Analysing an ABG

Question 1

What are the 5 steps to analysing an ABG?

Answer 1

1. Is the patient hypoxic?
2. Is there a significant degree of lung injury? (A-a gradient)
3. Does the patient have an acidosis or alkalosis?
4. Is the cause respiratory or metabolic?
5. Is there any attempt at compensation?

Question 2

How do you know if there is lung injury?

Answer 2

E.g. PaO2 is normal but patient taking on 60% oxygen so actually the lungs aren’t working well

Question 3

What is the blood gas analysis chart?

Answer 3

IN FOLDER

Question 4

How low should oxygen be to be hypoxic?

Answer 4

<8kPa

Question 5

What is the A-a gradient?

Answer 5

The alveolar-arterial gradient. In 21% oxygen PAO2 is 21kPa where the PaO2 will usually be above 10.6kPa
21-10.6 = 10.4
Therefore 10.4 is the normal A-a gradient. If this number is significantly greater than 10 then there is a problem with the patient’s lungs.

Question 6

How can the A-a gradient help if you are giving the patient oxygen?

Answer 6

If you are giving 35% and the PaO2 is still only 8kPa, then the A-a gradient is 35-8 = 27. This means that although they are only just hypoxic, there is something v bad going on.

Question 7

Explain the values of respiratory acidosis/alkalosis.

Answer 7

If the pH rises above 7.45 and the PaCO2 drops below 4.5kPa, then it is a respiratory alkalosis.
If the pH drops below 7.35 and the PaCO2 rises above 6kPa, then it is a respiratory acidosis.

Question 8

Explain the values of a metabolic acidosis/alkalosis.

Answer 8

If the pH rises above 7.45 and the base excess/HCO3 is more than +2 or 26, then it is a metabolic alkalosis
If the pH drops below 7.35 and the base excess/HCO3 is less than -2 or 22, then it is a metabolic acidosis

Question 9

Explain the values of a mixed acidosis/alkalosis.

Answer 9

If the pH is less than 7.35, the PaCO2 is above 6kPA and the HCO3 is below 22 then it is a mixed acidosis.
If the pH is more than 7.45, the PaCO2 is below 4.5kPA and the HCO3 is more than 26 then it is mixed alkalosis (very rare).

Question 10

What to do first when trying to read the ABG?

Answer 10

AFTER LOOK AT OXYGEN THEN IGNORE IT
1. Is it alkalosis or acidosis?
2. Which of the PaCO2 and HCO3 match the pH?
3. Is the other of the PaCO2/HCO3 going opposite to pH?
THINK OF NICK SMITH’S DIAGRAM