1. Blood Gas analysis DR. MANSOOR AQILASSOCIATE PROFESSOR,KING SAUD UNIVERSITY HOSPITALSRIYADH.

2. Clinical case

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

5. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

6. BLOOD pH

7. The challenge 7.4 ACIDOSIS 7.8 7.0 Volatile ACID (CO2) & Fixed acids Defense of normal alkalinity

8. Types of Acids Volatile acids Easily move from liquid to gas state within the body Lung can remove H2CO3 + renal enzyme  H2O + CO2 (both of which are exhaled) Carbon dioxide is therefore considered an acid

10. The challenge Sources of acids: Volatile acid CO2 + H2O  H2CO3  H+ + HCO3 Fixed acids Organic and inorganic source Lactic acid, ketones, Sulfuric and phosphoric acid Kidney plays an important role handling fixed acids.

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

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+ Buffer OH- OH- H+

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

17. 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.

18. Bicarbonate buffer systems: CO2 + H2O  H2CO3  H+ + HCO3- Maintains a ratio of 20 parts bicarbonate to 1 part carbonic acid

19. 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

20. BICARBONATE BUFFER SYSTEM H+ H2CO3H+ + HCO3- Hydrogen ions generated by metabolism or by ingestion react with bicarbonate base to form more carbonic acid H2CO3 HCO3- 20

21. BICARBONATE BUFFER SYSTEM H+ Equilibrium shifts toward the formation of acid Hydrogen ions that are lost (vomiting) causes carbonic acid to dissociate yielding replacement H+ and bicarbonate H2CO3 HCO3-

22. 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

23. Bicarbonate buffer systems: CO2 + H2O  H2CO3  H+ + HCO3- pK = 6.1 [HCO3-] = 24 mmol/L

24. Bicarbonate buffer systems:

25. Phosphate buffer systems Phosphate buffer H2PO4- / HPO4 pK = 6.8 and has a low concentration. Role as intracellular and urinary buffer.

26. Phosphate buffer systems H2PO4- / HPO4-2

27. Protein buffers: A. Amino acid residues of proteins take up H+ (pK=7.0) are most important NH2 NH3- B. Hemoglobin is important due to high concentrationand its increased buffering capacity when deoxygenated.

28. Relative Buffering power:

29. Relative Buffering power:

30. Compensation

31. Renal buffering mechanisms Renal - kidney excretes H+ and replenishes [HCO3-] . But, this is a slow process taking hours to days.

32. Renal buffering mechanisms

33. Renal buffering mechanisms

34. METABOLIC DISORDERS

36. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

37. RESPIRATORY ACIDOSIS H2O + CO2  H2CO3  H+ + HCO3- Cause - hypoventilation Retention of CO2 Drives equation rightward Increases both [H+] and [HCO3-]

39. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

40. RESPIRATORY ALKALOSIS H2O + CO2  H2CO3  H+ + HCO3- 2. Respiratory Alkalosis cause - hyperventilation Blows off CO2 Drives equation leftward decreasing both [H+] and [HCO3-]

42. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

43. Metabolic Acidosis Deficit in HCO3- and decreased pH Causes: Increased production of nonvolatile acids. Decreased H+ secretion in the kidney Increased HCO3- loss in kidney Increased Cl- reabsorption by the kidney.

44. Metabolic Acidosis Body response is increased ventilation to blow off excess CO2

46. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

48. Increased secretion of H+ by kidney and gut

50. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

52. (HCO3)METABOLIC COMPONENT RESPIRATORY COMPONENT

54. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT PARTIALLY COMPENSATED METABOLIC ACIDOSIS

56. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

58. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

60. (HCO3)RESPIRATORY COMPONENT METABOLIC COMPONENT

61. Acute ventilatory failure (acute respiratory acidosis) N

62. Chronic ventilatory failure (compensated respiratory acidosis) Normal

63. Acute alveolar hyperventilation(acute respiratory alkalosis) Normal

64. Chronic alveolar hyperventilation(compensated respiratory alkalosis) Normal

65. Acute metabolic acidosis Normal

66. Chronic metabolic acidosis Normal

67. Acute Metabolic Alkalosis Normal

68. Chronic metabolic alkalosis Normal Normal

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

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

71. 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

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

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

74. Diagnosis of acid base disturbance

75. 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

76. 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

77. 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

78. 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

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

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

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

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

83. 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

84. 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

85. Determining the Metabolic component 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

86. 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).

87. 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.

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93. Clinical case

94. Thank U

95. 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 370 C