O2 and CO2
And a bit about FiO2
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.
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.