.Vasodilationofthelowerlimbscanproducehypothermia.

C.Technique

1.Spinalneedle.NewerneedlessuchastheSprotteandWhitacrefeatureapencil-pointdesignwithalateralopening.Theseneedlesmayreducetheincidenceofpostduralpunctureheadache（to≤1%）comparedwithtraditional“cuttingtip”needlesbysplittingratherthancuttingduralfibersduringinsertion.Needlesthatare24and25gaugeareeasilybentandareofteninsertedthrougha19-gaugeintroducerneedle.The22-gaugeQuinckeneedleismorerigidandismoreeasilydirectedandinserted.Itcanbeusefulinolderpatientsinwhomaccessmaybemoredifficultandtheincidenceofpostduralpunctureheadacheislow.

2.Patientposition.Thelateraldecubitus,prone,andsittingpositionscanbeusedforadministrationofspinalanesthesia.

a.Inthelateralposition,thepatientisplacedwiththeaffectedsideupifahypobaricorisobarictechniqueistobeusedandwiththeaffectedsidedownifahyperbarictechniqueistobeused.Thespineishorizontalandparalleltotheedgeofthetable.Thekneesaredrawnuptowardthechestandthechinflexeddownwardontothechesttoobtainmaximalflexionofthespine.

b.Thesittingpositionisusefulforlowspinalblocksrequiredincertaingynecologicandurologicproceduresandiscommonlyusedinobesepatientstoassistinidentificationofthemidline.Itisusedinconjunctionwithhyperbaricanesthetics.TheheadandshouldersareflexeddownwardontothetrunkwiththearmsrestingonaMayostand.Anassistantshouldbeavailabletostabilizethepatient,andthepatientshouldnotbeoversedated.

c.Thepronepositionisusedinconjunctionwithhypobaricorisobaricanestheticsforproceduresontherectum,perineum,andanus.Apronejackknifepositioncanbeusedforbothadministrationofspinalanesthesiaandthesubsequentsurgery.

3.Procedure

a.TheL2-3,L3-4,orL4-5interspacesarecommonlyusedforspinalanesthesia.TheL3-4interspaceorthespinousprocessofL-4arealignedwithupperbordersofthesuperioriliaccrests.

b.Disinfectalargeareaofskinwithanappropriateantisepticsolution.Caremustbetakentoavoidcontaminationofthespinalkitwithantisepticsolution,whichispotentiallyneurotoxic.

c.Checkthestyletforcorrectfitwithintheneedle.

d.Raiseaskinwhealwith1%lidocaineanda25-gaugeneedleatthespinalpuncturesite.

e.Approaches

1.Midline.Placethespinalneedle（orintroducer）throughtheskinwhealandintotheinterspinousligament.Theneedleshouldbeinthesameplaneasthespinousprocessesandangulatedslightlycephaladtowardtheinterlaminarspace.

Spinalneedleinsertion,lateralview.Fortheclassicmidlineapproach,theneedleisintroducedinthemiddleoftheinterspaceandadvancedwithaslightcephaladangulation.Ifcorrectlyangled（A）,itwillentertheinterspinousligament,ligamentumflavum,andepiduralspace.Ifboneiscontacted,itmaybetheinferiorspinousprocess（B）,andcephaladredirectionwillidentifythecorrectpath.Ifanglingcephaladcausescontactwithboneagainatashallowerdepth（C）,itisprobablythesuperiorspinousprocess.Ifboneisencounteredatthesamedepthafterseveralattemptsatredirection（notshown）,theneedleismostlikelyonthelaminalateraltotheinterspace,andthepositionofthetruemidlineshouldbereassessed.（FromMulroyMF.Regionalanesthesia:

anillustratedproceduralguide,2nded.Boston:

Little,BrownandCompany,1996:

79,withpermission.）

2.Paramedian.Thisapproachisusefulinpatientswhocannotadequatelyflextheirbackbecauseofpainorwhoseinterspinousligamentsmaybeossified.Placethespinalneedle1.5cmlateralandslightlycaudad（approximately1cm）tothecenteroftheselectedinterspace.Aimtheneedlemediallyandslightlycephalad,passinglateraltothesupraspinousligament.Ifthelaminaiscontacted,redirecttheneedleandwalkthetipoffthelaminainamedialandcephaladdirection.

3.Needleplacement.Alwayskeepthestyletinplacewhenadvancingtheneedlesothattheneedle'slumendoesnotbecomepluggedwithtissue