Arterial Blood Gas (ABG)

Question 1

What is an ABG test?

Answer 1

An arterial blood gas (ABG) test measures oxygen and carbon dioxide levels in your blood. It also measures your body’s acid-base (pH) level, which is usually in balance.

Question 2

How do you assess ABG?

Answer 2

A systematic approach:

Focus on the PH then PaCO2 then HCO3

Question 3

Changes in pH

Answer 3

Normal values- 7.35-7.45
pH <7.35 shows an acidosis picture
pH > 7.45 shows an alkalosis picture

Question 4

Changes in PaCO2

Answer 4

pCO2: 4.6 – 6.4 kPa
If PCO2 is raised and there is acidosis, then this is respiratory acidosis.
If PCO2 is low and there is alkalosis, then the lack of acid gas has led to respiratory alkalosis.
If PCO2 is low and there is acidosis, then this is a metabolic acidosis (HCO3 will be low)
If PCO2 is high/normal and there is alkalosis, there must be metabolic alkalosis. HCO3 should be raised.

Question 5

Changes in PaO2

Answer 5

pO2: 11.0 – 14.4 kPa.
Hypoxia is when PO2 is lower than the range and can result from a ventilation-perfusion mismatch (PE) or from alveolar hypoventilation (COPD, pneumonia).

Question 6

What is type 1 respiratory failure?

Answer 6

Type 1 respiratory failure- hypoxia and PCO2 < 6.4kPa

If the PO2 is very low, consider venous blood contamination.

Question 7

What is type 2 respiratory failure?

Answer 7

Hypoxia and PCO2 > 6kPa

Question 8

Compensatory mechanisms to maintain a normal pH

Answer 8

Mechanisms controlled pH are activated when acid-base imbalance threatens.
Renal control of H+ and HCO3- ion excretion can result in compensatory metabolic changes.
Similarly, ‘blowing off’ or retaining CO2 via control of respiratory rate can lead to compensatory respiratory changes.
A compensated picture suggests chronic disease.

Question 9

What is acidosis?

Answer 9

`Excess cation, unless adequately compensated will result in acidosis (acidaemia).

Question 10

Causes of respiratory acidosis

Answer 10

COPD
Asthma
Pneumonia
Pneumothorax
Obstructive sleep apnoea
Opiate overdose (causing respiratory depression)
Neuromuscular disorders (e.g. Guillain-Barre)
Skeletal abnormalities (e.g. kyphoscoliosis)
Congestive cardiac failure

Question 11

Causes of metabolic acidosis

Answer 11

Raised anion gap is caused by: 
DKA
Renal failure 
Lactic acidosis 
Salicylates 
Ethylene glycol
Biguanides
Normal anion gap is caused by: 
Chronic diarrhoea, ileostomy (loss of HCO3-)
Addison's disease
Pancreatic fistulae
Renal tubular acidosis 
Acetazolamide treatment (loss of HCO3-)

Question 12

What is alkalosis?

Answer 12

A relative excess of anions, unless adequately compensated.

Question 13

Causes of respiratory alkalosis

Answer 13

Hyperventilation secondary to: a panic attack, pain.
Meningtis 
Stroke 
Subarachnoid haemorrhage 
High altitude

Question 14

Causes of metabolic alkalosis

Answer 14

Diuretic drugs (loss of K+)
Prolonged vomiting (via acid replacement and release of HCO3-)
Burns
Base ingestion