04/12/23 1

Arterial blood gas analysis

Dr. Mashfiqul Hasan

5 steps

04/12/23 2

Step 1

Determine the primary disorder

Acidosis or alkalosis?

Metabolic or respiratory?

04/12/23 3

Example - 1

PH = 7.32 HCO3 = 40

meq/L (High) PaCO2 = 60

mm of Hg (High)

WHAT IS THE DIAGNOSIS?

04/12/23 4

Respiratory acidosis

04/12/23 5

Example - 2

PH = 7.22 HCO3 = 14 meq/L (Low)

PaCO2 = 27 mm of Hg (Low)

WHAT IS THE DIAGNOSIS?

04/12/23 6

Metabolic acidosis

04/12/23 7

Step - 2

Calculate the range of compensatory mechanism to determine the presence of mixed acid base disorder.

04/12/23 8

Example - 1

PH = 7.32 HCO3 = 40 meq/L (High) PaCO2 = 60 mm of Hg

(High) Respiratory acidosis

How to calculate the compensatory change?

04/12/23 9

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

04/12/23 10

But here HCO3 is 40 meq/L.

What does it indicate?

04/12/23 11

It indicates mixed acid base disorder.

Superimposed metabolic alkalosis which causes rise of HCO3 disproportionately

04/12/23 12

Final diagnoses

Chronic respiratory acidosis with metabolic alkalosis

Which may develop in a patient of Corpulmonale getting diuretics therapy

04/12/23 13

Example - 2

PH = 7.22 HCO3 = 14 meq/L (Low)

PaCO2 = 27 mm of Hg (Low)

Metabolic acidosis

How to calculate the compensatory change?

04/12/23 14

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

04/12/23 15

PaCO2 is consistent with calculation

So there is no mixed acid base disorder according to step 2.

Final diagnosis is metabolic acidosis.

04/12/23 16

Step - 3

Calculate the anion gap.

04/12/23 17

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

04/12/23 18

Calculation of anion gap

Na+ - (Cl- + HCO3-) = anion

gap 140 – (90 + 40) = 10

mmol/L

Anion gap is normal

04/12/23 19

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?

04/12/23 20

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

04/12/23 21

Step - 4

Calculate the HCO3-

concentration if the anion gap is increased.

04/12/23 22

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.

04/12/23 23

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

04/12/23 24

Calculation of HCO3-

Corrected HCO3- = Measured

HCO3- + the increase in anion

gap Corrected HCO3

- = 14+15 = 29 mmol/L

It is higher than normal.

04/12/23 25

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.

04/12/23 26

Final diagnosis of example 2 High Anion gap Metabolic

acidosis with concomitant metabolic alkalosis

Which may be present in alcoholic ketoacidosis with excessive vomiting

04/12/23 27

Final step

04/12/23 28

Final step

Examine the patient to determine whether the clinical signs are compatible with the acid base analysis thus obtained.

04/12/23 29

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.

04/12/23 30

Thanks for patient hearing