Understanding PaO₂ for Student Nurses
Learn what PaO₂ means on an arterial blood gas, how it reflects the amount of oxygen dissolved in arterial blood and how to connect the result with oxygen saturation, respiratory assessment and deterioration.
What does PaO₂ measure?
PaO₂ is the partial pressure of oxygen in arterial blood. It provides information about how effectively oxygen has moved from the lungs into the bloodstream at the time the arterial blood gas was taken.
Arterial oxygen
PaO₂ measures dissolved oxygen in arterial blood and forms part of assessment of oxygenation.
Oxygen saturation
SpO₂ estimates the percentage of haemoglobin carrying oxygen and is measured non-invasively.
Always assess the patient
Neither PaO₂ nor SpO₂ replaces examination of respiratory rate, effort, consciousness and circulation.
They are related, but they are not the same measurement
| Measurement | What it tells you | Important limitation |
|---|---|---|
| PaO₂ | Partial pressure of oxygen dissolved in arterial blood. | Requires an arterial blood sample and must be interpreted with oxygen therapy and context. |
| SpO₂ | Estimated percentage of haemoglobin saturated with oxygen. | Can be affected by poor perfusion, movement and other factors. |
| Respiratory rate | How frequently the patient is breathing. | Does not by itself show oxygenation or ventilation effectiveness. |
| PaCO₂ | Provides information about carbon dioxide and ventilation. | Oxygenation and ventilation are different physiological processes. |
Think oxygenation and ventilation separately
PaO₂ helps you understand oxygenation. PaCO₂ helps you understand ventilation. A patient may have problems with one, the other or both.
What may accompany impaired oxygenation?
Tachypnoea
Respiratory rate may rise as the patient attempts to compensate for impaired oxygenation or underlying illness.
Work of breathing may increase
Accessory muscle use, difficulty speaking and visible respiratory distress can accompany deterioration.
Consciousness may change
Agitation, confusion, drowsiness or reduced responsiveness can be important warning signs.
Always ask: how much oxygen is the patient receiving?
The clinical meaning of PaO₂ depends partly on the inspired oxygen concentration. Recording and communicating oxygen delivery is therefore an important part of interpreting respiratory blood gases.
No supplemental oxygen
A PaO₂ obtained while breathing room air has a different context from one obtained during supplemental oxygen therapy.
Record the delivery
Note the device and prescribed oxygen therapy so the result can be interpreted properly by the clinical team.
A clinical trend
Needing progressively more oxygen to maintain an appropriate target can itself indicate respiratory deterioration.
The same PaO₂ can mean different things
Always communicate the oxygen therapy being used when discussing an arterial blood-gas result.
How should you assess abnormal oxygenation?
Check respiratory rate
Compare the current rate with earlier observations and look for a changing trend.
Assess work of breathing
Look for accessory muscle use, recession, posture changes and difficulty speaking.
Check oxygen saturation
Review SpO₂ against the patient's prescribed target and note the oxygen therapy being used.
Review the full blood gas
Connect PaO₂ with PaCO₂, pH, bicarbonate and other relevant findings.
Assess consciousness
Look for agitation, confusion, drowsiness or reduced responsiveness.
Escalate deterioration
Significant hypoxaemia or associated respiratory deterioration requires prompt clinical review.
Use ABCDE to assess the whole patient
Airway
Confirm airway patency and identify obstruction or secretion problems.
Breathing
Assess respiratory rate, effort, oxygen saturation, chest movement and oxygen therapy.
Circulation
Assess pulse, blood pressure and perfusion because oxygen delivery depends on circulation as well as oxygenation.
Disability
Assess consciousness and identify neurological changes that may accompany significant deterioration.
Exposure
Consider infection, fever, pain, underlying respiratory disease and other relevant clinical findings.
Escalate
Follow local emergency procedures and seek appropriate registered and medical support when significant deterioration is present.
Look at oxygen requirement as well as PaO₂
Stable oxygenation
The patient's oxygen requirement and respiratory observations are relatively stable.
Requirement increases
More supplemental oxygen is required to maintain the prescribed saturation target.
Blood gas worsens
PaO₂ remains concerning despite increased oxygen and respiratory effort has worsened.
When should PaO₂ increase your concern?
- A low or worsening PaO₂ alongside respiratory distress.
- An increasing oxygen requirement.
- Falling oxygen saturation despite prescribed oxygen therapy.
- Increasing respiratory rate or work of breathing.
- Difficulty speaking because of breathlessness.
- New agitation, confusion or drowsiness.
- Abnormal PaCO₂ or pH alongside worsening oxygenation.
- A significant change from previous blood-gas results.
- A patient whose overall respiratory condition is deteriorating.
Think oxygen + oxygen requirement + patient
PaO₂ becomes clinically meaningful when you connect it with how much oxygen the patient is receiving and what the bedside assessment shows.
Putting PaO₂ into context
Example
A patient with an acute respiratory illness is receiving supplemental oxygen.
During the shift their oxygen requirement increases, their respiratory rate rises and they are now using accessory muscles to breathe.
Their latest arterial blood gas shows that PaO₂ remains low despite the increased oxygen therapy.
They also appear more tired and are becoming mildly confused.
The concern is the combined pattern of impaired oxygenation, increasing oxygen requirement, respiratory distress and neurological deterioration.
As a student nurse, recognise the deterioration, assess within ABCDE and promptly communicate the blood-gas result, oxygen therapy and clinical changes to the registered and medical team.
PaO₂ interpretation errors to avoid
- Looking at PaO₂ without knowing how much oxygen the patient is receiving.
- Assuming PaO₂ and SpO₂ are the same measurement.
- Ignoring increasing oxygen requirements.
- Focusing on oxygenation while ignoring ventilation and PaCO₂.
- Ignoring respiratory effort because oxygen saturation looks acceptable.
- Missing confusion or drowsiness as signs of deterioration.
- Delaying escalation while trying to interpret every ABG value independently.
Report PaO₂ together with oxygen therapy
Example escalation
“I'm concerned about Mrs Roberts. Her oxygen requirement has increased during the shift and she is now tachypnoeic with increased work of breathing. Her latest arterial blood gas shows a low PaO₂ despite the increased oxygen, and she is becoming mildly confused.”
This communicates the oxygenation problem, oxygen requirement, respiratory deterioration and neurological change.
Recognise → assess → communicate → escalate
Notice impaired oxygenation
Identify a concerning PaO₂ and connect it with oxygen therapy and the saturation trend.
Look at breathing
Assess respiratory rate, work of breathing, oxygen saturation, consciousness and the wider ABCDE picture.
Report the full picture
Communicate the PaO₂ result together with oxygen delivery and clinical deterioration.
Build your respiratory blood-gas skills
Continue developing your ability to connect PaO₂, PaCO₂, oxygen saturation, oxygen therapy and respiratory deterioration through the NurseNet Clinical Confidence pathway.
Explore Clinical Confidence →