birds eye view on understanding acid base balance for postgraduates

1. Basics for ABG analysis
PROF. N.KRISHNAN
Prof of Anaesthesiology
Madras Medical College
Chief Anaesthesiologist
Institute of Child Health & Hospital for Children
chennai

2. Arterial Blood gas analysis is important
Adequacy of oxygenation
Adequacy of ventilation
Acid base status

3. Acid base balance
• It is a regulatory process ,whose aim is to maintain optimal
H+ ion conc for body’s metabolic processes
• Biochemical and enzymatic processes of cellular
mechanism depends on hydrogen activity.

4. Changes in Hydrogen ions conc
CAN PRODUCE CHANGES IN
Enzyme systems
Electro physiology of myocardium and CNS
Electrolyte balance
Response to exogenous drugs
Alteration in blood flow to organs

5. H+ ion activity
It reflects a balance between production and excretion of acids
Normal value is 40 nanomoles/ liter
(40 x 10-9 ) moles / liter.
It yields a pH of 7.40.

7. Ion
• Atoms give up, accept or share
electrons with other atoms to
produce an ion
• Atoms gaining an electron is called
an
• An i on – ve
• Atom which loses an electron is a
• Ca t ion +Ve.

8. Acid
• It’s a substance which
liberates H ions

9. Base
It’s a substance that accepts H ions

10. ACID PRODUCTION
Breakdown of food
Cell metabolism.

11. • Intra cellular Aerobic
Metabolism of
carbohydrates ,fat and
protein yields
• VOLATILE ACID ( C02)
• Anaerobic Metabolism, of
carbohydrates, fat and
protein yields
• NON VOLATILE
ACIDS ( lactic acid)

12. Common Metabolic acids in the body
• Phosphoric acid from ingested food
• Sulphuric acid from metabolism
• Carbonic acid from Co2
• Beta hydroxyl butric acid
• Acetoacetic acid
• Lactic acid.

13. FATE OF ACID
• Dilution
• Buffers
• Respiratory elimination
• Renal elimination

14. FATE OF ACID
• DILUTION
• EXTRA CELLULAR :Rapid process
• INTRA CELLULAR : Slow process.

15. FATE OF ACID
• Buffers in blood
• H+ combines with buffer base
• H+ exchanges with cellular
cation like Na ,K
• H+ Exchanges with Ca in bone

17. Buffers in body
• H2Co3 / HCo3 – pk 6.1
• HHB / HB – pK 6.8
• HPR / PR – pk 6.8
• H2P04 /HPo4 – pk 6.8

22. Bone
• Bone can take up H+ in exchange for Ca++, Na+ and K+ (ionic exchange) .
Bone can release HCO3
-, CO3
- or HPO4
-2.
• In acute metabolic acidosis uptake of H+ by bone in exchange for
Na+ and K+ is involved in buffering as this can occur rapidly without any
bone breakdown.

23. Bone
• In chronic metabolic acidosis, the major buffering mechanism by far is
release of calcium carbonate from bone. The mechanism by which this
dissolution of bone crystal occurs involves two processes:
• Direct physicochemical breakdown of crystals in response to [H+]
• Osteoclastic reabsorption of bone.

24. FATE OF ACID- Respiration
• Fall in pH stimulates chemoreceptors
• Increase in ventilation
• Leads to drop in pco2.

25. FATE OF ACID - Kidneys
• Kidneys maintain acid base balance
• Slow process
Excretion of H+ IONS
Generation of New HCo3
Reclamation of filtered HCo3

26. Kidney

27. Acid excretion by kidneys
PCT
H+ is produced in the tubular cell in the presence of
CARBNIC ANHYDRASE enzyme ,
.

28. Acid excretion by kidneys
DCT & CT
H+ produced is secreted in to lumen of the tubule
ACTIVE ATP DRIVEN PUMPS CALLED PROTON PUMPS .

29. Reclamation of filtered HCo3
In PCT
filtered HCo3 combines with H+ which is secreted from the tubular cell
in exchange for sodium to form H2Co3

30. Generation of new HC03
In DCT via H2Po4 / HPo4 mechanism
Limited to the supply of dibasic phosphate.
HPO4 Combine with H+ to becomes H2PO4 &
1 H+ IS LOST
Na2HPO4
Dissociates
into
2 Na & HPO4

31. Metabolism of amino acid GLUTAMINE in PCT
• The generates 2 NH+
4
within the tubular cells
and two HCO-
3 ions,
which are then returned
to the systemic
circulation
Glutamine is converted to 2 mole of NH4 and 2 moles of
HCo3

32. KIDNEY DURING ACIDOSIS

33. Three mechanisms act to excrete this excess amount of H+
from lumen.
1.The number of proton pump in membrane increases
2.Phosphate mechanism
3.Ammonia mechanism .

34. Both Generation and Reclamation of HCo3 is regulated
•Blood pH
•Serum HCo3
•GFR
•Filtered load of HCo3
•PCo2
•Renin / Angio / Aldosterone Mechanism
•K load

36. pH

37. pH
S.P.L Sorenson referred Hydrogen ion concentration of 10-7 moles per liter
as
7 Puissances Hydrogen.

38. H+ ion activity
It reflects a balance between production and
excretion of acids
Normal value is 40 nanomoles/ liter
(40 x 10e-9) moles / liter.
It yields a pH of 7.40.

39. pH
pH is the negative logarithm to base 10 of the
hydrogen ion concentration in nanomoles per liter
The raise in pH indicates a proportionate fall in hydrogen
ion concentration and vice versa.

40. pK
pK represents the pH value at which the solute is 50% dissociated
It represents the pH value at which maximum buffering can be achieved.

41. Buffer
Buffer is a substance that resists changes in pH
( hydrogen concentration) by undergoing a
reversible reaction.
A buffer system consists of a weak acid with its strong base or salt.

42. HCo3 buffer
H2Co3 (acid) and NaHCo3 (Salt)
If HCl is added
HCl + NaHCO3 produces H2Co3 + NaCl
H2Co3 is a weak acid compared to the added strong HCL

43. It’s a weak but powerful buffer because
High solubility of Co2
Presence of Carbonic anhydrase enzymes in RBC and renal tubular
cells.
The ability of kidney to synthesize new bicarbonate
Efficient removal of carbon dioxide by lung
HCo3 buffer

44. Henderson Hasselbach Equation 1
It expresses acid base relationship by
looking at one component of the
body buffer system
H2Co3 ↔ H+ + HCo3- (.1)

45. H2Co3 ↔ H+ + HCo3-
• HH-2
• Law of mass action
• “states that product of the concentration of
the substance on the right
• Divided By
• the concentration of the substance on the
left is equal to a constant” – Ka
• H2Co3 ↔ H+ + HCo3-
•

46. HH-3
(H)(HCo3)
Ka = ……………………( 2 )
(H2Co3)
Log of both sides
(H)(HCo3)
log(Ka) = log […………………… ] (3 )
(H2Co3)
H2Co3 ↔ H+ + HCo3-

47. HH-4
(HCo3)
log (Ka) = log (H) + log ………… (4 )
(H2Co3)
Shifting log (H) to the Left
+ ve Becomes - ve
(HCo3)
- log (H) = -log(Ka) + log ………
(H2Co3)
… (5 )
(H)(HCo3)
log(Ka) = log […………………… ]
(H2Co3)

48. HH-5
(HCo3)
-log (H) = -log(Ka) + log ……………..(5)
(H2Co3)
-log (H) = pH -log(Ka) = pK
HCo3
pH = pK + _______
H2Co3

49. HH-6
• H2Co3 depends on dissolved Co2
• Dissolved Co2 depends on its solubility co-efficient – “s”
HCo3
pH = pK + log ___________
s x PCo2
[ pK = 6.1 , s = 0.03 , PCo2 = 40 , Hco3 =24 ]
HCo3
pH = pK _______
H2Co3

50. HH-7
HCo3 24
pH = 6.1 + log ______ ________ = 20
H2Co3 0.03 x 40
(1.2 )
pH = 6.1 + log of 20 = (1.3)
pH = 6.1 + 1.3 = 7.4

51. HH-8
HCo3
pH = pK ___________
H2Co3
kidney
pH = pk __________
Lung

52. Hco3 [ kidneys ]
pH ----------------------------
pco2 [ lung]
pH is directly related to HCO3.
pH is inversely related to PCO2

53. Buffer Ratio
BASE HCO3- 24
------------- ------------
Acid H2CO3 0.03 X40
Buffer ratio is 20 :1 for HCO3 / H2CO3 BUFFER.
as long as the buffer ratio is maintained the pH will not
change,
even though the amount of hydrogen changes.

54. Key points in H-H Equations
• pH is Directly related to HCO3
• pH is Inversely related to pCO2
• The Ratio of HCO3 : Pco2 defines the pH .

55. pH : is measured by determining the potential difference between
Ag /Agcl reference cell and Hg /Hgcl test cell separated by a pH
sensitive glass membrane.
Pco2 : one electrode is coated with Teflon and is permeable only to
co2.

56. BG machine measures ONLY Pco2 and pH.
Hco3 , Std Hco3 ,Total Co2 ,Buffer Base, Base Excess
are all calculated from nomograms based on H-H equation in vitro .

57. Plasma Hco3 : measured from serum during electrolyte
investigation
. N = 22 – 26 meq / L
Std Hco3 : Hco3 of whole blood equilibrated under std
conditions of pco2 and temp . N : 21 – 25 meq /L.
Tco2 : total co2 = Hco3 + dissolved co2 N =23 – 27 meq / L .
Whole buffer base is the “ unmeasured ” buffers [amino
acid in proteins ] and plasma Hco3 . N: 45 - 50 meq / L .

58. ANION GAP
• It measures the
UNMEASURED
ANIONS [A-]
accompanying
non volatile acids.
Whole buffer base is the “ unmeasured ” buffers [amino acid in proteins ] and
plasma Hco3 . N: 45 - 50 meq / L .

59. ANION GAP
ANION GAP
is the calculated difference between
negatively charged (anion)and positively
charged (cation) electrolytes.
= [Na+] − ([Cl−] + [HCO3
−]) =12 mEq/L
(if using with potassium then 16 )

60. ANION GAP

62. BASE EXCESS
BE is the amount of acid or base needed to return a sample of whole
blood to normal pH 7.4 under standard conditions of Pco2 [40] , Po2
[100] and temp of 37c .

63. Normal BE: + or – 2 meq / L.
BE quantifies the metabolic component.
+ve BE means alkalosis.
-ve BE means acidosis.

64. ACIDEMIA: [H+] ion conc above normal range of 44
nmol /L & pH < 7.36
ACIDOSIS: a process that would cause acidemia if it
were not compensated.

65. ALKALOSIS : a process that would cause
alkalemia if it were not compensated
ALKALEMIA : [H+] ion conc below normal
range of 36nmol /L & pH > 7.44

66. Normal values
• Serum pH =7.36-7.44
• Serum H+ = 40 nEq / L
• Serum Hco3 = 24 mEq / L
• Serum Pco2 = 40 mm Hg

68. METABOLIC MAINTAINS.
RESPIRATION –IRREGULAR.

69. Types of Acid – Base disorders
•Primary
•Mixed

70. Primary metabolic disorder
•METABOLIC ACIDOSIS
Is a primary process that causes a fall of HCO3
•METABOLIC ALKALOSIS
Is a primary process that causes a rise of HCO3

71. Primary respiratory disorder
•RESPIRATORY ACIDOSIS
Is a primary process that causes a RISE in Pco2
•RESPIRATORY ALKALOSIS
Is a primary process that causes a FALL in pCO2

72. Types of acid – base disorders
•Primary- single disorders
• Met- Acidosis : Reduction Of HCO3-
• Met- Alkalosis: Increase In HCO3-
• Resp- Acidosis : Increase In Pco2
• Resp- Alkalosis : Decrease In Pco2

73. Met-acidosis + Met-alkalosis
Met-acidosis + Resp Alkalosis
Met-alkalosis +Resp –Acidosis
Met-acidosis + Resp-acidosis
Met –Alkalosis + Resp -Alkalosis
MIXED DISORDERS

75. Compensation
• It is the body’s response to a pathophysiological
process which produced a change in serum pH .
• It doesn't return the pH to 7.40

76. Kidney senses the change in pH
Adjusts The Reclamation And Regeneration Of Hco3
COMPENSATION FOR PRIMARY RESPIRATORY DISORDER

77. Kidney
• In Respiratory Acidosis : High pco2 , low pH
• [ HIGH ACID}
• Stimulates Tubular Reclamation & Generation of Hco3

78. Kidney
• In Respiratory Alkalosis : low pco2 , high pH
• {LOW ACID}
• Inhibits tubular Reclamation & Generation of Hco3

79. Brain senses the change in pH
Adjusts the ventilation
Compensation for Primary metabolic disorder

80. In Metabolic Acidosis : pH is low , H+ is high
{ HIGH ACID}
Ventilation is increased to Decrease Pco2

81. In Metabolic Alkalosis: pH is high , H+ is low
{ LOW ACID}
Ventilation is decreased to increase PCO2

82. COMPENSATION FORMULAE

83. Use of compensation formulae
The degree to which compensation can occur to a met
or resp process can be calculated using compensation
formulae
Works well in range 7.10 -7.60
Formula gives PREDICTED OR EXPECTED values

84. Use of compensation formula
• If predicted value doesn’t match measured value [ABG slip ]
Two primary disorder
Insufficient time to achieve full compensation

85. Formulas for compensation in Metabolic Acidosis
• EXPECTED FALL IN PCO2 [ Exp PCO2]
WINTERS FORMULA
• Exp PCO2 = 1.5 [HCO3] + 8 (± 2 )
Resp system tries to expel more CO2 -
COMPENSATES

86. Formulae for compensation in metabolic alkalosis
• Expected rise of pco2 [exp pco2]
•Exp [ pco2 ] = 0.9 [Hco3] + 16
Resp system tries to expel LESS CO2 - COMPENSATES

87. Formulae for compensation ACUTE respiratory alkalosis
• Expected fall in HCO3 : [exp HCO3 ]
• ACUTE [ < 8 hrs ]
• Every 10 mm fall in pco2 below 40
• Hco3 decreases by 2 meq / L
• pH increases by 0.08
KIDNEY TRIES TO LOOSE HCO3

88. Formulae for compensation CHRONIC respiratory alkalosis
• Expected fall in HCO3 : [exp HCO3 ]
Chronic [ > 24 hrs ]
• Every 10 mm fall in pco2 below 40
• Hco3 decreases by 5 meq / L
• pH increases by 0.03
KIDNEY TRIES TO LOOSE HCO3

89. Formulae for compensation in ACUTErespiratory acidosis
• Expected rise in HCO3 : [exp HCO3 ]
• ACUTE [ < 8 hrs ]
• Every 10 mm rise in pco2 above 40
• Hco3 increases by 1 meq / L
pH decreases by 0.08
Kidney tries to save Hco3

90. Formulae for compensation in CHRONIC respiratory acidosis
• Expected rise in HCO3 : [exp HCO3 ]
Chronic [ > 24 hrs ]
• Every 10 mm rise in pco2 above 40
• Hco3 increases by 4 meq / L
• pH decreases by 0.03
Kidney tries to save Hco3

91. Respiratory acidosis
If measured pH or Hco3 is LOWER than expected value
[Associated METABOLIC ACIDOSIS ]
If measured pH or Hco3 is Higher than expected value
[Associated METABOLIC ALKALOSIS ]
if both measured and expected values are same
[Pure Primary Respiratory Acidosis ]

92. Thank you