This document summarizes blood gas analysis and acid-base balance. It describes how pH is maintained between 7.36-7.44 through bicarbonate and phosphate buffer systems. Respiratory and metabolic acidosis and alkalosis are explained in relation to changes in CO2 and bicarbonate levels. Key factors in analyzing acid-base disturbances including anion gap, predicted respiratory pH, and metabolic components are outlined. Different types of acid-base disorders and their diagnoses are also summarized.

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

2. Clinical case

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

4. NORMAL7.4ACIDOSISALKALOSIS7.87.0ACID(CO2)BASE

5. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

6. BLOOD pH

7. The challenge7.4 ACIDOSIS7.87.0Volatile ACID (CO2) & Fixed acidsDefense of normal alkalinity

8. Types of AcidsVolatile acidsEasily move from liquid to gas state within the bodyLung can removeH2CO3 + renal enzyme  H2O + CO2 (both of which are exhaled)Carbon dioxide is therefore considered an acid

9. Types of AcidsNonvolatile acids(Fixed acids)Cannot be changed to gas state within the bodyExamplesKeto acidsLactic acids

10. The challengeSources of acids:Volatile acidCO2 + H2O  H2CO3  H+ + HCO3Fixed acidsOrganic and inorganic sourceLactic acid, ketones, Sulfuric and phosphoric acid Kidney plays an important role handling fixed acids.

11. HYDROGEN ION SOURCESCO2 15000 mmol/dayCO2 + H2O H2CO3 H+ + HCO3-Noncarbonic acids 70 mmol/dayDEFENCE AGAINST pH CHANGEAcute (minutes to hours)VentilationBuffering

12. DEFENCE AGAINST pH CHANGEAcute (minutes to hours)Long termRenal excretionHepatic metabolism

13. Chemical BuffersThe body uses pH buffers in the blood to guard against sudden changes in acidityA pH buffer works chemically to minimize changes in the pH of a solutionH+OH-H+BufferOH-OH-H+

15. BUFFERSIntracellular BuffersProteinsHaemoglobinPhosphateExtracellular BuffersProteinsBicarbonate

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 SystemIf strong acid is added:HCl + NaHCO3 = H2CO3 + NaClHydrogen 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 SYSTEMH+H2CO3H+ + HCO3-Hydrogen ions generated by metabolism or by ingestion react with bicarbonate base to form more carbonic acidH2CO3HCO3-20

21. BICARBONATE BUFFER SYSTEMH+Equilibrium shifts toward the formation of acidHydrogen ions that are lost (vomiting) causes carbonic acid to dissociate yielding replacement H+ and bicarbonateH2CO3HCO3-

22. Bicarbonate Buffer SystemIf strong base is added:NaOH + H2CO3 = NaHCO3 + H2OIt reacts with the carbonic acid to form sodium bicarbonate (a weak base)The pH of the solution rises only slightlyThis 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 systemsPhosphate buffer H2PO4- / HPO4pK = 6.8 and has a low concentration.Role as intracellular and urinary buffer.

26. Phosphate buffer systemsH2PO4- / 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

35. RESPIRATORY ACIDOSIS7.47.87.0ACID(CO2)BASE

36. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

37. RESPIRATORY ACIDOSISH2O + CO2  H2CO3  H+ + HCO3- Cause - hypoventilationRetention of CO2 Drives equation rightwardIncreases both [H+] and [HCO3-]

38. RESPIRATORY ALKALOSIS7.47.87.0ACID(CO2)BASE

39. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

40. RESPIRATORY ALKALOSISH2O + CO2  H2CO3  H+ + HCO3- 2. Respiratory Alkalosis cause - hyperventilationBlows off CO2 Drives equation leftward decreasing both [H+] and [HCO3-]

41. METABOLIC ACIDOSIS7.47.87.0ACID(CO2)BASE

42. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

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

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

45. METABOLIC ALKALOSIS7.47.87.0ACID(CO2)BASE

46. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

47. Metabolic AlkalosisPrimarily due to Increased HCO3- , increased pH CausesAdministration of excess HCO3-

48. Increased secretion of H+ by kidney and gut

49. Sudden volume contraction which leads to increased Na+retention.This leads to water and HCO3- to follow the Na+PARTIALLY COMPENSATED RESPIRATORY ACIDOSIS7.47.07.8ACID(CO2)BASE

50. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

51. PARTIALLY COMPENSATED RESPIRATORY ALKALOSIS7.47.87.0ACID(CO2)BASE

52. (HCO3)METABOLIC COMPONENTRESPIRATORY COMPONENT

53. 7.47.07.8ACID(CO2)BASE

54. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENTPARTIALLY COMPENSATED METABOLIC ACIDOSIS

55. PARTIALLY COMPENSATED METABOLIC ALKALOSIS7.47.87.0ACID(CO2)BASE

56. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

57. MIXED ACIDOSIS7.47.87.0ACID(CO2)BASE

58. (HCO3)RESPIRATORY COMPONENTMETABOLIC COMPONENT

59. COMPENSATED STATE7.4ACIDOSISALKALOSIS7.87.0ACID(CO2)BASE

60. (HCO3)RESPIRATORY COMPONENTMETABOLIC 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 acidosisNormal

66. Chronic metabolic acidosisNormal

67. Acute Metabolic AlkalosisNormal

68. Chronic metabolic alkalosis Normal Normal

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

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

71. Anion gapUnmeasured AnionsvsUnmeasured CationsProteins, 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 acidosisElevated anion gapUremiaKetoacidosisLactic acidosisMethanol toxicityEthylene glycol toxicitySalicylateParaldehyde Normal anion gapRenal tubular acidosisDirrhoeaCarbonic anhydrase inhibitionUreteral diversionEarly renal failureHydronephrosisHCL administrationSaline 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 unitsAcute 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 0.1040 – 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 = 7.3360 - 40 = 20 X ½ =10 = 0.10pH = 7.40 – 0.10 = 7.30

78. Determining the predicted “Respiratory pH”If the PaCO2 is less than 40 add the difference to 7.4040 - 30 = 10 0.10pH = 7.40 + 0.10 = 7.50

79. Determining the predicted “Respiratory pH”pH 7.04PCO2 7676 - 40 = 36 X ½ = 18 0.187.40 - 0.18 = 7.22

80. Determining the predicted “Respiratory pH”pH 7.21PCO2 9090 - 40 = 50 X ½ = 25 0.257.40 – 0.25 = 7.15

81. Determining the predicted “Respiratory pH”pH 7.47PCO2 1840 – 18 = 22 0.227.40 + 0.22 = 7.62

82. Determining the Metabolic componentRULE10 mmol/L variance from the normal buffer base represents a pH change of approximately 0.15 units.

83. pH 7.21PCO2 9090 - 40 = 50 X ½ = 0.257.40 – 0.25 = 7.15Determining the Metabolic component 7.21 -7.15 = 0.06 X 2/3 = 0.04 = 4 mmol/L base excess

84. pH 7.04PCO2 7676 - 40 = 36 X ½ = 0.187.40 - 0.18 =7.22Determining the Metabolic component 7.22 -7.04 = 0.18 X 2/3 =12 mmol/L base deficit

85. Determining the Metabolic componentpH 7.47PCO2 1840 – 18 = 22 = 0.227.40 + 0.22 = 7.62Determining the Metabolic component 7.62-7.47 = 0.15 X 2/3 =10 mmol/L base deficit

86. Diagnosis of acid base disturbanceExamine 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 disturbanceCompare 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.

89. Clinical case

90. Clinical case

91. Clinical case

92. Clinical case

93. Clinical case

94. Thank U

95. Base Excess/ DeficitThe degree of deviation from normal total body buffer base can be calculated independent of compensatory PCO2 changesThe 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