must know on how to interpret ABG for Anaesthesiology residents

1. Interpretation of ABG
PROF. N.KRISHNAN
Prof of Anaesthesiology
Madras Medical College
Chief Anaesthesiologist
Institute of Child Health & Hospital for Children
chennai
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2. Arterial Blood gas analysis is important
Adequacy of oxygenation
Adequacy of ventilation
Acid base status
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3. 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
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4. 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.
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5. VOLATILE ACID ( C02)
• Intra cellular Aerobic
Metabolism of
carbohydrates ,fat and
protein yields
NON VOLATILE ACIDS (
lactic acid)
• Anaerobic Metabolism, of
carbohydrates, fat and
protein yields
•
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6. 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.
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7. FATE OF ACID
• Dilution
• Buffers
• Respiratory elimination
• Renal elimination
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8. kidney physiology
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9. Acid excretion by kidneys
PCT
H+ is produced in the tubular cell in the presence of
CARBNIC ANHYDRASE enzyme ,
.
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10. Acid excretion by kidneys
DCT & CT
H+ produced is secreted in to lumen of the tubule
ACTIVE ATP DRIVEN PUMPS CALLED PROTON PUMPS .
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11. 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
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12. Generation of new HC03
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13. KIDNEY DURING ACIDOSIS
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14. 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 .
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15. Brain senses the change in pH
Adjusts the ventilation
Respiratory physiology
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17. 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-
•
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18. 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
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19. HH-8
HCo3
pH = pK ___________
H2Co3
kidney
pH = pk __________
Lung
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20. Hco3 [ kidneys ]
pH ----------------------------
pco2 [ lung]
pH is directly related to HCO3.
pH is inversely related to PCO2
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21. 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.
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22. 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 .
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23. 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 .
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24. 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 .
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25. UNMEASURED ANIONS [A-]
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26. Whole buffer base
• Whole buffer base is the
“ Unmeasured ” buffers = 23
+
• Plasma Hco3 = 24
• Normal : 45 - 50 meq / L .
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27. ANION GAP
is the calculated difference between positively charged
(cation) electrolytes and negatively charged (anion)
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.
= [Na+] − ([Cl−] + [HCO3
−]) =12 mEq/L
(if using with potassium then 16 )

28. Increased ANION GAP
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29. 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 .
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30. Normal BE: + or – 2 meq / L.
BE quantifies the metabolic component.
+ve BE means alkalosis.
-ve BE means acidosis.
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33. 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.
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34. 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
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35. Normal values
• Serum pH =7.36-7.44
• Serum H+ = 40 nEq / L
• Serum Hco3 = 24 mEq / L
• Serum Pco2 = 40 mm Hg
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37. 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
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38. 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
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39. 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
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40. Met-acidosis + Met-alkalosis
Met-acidosis + Resp Alkalosis
Met-alkalosis +Resp –Acidosis
Met-acidosis + Resp-acidosis
Met –Alkalosis + Resp -Alkalosis
MIXED DISORDERS
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42. 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
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43. Kidney senses the change in pH
Adjusts The Reclamation And Regeneration Of Hco3
COMPENSATION FOR PRIMARY RESPIRATORY DISORDER
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44. Brain senses the change in pH
Adjusts the ventilation
Compensation for Primary metabolic disorder
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45. Compensation for primary respiratory disorder
by Kidney
• In Respiratory Acidosis : High pco2 , low pH
• [ HIGH ACID}
• Stimulates Tubular Reclamation & Generation of Hco3
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46. Compensation for primary respiratory disorder
by Kidney
• In Respiratory Alkalosis : low pco2 , high pH
• {LOW ACID}
• Inhibits tubular Reclamation & Generation of Hco3
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47. Brain senses the change in pH
Adjusts the ventilation
In Metabolic Acidosis : pH is low , H+ is high
{ HIGH ACID}
Ventilation is increased to Decrease Pco2
Compensation for Primary Metabolic Acidosis
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48. Brain senses the change in pH
Adjusts the ventilation
In Metabolic Alkalosis: pH is high , H+ is low
{ LOW ACID}
Ventilation is decreased to increase PCO2
Compensation for Primary Metabolic Alkalosis
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50. COMPENSATION FORMULAE
PREDICTS THE CHANGES IN OPPOSITE SYSTEM
PREDICTED OF EXPECTED VALUE
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51. 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
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52. Use of compensation formula
• If predicted value doesn’t match measured value [ABG slip ]
Two primary disorder
Insufficient time to achieve full compensation
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53. Formulas for compensation in Metabolic Acidosis
Resp system: hyper ventilates : Tries to expel more CO2
COMPENSATES
Expect a FALL in PCO2
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54. Formulas for compensation in Metabolic Acidosis
WINTERS FORMULA
• EXPECTED FALL IN PCO2 [ Exp PCO2]
• Exp PCO2 = 1.5 [HCO3] + 8 (± 2 )
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55. Formulae for compensation in metabolic alkalosis
Resp system hypoventilation - tries to retain CO2
COMPENSATES
Expect a RAISE in PCO2
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56. Formulae for compensation in metabolic alkalosis
• Expected rise of pco2 [exp pco2]
•Exp [ pco2 ] = 0.9 [Hco3] + 16
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57. Respiratory alkalosis [ low PCO2]
KIDNEY TRIES TO LOOSE HCO3
Inhibits tubular Reclamation & Generation of Hco3
Expect fall in HCO3 : [exp HCO3 ]
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58. Formulae for compensation respiratory alkalosis
BASED ON THE DURATION OF INSULT
ACUTE [ < 8 hrs ]
Chronic [ > 24 hrs ]
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59. Formulae for compensation ACUTE respiratory alkalosis
• Expected fall in HCO3 :
• Every 10 mm fall in pco2 below 40
• Hco3 decreases by 2 meq / L
• pH increases by 0.08
ACUTE [ < 8 hrs ]
Inhibits tubular Reclamation & Generation of Hco3
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60. Formulae for compensation ACUTE respiratory alkalosis
• Expected fall in HCO3 :
• Every 10 mm fall in pco2 below 40
• Hco3 decreases by 2 meq / L
• pH increases by 0.08
ACUTE [ < 8 hrs ]
Inhibits tubular Reclamation & Generation of Hco3
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61. 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
Chronic [ > 24 hrs ]
Inhibits tubular Reclamation & Generation of Hco3
Fall in HCO3
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62. Formulae for compensation respiratory Acidosis
Kidney tries to save Hco3
Stimulates Tubular Reclamation & Generation of Hco3
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63. Formulae for compensation ACUTE respiratory Acidosis
• Expected rise in 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
Stimulates Tubular Reclamation & Generation of Hco3
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64. 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
Stimulates Tubular Reclamation & Generation of Hco3
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65. INTERPRETATION
OF
ABG
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66. INTERPRETATION OF ARTERIAL BLOOD GAS
History
Duration of the disease process
Clinical examination
Serum Electrolytes
Stepwise Repeated ABG evaluation
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67. Step 1 - CHECK pH
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68. STEP 1
• CHECK pH
• Normal range = 7.36 -7.44
• < 7.36 = acidemic [ resp / met ]
• >7.44 = alkalemic [ resp / met ]
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69. Step 2 - Evaluate pco2
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70. 2a STEP if pH less than 7.36
• Evaluate pco2
• pH less than 7.36, Pco2 more than 40 mm
- Respiratory Acidosis
• pH less than 7.36, Pco2 less than 40 mm ,
- Metabolic Acidosis
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71. STEP 2b if pH more than 7.44
• Evaluate pco2
• pH more than 7.44 , Pco2 Less than 40 mm ,
Respiratory alkalosis
• pH more than 7.44 , Pco2 more than 40 mm
Metabolic alkalosis
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72. Step 3 - COMPENSATION FORMULA
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73. STEP 3a – metabolic process
• COMPENSATION FORMULA
•Metabolic acidosis
•Exp pco2 = 1.5 (HCO3) + 8
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74. STEP 3a – metabolic process
• COMPENSATION FORMULA
•Metabolic alkalosis
•Exp pco2 = 0.9 (HCO3) +16
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75. STEP 3b - Respiratory disorder
• Compensation formula for respiratory disorder
• Acute resp acidosis
• Expected pH decreases by 0.08
for 10mm increase in pco2 above 40
• Expected Hco3 increases by 1 meq
for 10mm increase in pco2 above 40
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76. Step 3c
• Compensation formula for respiratory disorder
• Chronic resp acidosis
• Expected pH decreases by 0.03
for 10mm increase in pco2 above 40
• Expected Hco3 increases by 4 meq
for10mm increase in pco2 above 40
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77. Step 3d
• Compensation formula for respiratory disorder
• Acute resp alkalosis
• Expected pH increases by 0.08
for 10mm decrease in pco2 below 40
• Expected Hco3 decreases by 2 meq
for10mm decrease in pco2 below 40
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78. Step 3e
• Compensation formula for respiratory disorder
• Chronic resp alkalosis
• Expected pH increases by 0.03
for 10mm decrease in pco2 below 40
• Expected Hco3 decreases by 5 meq
for 10mm decrease in pco2 below 40
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79. MIXED DISORDER
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80. STEP 4
• MIXED DISORDER
• Compare between measured value with expected
values.
• If both are same [ pure primary disorder ]
• If not [ associated another primary disorder ]
( mixed disorder )
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81. Winters formula for met acidosis
EXP PCO2 = 1.5 [HCO3 ] + 8 ( ± 2 )
If measured pco2 is lower than expected pco2
[ASSOCIATED respiratory alkalosis # mixed ]
If measured pco2 is higher than expected pco2
[ASSOCIATED respiratory acidosis # mixed ]
If measured pco2 is same as expected pco2
[ pure primary metabolic acidosis]
Step 4a
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82. Formula for met alkalosis
EXP PCO2 = 0.9 [HCO3 ] + 16
If measured pco2 is lower than expected pco2
[ASSOCIATED respiratory alkalosis # mixed ]
If measured pco2 is higher than expected pco2
[ASSOCIATED respiratory acidosis # mixed ]
If measured pco2 is same as expected pco2
[ pure primary metabolic acidosis] ]
Step 4b
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83. 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 ]
Step 4c
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84. RESPIRATORY ALKALOSIS
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 Alkalosis ]
Step 4d
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85. Use of serum electrolytes
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86. STEP 5a
• Use of serum electrolytes in ABG analysis
• HCO3 :
•↑ met alkalosis / compensatory resp acidosis
• ↓ met acidosis / compensatory resp alkalosis
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87. STEP 5b
• K+
• ↑ acidosis
• ↓ alkalosis
• Cl-
• ↑ in Hyperchloremic met acidosis
• [ normal gap met acidosis ]
• ↓ in met-alkalosis
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88. Step 5 a
ANION GAP
Na + [ Hco3 + cl ] = normal 12 ± 3
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89. STEP 5 b
• BASE EXCESS
• Positive = met-alkalosis
• Negative = met-acidosis
• Useful for Metabolic Disorders
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90. Delta Ratio
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91. Delta Ratio
• If one molecule of metabolic acid (HA) is added to the
ECF and dissociates.
• HA = H+ / A-
• H+ + Hco3- = H2CO3 = H2O +CO2
• A- = UNMEASURED ANION
• This is the process of buffering.
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92. Delta ratio
• The net effect will be an increase in unmeasured
anions by the one acid anion A- (ie anion gap
increases by one)
• a decrease in the HCO3- by one meq.
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93. 12-12
Normal : -----
24-24
12-12 1
met acidosis : ----- = ---- = 0.7
24-18 1.3
12-12 1
met alkalosis ----- = --- 1.2
24-30 0.8
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A value below 1:1
a greater fall in [HCO3-]
Associated non gap met acidosis
A value above 2:1
a lesser fall in [HCO3-]
a concurrent metabolic alkalosis.

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95. STEP 6
• URINE pH
• Normal acidic [< 5.0] due to non volatile acids
produced in protein metabolism
• In acidemia : more acidic
• In alkalemia: more alkaline
[ in RTA alkaline urine despite of acidemia]
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96. CASE
• 24 yr old girl had taken some drugs to commit suicide
• Clinical examination
Nausea , vomiting ,dehydration , rapid respiration
• ABG VALUES
• pH = 7.32
• pCo2 = 15
• Hco3 = 8
• Na =140 , Cl = 104
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97. STEP 1
• CHECK pH
• Normal range = 7.36 -7.44
• Pt’s pH = 7.32
• < 7.36 = acidemic [ resp / met ]
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98. STEP 2
• Evaluate pco2
• Pt’s Pco2 = 15
• pH less than 7.36, Pco2 less than 40 mm ,
• METABOLIC ACIDOSIS
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99. STEP 3
• COMPENSATION FORMULA
• Metabolic acidosis
• Exp decrease in pco2 = 1.5 (HCO3) + 8
• Exp pco2 = 1.5 (8) + 8 = 20
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100. STEP 4
• MIXED DISORDER
• Compare between measured value with expected
values.
• Pt’s pco2 = 15
• Exp pco2 = 20
• measured pco2 is lower than expected pco2
[ Associated Respiratory Alkalosis # Mixed ]
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101. STEP 5
• ANION GAP
• Na - [ Hco3 + cl ] = normal 12 ± 3
• 140 - [ 8 +104 ] = 28
• Elevated gap metabolic acidosis
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102. • Elevated gap metabolic acidosis
With
• Respiratory alkalosis
With
• History of ingestion of tablets
• Probable diagnosis:
• SALICYLATE POISONING
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103. • A 50 year old insulin dependent diabetic woman was
brought to the ED by ambulance. She was semi-
comatose and had been ill for several days. Current
medication was digoxin and a thiazide diuretic for CHF.
•
Lab results Serum chemistry:
• Na 132, K 2.7, Cl 79, HCO3- 19
• Glu 815, Lactate 0.9 urine ketones 3+
•
ABG: pH 7.41 PCO2 32 HCO3- 19 pO2 82
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