Arterial Blood Gases- ABG

easy way to interpret the ABG

1. Dr. Mohamed Abbass
Consultant Nephrology
PGDD,CARDIFF,UK
Nephro-Medical Tube
Arterial Blood Gases

2. Arterial Venous
PH 7.35-7.45
(7.40)
7.32-7.43
PaCO2 35–45 (40)40 – 50 mmHg
PaO2 >80 mmHg 25 – 40 mmHg
HCO3a - (MEq/l) 22-26mmol/L (24)23 – 27 mmol/L
Base excess ± 2 mmol/L ± 2 mmol/L
O2 saturation >95 % 30-40 %
H
+
37−43 nEq/L 42−48 nEq/L
Normal value of ABG

3. Steps of interpretation of ABG ( 4
STEPS )
1.Authentic
2.Acid or Alkalosis
3.Compensation
4.Metabolic or Respiratory pathway
Metabolic pathway:
 A,B,C,D,E
Respiratory pathway
O,1,2,4,5

4. STEP 1: Authentic

8. STEP3 : Compensation

9. STEP3 : Compensation

10. STEP3 : Compensation

11. STEP3 : Compensation

12. STEP 4 : Metabolic or Respiratory pathway
Metabolic Pathway

13. Metabolic Pathway

14. Metabolic Pathway
5. Electrolytes, urinary:
To determine the causes of Metabolic acidosis
oMeasure the urinary anion gap (UAG) is relevant in
hyperchloremic metabolic acidosis. UAG = [Na+] +
[K+] – [Cl -]
Negative UAG: bowel loss of bicarbonate
Positive UAG: renal loss of bicarbonate
To determine the causes of Metabolic alkalosis
o Measure Urinary chloride

16. Respiratory Pathway : O-1-2-3-4-5
Oxygenation, assessment of: PaO2/FIO2 ratio is the ratio of
arterial oxygen partial pressure to fractional alveolar oxygen,
clinical indicator of hypoxaemia
Acute Respiratory acidosis:
Expected HCO3 =24+(PCO2-40/10 )X 1
Chronic Respiratory acidosis:
Expected HCO3 =24+(PCO2-40/10 )X 2
Acute Respiratory alkalosis:
Expected HCO3 =24-(40-PCO2/10) X 4
Chronic Respiratory alkalosis:
Expected HCO3 =24-(40-PCO2/10) X 5

18. pH 7.18
PCO2 58 mm Hg
HCO3 26 mEq/L
Respiratory acidosis (acute) due to no renal compensation.
Expected HCO3 =24+(PCO2-40/10 )X 1= 25.5
pH 7.39
PCO252 mm Hg
HCO329 mEq/L.
Respiratory acidosis (chronic) with renal compensation.
Expected HCO3 =24+(PCO2-40/10 )X 2= 26.4

19. pH 7.57
PCO2 25 mm Hg
HCO3 23 mEq/L
Respiratory alkalosis (acute) due to no renal compensation.
 Expected HCO3 =24-(40-PCO2/10) X 4= 18
pH 7.44
PCO224 mm Hg
HCO316 mEq/L
Respiratory alkalosis (chronic) with renal compensation.
 Expected HCO3 =24-(40-PCO2/10) X 5=

20. Ph 7.50 (7.35–7.45)
PCO2 28.1 mmHg (35–45)
HCO3 23.9 mmol/L (22–28)
Acute (uncompensated) respiratory alkalosis
Excepted HCO3= 24-(40-PCO2/10) X 4= 19.16
Ph 7.35 (7.35–7.45)
PCO2 54.8 mmHg (35–45)
HCO3 29 mmol/L (22–28)
Chronic (compensated) primary respiratory acidosis
Expected HCO3 =24+(PCO2-40/10 )X 2= 26.96