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Energy and Built Environment
journalhomepage:http://www.keaipublishing.com/en/journals/energy-and-built-environment/
Experimental measurements of surgical microenvironments in two operating rooms with laminar airflow and mixing ventilation systems
Guangyu Cao
a,∗, Ingeborg Kvammen
a, Thea Amalie Solberg Hatten
a, Yixian Zhang
b, Liv-Inger Stenstad
c, Gabriel Kiss
c, Jan Gunnar Skogås
caDepartment of Energy and Process Engineering, Norwegian University of Science and Technology, Kolbjørn Hejes Vei 1B, 7491 Trondheim, Norway
bFaculty of Urban Construction and Environmental Engineering, Chongqing University, Chongqing 40045, China
cOperating Room of the Future at St. Olavs Hospital, St. Olavs Hospital, Olav Kyrres Gate 10, 7030 Trondheim, Norway
a r t i c le i n f o
Keywords:
Operating room Laminar airflow Mixing ventilation Surgical microenvironment Thermal environment
a b s t r a ct
Atpresent,laminarairflow(LAF)systemsandmixingventilation(MV)systemsaretwocommonlyusedventila- tionsolutionsforoperatingrooms(ORs)toensuretherequiredindoorairquality.However,recentstudieshave shownthatthereislittledifferenceintheprevalenceofsurgicalsiteinfection(SSI)fortheLAFsystemsandMV systems.TheobjectiveofthisstudywastocomparetheperformanceofanLAFsystemwithanMVsystemin ORsatSt.Olavshospital,Norway.Inthisstudy,alltheexperimentalmeasurementswereconductedinrealORs withLAFandMVsystems.Thisstudyfoundthattheairvelocityabovethesurgicalincisionisapproximatelytwo timeshigherintheORwithLAFthanthatintheORwithMV.Theuseofsurgicallampsanddifferentairflow patternsmaycontributetothedifferentsurgicalmicroenvironmentofORswithLAFandMV.
1. Introduction
Asurgical siteinfection(SSI) isaninfectionwithin 30 dayspost surgery.SSIsaccountfor36%ofnosocomialinfectionsandarethemost commonhospital-acquiredinfections for surgical patientsin modern hospitals[1].SSIscanbeclassifiedbytheirlocation,whichindicates theirseverity.Superficialinfectionsinvolveonlytheskinorsubcuta- neoustissue,whilethoseinvolvingdeepsofttissuesarereferredtoas deepincisionalinfections.Themostsevereinfectionsinvolveorgansor bodyspaces[2].InNorway,theaverageSSIrateofhipsurgeryranged from3.3%to3.6%between2015and2018.However,moreseverevari- ationscanbeobservedforSt.Olav’sHospitaloverthesametimeperiod [3].Thegeneralhealthanddiseasestatesofthepatient,aswellasproper techniqueandsoundjudgmentbeingexercisedbythesurgicalteam,are themostcriticalfactorsinavoidingpostoperativeinfectionsandaredif- ficulttoquantify.However,especiallyforprocedureswithlowinfection rates(<3%),thedevelopmentofSSIsisrelatedtoairborneexogenous microorganisms[4].
ASpanishstudyincluding18,910patientsinvestigatedbothenvi- ronmentalandpatientvariationsinrelationtoSSIs[5].Apercentage of6.7% experiencedSSIs, butthedefinitionsandproceduresrelated totracking SSIsvary,causing uncertaintywhenperformingcompar- isons.SuperficialSSIswereassociatedwithenvironmentalfactors,such astemperature,humidity andsurfacecontamination.Higher relative
∗Correspondingauthor.
E-mailaddress:[email protected](G.Cao).
humiditywaslinkedtoahigherriskofSSIs.However,thesewereroom characteristicsandnotdirectlylinkedtothesurgicalwoundenviron- ment.Anotherstudyregardinghumidityinoperatingroomsalsofound anincreaseinSSIrateswithincreasedhumidity,althoughthediffer- encesinthestudywerenotstatisticallysignificant[6].
Thermalcomfortisdefinedasthatconditionofmindthatexpresses satisfactionwiththethermalenvironmentandisassessedbysubjective evaluation[7].Anoperatingroomisoneofthemostcontrolledwork environments,anditisimportantthattheenvironmentisperceivedas comfortableandhealthyforboththesurgicalstaff andthepatient[8]. Forthesurgicalstaff,itisimportanttomaintainthermalcomfortsothat theycanperformtheirwork.Ifthesurgicalstaff experiencethermaldis- comfort,theyareeithertoocoldortoowarmintheirworkingenviron- ment.Thesensationofthermaldiscomfortcanaffecttheirwell-being andleadtopoorworkefficiency,headacheanddizziness.Thermaldis- comfortforthepatientcouldmeanthatthethermoregulatoryresponses ofthehumanbodyaresuppressed,whichcancauseillnessand,insome cases,death[9].
Thermalcomfortdependsonsixparameters.Theyaredividedinto twogroups:environmentalparameters,whichconsistoftheairtempera- ture,meanradianttemperature,relativeairvelocityandrelativehumid- ityoftheair,andpersonalparameters,whichconsistofthemetabolic rateandclothinginsulation[7].AccordingtoASHRAE[7],anaccept- ablethermalenvironmentisanenvironmentthat80%ofoccupantsfind thermallyacceptable.Thefocusshouldbetoachievetheenvironmen-
https://doi.org/10.1016/j.enbenv.2020.08.003 Availableonline25August2020
2666-1233/Copyright© 2019SouthwestJiatongUniversity.PublishingservicesbyElsevierB.V.onbehalfofKeAiCommunicationCo.Ltd.Thisisanopenaccess articleundertheCCBY-NC-NDlicense(http://creativecommons.org/licenses/by-nc-nd/4.0/)
Nomenclature
A woundarea(m2) B temperaturefactor(K)
cp specificheatatconstantpressure(J/kg•K) hc convectionheattransfercoefficient(W/m2•K) hr radiationheattransfercoefficient(W/m2•K) hm convectionmasstransfercoefficient(m/s)
hfg thelatentheatofvaporization(J/kg) kair conductivityofair(W/m•K) L characteristiclength(m) Le Lewisnumber
Mw molecularweightofwater(kg/kmol) 𝑛′′𝑤 vaportransfer(kg/m2•s)
Pw,i waterpressureatthetemperatureofthepointabovethe incision(N/m2)
Pw,sat saturatedwaterpressureatthetemperatureofthepoint abovetheincision(N/m2)
𝑞′′𝑐𝑜𝑛𝑣 convectiveheattransfer(W) 𝑞′′𝑟𝑎𝑑 radiantheattransfer(W) q heattransfer(W) R gasconstant(J/kg•K)
RaL Rayleighnumberbylengthscale.
Nu Nusseltnumber T temperature(K)
Ti airtemperatureabovewound(K) Ts surfacetemperature(K)
Tsur surroundingtemperature(K) 𝜀 emissivity(0<𝜀<1)
𝜎 Stefan–Boltzmanconstant(5.67×10−8W/m2•K4) 𝜌 densityoffluid(kg/m3)
𝜌w,s densityatthetemperatureofthesurface(kg/m3) 𝜌w,i densityatthetemperatureofthepointabovetheinci-
sion(kg/m3)
talconditionswherethehighestpossiblepercentageoftheoccupants feelthermallycomfortable[10].Standardsandguidelinesregardingthe ventilationofoperatingroomsoftenproviderangesforenvironmental parametersratherthanonespecificvalue.Thishastodowiththedif- ferentsurgicalproceduresbeingperformedatahospitalandeachpro- cedure’srequirementsfortheindoorenvironment.Whendetermining therequirementsforonespecificOR,theneedsofthepatientandthe surgicalteamandthesecurityaspectsofinfectioncontrolneedtobe considered[11].Twoventilationstrategiesusuallyusedinoperating roomsaremixingventilation(MV)andlaminarairflow(LAF).
TheworkingprincipleofMVistosupplyairtoaroomwithanair velocityhighenoughtocreatefullmixingthroughouttheroom[12]. Theairvelocitymustbehighenoughsothatthetotalairvolumeinthe roomismoved[13].Itisimportanttosupplyairatavelocitythatcan createfullmixingintheroomwhileconsideringthatnoisemightbe generated.Thepurposeofcreatingfullmixingthroughouttheroomis tomixthesupplyairwiththeexistingairtodilutewhateverthecon- taminantsarepresent.Toavoiddraughtinthezoneofoccupancy,the supplydiffusersareusuallylocatedintheceilingoronthewall.
LAFisnormallyusedincleanroomssuchasoperatingroomstopre- ventbackswirlingofpollutedair.Cleanroomventilationrequireshigh airflowrates,whichiswhytheventilationistypicallyarrangedbyre- circulatingtheairthroughabankofhighefficientparticulateairfilters (HEPA)[13].Inahospitalenvironment,theventilationisaunidirec- tionalairflowthroughthecleanzoneorroom.Thisunidirectionalair- flowtypicallyhasavelocitybetween0.3and0.5m/s[13].Theairflow ishighestatthecenteroftheHEPAfilteranddecreasestowardsthepe- riphery.Themovementofthesurgicalstaff isanimportantfactorwith LAFventilationandcantransportbacteriatoasterilezone[1].Fig.1
showstheworkingprincipleofbothMVandLAF.Asamatterasfact, using LAFhasbeen recommendedinseveralnational guidelinesand standards[14–17].Thegreateffortofpreviousstudieshavebeenmade ontheperformanceofvariousventilationsolutionsregardingairborne contaminationlevelsandthewholeairflowpatternintheroom[18– 22].However,verylittlestudieshavebeendoneregardingthesurgical microenvironmentundervariousventilationstrategies[23].Theobjec- tiveofthisstudywastoinvestigatetheeffectsofLAFsystemsandMV systemsonthesurgicalmicroenvironmentsinORsatSt.OlavsHospital.
Asurgicalmicroenvironmentisdefinedastheareaclosetothesurgical incision,illustratedinFig.1.
2. Theoreticalmodeling
Roomairflowdistributionmayaffecttheheattransferfromthesur- gicalincisionbyconvectiveheattransfermechanisms.Inaddition,ra- diationfromsurfaces,includingequipmentandpersonnel,induceheat transfer.Wetsurfacescancauseadditionalheatlossduetotheevapo- rationoffluids[24].
Thetotalheattransferfromthesurgicalwoundcanbedenotedas showninEq.(1)
𝑞=𝑞′′𝑐𝑜𝑛𝑣+𝑞′′𝑟𝑎𝑑 (1)
whereAsisthewoundarea,𝑞′′𝑐𝑜𝑛𝑣istheconvectiveheattransfer,𝑞𝑟𝑎𝑑′′ is theradiantheattransfer.Theconvectiveandradiantheattransferrates areshowninEqs.(2)and(3)[24]:
𝑞′′𝑐𝑜𝑛𝑣=ℎ𝑐( 𝑇𝑠−𝑇𝑖)
∗𝐴𝑠 (2)
𝑞′′𝑟𝑎𝑑=ℎ𝑟( 𝑇𝑠−𝑇𝑠𝑢𝑟)
∗𝐴𝑠 (3)
ℎ𝑟=𝜖𝜎(
𝑇𝑠+𝑇𝑠𝑢𝑟)(
𝑇𝑠2+𝑇𝑠𝑢𝑟2 )
(4) Theradiationheattransfercoefficient,hr,isdeterminedfrom the surfacetemperatureandsurroundingtemperature.Theconvectiveheat transferisdeterminedfromthesurfacetemperatureandtemperature directlyabovethewound,Ti.
Assumingthewoundgeometryisnearlyaflatplatewithverylow velocities(<0.08m/s),withthecharacteristiclength𝐿=𝐴𝑠∕𝑃,thecon- vectiveheattransfercoefficient,hc,canbe foundfromthefollowing NusseltnumbercorrelationinEq.(5):
𝑁𝑢𝐿=ℎ𝑐𝐿
𝑘𝑎𝑖𝑟 =0.52𝑅𝑎𝐿15 (5)
wherekairistheconductivityofair,andRaListheRayleighnumberby lengthscale.
Inthisstudy,masstransferislimitedtomoisturetransportation.Wa- tervaportransfercanbeexpressedinamannersimilartothatofheat transferbyEq.(6)[24]:
𝑛′′𝑤=ℎ𝑚(
𝜌𝑤,𝑠−𝜌𝑤,𝑖)
(6) where𝜌isthedensityatthetemperatureofthesurfaceorthepoint abovetheincision.Theheatandmasstransferrelationsforaparticular geometryareinterchangeable,resulting inthefollowingrelationship betweentheheatandmasstransfercoefficientsasshowninEq.(7):
ℎℎ𝑚 =𝜌∗𝑐𝑝∗𝐿𝑒1−𝑛 (7)
Neglectingthenetradiativeheattransferundersteady-statecondi- tionsandtreatingtheairasanidealgas,thecoolingeffectofevapora- tioncanbedeterminedfromEq.(8)
(𝑇𝑖−𝑇𝑠)
= 𝑀𝑤∗ℎ𝑓𝑔 𝑅∗ρ𝑎𝑖𝑟∗𝑐𝑝∗𝐿𝑒23
∗
[𝑝𝑤,𝑠𝑎𝑡(𝑇𝑠)
𝑇𝑠 −𝑝𝑤𝑎𝑡𝑒𝑟,𝑖 𝑇𝑖
]
(8)
Retrievedbytheheatandmasstransferrelation ℎℎ
𝑚 =𝜌𝑐𝑝𝐿𝑒1−𝑛,n isassumedtobe1/3,where𝜌,cpandLeareallairproperties.𝜌isthe
Fig.1. PrincipleofventilationsystemsinORs:(a)averticalLAFsystem and(b)aMVsystem[13].
Fig.2. Experimentalsetupwithmeasurement points:(a)photooftheLAFOR;(b)photoof theMVOR.
density,cpisthespecificheatatconstantpressureandLedenotesthe Lewisnumber.Mwisthemolecularweightofwater,andhfgisthelatent heatofvaporization.AllthepropertiesareevaluatedatTi.Insituations ofverylowhumidity,Pw,icanbeneglected,andthesurfacetemperature iscalculatedfromEq.(9):
𝑇𝑠= 𝑇𝑖+√ 𝑇𝑖−4𝐵
2 (9)
where
𝐵=𝑀𝑤ℎ𝑓𝑔𝑃𝑤,𝑠𝑎𝑡 𝑅𝜌𝑐𝑝𝐿𝑒23
(10)
TheevaporativeheatlosscanbeshowninEq.(11):
𝑄𝑒𝑣𝑎𝑝=ℎ𝑓𝑔𝑛′′𝐴𝑠 (11)
3. Experimentalsetup
Inthis study,all themeasurementsweretaken from twoORs at St.Olavs hospital in Trondheim,Norway.The OR withan LAFsys- temhadanareaof56m2withalaminarairflowzoneof11m2and wassurroundedby1.1mlongpartialwalls,asshowninFig.2.Dur- ingtheexperimentalmeasurements,theventilationsystemwasoper- atedatthefullload,andtheroomtemperaturewascommonlysetto 22.4°C.Duringtheexperiments,thesupplyairtemperaturewasmea- suredas20±1°C.ThedesignedsupplyairintheorthopedicLAFORwas 10,580m3/h,comprising4280m3/hofoutdoorairand6300m3/hof recirculatedair.
TheORwithanMVsystemwasequippedwithfourceiling-mounted diffusers.Fortheexhaust,thereweretwowall-mountedexhaustoutlets andoneneartheceiling.TheMVORhadanareaof59.7m2.Thesupply airtemperaturewassetto23.0°Cinallthescenarios.Thesupplyairflow ratewas3700m3/h,andtheexhaustairflowwas3600m3/h.During measurement,anadjustablestandwasusedtocarrytheanemometers.
Inthisstudy,threescenarios(seeTable1)thatincludedsixdiffer- entcaseswereinvestigated.Scenario1(cases1and2)investigatedthe thermalenvironmentintheORs.Scenario2(cases3and4)measured thetemperatureandrelativehumidityintheORstocalculatetheheat andmasstransfer.Scenario3(cases5and6)measuredtheairvelocity ofsurgicalmicroenvironmentintheORs.
4. Measurementinstruments
Avarietyofmeasuringdeviceswereusedtoobtainvalidresultsfor temperature,relativehumidityandvelocitybothinthemacro-andmi- croenvironments.Tomeasuretemperatureandrelativehumidityclose to thesurgical incision,the humidity andtemperatureprobe HMP9 (Vaisala, Finland)for rapidlychanging environmentswas used,with adiameterof 5mm,ameasurementrangeof−40to120°Cand0–
100%RH, andmeasurementaccuraciesof ±0.8%RHand± 0.1°Cat 23°C.ThemanufacturecalibrationofHMP9instrumentwasstillvalid.
ABoschPTD1isathermaldetectorbasedoninfraredtechnology thatdetectsthesurfacetemperatureofthesurgicalincision.Themea- suringrangeforsurfacetemperaturesis−20to200°Cforambienttem- peratures between −10and40 °C.Theaccuracyatameasuringdis- tanceof0.75–1.25m,inanambientenvironmentof22°C,is±1°C forsurfacetemperaturesbetween10and30°Cand±3°Cforatem- peraturerangeof30–90°C.AFlirE60,displayingIRimagesinaddi- tiontothesurfacetemperaturesoftheassignedspots,recordedthermal imagesandsurfacetemperatures.Temperaturemeasurementsbythe devicehaveanaccuracyof±2°Cforambienttemperaturesbetween 10and35°CSurfacetemperaturesrangefrom−20to120°C,witha thermalsensitivityof0.05°Cat30°C.Theminimumfocusdistanceis 0.4m.
TheTSIvelocitymeterwasusedtomeasurethevelocityinagivendi- rection,whichwasdeterminedbytherotationofthetelescopingprobe.
Forairtemperatureswithin−10to60°C,thereadingshaveanaccu- racyof3%readvalueor0.02m/s,whicheverisgreater.TinyTaglog- gerswereusedtorecordthetemperatureandrelativehumidityroom conditionsintherealoperatingroomsatintervalsof5min.APegasor IndoorQuality,withanoperatingtemperaturerangeof0–40°C,was usedtomeasuretheroomconditionsincases3and4.Thedevicehas anaccuracyof±2°Cand±1.5%RH.
Theairvelocitywasmeasuredattwopointsnearthewoundbyus- ingaSwema03+anemometer:Therangeofairvelocitymeasuredwas 0.05–3m/sat15–30°C.At20–25°C,themeasurementuncertaintywas
±0.03m/sinthevelocityrangeof0.05–1m/sorand±3%readvaluein thevelocityrangeof1.0–3.0m/s.At15–30°C,themeasurementuncer- taintywas±0.04m/sat0.05–1m/sor±4%readvalueat1.0–3.0m/s.
Theloggingtimeforeachpointwas10min,withatimeintervalof1s.
Themanufacturecalibrationwasstillvalid.
Table1
Scenariosoftheexperimentalmeasurements.
Scenario Case
Number of people
Ventilation
mode Remarks
S1 – real surgeries Case 1 6 LAF Thermal images were taken during:
1 h 24 min (LAF), 3 h (MV)
Case 2 10–12 MV
S2 – simulated surgeries
Case 3 6 LAF Parameters in surgical environment:
relative humidity, air temperature
Case 4 6 MV
S3 – simulated surgeries
Case 5 6 LAF Air velocity
Case 6 6 MV
Fig.3. Thermalimagesofthesurgeon,assis- tantsurgeonandsterilenurseinMVOR:(a) After40min;(b)After1hand40min;(c)Af- ter2hand40min.
5. Resultsanddiscussion
5.1. Thermalimagesofthesurgicalmicroenvironmentsintwooperating rooms
Thefootagefromthethermalcameraisusedtoevaluatethesurface temperaturedistributionsinbothoperatingrooms.Figs.3–5showthe temperaturedistributionof thesurgeon,assistantsurgeonandsterile nurseinbothoperatingrooms.ThesurgeryintheMVORwastheinser- tionofastentgrafttopreventananeurysmfromgrowing.Thissurgery lastedforapproximately3h.Thesurfacetemperaturesofthesurgeon andassistantsurgeonaregenerallyhigherthanthesurfacetemperature ofthesterilenurse(Fig.3).Thiscanbeexplainedintwoways.Thefirst isthatthesurgeonshaveahigheractivitylevelthanthesterilenurse, whichleadstomoresweatingandheatreleasedfromthebody.Thesec- ondaspectisthefactthatthesurgeonsarelocatedclosertothesurgical lampsandmedicalequipment.Theequipmentreleasesheat,whichcan beabsorbedbytheclothingofthesurgeons,thusincreasingthesurface temperature.Itcanalsobeobservedthatthesurfacetemperaturesof allthreemembersof thesurgicalstaff isincreaseduringthesurgery, whichistheexpectedresult.Theworkloadduringthesurgery,inad- ditiontobeinginthesameroomwithhighairtemperatureandlow relativehumidity,leadstoanincreasingsurfacetemperaturethrough- outthesurgery.
FortheLAFOR,akneereplacementwasconducted,whichlasted forapproximately1.5h.Thetendenciesobserved(Figs. 4and5)are thesameasthoseintheMVOR.Generally,thesurfacetemperatureof thesurgeonsishigherthanthatofthesterilenursebutnotasclearly asFig.3shows.Mainlytheheadandfacialregionhasahighersurface temperature.Thiscouldbebecauseofsweatfromtheforeheaddueto hardandtiresomework.Oneexplanationforwhythetemperaturedif-
ferencebetweenthesurgeonsandthesterilenurseissmallerunderLAF couldbe theimpactofthelamps.Thefieldmeasurementsshowthat thelampsintheMVORemitmoreheatthanthelampsintheLAFOR.
Becauseofthis,thesurgeonsintheLAFORabsorblessheatfromthe lamps.Thiscouldaffectthesurfacetemperatureoftheclothingandbe acausativefactorastowhythedifferencebetweenthesurgeonsand sterilenursesissmaller.FortheMVOR,thesurfacetemperaturesforall threeindividualsincreaseduringthesurgery,asexpected.
5.2. Measuredtemperatureandrelativehumidity
Thesurgicalmacroenvironmentparameters,includingairtempera- tureandrelativehumidity(seeinFig.2),weremeasuredbyaPegasor IndoorQualityandwereverystablethroughouttheexperiments.The measuredaverageroomtemperatureinLAFORis21.2±0.47°C,and themeasuredaveragerelativehumidityis14.6±0.73%.Themeasured averageroomtemperatureinMVORis24.6±0.17°C,andthemeasured averagerelativehumidityis21.7±0.55%.
Inthesurgicalmicroenvironment,thetemperatureandrelativehu- midityweremeasuredapproximately1–2cmabovethesimulatedsur- gicalincisionbyaVaisalaHMP9.Fig.6(a)showsthatthemeasured airtemperatureabovethesimulatedincisionincase3isstable,withan exceptionimmediatelyafterapproximately3000s.Adropinthemea- suredsurfacetemperatureandtemperaturedirectlyabovetheincision canbeobservedsimultaneouslyastherelativehumidityincreases.The surfacetemperaturewas recordedeveryminutebytheBoschPTD1, whiletheVaisalaHMP9placedapproximately2cmabovetheincision measuredtherelativehumidityandtemperatureclosetotheincision.
Fig.6(b)showsthatthesurfacetemperatureisalreadybelowareal- isticvalueandenvironmentaltemperature;nevertheless,itisdecreasing steadily.Towardstheendofthesimulatedsurgery,theairtemperature
Fig.4. Thermalimagesofsurgeonandassis- tantsurgeoninLAFOR:(a)After20min;(b) After1hand20min.
Fig.5. ThermalimagesofsterilenursesinLAF OR:(a)After1min;(b)After1h.
Fig.6. Surfacetemperatureoftheincision,inadditiontotemperatureandrelativehumiditymeasuredclosetothesurgicalwoundduring(a)case3intheORwith LAF(b)case4intheORwithMV.
isapproximately8∘Chigherthanthesurfacetemperature.Thehigher airtemperature inthesurgical microenvironmentmaybe causedby lowairvelocitycomparingwiththesituationwithLAF.Nevertheless, thesurfacetemperatureisstilldecreasing.Theliquidalwaysremaining atthesurfacesuggeststhatthecoolingeffectofevaporationislarger thantheheatingfrom thehigherroomtemperatureunderthegiven conditions.
Priortothe"start",surgicallampsareturnedoff incase4.Aslight decreaseinthetemperatureabovetheincisioncanbeobservedatthis stage.However,whenthesurgicallampsareturnedon,arapidchange intheairtemperatureoccurs.Thesurfacetemperaturealsoincreases, butnaturallywithaslowerpace.Therelativehumiditylevelsfollow aninversepattern,resultinginahumiditypeakatthelowestairand surfacetemperaturemeasured.Anexplanationfortheinversepatternis thecapabilityofwarmerairtoholdmoremoisture.Thisimpliesthatfor
thesameabsolutehumiditylevel,lowerrelativehumidityisreachedin warmerair.Thisjustificationsuggeststhattheabsolutehumiditylevel doesnotincreaseenoughtoobtainthesamerelativehumidity,even whenevaporationfromtheincisionoccurs.Aftersometime,thesur- facetemperatureconvergestowardsavalueofapproximately28–29°C.
Beingabletohaveasignificantlyhigherandmorerealisticsurfacetem- peraturein thebeginningwouldprobablycauseamore stablevalue throughoutthesurgery.
Thecorrelationbetweentherelativehumidityandtemperaturesug- geststhatlowhumiditylevelsappearforhighertemperatures.Further investigationshowsthatalmostone-thirdofallthemeasuringpointsare belowtherecommendedRHvalue,asshowninFig.7.Forhighertem- peratures,evenlowerRHvaluesaremeasured.Near37∘C,thelowest RHvalueisobserved,slightlybelow13%.Thegoalofthemixingair- flowventilationprincipleisauniformairdistribution.However,themi-
Fig.7. Relativehumidityplottedwiththecorrespondingtemperature.
croenvironmentdifferssignificantlyfromtheoverallroomconditions.
Theresultssuggestthattoobtainacertainrelativehumidityintheoper- atingmicroenvironment,temperatureiscritical,whichhereisaffected bythesurgicallamps.
5.3. Calculatedincisionsurfacetemperature
Thetheoreticalmodelingappliedtocase3suggeststheintroduction ofa time-dependentvariableforbetterapproximationofthesurface temperature.Asalmostalineartrendisobservedforthesurfacetem- perature,alineartimeparametershouldbefurtherinvestigated.
Appliedtocase4,anothertrendcanbeobserved.Goingtowardsthe steadysurfacetemperature,theapproximationisclose.However,the inertiainsurfaceheating,duetothethermalpropertiesoftheincision, isnot sufficientlyconsideredandshould bestudiedin furtherwork.
Moreover,thedynamicprocessofevaporationofthesurgicalincision maynotbeaccuratelyexpressedandshouldbestudiedinfurtherwork.
Fig.8presentstheresultscalculatedbyEqs.(9)and(10),whileallair andwaterpropertiesarefoundintables[24].
5.4. Measuredairflowvelocities
Fig.9showsthattheairvelocityfluctuatesovertime.Point1was abovethewound,andPoint2wasabovethekneeataheightof3.3cm fromthewoundandknee.Thepointclosetothewoundexperiencesa higherairvelocitythanthepointclosetotheknee.IntheLAFOR,the verticallaminarairflowdirectlyflowstothesurgicalmicroenvironment.
IntheMVOR,thesupplyairswirlsintotheroomfromfourceiling- mounteddiffusers,andtheairflowvelocityisdecreasinginthesurgical microenvironment.Hence,theairvelocityabovethewoundandkneeis higherintheLAFORthanthatintheMVOR.Thismaysupportoneof thelateststudieswhichfoundthatinORswithhigh-volume,unidirec- tionalverticalairflowsystemshadlowerriskofrevisionduetoinfection thaninORswithMVsystems[25].
6. Conclusion
This studyfocusedon thesurgicalmicroenvironmentin twoORs withLAFandMVsystems.Byusingathermalcamera,thethermalenvi- ronmentandcomfortofthesurgeon,assistantsurgeonandsterilenurse wereinvestigated.Basedonthemeasurementresults,conclusionsre- gardingthesurgicalmicroenvironmentcanbedrawnasfollows:
(1) Thesurfacetemperaturesofthesurgeonandassistantsurgeonare higherthanthatofthesterilenurseinbothORs.
(2) Ahigher surfacetemperatureovertimeleads tothe sensationof beingwarmerintheORwithMVthanintheORwithLAFandthus causesthermaldiscomfort.
(3) Thetemperatureof surgicalincisionmicroenvironmentintheOR withMVbecomeswarmerthanin theOR withLAFduetolower airflowvelocity.
(4) Theairvelocityatapointof3.3cmfromthesurgicalincisionis approximatelytwotimeshigherintheORwithLAFthanthatinthe ORwithMV.
(5) Theuseofsurgicallampsanddifferentairflowpatternsmaycon- tributetothedifferentsurgicalmicroenvironmentofORswithLAF andMV.
Fig.8. Measuredtemperatureapproximately2cmabovetheincision,comparedwithcalculatedandmeasuredsurfacetemperaturesfor(a)case3(b)case4.
Fig.9. ThevelocitymeasurementsattwopointsclosetothewoundintwoORsfor(a)case5(b)case6.
Thesurfacetemperaturesofthesurgicalstaff differbecauseofdiffer- encesinmovementandlocationinrelationtomedicalequipment.The factthatoneORexperiencedmoreheatemittedfromthesurgicallamps couldhaveanimpactontheresultsofthermalcomfortandthesurface temperaturedistributionobtainedfromobservationswiththethermal camera.
Theresultsobtainedfrommeasurementsinthesurgicalmicroenvi- ronmentareconsistentwiththoseofthethermalmacroenvironment.
Incase4,theemittedheatcausedtemperaturesfarabovetherecom- mendedvalues,whilethecorrespondingrelativehumidityvalueswere belowtherecommendations. Thegoalof themixingairflowventila- tionprincipleisauniformairdistribution.However,themicroenviron- mentdifferssignificantlyfromtheoverallroomconditions.Theresults suggestthattoobtainacertainrelativehumidityintheoperatingmi- croenvironment,onecriticalfactorislocaltemperature,whichwillbe affectedbythesurgicallamps.
Incase3,lessheatingfromsurgicallampscausesaslowerevapo- rationofwoundmoisture.However,theevaporativecoolingeffectis suggested tobegreater thanthe netheatgaindue toradiationand convectionfromwarmer,ambientenvironments.Asthesetvaluesin theinvestigatedoperatingroomarebelowrecommendedvalues,fur- therinvestigationisneededtoevaluatetheimpactoftheseparameters.
Thepresentedequationsprovideareasonableestimateofsurfacetem- peratureinthesurgicalmicroenvironment.Nevertheless,furtherinves- tigationsandconfirmationoftheseresultsarenecessary.Inparticular, theoreticalmodelsrelatedtomoisturetransferneedmorevalidation.
DeclarationofCompetingInterest None.
Acknowledgments
TheauthorsgreatlyappreciatethecollaborationwiththeOperating RoomofTheFuture(FOR)– St.Olavshospital,whichprovidedrealop- eratingroomsforfieldmeasurements.NorwegianUniversityofScience andTechnologyprovidedin-kindfundingtosupportthefieldmeasure- mentsatSt.Olavshospital.
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