ABG interpretation

Table of Contents

Learn how to perform one too…

Performing an ABG is covered here!

Normal values
pH	7.35-7.45
pCO₂	4.5-6.0 kPa
pO₂	11-13 kPa
HCO₃^–	22-26 mmol/l
BE	-2 to +2
SaO₂	>95%
Lactate	<2

Step 1 – Oxygenation

– Hypoxaemic? (i.e. Is the PaO₂ <11kPa?)
– Is there impaired oxygenation?
– Oxygenation takes into account the percentage of oxygen the patient is on (the fraction of inspired oxygen or FiO₂)
– PaO₂ should be approximately 10kPa less than the FiO₂ percentage

Respiratory failure:
– Type 1 = 1 gas abnormal (↓O₂)
– Type 2 = 2 gases abnormal (↓O₂ + ↑CO₂)

Step 2 – pH status

Determine pH status:
– Acidosis (pH↓)
– Alkalosis (pH↑)

Step 3 – Respiratory component

Determine respiratory component (PaCO2):
– Respiratory acidosis (pH↓, PaCO₂↑)
– Respiratory alkalosis (pH↑, PaCO₂↓)

If the PaCO₂ doesn’t agree with the pH, ignore it until step 5

Step 4 – Metabolic component

Determine the metabolic component (HCO3- or BE):
– Metabolic acidosis (pH↓, HCO₃^–↓)
– Metabolic alkalosis (pH↑, HCO₃^–↑)

If the HCO₃^– doesn’t agree with the pH, ignore it until step 5

Step 5 – Combine

– Primary disturbance
– Compensation

See below

Step 5 (continued)

Primary disturbance: you know this from the above steps
- NB. both the respiratory and metabolic component may have agreed with the pH ie. a ‘mixed respiratory and metabolic acidosis/alkalosis’
Compensation: If either the respiratory/metabolic component was not consistent with the pH, there is compensation…
- Acidosis may be compensated by
  - Respiratory compensation: increasing respiratory rate to blow off CO₂ (will result in ↓CO₂)
  - Metabolic compensation: increased bicarbonate production by kidney (will result in ↑HCO₃^–)
- Alkalosis may be compensated by
  - Respiratory compensation: decreasing respiratory rate to retain CO₂ (will result in ↑CO₂)
  - Metabolic compensation: decreased bicarbonate production by kidney (will result in ↓HCO₃^–)
Partial compensation = pH not quite back to normal yet; full compensation = pH normal (you cannot over-compensate)
- NB. You can’t fully compensate metabolic alkalosis (you can only hypoventilate a bit). Metabolic compensation by the kidneys takes 3 days, respiratory compensation is fast.

± Respiratory failure
1. Type 1 = 1 gas abnormal (↓O₂)
2. Type 2 = 2 gasses abnormal (↓O₂ + ↑CO₂)

Causes of Acid-Base Mismatch

Causes of Respiratory Failure

Type 1 = 1 gas abnormal = ↓O₂, normal CO₂

Caused by impaired diffusion (e.g. pneumonia, ARDS, pulmonary fibrosis) or ventilation-perfusion (V/Q) mismatch, ie. either:

Low V/Q: areas of lung are perfused with deoxygenated blood but not ventilated with oxygen (ie. airway obstruction)
- Causes: mucus plug in asthma/COPD, airway collapse in emphysema
High V/Q: areas of lung are ventilated with oxygen but not perfused with deoxygenated blood (ie. block in blood flow)
- Causes: PE

The reason CO₂ is normal is that the areas of the lung which are perfused and ventilated can blow off extra CO₂ by increasing ventilation rate (making CO₂ low in this area and high in the area with V/Q mismatch which makes it normal overall). Extra oxygen, however, cannot be absorbed (without giving a higher oxygen concentration) because the maximum amount of oxygen diffuses across the alveolar membrane in normal circumstances anyway.

Type 2 = 2 gasses abnormal = ↓O₂, ↑CO₂

Caused by alveolar hypoventilation. This means oxygen cannot get into alveoli and carbon dioxide cannot get out.

Causes: obstructive lung diseases (e.g. COPD), restrictive lung diseases, decreased respiratory drive, neuromuscular disease, thoracic wall disease

Lactic Acidosis

Lactic acid = a product of anaerobic metabolism
Types of lactic acidosis
- TYPE 1 (hypoxic) = produce too much lactic acid (e.g. DKA, starvation, cardiovascular/respiratory depression)
- TYPE 2 (non-hypoxic) = cannot break down lactic acid (e.g. secondary to metformin or poisoning)

Common ABG patterns

NB. Respiratory alkalosis ∆∆ = hyperventilation (↑O₂), asthma exacerbation (normal O₂), PE (↓O₂)

Why don’t you try an example?

A 49 year old patient has been brought to the emergency department with breathlessness and a reduced GCS. Their chest sounds wheezy on auscultation. Please review the patient’s ABG :

pH 7.25 (7.35-7.45)
pCO2 7.7 (4.5-6)
pO2 7.6 (11-13)
HCO3- 14.7 (22-26)
BE -6 (-2 to +2)

What does the ABG show?