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Blood Gas Analysis

Blood Gas Analysis. Dr. Prakash Mohanasundaram Department of Emergency & Critical Care medicine Vinayaka Missions University. Primary & Compensatory changes. THE SIX STEP APPROACH. The 6 step approach. Acidemic or Alkalemic Is the primary disturbance respiratory or metabolic

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Blood Gas Analysis

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  1. Blood Gas Analysis Dr. Prakash Mohanasundaram Department of Emergency & Critical Care medicine Vinayaka Missions University

  2. Primary & Compensatory changes

  3. THE SIX STEP APPROACH

  4. The 6 step approach • Acidemic or Alkalemic • Is the primary disturbance respiratory or metabolic • If Respiratory, is it acute or chronic • If Metabolic, is there an anion gap?

  5. The 6 step approach • Are there any other metabolic disturbances with the anion gap acidosis? • Assess normal respiratory compensation for a metabolic disturbance.

  6. Step - 1 • Acidemic or Alkalemic • Acidemic : pH < 7.35 • Alkalemic : pH > 7.45

  7. Step - 2 • Is the primary disturbance Respiratory or Metabolic • Respiratory disorder will alter thePaCO2 High CO2 in acidosis ,LowCO2 in alkalosis Go to Step – 3

  8. Step - 2 • Metabolic disorder will alter the[HCO3] HCO3 < 22, metabolic acidosis “Go to Step-4” • HCO3 >26 ,metabolic alkalosis

  9. Step - 3 RESPIRATORY ACUTE CHRONIC

  10. Step - 3 • Respiratory Acidosis Acute pH = 0.08 x (PaCO2 – 40 ) /10 Chronic pH = 0.03 x (PaCO2 – 40 ) /10

  11. Step - 3 • Respiratory Alkalosis Acute pH = 0.08 x (PaCO2 – 40 ) /10 Chronic pH = 0.03 x (PaCO2 – 40 ) /10

  12. Respiratory Acidosis • Hypoventilation : PaCO2 • CNS depression : Sedatives , CNS disease • Lung disease : Asthma , Chronic lung disease • Musculoskeletal disorders : GBS , Myasthenia

  13. Respiratory Alkalosis • Hyperventilation : PaCO2 • CNS events ( hemorrhage) • Drugs ( salicylates) • Anxiety

  14. Step - 4 METABOLIC ACIDOSIS CHECK ANION GAP

  15. Step - 4 • Anion Gap= Na- (Cl + HCO3) • Usually 12 +- 2 • Anion Gap Metabolic acidosis : AG > 12 • Non Anion Gap acidosis : AG < 12

  16. Anion Gap Metabolic Acidosis • HCO3 & AG Methanol Uremia DKA Paraldehyde Infection Lactic acid Ethylene glycol Salicylates

  17. Non-Anion Gap Metabolic acidosis Loss of HCO3, or external acid infusion Low HCO3 but AG < 12 • GI loss of HCO3 (Diarrhea) • Renal losses • Renal tubular necrosis • Carbonic anhydrase inhibitors • Ureteral diversion

  18. 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 .

  19. Example Anion Gap Metabolic acidosis HCO3 10 , AG 26 Corrected HCO3 = 1O + (26-12) =24 No additional disturbance HCO3 15 , AG 26 Corrected HCO3 = 15 + (26-12) =29 Co-existing metabolic alkalosis

  20. Step-6 • Assess normal respiratory compensation for a metabolic acidosis Winter’s formula • ExpectedPaCO2 = (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

  21. Step - 6 • Assess normal respiratory compensation for a metabolic disturbance • In metabolic Alkalosis winters formula not applicable • PaCO2 will rise, but not over 50-55

  22. Measured pH 7.24 PCO2 60 mm Hg PO2 56 mm Hg Calculated HCO3 24mmol/L BE +2 SaO2 85 % Case 1 FiO2 70% 5 Month old baby with Bronchiolitis

  23. pH 7.24 PCO2 60 mm Hg PO2 56 mm Hg HCO3 24mmol/L 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

  24. Measured pH 7.10 PCO2 20 mm Hg PO2 87 mm Hg Calculated HCO3 8mmol/L BE - 15 SaO2 96% Case 2 3 year old with severe diarrhea , diminished urine output, lethargy, Na-135 , Cl- 115 FiO2 24%

  25. pH 7.10 PCO2 20 mm Hg PO2 87 mm Hg HCO3 8 mmol/L Na-135 , Cl- 115 Case 2 • Step-1 Acidemia or Alkalemia ? Acidemia • Step-2 Primary disturbance Metabolic • Step-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

  26. Measured pH 7.1o PCO2 60 mm Hg PO2 60 mm Hg Calculated HCO310mmol/L BE - 15 SaO2 87 % Case 3 FiO2 100% 50 Year old resuscitated after a cardiac arrest. Na-142 , Cl- 108

  27. pH 7.1o PCO2 60 mm Hg PO2 60 mm Hg HCO3 10 mmol/L Na-142 , Cl- 108 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

  28. Case 3 • Step – 5 What is the corrected HCO3 • Corrected HCO3 = HCO3 + (AG – 12 ) 10 + ( 24 – 12 ) = 22 • Step – 6 What is the expected CO2 ? ExpectedPaCO2 = (1.5 x HCO3) +( 8 +- 2 ) ( 1.5 x 10 ) + ( 8 +- 2 ) = 21 – 25 Mixed metabolic and respiratory acidosis

  29. Measured pH 7.52 PCO2 26 mm Hg PO2 112mm Hg Calculated HCO3 22 mmol/L BE +2 SaO2 99% Case 4 FiO2 33% 4 year girl with lethargy, irritability, jaundice

  30. pH 7.52 PCO2 26 mm Hg PO2 112mm Hg HCO3 22 mmol/L Case 4 • Step-1 Acidemia or Alkalemia ? Alkalemia • Step-2 Primary disturbance Respiratory • Step-3 Acute or Chronic pH = 0.08 x (26-40) / 10 = 0.11 7.4 + 0.11 = 7.51 Acute Respiratory Alkalosis Uncompensated

  31. THANK YOU

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