This document discusses arterial blood gases (ABG) analysis and interpretation. It provides the normal ranges for pH, PaCO2, PaO2, HCO3, and other values in ABGs. It outlines a 4-step process for interpreting ABGs: 1) determine if values are authentic, 2) identify if there is acidosis or alkalosis, 3) assess for compensation, 4) determine if the cause is metabolic or respiratory. Formulas are provided to calculate expected bicarbonate levels in respiratory acidosis and alkalosis based on the degree of compensation. Examples of interpreting ABG results are given for various acid-base disorders.

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

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
o Measure 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

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:

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
 PCO2 52 mm Hg
 HCO3 29 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
 PCO2 24 mm Hg
 HCO3 16 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