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Blood Gas Analysis

Dr. Prakash Mohanasundaram

Department of Emergency & Critical Care medicineVinayaka Missions University

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Primary & Compensatory changesPrimary change

Compensation

Respiratory acidosis

PaCO2 [HCO3]

Respiratory alkalosis

PaCO2 [HCO3]

Metabolic acidosis

[HCO3] PaCO2

Metabolic alkalosis

[HCO3] PaCO2

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THE SIX

STEP APPROACH

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The 6 step approach

1) Acidemic or Alkalemic2) Is the primary disturbance

respiratory or metabolic3) If Respiratory, is it acute or

chronic4) If Metabolic, is there an anion

gap?

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The 6 step approach

5) Are there any other metabolic disturbances with the anion gap acidosis?

6) Assess normal respiratory compensation for a metabolic disturbance.

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Step - 1Acidemic or AlkalemicAcidemic : pH < 7.35Alkalemic : pH > 7.45

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Step - 2Is the primary disturbance Respiratory or Metabolic

Respiratory disorder will alter the

PaCO2 High CO2 in acidosis ,Low CO2 in alkalosis

Go to Step – 3

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Step - 2

Metabolic disorder will alter the [HCO3] HCO3 < 22, metabolic acidosis

“Go to Step-4”

HCO3 >26 ,metabolic alkalosis

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Step - 3

RESPIRATORY

ACUTE CHRONIC

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Step - 3

Respiratory Acidosis Acute pH = 0.08 x (PaCO2 – 40 ) /10

Chronic pH = 0.03 x (PaCO2 – 40 ) /10

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Step - 3

Respiratory Alkalosis Acute pH = 0.08 x (PaCO2 – 40 ) /10

Chronic pH = 0.03 x (PaCO2 – 40 ) /10

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Respiratory Acidosis

Hypoventilation : PaCO2

CNS depression : Sedatives , CNS disease

Lung disease : Asthma , Chronic lung disease

Musculoskeletal disorders : GBS , Myasthenia

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Respiratory Alkalosis

Hyperventilation : PaCO2

CNS events ( hemorrhage)

Drugs ( salicylates)

Anxiety

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Step - 4

METABOLIC ACIDOSIS

CHECK ANION GAP

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Step - 4

Anion Gap = Na- (Cl + HCO3)Usually 12 +- 2

Anion Gap Metabolic acidosis : AG > 12

Non Anion Gap acidosis : AG < 12

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Anion Gap Metabolic Acidosis

HCO3 & AG

Methanol Uremia DKA Paraldehyde Infection Lactic acid Ethylene glycol Salicylates

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Non-Anion Gap Metabolic acidosis

Loss of HCO3, or external acid infusion Low HCO3 but AG < 12

GI loss of HCO3 (Diarrhea)Renal lossesRenal tubular necrosisCarbonic anhydrase inhibitorsUreteral diversion

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Step - 5

Is there another metabolic disturbance co-existing with an anion gap acidosis

Corrected HCO3 = HCO3 + (AG – 12) Corrected HCO3 > 24 : Additional metabolic alkalosis Corrected HCO3 < 24 : Non AG acidosis co-exists .

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Example

Anion Gap Metabolic acidosis HCO3 10 , AG 26 Corrected HCO3 = 1O + (26-12) =24

No additional disturbance

HCO3 15 , AG 26Corrected HCO3 = 15 + (26-12) =29 Co-existing metabolic alkalosis

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Step-6Assess normal respiratory compensation for a metabolic acidosis

Winter’s formulaExpected PaCO2 = (1.5 X HCO3) + (8 +- 2)

If HCO3 is 10 , PaCO2 should be 21-25 If PaCO2 < 21, additional respiratory alkalosis

If PaCO2 >25, additional respiratory acidosis

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Step - 6Assess normal respiratory compensation for a metabolic disturbanceIn metabolic Alkalosis winters formula not applicablePaCO2 will rise, but not over 50-55

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Case 1

MeasuredpH 7.24PCO2 60 mm Hg PO2 56 mm Hg

CalculatedHCO3 24mmol/LBE +2SaO2 85 %

5 Month old baby with Bronchiolitis

FiO2 70%

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Case 1

Step-1 Acidemia or Alkalemia ? Acidemia

Step-2 Primary disturbance Respiratory

Step-3 Acute or Chronic pH = 0.08 x (60-40) / 10 =

0.16 7.4 – 0.16 = 7.24 Acute Respiratory Acidosis

Uncompensated

pH 7.24PCO2 60 mm Hg PO2 56 mm HgHCO3 24mmol/L

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Case 2

MeasuredpH 7.10PCO2 20 mm Hg PO2 87 mm Hg

CalculatedHCO3 8 mmol/L

BE - 15SaO2 96%

3 year old with severe diarrhea , diminished urine output, lethargy, Na-135 , Cl- 115

FiO2 24%

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Case 2 Step-1 Acidemia or Alkalemia ?

AcidemiaStep-2 Primary disturbance

MetabolicStep-4 What is the Anion Gap ?

Na- (Cl + HCO3) 135-(115 + 8) = 12 Step-6 What is the expected CO2 ?

Expected PaCO2 = (1.5 x HCO3) +( 8 +- 2 )

=( 1.5 x 8 ) + ( 8 +- 2 ) = 18 – 22

Compensated metabolic acidosis

pH 7.10PCO2 20 mm Hg PO2 87 mm HgHCO3 8 mmol/LNa-135 , Cl- 115

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Case 3

Measured

pH 7.1oPCO2 60 mm Hg PO2 60 mm Hg

Calculated

HCO3 10 mmol/LBE - 15SaO2 87 %

50 Year old resuscitated after a cardiac arrest. Na-142 , Cl- 108

FiO2 100%

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Case 3

Step-1 Acidemia or Alkalemia ? Acidemia

Step-2 Primary disturbance Metabolic

Step-4 What is the Anion Gap ?

Na- (Cl + HCO3) 142-(108 + 10) = 24

pH 7.1oPCO2 60 mm Hg PO2 60 mm Hg

HCO3 10 mmol/L

Na-142 , Cl- 108

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Case 3Step – 5 What is the corrected HCO3

Corrected HCO3 = HCO3 + (AG – 12 ) 10 + ( 24 – 12 ) = 22

Step – 6 What is the expected CO2 ?

Expected PaCO2 = (1.5 x HCO3) +( 8 +- 2 ) ( 1.5 x 10 ) + ( 8 +- 2 ) = 21 – 25

Mixed metabolic and respiratory acidosis

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Case 4

MeasuredpH 7.52PCO2 26 mm Hg PO2 112mm Hg

CalculatedHCO3 22 mmol/LBE +2SaO2 99%

4 year girl with lethargy, irritability, jaundice

FiO2 33%

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Case 4Step-1 Acidemia or Alkalemia ?

AlkalemiaStep-2 Primary disturbance

RespiratoryStep-3 Acute or Chronic

pH = 0.08 x (26-40) / 10 = 0.11

7.4 + 0.11 = 7.51

Acute Respiratory Alkalosis Uncompensated

pH 7.52PCO2 26 mm Hg PO2 112mm HgHCO3 22 mmol/L

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THANK YOU