abg und re standing the principles

8/3/2019 ABG Und Re Standing the Principles

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ACID BASE EVALUATION:ACID BASE EVALUATION:

Nauman Tarif, MDNauman Tarif, MD

Associate ProfessorAssociate Professor

SIMSSIMS


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ACIDS AND BASESACIDS AND BASES

Using the definitions proposed byUsing the definitions proposed by

BronstedBronsted

An acid is a substance that canAn acid is a substance that candonate H+ ionsdonate H+ ions

A base is a substance that canA base is a substance that can

accept H+ ionsaccept H+ ions


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H2CO3 H+ + HCO3-

HCl H+ + Cl-

NH4+ H+ + NH3

H2PO4- H+ + HPO42-

Acid Base


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pHpH

What is pH ?What is pH ?

P French word Pvissance (power)P French word Pvissance (power)

Meaning power of hydrogenMeaning power of hydrogen

Def: -ve log of [H+] conc. i.e.minus no. toDef: -ve log of [H+] conc. i.e.minus no. to

which 10 must be raised to get that no.which 10 must be raised to get that no.


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pH ScalepH Scale

7 (neutral)7 (neutral)

||0 --------------------------------------------------140 --------------------------------------------------14

|| alkalinealkaline

7.40(Blood)7.40(Blood)pH< 7.35Acidosis

pH > 7.45

Alkalosis


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Acid-Base DisordersAcid-Base Disorders

Blood PH 7.4Blood PH 7.4

HH++ = 40 x 10= 40 x 10-9-9 = 40 nmol/L= 40 nmol/L


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AcidsAcidsHH++ ionsions or proton donoror proton donor

Two types of acids are formed byTwo types of acids are formed by

metabolic processesmetabolic processes

Volatile acids:Volatile acids: liquidliquid gas. COgas. CO22eliminated by lungs.eliminated by lungs.

COCO22 + H+ H22OO HH22COCO33 HH++ + HCO+ HCO33 NonvolatileNonvolatile or fixed acids: are eliminatedor fixed acids: are eliminated

by the kidneysby the kidneysExamples: SOExamples: SO44, PO, PO44, lactic acid, ketoacids, lactic acid, ketoacids

TheThe non-volatile portion is trivialnon-volatile portion is trivial whenwhen

compared to the volatile COcompared to the volatile CO22.About 50-100.About 50-100


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H+ isH+ is maintained within narrowmaintained within narrowlimits.limits.

Normal LevelNormal Level approx.40 nanomol/Lapprox.40 nanomol/LLow Conc.Low Conc. Essential for normalEssential for normalcellular fxn.cellular fxn.

Changes in the H+ conc.,Changes in the H+ conc., proteinsproteinsgain or lose H+ ions.gain or lose H+ ions.

resulting in:resulting in:

alteration in charge distributionalteration in charge distributionmolecular configurationmolecular configuration

protein functionprotein function


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pH change RegulationpH change Regulation

The bodyThe body constantly produces acidsconstantly produces acids throughthroughmetabolismmetabolism

These acids must beThese acids must be constantly eliminatedconstantly eliminatedfrom the bodyfrom the body

BuffersBuffersChemical substance that prevents largeChemical substance that prevents largechanges in pHchanges in pH

VentilationVentilation

Can handle ~75% of most pH disturbancesCan handle ~75% of most pH disturbances

Renal regulationRenal regulation of Hof H++ & HCO& HCO33--

slow but very effectiveslow but very effective


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Acid-Base LoadAcid-Base Load

Addition from extrinsic source:Addition from extrinsic source:

Infusions [eg HCl, NH4Cl]Infusions [eg HCl, NH4Cl]

Intrinsic sources:Intrinsic sources: Acid generation: ketoacidosis, LacticAcid generation: ketoacidosis, Lacticacidosisacidosis

Acid loss: VomitingAcid loss: Vomiting

Base loss: DiarrheaBase loss: Diarrhea


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HENDERSON-HASSELBACHHENDERSON-HASSELBACH

EQUATIONEQUATION

pH = pKa + logpH = pKa + log basebase

acidacid

When ph and pKa are equal: 50% exists as Base

& 50% as Acid

HA H+ + A-Law of Mass Action: at Equilibrium reactions are equal

Strong Acid: Completely ionized at ph= 7.4


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weak acids, are able to

take up or release H+ sothat changes in the free H+

concentration areminimized


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HPO4(2-) + H+ H2PO4-

pKa= 6.8


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[HPO4-]

pH = PKa [H2PO4

(2-) ]

10

= 160 x = 160 nanomol/L (pH=6.80)

10

pKa= 6.8 At pH=7.4[HPO4-] : 80%

[H2PO4-]: 20%


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Addition of Acid:Large changes in H+

concentration are prevented by:

Buffering


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HCl + Na2HPO4 > NaCl + NaH2PO4

It was possible because the Na2HPO4 can beionized as pk is close to the physiological Ph 7.4

pK= 6.8 and so can bind the H ions and make aweaker acid and thus nullify the effect of HCL


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12

H + = 6.8 x = 240 nanomol/L (pH =6.62)8

pH = 2.7

If all H+ taken up by HPO4 then

[HPO4-][H2PO4

(2-) ]

If No Buffer


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[CO2]dis + H2O H2CO3 H+ + HCO3-

[H+] [HCO3-]

Ka =

[CO2]dis [H2O]

[H+] [HCO3-]

K'a =

[CO2]dis

PCO2

pH = 24 x

[HCO3-]

pKa x [CO2]dis

pH =

[HCO3-]

At Physiological pH


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BufferBuffer

There are four main buffer systems inThere are four main buffer systems inthe body:the body: Bicarbonate buffer system. (the MAIN one)Bicarbonate buffer system. (the MAIN one)

64%64%

NaHCONaHCO33 HH22COCO33COCO22

Hemoglobin buffer system. 29%Hemoglobin buffer system. 29%

HbOHbO22-- HHbHHb

Protein buffer system. 6%Protein buffer system. 6%PrPr-- HPrHPr

Phosphate buffer system. 1%Phosphate buffer system. 1%

NaNa22

HPOHPO44

NaHPONaHPO44


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Open buffer systemOpen buffer system

[CO2]dis + H2O H2CO3 H+ + HCO3-

Add AcidExpired Gas

At physiologic pH of 7.4: HCO3 : H+ 20:1


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Primary and Secondary DisordersPrimary and Secondary Disorders

Primary:Primary: Respiratory:Respiratory: AcidosisAcidosis

AlkalosisAlkalosis

Metabolic:Metabolic: AcidosisAcidosisAlkalosisAlkalosis

Secondary:Secondary: Respiratory:Respiratory: AcidosisAcidosis

AlkalosisAlkalosis Metabolic:Metabolic: AcidosisAcidosis

AlkalosisAlkalosis


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Two classes of acids are physiologically

important:

Carbonic acid (H2CO3) &

non-carbonic acids.

Metabolism ofcarbohydrates and fats

results in the generation of

approximately 15,000 mmol ofCO2/Day


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COCO22 + H+ H22OOHH22COCO33 H+ + HCOH+ + HCO33

This is prevented by the loss of CO2 via

Via lungs


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Noncarbonic acids:

Primarily derived from themetabolism of proteins


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CO2 + H2O H2CO3 H+ + HCO3 HCO3-

H2CO3 : HCO3= 1 : 6800

At Physiological pH

H2CO3 : PCO2= 1: 340


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The primary intracellular buffers are proteins,

organic and inorganicphosphates,

and,

in the erythrocyte, hemoglobin (Hb-):

H+ + Hb- HHb

H+ + Pr- HPr


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H+ + Hb- HHb

H+ + Pr- HPr


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Bone:

Important site of acid and base buffering

Exchange for surface Na+ and K+,Dissolution of bone mineralRelease of buffer compounds:

NaHCO3 & KHCO3 initiallyThen CaCO3 and CaHPO4,

This buffering reaction appears to be initiated

in part by the fall in the plasma HCO3-concentration


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Methionine > glucose + urea + SO4(2-) + 2 H+

Arginine+ > glucose (or CO2) + urea + H+

R-H2PO4 + H2O > ROH + 0.8 HPO42- / 0.2 H2PO4- + 1.8 H+

Glutamate- + H+ > glucose + urea

Citrate- + 4.5 O2 > 5 CO2 + 3 H2O + HCO3-

Lactate- + H+ > glucose + CO2

Increasing the pH/ Alkalosis/ Decreasing H+


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Glutamate- + H+ > glucose + urea

Citrate- + 4.5 O2 > 5 CO2 + 3 H2O + HCO3-

Lactate- + H+ > glucose + CO2

Increasing the pH/ Alkalosis/ Decreasing H+


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(1)Reabsorption of the filtered HCO3-

(1)Excretion of the 50 to 100 meq of H+

produced per day

Renal Actions


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A normal subject

GFR: 180 L/day (125 mL/min)

plasma HCO3- concentration of24 meq/L

filters & then must reabsorb

approximately 4300 meq of HCO3- each day


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The lowest urine pH that can be achieved inhumans is 4.5.

Almost 1000 times (3 log units) more acid thanthe extracellular pH,

Still extremely low free H+ concentration of lessthan 0.04 meq/L.


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Secretion of each H+ ion is associated with

the generation of one HCO3- ion in the plasma.


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steady state, the net amount of H+ excreted is equal to the

normal H+ load of50 to 100 meq/day

This value can exceed 300 meq/day (primarily due to enhanced

NH4+ excretion) if acid production is increased

Secretion of each H+ ion is associated with

the generation of one HCO3- ion in the plasma.


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Proximal Tubular CellProximal Tubular Cell

2K

H2O OH + CO2

H+H+ CA

3HCO3 HCO3

BloodTubular Lumen

Na+

Na+

3Na+

2K

Na K AtPase

HCO3 +

H2O + CO2

Renal Tubular Acidosis: Proximal RTA


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Distal acidification

H+ secretion in the distal nephron primarily

occurs in the

intercalated cells in the cortical collecting

tubule

and in the cells in the outer and innermedullary collecting tubules


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-Intercalated Cells of Cortical-Intercalated Cells of CorticalCollecting Collecting TubuleCollecting Collecting Tubule

H+ H+

K K

H2O OH

H+H+

H-ATPase

CA

HCO3

Cl

Cl

HCO3

BloodTubular Lumen

Cl

Renal Tubular Acidosis: Distal RTA


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H2OOH

H+H+

H-ATPase

CA

HCO3

Cl

HCO3

BloodTubular Lumen

Cl Cl

-Intercalated Cells of Cortical-Intercalated Cells of CorticalCollecting Collecting TubuleCollecting Collecting Tubule


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Glutamine NH4+ + glutamate-

NH4+ + alpha-ketoglutarate(2-)

AMMONIUM EXCRETION


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AcidosisAcidosis

So what kind of metabolic acidosis is it?So what kind of metabolic acidosis is it?

LossLoss of HCO3 orof HCO3 orgaingain of intrinsic orof intrinsic or

extrinsic Acidsextrinsic Acids

CalculateCalculate anion gapanion gap: 8-16(normal): 8-16(normal)


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Anion Gap [AG]

Na= 139 Cl= 103 HCO3=24

AG = [+VE] [-VE]

Ca, Mg, K= So4 , PO3

Na - [Cl+HCO3] =

139 - [103+ 24] =

139 - 127 = 12


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What is AG of 12 ?

All charges POSITIVE = Negative

Some other [ ve] charges

that we could not estimate from our lab

These [ ve] charges are forAlbumin


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Causes of High AG MA:

M

U

D

P

I

L

E

S

Methanol [Formate]

Uremia [SO4, Pho3, other]

DKA [Acetoacetate, -OH Butyrate]

Paraldehyde [unknown/Acetic Acid]

Isopropyl Alcohol [Lactate]

Lactic Acidosis [Lactate]

Ethylene Glycol [Oxalate, Glycolate]

ASA [Lactate & Ketoacids]


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Osmolal Gap:

Addition of solute the P Osmolality

Measured Osmol - Calculated Osmol = < 20[Nml]

Calculated Osmol =

2 x Na + Urea + Glucose= mmol/L

Causes ofCauses ofOsmol GapOsmol GapEthanol

Methanol

Ethylene Glycol

Isopropyl Alcohol

B i R l f thAcid Base Evaluation: Summary


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Basic Rules of the game

AG, CO2, HCO3, pH

1: PH=7.4 CO2 =40 HCO3 =24

2: HCO3 < 13 Definitely Metabolic Acidosis

3: Anion Gap [AG]

AG >20 Metabolic Acidosis Definitely

AG 12-20 MA Or MAlk

IF YOU FOLLOW THE RULES

Mixed Acid Base Disorder evaluation is easy!

Clinical Picture

AG, HCO3, PCO2, pH, Osmolal Gap

Osmolal Gap: Addition of solute will the plasma OsmolalityMeasured Osmolality - Calculated Osmolality = Nml < 20

Calculated Osmolality=2Na + Urea + Glucose= All in mmol/L

Metabolic Acidosis

1. Last 2 digits of pH 7.25: CO2 = 25

2. Expected PCO2 = HCO3 X 1.2[Limit: Max PCO210 mm of Hg]

12-24 Hours for compensation

Metabolic Alkalosis

Expected PCO2 = 0.7 X HCO324-36 hrs for respiratory compensation

Limit: Max PCO2 55 mm of Hg

Chronic Respiratory Acidosis

10 PCO2 = 4 HCO3- [ Chronic RA]5-10 Days for Metabolic Compensation Max : 45 meq/L

Acute Respiratory Alkalosis

10 PCO2 = 2 HCO3 ,5-10 minutes for Compensation Max : 18 meq/L

Acute Respiratory Acidosis

10 PCO2 = by 1 HCO35-10 minutes for Compensation Max : 30 meq/L

Chronic Respiratory Alkalosis

10 PCO2 = 4 HCO32-3 Days for Metabolic Compensation Max : 14 meq/L

Acid Base Evaluation: Summary

Nauman Tarif, MD

AG / HCO3 = 1-1.6


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Primary Disorder

Acidosis or Alkalosis

If it is Respiratory thenRenal [metabolic] compensation

Respiratory Disorders:

Breathe Too MUCH!

Breathe Too LESS!

Respiratory Alkalosis

Respiratory Acidosis

H2O + CO2 H2CO3 H+ + HCO3

H+ + HCO3 H2CO3 H2O + CO2

M b li Di d


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Metabolic Disorders

Add Acid or Lose HCO3 Metabolic Acidosis

Add Alkali or Lose Acid Metabolic Alkalosis

If it is Metabolic then

Respiratory compensation

Compensatory responses

will not bring the pH to normal;

always close towards normal.


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In the presence ofNORMAL pH

IT IS A MIXED Disorder!

AT least 2 primary disorders

Human environment is Dynamic !

Abnormal CO2 or HCO3


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Pulmonary Embolus Respiratory Alkalosis

Shock Metabolicacidosis

Pulmonary edema RespiratoryAlkalosis

Renal Failure MetabolicAcidosis

Sepsis RespAlk+ MetAcid

COPD Respiratoryacidosis

Diuretic Use MetabolicAlkalosis

Cirrhosis RespiratoryAlkalosis


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42 y/o male found unconsciuos in the desert

[lost his way for the last 2 days] with an empty bottle

BP& bilateral chest creptsAG 144 -[97+10]= 37 HAGMA

PCO2 35 14 X 1.2 = 16.8 , [40-17 = 23 ]

So PCO2 should be 25 Here PCO2 is 35

So Something is keeping the PCO2RA


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In an unconscious pt with no clear cut history and MA

MUST Exclude: Ingestion [empty bottle!]

Gap: Osmolal Gap

Measured - Calculated Osmolality > 20 [Ingestion]

Methanol, Ethylene Glycol and Isopropyl Alcohol

Osmolal gap = 29 MSU : CaOxalate Crystals

Ethylene Glycol Intoxication


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Gap of the Gaps

Delta AG : Delta HCO3

37-12 : 24-10

25 : 14

1. 8 : 1 [ >1.6]

So Something is keeping the HCO3MAlkHe apparently had Vomiting after ingestion of EthGlycol

HAGMA + RA + MAlk


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pH 7.28


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Management Guidelines:

1. Treat the Clinical state

2. Metabolic Acidosis:

pH to 7.2 & HCO3 >10

[Prevent CVS Instability]

3. Metabolic Alkalosis

Fluid for Cl responsive

Acetazolamide

IV HCL, NH4Cl, ArgHCl

4. Respiratory Acidosis :

5. Respiratory Alkalosis


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Management Guidelines:

1. Treat the Clinical state

2. Metabolic Acidosis:

pH to 7.2 & HCO3 >10

[Prevent CVS Instability]

3. Metabolic Alkalosis

Fluid for Cl responsive

Acetazolamide

IV HCL, NH4Cl, ArgHCl

4. Respiratory Acidosis :

5. Respiratory Alkalosis

abg und re standing the principles

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