arterial blood gas analysis
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04/12/23 1
Arterial blood gas analysis
Dr. Mashfiqul Hasan
5 steps
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Step 1
Determine the primary disorder
Acidosis or alkalosis?
Metabolic or respiratory?
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Example - 1
PH = 7.32 HCO3 = 40
meq/L (High) PaCO2 = 60
mm of Hg (High)
WHAT IS THE DIAGNOSIS?
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Respiratory acidosis
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Example - 2
PH = 7.22 HCO3 = 14 meq/L (Low)
PaCO2 = 27 mm of Hg (Low)
WHAT IS THE DIAGNOSIS?
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Metabolic acidosis
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Step - 2
Calculate the range of compensatory mechanism to determine the presence of mixed acid base disorder.
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Example - 1
PH = 7.32 HCO3 = 40 meq/L (High) PaCO2 = 60 mm of Hg
(High) Respiratory acidosis
How to calculate the compensatory change?
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Calculation Primary change in
PaCO2 is : 60-40=20mm of Hg raised
Compensatory change in HCO3 should be 3.5 meq/L rise for per 10 mm of Hg rise in PaCO2
So, HCO3 should rise by 7 meq/L here.
HCO3 level should be 24+7=31 meq/L
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But here HCO3 is 40 meq/L.
What does it indicate?
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It indicates mixed acid base disorder.
Superimposed metabolic alkalosis which causes rise of HCO3 disproportionately
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Final diagnoses
Chronic respiratory acidosis with metabolic alkalosis
Which may develop in a patient of Corpulmonale getting diuretics therapy
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Example - 2
PH = 7.22 HCO3 = 14 meq/L (Low)
PaCO2 = 27 mm of Hg (Low)
Metabolic acidosis
How to calculate the compensatory change?
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Calculation
Primary change in HCO3 is : 24-14=10 meq fall
Compensatory change in PCO2 should be 1.3 mm of Hg fall for per 1 meq fall in HCO3
So, PCO2 should fall by 13 mm of Hg here.
PCO2 level should be 40-13=27 meq/L
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PaCO2 is consistent with calculation
So there is no mixed acid base disorder according to step 2.
Final diagnosis is metabolic acidosis.
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Step - 3
Calculate the anion gap.
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Example -1
PH = 7.32 HCO3 = 40 meq/L (High)
PaCO2 = 60 mm of Hg (High) Na+ = 140 mmol/L K+ = 4 mmol/L Cl- = 90 mmol/L
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Calculation of anion gap
Na+ - (Cl- + HCO3-) = anion
gap 140 – (90 + 40) = 10
mmol/L
Anion gap is normal
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Example - 2
PH = 7.22 HCO3 = 14 meq/L (Low)
PaCO2 = 27 mm of Hg (Low) Na+ = 142 mmol/L K+ = 4.9 mmol/L Cl- = 101 mmol/L
What is the anion gap?
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Calculation of anion gap
Na+ - (Cl- + HCO3-) = anion gap 142 – (101 + 14) = 27 mmol/L
Anion gap is increased So the diagnosis is high anion
gap metabolic acidosis. Change of anion gap is 27-
12=15
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Step - 4
Calculate the HCO3-
concentration if the anion gap is increased.
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Calculation of HCO3-
Corrected HCO3- = Measured
HCO3- + the increase in anion
gap Corrected HCO3
- should be the normal value for HCO3
-, that is 24 mmol/L if there is no mixed acid base disorder.
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Example 2
PH = 7.22 HCO3 = 14 meq/L (Low)
PaCO2 = 27 mm of Hg (Low) Na+ = 142 mmol/L K+ = 4.9 mmol/L Cl- = 101 mmol/L Anion gap = 27 mmol/L Change in anion gap = 15 mmol/L
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Calculation of HCO3-
Corrected HCO3- = Measured
HCO3- + the increase in anion
gap Corrected HCO3
- = 14+15 = 29 mmol/L
It is higher than normal.
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What does it indicate?
Higher corrected HCO3-
indicates concomitant presence of metabolic alkalosis.
Lower corrected HCO3-
indicates concomitant presence of non anion gap metabolic acidosis.
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Final diagnosis of example 2 High Anion gap Metabolic
acidosis with concomitant metabolic alkalosis
Which may be present in alcoholic ketoacidosis with excessive vomiting
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Final step
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Final step
Examine the patient to determine whether the clinical signs are compatible with the acid base analysis thus obtained.
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5 steps at a glance
1. Determine the primary disorder.2. Calculate the compensatory
change to determine the presence of mixed acid base disorder.
3. Calculate the anion gap.4. If anion gap is high, calculate the
corrected HCO3- level.5. Correlate with the patient.
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Thanks for patient hearing
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