acid based disorders med07

8/6/2019 Acid Based Disorders Med07

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Terminology.

pH-Thisisthenega5velogarithmtothebaseof10oftheH+ionconcentra5on.

PaCO2Thepar5alpressureofarterialCO2.Itcons5tutestherespiratorycomponentsofacid-baseregula5onrelatedtopulmonaryven5la5on.

PaO

2Thepar5alpressureofarterialO2hasnoprimaryroleinacid-baseregula5on.

SaO2ArterialO2satura5on;thedegreetowhichtheHbissaturatedbyO2. HCO3Thiscons5tutesthemajorbufferofthebody. AnionsNega5velychargedionse.g.Cl-,HCO3-andPO4-. BufferAsubstanceorgroupofsubstancesthatbindreversiblywith[H+]

andeffec5velyneutralizetheminordertocorrectacid-baseimbalances.

Themajorbufferscountsofbicarbonate,phosphate,protein,andHb. BaseExcess:2.

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NORMALARTERIALANDMIXEDVENOUSSAMPLEBLOODGASES .

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Acid-BaseDisorders

The[H+]concentra5onintheECFisdeterminedbybalancebetweenpar5alpressureofCO2

(PCO2)andtheconcentra5onof[HCO3-]inthefluid

[H+]nEq/L=24x(PCO2)

HCO3-

pH=-log[H+]

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H+Concentra5onintheECFisexpressedin

nonequivalent'snEq/L.Anormal[H+]of40

nEq/LcorrespondstoapHof7.40.Changesin

pHareinverselyrelatedtochangesin[H+].


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TypesofAcid-BaseDisorders

ThedifferenttypesofAcid-BasedisorderscanbedefinedusingthenormalrangesofpH,

PCO2andHCO3-concentra5onsintheECF.

Normalrangesareasfollows;-pH=7.3(Av.pH7.4)

-pCo2=34mmHg(Av.pCO240)

-HCo3=22-26mEq/L(Av.242)

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Acid-BaseControl

Achangeinoneofthedeterminants(PCO2orHCO3-)mustbeaccompaniedbyapropor5onalchangeintheotherdeterminantstokeepthePCO2/HCO3ra5o(andthepH)constant.ThusanincreaseinPCO 2respiratory

acidosis)mustbeaccompaniedbyanincreaseinHCO 3(metabolicalkalosis)tokeepthepHconstant.

Theini5alchangeinPCO2orHCO3iscalledprimaryacid-basedisorderandthesubsequentresponseiscalledcompensatoryorsecondaryacid-basedisorder.

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PrimaryAcid-BaseDisordersandAssociated

CompensatoryChanges

[H+]=24x(PaCO2)

HCO3- CompensatorychangesaredesignedtokeepthePaCO2ra5oconstant

HCO3-

Metabolic

Acidosis

Metabolic

Alkalosis

Respiratory

Acidosis

Respiratory

Alkalosis

Primary

Change

HCO3- HCO3 PaCO2 PCO2

EffectonpH pH pH pH pH

Compensator

yChange

PCO2 PaCO2 HCO3- HCO3-

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ACIDBASEABNORMALITIESANDAPPROPRIATECOMPENSATORY

RESPOPNSESFORSIMPLEDISORDERS.

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MetabolicAcidosisPresentwhenthereisadecreaseinPlasmaHCO3-orincreasein[H+]

CausesofMetabolicAcidosis(HighAnionGap):

1. EndstageRenalfailure2. Ketoacidosis(DKA,Alcohol,starva5on)3. Lac5cAcidosis4. Toxins(Salicylates,Methanol,EthyleneGlycol). Rhabdomyolysis.

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CausesofaNormalAGmetabolicAcidosis

1. DiarrhealossofHCO3-instools2. EarlyRenalFailureHCO3-lossesinurine3. Saline(isotonic)infusionCl-conc.InECFHCO3-

lossesinurine.

4. RenalTubularAcidosisImpairedrenalacidexcre5on-withK+-(ClassicDistal[Type1]RTA)-withK+[Type4]DistalRTA.

. Acetazolamide(aCarbonicanlydroseinhibitonthatincreasesHCO3-lossesinurine.

6. FistulasbetweenGITandureterswhichpromoteHCO3

-lossesinstools.

7. Pancrea5c/BiliaryDrainage.11


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TheSerumAnionGap:

AG=[Na+](Cl-+HCO3-)=124mEq/L

TheA.Gisspecificallyusedtodifferen5atebetweenm.acidosiscausedbyaccumula5onof[H+]e.g.(DKA,LA)andm.acidosiscausedbylossofHCO3e.g.GITorRenalloss.Whenm.acidosisisduetoaccumula5on[H+}inECF,H+combineswiththeHCO3toformCarbonicAcid,thisHCO3-conc.InECFwhichinturn

theA.G.

Alltheionspar5cipateinthisequa5on.Theunmeasuredcau5ons(U.C)andtheunmeasuredanions(U.A)areincludedasshown;

Na+U.C=(Cl-+HCO3)+U.Aor

Na+(Cl-+HCO3-)=U.A-U.C

Thedifference(U.A-U.C)isameasureoftheunmeasuredanionsandiscalledtheA.G.

Thedifferencebetweenthe2groupsrevealsanA.G.of12mEq/L.

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DeterminatesoftheA.G.

UnmeasuredAnions

-Albumin1mEq/L

-OrganicacidsmEq/L

-Phosphate2mEq/L

-Sulphate1mEq/L

TotalU.A23mg/L

AG=U.A-U.C=12mEq/L

UnmeasuredCa5ons

-CalciummEq/L

-K+4.mEq/L

-Mg+1.mEq/L

TotalU.C11mEq/L

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Correc5onofAGwithAlbuminLevel

AlbuminisamajorsourceofunmeasuredanionAinS.Albuminof1gm/dlfroma

normalof4.gm/dltheA.Gby2.mEq/L

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Interpre5ngtheA.G.

A) HighA.G.M.Acidosis: MetabolicacidosiswithanAGarecaused

byaddi5onofafixedacidtotheECFandthosewithanormalAGarearesultofanetin

chlorideconcentra5ontoECF.B)NormalAGM.Acidosis:

Whenm.acidosisiscausebylossofHCO3-formtheECF,theHCO3-lossiscounterbalanced

byagainofchlorideionstomaintainelectricalchargeneutrality(Hyperchloremicmetabolicacidosis).

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MetabolicAlkalosis

CharacterizedbyHCO3-,ph,K+,Cl-,Mg

I. VolumeDepletedType(ChlorideResponsive)UrineCl-


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Cont.

MetabolicAlkalosis

-K+lossisenhancedbydiure5cs.TheresultantK+deple5on

canthenproduceECFalkolosisbypromo5ngI.C.accumula5onofH+in

theDCT(transcellularH+K+exchange).

3.Posthypercepnoicstate.

4.HypovolemiaContrac5onAlkalosisin2ways

(i) lossoffreewaterconcentratestheexis5ngHCO 3-andraisestheHCO3-conc.IntheECF.

(ii) Adecreaseincircula5ngbloodvolumeRenin-Ang.AldosteroneAxislossofK+&H+inD.C.T

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II.VolumeRepleteType

(ChlorideUnresponsiveUrineCl->1mEq/L

1.Mineralocor5coidExcess.

2.Hyperaldosteronism

3.CushingsSyndrome

4.ProfoundHypokalemia. .Barer'sSyndrome(lossoffunc5oninTAHLK+

andmetabolicalkalosis)

6.GitelmansSyndrome-K+,Mg+,metabolicalkalosisandhypocalciuria.

7.Excessivealkali-administra5on(milkalkalisyndromemassivecitrateinfusionwithbloodtransfusion).

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Differen5alDiagnosisofRespiratoryAcidosis

1. AbnormalRespiratoryDrive- Centralalveolarhypoven5la5on- Brainsteminfora5on- Myxedema- SleepApnoea- Seda5vemedica5onsopioids,anesthe5cs

2. AbnormalNerveConducon- Polionyeli5s- Mul5pleSclerosis- GuillainBavreSyndrome- Amyotropiclateralsclerosis- Severehypokalemia,hypophosphotemia.

3. ThoracicCageProblems- Kyphoscoliosis- MuscularDystrophy- Polymyosi5s- Ankylosingspondyli5s

4. ChronicAirwayDisease/PulmonaryDisease COPD,Emphysema,inters55allungdisease,severeasthma,ARDS,lungfibrosis.

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DifferenalDiagnosisofRespiratoryAlkalosis

1. CNSsmulaonpain,anxiety,Psychosis,Fever,Meningi5s,Encephali5s,trauma,stroke.

2. Hypoxemia-Tissuehypoxia.Highal5tude,Pneumonia,Pulmonaryoedema.P.E,SevereAnemia,Aspira5on

3. DrugsorHormonesSalicylatepoisoning,progesterone,xanthines.

4. MiscellaneousSep5cemia,Hepa5cfailure,Heatexposure,pregnancy.

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MixedAcidBaseDisorders

1. MetabolicAcidosis&RespiratoryAcidosis Apa5entwithCOPDwithCO2reten5on(Chr.Resp.Acidosis)

developsdiarrhea(metabolicacidosis)

2. MetabolicAlKalosis&RespiratoryAcidosis

Samepa5entwithCOPDisgivenDiure5cforCor-pulmonale.HCO3-from40to48mmol/LwhichwiththePCO2ofat80mHgsetsthepHat7.40.

3.MetabolicAcidosis,MetabolicAlkalosisandRespiratoryAlkalosis

e.g.DKApa5entmaydevelopmetabolicalkalosisduetovomi5ngandsuperimposedrespiratoryalkalosisduetohyperven5la5onofhepa5cdysfunc5on.

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ACID-BASEINTERPRETATION

STAGEIIden5fythemajoracid-basedisorderfromthepH,PCO2andHCO3inanABG.

STEP1AnAcid-BaseabnormalityispresentifeitherthePCO2orthepHisoutsidetheNR.

STEP2IfthepHandPaCO2arebothabnormal,compare

thedirec5onalchange.ifbothchangeinthesamedirec5on(bothincreaseordecrease)theprimaryacid-basedisorderisMETABOLIC.Ifbothchangeinoppositedirec5ons,theprimaryacid-basedisorderisRESPIRATORY.

Step3-ifeitherthepHorPaCO2isnormalthereisamixedmetabolicandrespiratoryacid-basedisorder.(oneisanacidosisandtheotherisanalkalosis).IfthepHisnormalthedirec5onof

changeinPaCO2iden5fiestherespiratorydisorderandifthePCO2isnormal,thedirec5onofchangeinthepHiden5fiesthemetabolicdisorder.

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Cont

ACID-BASEINTERPRETATION

STAGEIIDeterminethecompensatoryresponses. STAGEIIICalculatetheANIONGAPtoevaluatethemetabolicacidosis.

STAGEIVUsedifferen5aldiagnosisandsimplelabteststoestablishthecauseofacid-basedisorder. STAGEV-Directtreatmentofunderlyingdisorder.

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AdverseEffectsofMetabolicAcidosis.

Decreasedmyocardialcontrac5lityC.O. Shocksecondarytovasodila5on. Arrhythmias Hyperkalemia. Decreasedlevelofconsciousness Cardiorespiratoryarrest. KussmaulRespira5on.

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AdverseEffectsofMetabolicAlkolosis.

1. DecreasedLOC,Seizures,Carpopedalspasms.2.Hypoven5la5onfailedweaning.

3.Impaired5ssueoxygena5onshiinHBO2

curvetotheleO2delivery.

Increased5ssueO2consump5on.

4.Arrhythmias

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AcuteRespiratoryAcidosis

pH=0.008xPaCO2

ExpectedpH=7.40[0.008x(PaCO240)

AcuteRespiratoryAlKalosis

pH=0.008xPaCO2

ExpectedpH=7.40+[0.008x(40-PaCO2)

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ChronicRespiratoryAcidosis

pH=0.003xPaCO2

ExpectedpH=7.40[0.003x(PaCO240)

ChronicRespiratoryAlKalosis

pH=0.003xPaCO2

ExpectedpH=7.40+[0.003x(40-PaCO2)

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acid based disorders med07

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