Respiratory Alkalosis for Student Nurses
Learn how to recognise respiratory alkalosis on an arterial blood gas and connect low PaCO₂ with increased ventilation and the patient's clinical condition.
What is respiratory alkalosis?
Respiratory alkalosis develops when ventilation removes carbon dioxide faster than it is being produced. PaCO₂ falls and this contributes to an increase in pH.
Look at pH
The pH is increased when respiratory alkalosis produces alkalaemia.
Look at PaCO₂
A low PaCO₂ provides a respiratory explanation for the alkaline direction of the pH.
Look at HCO₃⁻
Bicarbonate helps you assess whether metabolic compensation may be occurring.
↑ pH + ↓ PaCO₂ = think respiratory alkalosis
Why does PaCO₂ fall?
Carbon dioxide is normally removed by ventilation. If ventilation increases substantially, carbon dioxide may be removed faster than the body produces it.
Ventilation increases
The patient breathes more frequently, more deeply or both.
More CO₂ is removed
Increased effective ventilation causes arterial carbon dioxide to fall.
PaCO₂ decreases
The respiratory component of the blood gas moves in an alkaline direction.
pH rises
If compensation is insufficient, the arterial pH becomes alkalotic.
Think: why is the patient hyperventilating?
Respiratory alkalosis describes the physiological pattern. The important clinical question is what is driving the increased ventilation.
What may cause increased ventilation?
A low PaCO₂ should not automatically be attributed to anxiety. Increased ventilation can occur in a range of important clinical situations.
Response to low oxygen
Impaired oxygenation may stimulate an increase in respiratory rate and ventilation.
Systemic illness
Fever, infection and systemic deterioration may be associated with increased respiratory drive.
Physiological stress
Significant pain can increase respiratory rate and alter the breathing pattern.
Acute illness
Some acute cardiac and respiratory conditions may present with tachypnoea and low PaCO₂.
Altered respiratory drive
Certain neurological processes can affect the control of ventilation.
One possibility
Anxiety can cause hyperventilation, but potentially serious causes should be considered before assuming symptoms are anxiety-related.
What might you notice?
- An increased respiratory rate.
- Deep or rapid breathing.
- Breathlessness or reported difficulty breathing.
- Dizziness or light-headedness.
- Tingling sensations around the mouth or in the hands.
- Chest discomfort or palpitations.
- Visible anxiety or distress.
- Signs of infection, pain, hypoxaemia or another underlying illness.
- A clinical picture that is changing from the patient's baseline.
Interpret respiratory alkalosis systematically
| Question | Finding | Interpretation |
|---|---|---|
| What is the pH? | Raised | The blood is moving in an alkaline direction. |
| What is the PaCO₂? | Reduced | The respiratory component explains the alkaline direction. |
| What is the HCO₃⁻? | Review the value | It may provide information about metabolic compensation. |
| What is the PaO₂? | Assess oxygenation | Consider whether hypoxaemia may be driving increased ventilation. |
| How is the patient? | ABCDE assessment | Identify the clinical reason for the abnormal breathing pattern. |
Use the direction test
If pH is moving up while PaCO₂ is moving down, the carbon dioxide change is pushing the pH in the alkaline direction. This supports a respiratory alkalosis pattern.
Low PaCO₂ does not necessarily mean the lungs are healthy
A patient can have significant respiratory disease while their PaCO₂ is low. Increased ventilation may be a response to hypoxaemia, inflammation, pain or physiological stress.
Ventilation
Low PaCO₂ tells you that carbon dioxide removal is increased relative to production.
Oxygenation
PaO₂ answers a different question. Oxygenation may still be significantly impaired.
Clinical condition
Respiratory rate, effort, saturation, consciousness and overall deterioration remain essential.
Return from the ABG to the patient
Airway
Confirm airway patency and identify any immediate airway threat.
Breathing
Assess respiratory rate, depth, work of breathing, oxygen saturation and prescribed oxygen therapy.
Circulation
Review pulse, blood pressure and perfusion for evidence of wider physiological stress.
Disability
Assess consciousness and recognise confusion, agitation or other neurological changes.
Exposure
Consider temperature, infection, pain and other clues to the cause of increased ventilation.
Escalate
Escalate significant abnormalities and clinical deterioration according to local procedures.
Follow the respiratory pattern over time
Baseline observations
The patient's respiratory rate and oxygen requirement are stable.
Respiratory rate rises
The patient becomes increasingly tachypnoeic and appears more distressed.
PaCO₂ falls
The blood gas develops an alkaline pH with a reduced PaCO₂.
Putting respiratory alkalosis into context
Example
A patient admitted with an acute illness becomes increasingly tachypnoeic during the shift.
Their respiratory rate has risen significantly and they appear more distressed than earlier. Their oxygen requirement has also changed.
An arterial blood gas shows an increased pH and reduced PaCO₂.
The important pattern is increasing respiratory rate + changing clinical condition + alkaline pH + reduced PaCO₂.
Rather than assuming the abnormal breathing is simply anxiety, assess the patient systematically and communicate the deterioration and ABG findings to the registered and medical team.
Respiratory alkalosis errors to avoid
- Seeing a low PaCO₂ without checking the pH.
- Assuming rapid breathing is caused by anxiety.
- Ignoring oxygenation because PaCO₂ is low.
- Missing infection, pain or other drivers of increased ventilation.
- Ignoring bicarbonate and possible compensation.
- Looking at the ABG without reviewing respiratory observations.
- Failing to compare the result with previous blood gases.
- Delaying escalation while trying to identify the exact cause independently.
Report the ABG and the clinical change together
Example escalation
“I'm concerned about Mr Patel. His respiratory rate has increased significantly and he is more breathless than earlier. His latest ABG shows an alkaline pH with a low PaCO₂, and his oxygen requirement has also changed.”
This communicates the respiratory alkalosis pattern and the patient's changing respiratory condition.
Recognise → assess → communicate → escalate
Spot the pattern
Connect an alkaline pH with a reduced PaCO₂ and recognise increased ventilation.
Find the clinical reason
Assess respiratory rate, oxygenation, work of breathing, pain, temperature and the wider ABCDE picture.
Report the whole pattern
Communicate the ABG result alongside observations, symptoms, oxygen therapy and the clinical trend.
Next pattern: metabolic acidosis
Continue your ABG pathway by learning how a low bicarbonate can contribute to an acidic pH and how to connect the result with the patient's wider clinical condition.
Explore Clinical Confidence →