This document provides information on interpreting arterial blood gas results. It discusses sampling arterial blood properly and transporting the sample quickly for analysis. The key components measured in an ABG - pH, PaCO2, PaO2, and HCO3 - are explained. A stepwise approach is outlined to interpret ABG results to determine if a patient has a respiratory or metabolic acid-base disorder, and if it is compensated or uncompensated. Case scenarios are used to demonstrate how to classify the acid-base disorder.

1. Arterial Blood Gas
Interpretation
By
Dr. Ahmed Galal Mohamed
PICU Specialist

2. Objectives
• ABG Sampling
• Interpretation of ABG
• Oxygenation status
• Acid Base status
• Case Scenarios

3. Sampling
Site:- (Ideally)
• Radial Artery
• Brachial Artery
• Femoral Artery
Heparinization:
• Syringe should be FLUSHED with
0.5ml of Heparin solution and
emptied.
• DO NOT LEAVE EXCESSIVE HEPARIN
IN THE SYRINGE
• As more HEPARIN makes more
dilutional effect.
• Syringes must have > 50% blood. Use
only 2ml or less syringe.

4. Sampling
ABG Syringe must
be transported at
the earliest to the
laboratory for EARLY
analysis via COLD
CHAIN
• Ensure No Air
Bubbles. Syringe
must be sealed
immediately after
withdrawing
sample.
• Contact with AIR
BUBBLES will
increase the PaO2
& decrease PaCO2

5. Sampling
o Patients Body Temperature affects the values
of PaCO2 and HCO3.
• ABG Analyser is controlled for Normal Body temperatures
• Any change in body temp at the time of sampling leads to
alteration in values detected by the electrodes
o The increase in cell count cause decrease in
PaO2.
o ABG Sample should always be sent with
relevant information regarding O2 status, FiO2
and Temp

6. ABG electrodes
• pH (Sanz Electrode)
o Measures H+ ion concentration of sample against a
known pH in a reference electrode, hence potential
difference.
• PaCO2 (Severinghaus Electrode):
o CO2 reacts with solution to produce H+
o higher C02- more H+  higher P CO2 measured
• PaO2 (Clark Electrode):
o O2 diffuses across membrane producing an electrical
current measured as PaO2.

7. Interpretation of ABG
Oxygenation
Acid-base
disorders

8. Oxygenation
PaO2
SaO2
Normally 60 – 95 on room air
Normally 85 – 100 on room air
PaO2/
FiO2
Normally > 450

9. Acid base disorders
Acid
Base
Any molecule that is able to release
hydrogen atoms “proton-donor substance.”
Any molecule that is able to receive
hydrogen atoms “proton-receptor
substance”

10. Regulation of acid base
CO2 + H2O carbonic anhydrase H2CO3 H+ + HCO3
-
CO2 HCO3

11. Normal acid base balance
pH AlkalosisAcidosis 7
Normal blood pH = 7.4
Acidemia & alkalemia
PaCO2
HCO3
35 – 45 mmHg
16 - 24

12. Stepwise approach
1. Acidemia or alkalemia.
2. Respiratory or metabolic.
3. Compensated or not?
4. Respiratory : acute or chronic.
5. Metabolic: anion gap.
6. High AG metabolic acidosis: gap
gap

13. Step1: acidemia VS alkalemia
• Look at pH:
<7.35 - acidemia
>7.45 – alkalemia
Step2: respiratory vs metabolic
• If CO2 is high so it is respiratory
• If HCO3 is low so it is metabolic

14. Step3: Compensated or not?
In metabolic disorders Expected PaCo2.
Metabolic acidosis:
• PCO2 = (1.5 X [HCO3
-])+8 + 2
• For every decrease by 1 in HCO3 the PCO2 falls
by 1.25 mm Hg
Metabolic alkalosis:
• PCO2 = (0.7 X [HCO3
-])+ 21 + 2
• For every increase by 1 in HCO3 the PCO2
increases by 0.75 mm Hg

15. Step3: Compensated or not?
In respiratory disorders Expected pH.
Respiratory acidosis:
• ACUTE :- pH=7.40–0.008( PCO2-40)
• CHRONIC :- pH=7.40–0.003(PCO2-40)
Respiratory alkalosis:
• ACUTE pH=7.40+0.008(40-PCO2)
• CHRONIC pH=7.40+0.003(40-PCO2)

16. Step4: respiratory: acute or chronic
In respiratory disorders Expected pH.
Compare pH measured VS pH expected
• pH m = pH e acute = acute respiratory disorder.
• pH m = between pH e of acute & chronic =
partially compensated disorder.
• pH m = pH e chronic = fully compensated disorder.