Blood Gas Analysis

1. Blood Gas analysis DR. MANSOOR AQIL ASSOCIATE PROFESSOR, KING SAUD UNIVERSITY HOSPITALS RIYADH.

2. Clinical case pH 7.62 pCO2 30mmHg PO2 80 mmHg HCO3 30 mmol /L Diagnosis

3. pH = Alkalemia (Alkalosis) pH = Acidemia (Acidosis) Maintained within narrow limits pH 7.36 to 7.44 BLOOD pH

4. NORMAL RESPIRATORY COMPONENT METABOLIC COMPONENT ACID (CO 2 ) BASE (HCO 3 ) 7.4 ACIDOSIS ALKALOSIS 7.8 7.0

5. BLOOD pH

6. The challenge Volatile ACID (CO 2 ) & Fixed acids 7.4 ACIDOSIS 7.8 7.0 Defense of normal alkalinity

7. Types of Acids Volatile acids Easily move from liquid to gas state within the body Lung can remove H 2 CO 3 + renal enzyme  H 2 O + CO 2 (both of which are exhaled) Carbon dioxide is therefore considered an acid

8. Types of Acids Nonvolatile acids (Fixed acids) Cannot be changed to gas state within the body Examples Keto acids Lactic acids

9. The challenge Sources of acids: Volatile acid CO 2 + H 2 O  H 2 CO 3  H+ + HCO 3 Fixed acids Organic and inorganic source Lactic acid, ketones , Sulfuric and phosphoric acid Kidney plays an important role handling fixed acids.

10. CO 2 15000 mmol/day CO 2 + H 2 O H 2 CO 3 H + + HCO 3 - Noncarbonic acids 70 mmol/day HYDROGEN ION SOURCES

11. Acute (minutes to hours) Ventilation Buffering DEFENCE AGAINST pH CHANGE

12. Acute (minutes to hours) Long term Renal excretion Hepatic metabolism DEFENCE AGAINST pH CHANGE

13. Chemical Buffers The body uses pH buffers in the blood to guard against sudden changes in acidity A pH buffer works chemically to minimize changes in the pH of a solution H + OH - H + H + OH - OH - Buffer

14. Intracellular Buffers Proteins Haemoglobin Phosphate Extracellular Buffers Proteins Bicarbonate BUFFERS

15. Biological systems and Buffering: The power of a buffer depends on : Concentration of the buffer. Whether the pK is close to the pH of the system.

16. Bicarbonate buffer systems : CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 - Maintains a ratio of 20 parts bicarbonate to 1 part carbonic  acid

17. Bicarbonate Buffer System If strong acid is added: HCl + NaHCO3 = H2CO3 + NaCl Hydrogen ions released combine with the bicarbonate ions and form carbonic acid (a weak acid) The pH of the solution decreases only slightly

18. 20 BICARBONATE BUFFER SYSTEM H 2 CO 3 H + + HCO 3 - Hydrogen ions generated by metabolism or by ingestion react with bicarbonate base to form more carbonic acid H + HCO 3 - H 2 CO 3

19. BICARBONATE BUFFER SYSTEM Equilibrium shifts toward the formation of acid Hydrogen ions that are lost (vomiting) causes carbonic acid to dissociate yielding replacement H + and bicarbonate H + HCO 3 - H 2 CO 3

20. Bicarbonate Buffer System If strong base is added: NaOH + H2CO3 = NaHCO3 + H2O It reacts with the carbonic acid to form sodium bicarbonate (a weak base) The pH of the solution rises only slightly This system is the only important ECF buffer

21. Bicarbonate buffer systems : CO 2 + H 2 O  H 2 CO 3  H + + HCO 3 - pK = 6.1 [ HCO 3 - ] = 24 mmol/L

22. Bicarbonate buffer systems :

23. Phosphate buffer systems Phosphate buffer H2PO 4 - / HPO 4 pK = 6.8 and has a low concentration. Role as intracellular and urinary buffer .

24. Phosphate buffer systems H2PO 4 - / HPO 4 -2

25. Protein buffers : A. Amino acid residues of proteins take up H + ( pK =7.0) are most important NH 2  NH 3 - B. Hemoglobin is important due to high concentration and its increased buffering capacity when deoxygenated.

26. Relative Buffering power:

27. Relative Buffering power:

28. Compensation

29. Renal buffering mechanisms Renal - kidney excretes H + and replenishes [HCO 3 - ] . But, this is a slow process taking hours to days.

30. Renal buffering mechanisms

31. Renal buffering mechanisms

32. METABOLIC DISORDERS

33. RESPIRATORY ACIDOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.8 7.4 7.0

34. RESPIRATORY ACIDOSIS H 2 O + CO 2  H 2 CO 3  H + + HCO 3 - Cause - hypoventilation Retention of CO 2 Drives equation rightward Increases both [H + ] and [HCO 3 - ]

35. RESPIRATORY ALKALOSIS ACID (CO 2 ) BASE (HCO 3 ) 7.0 7.4 7.8 RESPIRATORY COMPONENT METABOLIC COMPONENT

36. RESPIRATORY ALKALOSIS H 2 O + CO 2  H 2 CO 3  H + + HCO 3 - 2. Respiratory Alkalosis cause - hyperventilation Blows off CO 2 Drives equation leftward decreasing both [H + ] and [HCO 3 - ]

37. METABOLIC ACIDOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.8 7.4 7.0

38. Metabolic Acidosis D eficit in HCO 3 - and decreased pH Causes: Increased production of nonvolatile acids. Decreased H + secretion in the kidney Increased HCO 3 - loss in kidney Increased Cl - reabsorption by the kidney.

39. Metabolic Acidosis Body response is increased ventilation to blow off excess CO 2

40. METABOLIC ALKALOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.0 7.4 7.8

41. Metabolic Alkalosis Primarily due to Increased HCO 3 - , increased pH Causes Administration of excess HCO 3 - Increased secretion of H + by kidney and gut Sudden volume contraction which leads to increased Na + retention.This leads to water and HCO 3 - to follow the Na +

42. PARTIALLY COMPENSATED RESPIRATORY ACIDOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.4 7.8 7.0

43. PARTIALLY COMPENSATED RESPIRATORY ALKALOSIS METABOLIC COMPONENT ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT 7.4 7.0 7.8

44. ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.4 7.8 7.0 PARTIALLY COMPENSATED METABOLIC ACIDOSIS

45. PARTIALLY COMPENSATED METABOLIC ALKALOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.4 7.0 7.8

46. MIXED ACIDOSIS ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.8 7.4 7.0

47. COMPENSATED STATE ACID (CO 2 ) BASE (HCO 3 ) RESPIRATORY COMPONENT METABOLIC COMPONENT 7.4 ACIDOSIS ALKALOSIS 7.8 7.0

48. pH PaCO2 HCO3- Acute ventilatory failure (acute respiratory acidosis) N

49. Chronic ventilatory failure (compensated respiratory acidosis) pH PaCO2 HCO3- Normal pH PaCO2 HCO3- N

50. Acute alveolar hyperventilation (acute respiratory alkalosis) pH PaCO2 HCO3- Normal

51. Chronic alveolar hyperventilation (compensated respiratory alkalosis) pH PaCO2 HCO3- Normal pH PaCO2 HCO3- Normal

52. Acute metabolic acidosis pH PaCO2 HCO3- Normal

53. Chronic metabolic acidosis pH PaCO2 HCO3- Normal pH PaCO2 HCO3- Normal

54. Acute Metabolic Alkalosis pH PaCO2 HCO3- Normal

55. Chronic metabolic alkalosis pH PaCO2 HCO3- Normal pH PaCO2 HCO3- Normal

56. Anion Gap AG = [Na + ] - [Cl ‾ + HCO3‾ ] • AG represents unaccounted for anions (R ‾ ) • Normal anion gap = 10

57. Anion Gap Cations are Na + & K + Major Anions are Cl ‾ & HCO3 ‾

58. Anion gap Unmeasured Anions vs Unmeasured Cations Proteins, mostly albumin 15 mEq/L Calcium 5 mEq/L Organic acids 5 mEq/L Potassium 4.5 mEq/L Phosphates 2 mEq/L Magnesium 1.5 mEq/L Sulfates 1 mEq/L Totals: 23 mEq/L 11 mEq/L

59. Rules For Analyzing The ABG’s • Look at the anion gap.

60. Differential diagnosis of metabolic acidosis Normal anion gap Renal tubular acidosis Dirrhoea Carbonic anhydrase inhibition Ureteral diversion Early renal failure Hydronephrosis HCL administration Saline administration Elevated anion gap Uremia Ketoacidosis Lactic acidosis Methanol toxicity Ethylene glycol toxicity Salicylate Paraldehyde

61. Diagnosis of acid base disturbance

62. Determining the predicted “Respiratory pH” Acute 10 mmHg increase in PCO2 results in pH decrease of approximately 0.05 units Acute 10 mmHg decrease in PCO2 results in pH increase of approximately 0.10 units

63. Determining the predicted “Respiratory pH” First determine the difference between the measured PaCO2 and 40 mmHg and move the decimal point two places left. 60 - 40 = 20 X 1/2 0.10 40 – 30 = 10 0.10

64. Determining the predicted “Respiratory pH” If the PaCO2 is greater than 40 subtract half of the difference from 7.40 ? If this Pt has pH = 7.2 ? If this Pt has pH = 7.33 60 - 40 = 20 X ½ =10 = 0.10 pH = 7.40 – 0.10 = 7.30

65. Determining the predicted “Respiratory pH” If the PaCO2 is less than 40 add the difference to 7.40 40 - 30 = 10 0.10 pH = 7.40 + 0.10 = 7.50

66. Determining the predicted “Respiratory pH” pH 7.04 PCO2 76 76 - 40 = 36 X ½ = 18 0.18 7.40 - 0.18 = 7.22

67. Determining the predicted “Respiratory pH” pH 7.21 PCO2 90 90 - 40 = 50 X ½ = 25 0.25 7.40 – 0.25 = 7.15

68. Determining the predicted “Respiratory pH” pH 7.47 PCO2 18 40 – 18 = 22 0.22 7.40 + 0.22 = 7.62

69. Determining the Metabolic component RULE 10 mmol/L variance from the normal buffer base represents a pH change of approximately 0.15 units.

70. pH 7.21 PCO2 90 90 - 40 = 50 X ½ = 0.25 7.40 – 0.25 = 7.15 Determining the Metabolic component 7.21 -7.15 = 0.06 X 2/3 = 0.04 = 4 mmol /L base excess

71. pH 7.04 PCO2 76 76 - 40 = 36 X ½ = 0.18 7.40 - 0.18 =7.22 Determining the Metabolic component 7.22 -7.04 = 0.18 X 2/3 =12 mmol /L base deficit

72. pH 7.47 PCO2 18 40 – 18 = 22 = 0.22 7.40 + 0.22 = 7.62 Determining the Metabolic component 7.62-7.47 = 0.15 X 2/3 =10 mmol /L base deficit Determining the Metabolic component

73. Diagnosis of acid base disturbance Examine arterial pH: Is acidemia or alkalemia present? Examine PaCO2: Is the change in PaCO2 consistent with a respiratory component? If the change in PaCO2 does not explain the change in arterial pH, does the change in [HCO3–] indicate a metabolic component? Make a tentative diagnosis (see Table).

74. Diagnosis of acid base disturbance Compare the change in [HCO3–] with the change in PaCO2. Does a compensatory response exist (Table)? If the compensatory response is more or less than expected, by definition a mixed acid–base disorder exists. Calculate the plasma anion gap in the case of metabolic acidosis. Measure urinary chloride concentration in the case of metabolic alkalosis.

75. Table . Normal Compensatory Responses in Acid–Base Disturbances. Disturbance Response Expected Change Respiratory acidosis       Acute [HCO 3 – ]   1 mEq/L/10 mm Hg increase in Pa CO 2     Chronic [HCO 3 – ]   4 mEq/L/10 mm Hg increase in Pa CO 2   Respiratory alkalosis       Acute [HCO 3 – ]   2 mEq/L/10 mm Hg decrease in Pa CO 2 –     Chronic [HCO 3 – ]   4 mEq/L/10 mm Hg decrease in Pa CO 2   Metabolic acidosis PaCO 2   1.2 x the decrease in [HCO 3 – ]   Metabolic alkalosis PaCO 2   0.7 x the increase in [HCO 3 – ]  

76. Clinical case pH 7.62 pCO2 30mmHg PO2 80 mmHg HCO3 30 mmol /L Diagnosis

77. Clinical case pH 7.15 pCO2 22 mm Hg PO2 90 mmHg HCO3 9 mmol /L Partially compensated Metabolic acidosis Diagnosis

78. Clinical case pH 7.29 pCO2 33 mm Hg PO2 90 mmHg HCO3 19 mmol /L Partially compensated Metabolic acidosis Diagnosis

79. Clinical case pH 7.50 pCO2 29 mm Hg PO2 92 mmHg HCO3 24 mmol /L Acute respiratory Alklosis Diagnosis

80. Clinical case pH 7.30 pCO2 55 mm Hg PO2 74 mmHg HCO3 30 mmol /L Partially compensated Respiratory acidosis Diagnosis

81. Thank U

82. Base Excess/ Deficit The degree of deviation from normal total body buffer base can be calculated independent of compensatory PCO2 changes The amount of acid of base that must be added to return the blood pH to 7.4 and PCO2 to 40 at full O2 saturation and 37 C