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8/9/2019 Arterial Blood Gases (1)

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Arterial Blood GasesArterial Blood GasesSteve LanSteve Lan


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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 in

pH)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:

pHpH 7.357.35--7.457.45

PaO2PaO2 9595--100100mmHgmmHg

PaCO2PaCO2 3535--4545

mmHgmmHgHCO3HCO3 2222--2626

mmHgmmHg


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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


Blood (instantaneously)


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Lungs(within minutes)


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Excretion via kidneys(hours to days)


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Blood (instantaneously)

Lungs(within minutes)

Excretion via kidneys(hours to days)


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Respiratory AlkalosisRespiratory Alkalosis


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


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


Increased plasma HCO3Increased 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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CompensationCompensationDisorderDisorder Initial changeInitial change CompensationCompensation

MetabolicMetabolicAcidosisAcidosis

Decrease HCO3Decrease HCO3 1 mmHg decrease in1 mmHg decrease inPaCO2 for every 1PaCO2 for every 1decrease in HCO3decrease in HCO3

MetabolicMetabolicAlkalosisAlkalosis

Increase HCO3Increase HCO3 0.30.3--0.5 mmHg increase0.5 mmHg increasein PaCO2 for every 1in PaCO2 for every 1increase in HCO3increase in HCO3

RespiratoryRespiratoryalkalosisalkalosis

Decrease inDecrease inPaCO2PaCO2

Acute: 2 decrease inAcute: 2 decrease inHCO3 for every 10HCO3 for every 10

decrease in PaCO2decrease in PaCO2Chronic: 4Chronic: 4--5 decrease5 decrease

RespiratoryRespiratoryacidosisacidosis

Increase inIncrease inPaCO2PaCO2

Acute: 1 increase inAcute: 1 increase inHCO3 for every 10HCO3 for every 10increase in PaCO2increase in PaCO2

Chronic: 3 increaseChronic: 3 increase


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CompensationCompensationDisturbanceDisturbance PaCO2PaCO2 HCO3HCO3

Resp acidosisResp acidosis(acute)(acute)

1010 11

Resp acidosisResp acidosis

(chronic)(chronic)

1010 33

Resp alkalosisResp alkalosis(acute)(acute)

1010 22

Resp alkalosisResp alkalosis(chronic)(chronic)

1010 44

MetabolicMetabolicacidosisacidosis

11 11

MetabolicMetabolicalkalosisalkalosis

33 1010


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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


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


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


pH 7.08pH 7.08 PaCO2 80PaCO2 80

PaO2 37PaO2 37

HCO3 26HCO3 26

primarily respiratoryprimarily respiratory


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Case 1Case 1


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 mildcomponent 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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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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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 postprocedureprocedure

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