Chapter36-BarometricPressure,IncreasedWORKINGUNDERINCREASEDBAROMETRICPRESSUREEricKindwallTheatmospherenormallyconsistsof20.93%oxygen.Thehumanbodyisnaturallyadaptedtobreatheatmosphericoxygenatapressureofapproximately160torratsealevel.Atthispressure,haemoglobin,themoleculewhichcarriesoxygentothetissue,isapproximately98%saturated.Higherpressuresofoxygencauselittleimportantincreaseinoxyhaemoglobin,sinceitsconcentrationisvirtually100%tobeginwith.However,significantamountsofunburntoxygenmaypassintophysicalsolutioninthebloodplasmaasthepressurerises.Fortunately,thebodycantolerateafairlywiderangeofoxygenpressureswithoutappreciableharm,atleastintheshortterm.Longertermexposuresmayleadtooxygentoxicityproblems.Whenajobrequiresbreathingcompressedair,asindivingorcaissonwork,oxygendeficiency(hypoxia)israrelyaproblem,asthebodywillbeexposedtoanincreasingamountofoxygenastheabsolutepressurerises.Doublingthepressurewilldoublethenumberofmoleculesinhaledperbreathwhilebreathingcompressedair.Thustheamountofoxygenbreathediseffectivelyequalto42%.Inotherwords,aworkerbreathingairatapressureof2atmospheresabsolute(ATA),or10mbeneaththesea,willbreatheanamountofoxygenequaltobreathing42%oxygenbymaskonthesurface.OxygentoxicityOntheearth’ssurface,humanbeingscansafelycontinuouslybreathe100%oxygenforbetween24and36hours.Afterthat,pulmonaryoxygentoxicityensues(theLorrain-Smitheffect).Thesymptomsoflungtoxicityconsistofsubsternalchestpain;dry,non-productivecough;adropinthevitalcapacity;lossofsurfactantproduction.Aconditionknownaspatchyatelectasisisseenonx-rayexamination,andwithcontinuedexposuremicrohaemorrhagesandultimatelyproductionofpermanentfibrosisinthelungwilldevelop.Allstagesofoxygentoxicitythroughthemicrohaemorrhagestatearereversible,butoncefibrosissetsin,thescarringprocessbecomesirreversible.When100%oxygenisbreathedat2ATA,(apressureof10mofseawater),theearlysymptomsofoxygentoxicitybecomemanifestafteraboutsixhours.Itshouldbenotedthatinterspersingshort,five-minuteperiodsofairbreathingevery20to25minutescandoublethelengthoftimerequiredforsymptomsofoxygentoxicitytoappear.Oxygencanbebreathedatpressuresbelow0.6ATAwithoutilleffect.Forexample,aworkercantolerate0.6atmosphereoxygenbreathedcontinuouslyfortwoweekswithoutanylossofvitalcapacity.Themeasurementofvitalcapacityappearstobethemostsensitiveindicatorofearlyoxygentoxicity.Diversworkingatgreatdepthsmaybreathegasmixturescontainingupto0.6atmospheresoxygenwiththerestofthebreathingmediumconsistingofheliumand/ornitrogen.Sixtenthsofanatmospherecorrespondstobreathing60%oxygenat1ATAoratsealevel.Atpressuresgreaterthan2ATA,pulmonaryoxygentoxicitynolongerbecomestheprimaryconcern,asoxygencancauseseizuressecondarytocerebraloxygentoxicity.NeurotoxicitywasfirstdescribedbyPaulBertin1878andisknownasthePaulBerteffect.Ifapersonweretobreathe100%oxygenatapressureof3ATAformuchlongerthanthreecontinuoushours,heorshewouldverylikelysufferagrandmalseizure.Despiteover50yearsofactiveresearchastothemechanismofoxygentoxicityofthebrainandlung,thisresponseisstillnotcompletelyunderstood.Certainfactorsareknown,however,toenhancetoxicityandtolowertheseizurethreshold.Exercise,CO2retention,useofsteroids,presenceoffever,chilling,ingestionofamphetamines,hyperthyroidismandfearcanhaveanoxygentoleranceeffect.Anexperimentalsubjectlyingquietlyinadrychamberatpressurehasmuchgreatertolerancethanadiverwhoiswork...
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