This document provides information about arterial blood gas (ABG) analysis, including indications, technique, precautions, normal values, and a stepwise approach to interpretation. ABG analysis can monitor acid-base balance and provide information about oxygenation. It is indicated for patients with respiratory failure, metabolic acidosis related to conditions like renal failure, or to assess ventilation in mechanically ventilated patients. The preferred artery is the radial, and precautions must be taken to avoid air bubbles. A case example is then presented and interpreted step-by-step, identifying a mixed high and normal anion gap metabolic acidosis in a child with chronic kidney disease and dehydration from diarrhea.

1. ARTERIAL BLOOD GAS
ANALYSIS
Sasidharan, junior resident
Department of Pediatrics
Moderator : Dr. Ramesh Kumar

2. INTRODUCTION
• ABG is a routine investigation to monitor acid base balance in
patients
• It provides information about
- oxygenation
- acid base balance
- adequacy of ventilation
It helps in making a diagnosis ,assessing severity and titrating the
treatment

3. INDICATIONS OF ABG
clinical features of :
• Respiratory failure ( acute or chronic )
• Suspected metabolic acidosis
1. Renal failure
2. Cardiac failure
3. Hyperglycemic states associated with diabetes
 To assess ventilation and acid
base abnormalities in a ventilated patient

4. TECHNIQUE
• Preferred artery - RADIAL
• Arterial catheter can be left in place if repeated samples are required
• Needle size - 20 -26 G
• Angle - < 30 degrees
• 0.05 ml of heparin for 1 ml of
blood
- careful with excess heparin ( its weak acid and can alter ABG
values )

5. PRECAUTIONS
• Pull the syringe slowly to avoid air bubbles
• Air bubbles can alter paO2 by 30 mmHg
• Cap the needle immediately after sampling
• Store the sample if delay in processing is anticipated

6. Normal ABG values
• pH 7.35 – 7.45
• PCO2 35 – 45 mmHg
• PO2 80 – 100 mmHg
• HCO3 22 – 26 mmol/L
• BE -2 - +2
• SaO2 >95%

7. STEPWISE APPROACH OF ABG ANALYSIS
• STEP 1
Is the pH outside the normal range?
Acidemic – pH<7.35
Alkalemic – pH > 7.45
• STEP 2
Is the respiratory (PaCO2) or the metabolic (HCO3 -) component
abnormal?
Respiratory component alters the arterial paCO2 ( normal range
35-45)
Metabolic component alters the serum HCO3 (normal range 20-
26)

9. STEP 3
If its respiratory problem, is it acute or chronic.
This depends on pH
Acute respiratory acidosis – pH decrease = 0.08 * (paCO2
– 40)/10
Chronic respiratory acidosis – pH decrease = 0.03 * (
paCO2 – 40 )/10
Acute respiratory alkalosis – pH increase = 0.08 * ( 40 –
paCO2)/10
Chronic respiratory alkalosis – pH increase = 0.03 * ( 40-
paCO2) / 10

10. STEP 4
For respiratory component is metabolic component
adequate?
Respiratory acidosis :
Acute : for every 10 increase in CO2 , HCO3 will increase by
1
Chronic : for every 10 increase in CO2 , HCO3 will increase
by 3.5
Respiratory alkalosis :
Acute : for every 10 decrease in CO2, HCO3 will decrease by
2
Chronic : for every 10 decrease in CO2, HCO3 will decrease
by 4

11. STEP 5
If it’s a metabolic disorder, assess compensation by
respiratory system.
Metabolic acidosis : use winter’s formula: [(1.5 * HCO3 )
+ 8 ]± 2
CO2 concentration should be within this limits.
Metabolic alkalosis : For every 10 increase in HCO3 , CO2
increases by 7
If pH is <7.35 and CO2 more than 55, then it means
respiratory acidosis is co-existing

12. STEP 6
Anion gap calculation:
If its metabolic acidosis , calculate anion gap
AG = Na – ( HCO3 + Cl)
Normal AG : 12 ± 4
If > 12 = high anion gap acidosis
If < 12 = non anion gap acidosis

13. DELTA RATIO
<0.4 – pure Normal anion gap metabolic acidosis
0.4 to 0.8 – mixed normal anion gap with high anion
gap metabolic acidosis
0.8 to 2.0 – pure high anion gap metabolic acidosis
> 2.0 – mixed high anion gap metabolic acidosis with
metabolic alkalosis ( or pre existing compensated
respiratory alkalosis)

15. Case scenario
• A 6 year old male child k/ c/o ckd presented with complaints of
Loose stools 15- 20 episodes x 1 day
no blood in stool. Stool watery in consistency.
Past history of k/c/o ckd on CAPD.

16. Clinical examination
• O/E Child was irritable,lethargic
• Vitals
• PR-142/min CP/PP – ++/+- cold peripheries +
• RR-22/min
• SpO2 – 99 %
• BP – 69/ 37 mm Hg
• CFT – 4 sec
• Signs of dehydration :
• Sunken eyes +, dry tongue+, skin turgor release prolonged
• CVS – S1S2 normal.No murmur
• RS- B/L air entry equal.No added sounds
• P/A – Soft, no organomegaly
• CNS- E4V4M6
• RBS- 116mg/dl

17. • pH- 7.29
• pCO2-25.8
• paO2- 47.2
• HCO3- 12.3
• BE- -12.3
• Na- 135
• K-3.8
• Cl- 97
• AG- 25.5
Step 1 acidosis or alkalosis?
pH- 7.29---- ACIDOSIS.
Step 2
Metabolic or respiratory?
pCO2-25.8 and HCO3- 12.3.0 – METABOLIC
Step 3 Compensation?
paCO2 has decreased i.e. same direction change.
So compensation present.
Is the blood gas report
internally consistent?
{ [24*paCO2/HCO3]- 40 } *
0.01
here, { 24(25.8/12.3) -40 } *
0.01= 0.11
Subtracting this value from 7.4,
pH expected= 7.29

18. step 4 Is compensation adequate?
WINTER’S FORMULA (paCO2= 1.5*HCO3 + 8 +/- 2)
Here, 1.5x12.3 + 8 +/- 2= 26.25 +/ - 2 = 24.25- 28.25
Here Pco2 – 25.8 Adequate compensation present.
Step 5 Calculate anion gap
AG= 129-(97+12.3)= 19.7
HIGH ANION GAP METABOLIC ACIDOSIS PRESENT
Step 7 Calculate delta AG and delta HCO3
Delta AG= 7.7. delta HCO3= 11.7
Delta AG/ delta HCO3= 7.7/11.7 = 0.65
Mixed HAGMA, NAGMA.

19. interpretation
• There is metabolic acidosis
• Adequate compensation- acute- by respiratory alkalosis
• Values are internally consistent.
• There is HAGMA with NAGMA
• Probable cause is chronic renal failure causing high anion gap
metabolic acidosis with dehydration secondary to diarrhoea causing
normal anion gap metabolic acidosis.

20. Blood gas analysis after treatment
VBG DAY 1 DAY 2 DAY 3
pH 7.29 7.168 7.25
PCO2 25.8 31.8 23.5
PO2 47.2 49.5 45.1
HCO3 12.3 11.3 10.2
BE -12.3 -16.0 -15.4
Na 129 128 131.2
K 3.5 3.8 3.66
Cl 97 96 99
AG 19.7 20.7 21.8

21. THANK YOU