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Arterial Blood GasesArterial Blood Gases

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ObjectivesObjectives

Review of physiologyReview of physiology

Basic interpretation of ABGsBasic interpretation of ABGs

Radial ABG techniqueRadial ABG technique

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Some DefinitionsSome Definitions

pH: concentration of H+ in terms ofpH: concentration of H+ in terms of1010--xx per litreper litre

pH of 7.0 = 10pH of 7.0 = 10--77

= 0.0000001= 0.0000001 pH of 6.0 = 10pH of 6.0 = 10--66 = 0.000001= 0.000001

Acidosis: acidic blood; increase in H+Acidosis: acidic blood; increase in H+concentration (therefore decrease inconcentration (therefore decrease inpH)pH)

Alkalosis: alkalotic blood; increase inAlkalosis: alkalotic blood; increase inbase, therefore decrease in H+base, therefore decrease in H+

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Basic principlesBasic principles

Human bodies were designed toHuman bodies were designed tomaintain:maintain:

pH 7.35-7.45

PaO2 95-100mmHg

PaCO2 35-45

mmHgHCO3 22-26

mmHg

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BuffersBuffers

Limit pH changes when strongLimit pH changes when strongacids/bases are introducedacids/bases are introduced

Weak acid and its conjugate baseWeak acid and its conjugate base Addition of a strong acid is partlyAddition of a strong acid is partly

neutralized by the weak baseneutralized by the weak base

HB (weak acid) H+ (strong acid) B- (conjugate weak base)

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Bicarbonate BufferSystemBicarbonate BufferSystem

CO2+H20 H2CO3 H++HCO3-

Main extracellular bufferMain extracellular buffer

Also affected by lungs and kidneysAlso affected by lungs and kidneys

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Key EquationKey Equation

CO2+H20 H2CO3 H++HCO3-

Blood (instantaneously)

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Key EquationKey Equation

CO2+H20 H2CO3 H++HCO3-

Lungs

(within minutes)

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Key EquationKey Equation

CO2+H20 H2CO3 H++HCO3-

Excretion via kidneys(hours to days)

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Key EquationKey Equation

CO2+H20 H2CO3 H++HCO3-

Blood (instantaneously)

Lungs

(within minutes)

Excretion via kidneys(hours to days)

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Respiratory AcidosisRespiratory Acidosis

CO2+H20 H2CO3 H++HCO3-

Increase in CO2 pushes balance towardsIncrease in CO2 pushes balance towardsproducing more H2CO3producing more H2CO3

Concentration of H+ increases, loweringConcentration of H+ increases, loweringpHpH

CausesCauses Impaired lung functionImpaired lung function

Decreased respiratory driveDecreased respiratory drive

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Respiratory AlkalosisRespiratory Alkalosis

CO2+H20 H2CO3 H++HCO3-

Increased RR causes decreased CO2Increased RR causes decreased CO2

Balance shifts towards leftBalance shifts towards leftdecreasing concentration of H+decreasing concentration of H+(therefore increasing pH)(therefore increasing pH)

CausesCauses Hyperventilation (anxiety)Hyperventilation (anxiety)

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Metabolic AcidosisMetabolic Acidosis

CO2+H20 H2CO3 H++HCO3-

Abnormal metabolism releases H+,Abnormal metabolism releases H+,

increasing concentration of pH, andincreasing concentration of pH, anddecreasing pHdecreasing pH

CausesCauses

DKA, lactic acidosis (arrest)DKA, lactic acidosis (arrest) ToxinsToxins

Loss of HCO3 in diarrheaLoss of HCO3 in diarrhea

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Metabolic AlkalosisMetabolic Alkalosis

CO2+H20 H2CO3 H++HCO3-

I

ncreased plasma HCO3I

ncreased plasma HCO3-- neutralizesneutralizesH+H+

Decreased concentration of H+Decreased concentration of H+

Causes:Causes:

Loss of H+ in vomitLoss of H+ in vomit

Gain of HCO3 (ingestion, IV)Gain of HCO3 (ingestion, IV)

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Interpreting the ABGsInterpreting the ABGs

Remember the normalsRemember the normals

pH 7.4, PaCO2 40, HCO3 24pH 7.4, PaCO2 40, HCO3 24

Know the expected compensationKnow the expected compensation

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CompensationCompensationDisorder Initial change Compensation

MetabolicAcidosis

Decrease HCO3 1 mmHg decrease inPaCO2 for every 1decrease in HCO3

MetabolicAlkalosis

Increase HCO3 0.3-0.5 mmHg increasein PaCO2 for every 1increase in HCO3

Respiratoryalkalosis

Decrease inPaCO2

Acute: 2 decrease inHCO3 for every 10

decrease in PaCO2Chronic: 4-5 decrease

Respiratoryacidosis

Increase inPaCO2

Acute: 1 increase inHCO3 for every 10increase in PaCO2

Chronic: 3 increase

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CompensationCompensationDisturbance PaCO2 HCO3

Resp acidosis(acute)

10 1

Resp acidosis

(chronic)

10 3

Resp alkalosis(acute)

10 2

Resp alkalosis(chronic)

10 4

Metabolicacidosis

1 1

Metabolicalkalosis

3 10

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Basic ApproachBasic Approach

Is the pH acidemic or alkalemic?Is the pH acidemic or alkalemic?

What is the primary disturbance?What is the primary disturbance?

Metabolic: change in HCO3 and pH inMetabolic: change in HCO3 and pH insame directionsame direction

Respiratory: change in PaCO2 and pH inRespiratory: change in PaCO2 and pH inopposite directionopposite direction

Is the compensation appropriate?Is the compensation appropriate?

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Case 1Case 1

24 yo M hx of drug abuse, brought to24 yo M hx of drug abuse, brought toER cyanoticER cyanotic

pH 7.08pH 7.08 PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

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Case 1Case 1

24 yo M hx of drug abuse, brought to24 yo M hx of drug abuse, brought toER cyanoticER cyanotic

pH 7.08pH 7.08 PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

Acidemic or Alkalemic?Acidemic or Alkalemic?

AcidemicAcidemic

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Case 1Case 1

24 yo M hx of drug abuse, brought to24 yo M hx of drug abuse, brought toER cyanoticER cyanotic

pH 7.08pH 7.08 PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

pH in relation to PaCO2 and HCO3?pH in relation to PaCO2 and HCO3?

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Basic ApproachBasic Approach

Is the pH acidemic or alkalemic?Is the pH acidemic or alkalemic?

What is the primary disturbance?What is the primary disturbance?

Metabolic: change in HCO3 and pHMetabolic: change in HCO3 and pHin same directionin same direction

Respiratory: change in PaCO2 andRespiratory: change in PaCO2 andpH in opposite directionpH in opposite direction

Is the compensation appropriate?Is the compensation appropriate?

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Case 1Case 1

24 yo M hx of drug abuse, brought to24 yo M hx of drug abuse, brought toER cyanoticER cyanotic

pH 7.08pH 7.08 PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

primarily respiratoryprimarily respiratory

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Case 1Case 1

24 yo M hx of drug abuse, brought to24 yo M hx of drug abuse, brought toER cyanoticER cyanotic

pH 7.08pH 7.08

PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

Is the compensation adequate?Is the compensation adequate?

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Case 1Case 1

PaCO2 increased by 40PaCO2 increased by 40

For every 10 increase you wouldFor every 10 increase you would

expect 1 increase in HCO3expect 1 increase in HCO3 Expected HCO3 would be ~28Expected HCO3 would be ~28

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Case 1Case 1

24 yo M hx of drug abuse, brought to ER24 yo M hx of drug abuse, brought to ERcyanoticcyanotic pH 7.08pH 7.08

PaCO2 80PaCO2 80 PaO2 37PaO2 37

HCO3 26HCO3 26

Acidemic, primarily respiratory, but mildAcidemic, primarily respiratory, but mild

component of metaboliccomponent of metabolic Also hypoxemicAlso hypoxemic

Narcotic ODNarcotic OD

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Case 2Case 2

42 F IDDM, presents with 4d hx of unwell42 F IDDM, presents with 4d hx of unwell

pH 7.23pH 7.23

PaCO2 27PaCO2 27

PaO2 118PaO2 118

HCO3 12HCO3 12

Acidemia, metabolicAcidemia, metabolic

DKA, Na 135, Cl 99DKA, Na 135, Cl 99 AG = NaAG = Na ClCl HCO3 = 135HCO3 = 135 111 = 24111 = 24

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Case 3Case 3

71 m hx of COPD, c/o SOB71 m hx of COPD, c/o SOB

pH 7.21pH 7.21

PaCO2 75PaCO2 75 PaO2 41PaO2 41

HCO3 30HCO3 30

Acidemia, resp (acute on chronic),Acidemia, resp (acute on chronic),hypoxichypoxic

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Case 4Case 4

23 F c/o SOB, lightheaded and23 F c/o SOB, lightheaded andperioral tinglingperioral tingling

pH 7.54pH 7.54

PaCO2 22PaCO2 22

PaO2 115PaO2 115

HCO3 21HCO3 21

Alkalemia, resp (acute)Alkalemia, resp (acute)

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Case 5Case 5

32 M c/o vomitting x 5d, HR 110 BP32 M c/o vomitting x 5d, HR 110 BP90/50, dry MM90/50, dry MM

pH 7.50pH 7.50

PaCO2 47PaCO2 47

PaO2 80PaO2 80

HCO3 38HCO3 38

Alkalemia, metabolicAlkalemia, metabolic

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ABG SamplingABG Sampling

IndicationsIndications

Assessment of illness, or response toAssessment of illness, or response totherapytherapy

ContraindicationsContraindications

Positive Allens testPositive Allens test

Overlying infectionOverlying infection

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ABG samplingABG sampling

SitesSites

Radial artery, brachial, femoralRadial artery, brachial, femoral

ComplicationsComplications Arterial spasmArterial spasm

Hemorrhage/hematomaHemorrhage/hematoma

Nerve injuryNerve injury

infectioninfection

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Allens testAllens test

AnatomyAnatomy

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Allens testAllens test

Step 1: tight fist x 20 secStep 1: tight fist x 20 sec

Step 2: Occlude radial and ulnar arteriesStep 2: Occlude radial and ulnar arteries

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Allens testAllens test

Step 3: open hand and look forStep 3: open hand and look forblanchingblanching

Step 4: release ulnar artery and lookStep 4: release ulnar artery and look

for capillary refill (5for capillary refill (5--7 sec)7 sec)

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ABG techniqueABG technique

Position wristPosition wrist

Prep skinPrep skin

Insert needle ~45Insert needle ~45degrees, bevel updegrees, bevel up

Apply pressure xApply pressure x5min post5min post

procedureprocedure