O₂ and CO₂

Oxygen

Normal range: 10 - 13 kPa (on room air)

If the PaO₂ is <10kPa on room air the patient is considered hypoxaemic.

If it is <8kPa they are severely hypoxaemic – this indicates respiratory failure, which we will discuss in more detail later.

FiO₂

If the patient is receiving supplemental oxygen you must know the concentration of inspired oxygen (FiO₂), as the expected PaO₂ will change.

As a rule of thumb the PaO₂ should be approximately 10kpa less than the % inspired oxygen. For example, a patient breathing 30% oxygen would be expected to have a PaO₂ of approximately 20kPa.

It can be difficult to accurately determine the concentration of inspired oxygen. The table shows some rough guides, but you will note that a venturi mask is the only one that gives a reasonably accurate value. However it is important to be aware that as the inspired oxygen concentration increases so should the PaO₂.

These figures are an approximation.

Device	Flow rate	FiO2
Nasal cannula	1 L/min	24%
	2 L/min	28%
	4 L/min	36%
Simple face mask	15 L/min	40-60%
Venturi mask
Blue	2-4 L/min	24%
White	4-6 L/min	28%
Yellow	8-10 L/min	35%
Red	10-12 L/min	40%
Green	12-15 L/min	60%
Non-rebreathe mask	15 L/min	60-90%

Carbon Dioxide

Normal range: 4.7 - 6.0 kPa

If the PaCO₂ is high, this means the patient is not effectively exhaling CO₂ from their lungs (i.e. hypoventilation). This could be due to an obstructive lung condition (e.g. COPD), a neuromuscular disease, low haemoglobin concentration, or a central nervous system impairment.

If the PaCO₂ is low, this means the patient is exhaling increased amounts of CO₂ from their lungs (i.e. hyperventilation). This could be due to an increase in respiratory rate or volume or both, and has a variety of causes including anxiety, pain, drugs, head injury, etc.

We will discuss PaCO₂ in more detail when we come to interpreting ABGs.