Mixed Acid-Base Disorders for Student Nurses
Learn how to recognise when an arterial blood gas may contain more than one acid-base disturbance and why the patient's clinical condition remains central to interpretation.
What is a mixed acid-base disorder?
A mixed acid-base disorder occurs when more than one primary acid-base process is affecting the patient simultaneously.
One primary process
The other acid-base system may change appropriately as part of compensation.
More than one process
Respiratory and metabolic abnormalities may both be independently affecting the pH.
The pattern does not fit
Unexpected combinations of pH, PaCO₂ and bicarbonate should prompt further assessment.
the ABG values do not behave as expected for a straightforward primary disorder with appropriate compensation.
Know the four core patterns first
Mixed disorders become much easier to recognise once you are confident with the four basic acid-base disturbances.
| Primary disorder | pH direction | Primary abnormality |
|---|---|---|
| Respiratory acidosis | Acidic | Raised PaCO₂ |
| Respiratory alkalosis | Alkaline | Reduced PaCO₂ |
| Metabolic acidosis | Acidic | Reduced HCO₃⁻ |
| Metabolic alkalosis | Alkaline | Raised HCO₃⁻ |
Then add compensation
Once you know the primary disorder, ask whether the second acid-base component is changing in a direction that would appropriately reduce the pH disturbance.
When compensation does not make sense
Compensation should generally move pH towards the normal range rather than making the original disturbance worse.
Metabolic acidosis
A fall in bicarbonate may be accompanied by a fall in PaCO₂ as ventilation increases.
PaCO₂ also rises
A raised PaCO₂ would push pH further towards acidity rather than compensate for the metabolic acidosis.
Two acidifying processes?
The patient may have both a metabolic acidosis and a respiratory acidosis.
Respiratory acidosis + metabolic acidosis
In this pattern, both the respiratory and metabolic components are pushing the pH towards acidity.
PaCO₂ rises
Carbon dioxide retention contributes to respiratory acidosis.
HCO₃⁻ falls
Reduced bicarbonate contributes independently to metabolic acidosis.
Both push pH down
The combination can produce significant acidaemia and may occur in a seriously unwell patient.
↓ pH + ↑ PaCO₂ + ↓ HCO₃⁻
Respiratory alkalosis + metabolic alkalosis
Here, both the respiratory and metabolic components are pushing the pH towards alkalinity.
PaCO₂ falls
Increased ventilation contributes to respiratory alkalosis.
HCO₃⁻ rises
Increased bicarbonate contributes independently to metabolic alkalosis.
Both push pH up
The resulting alkalosis may be greater than would be expected from either disturbance alone.
↑ pH + ↓ PaCO₂ + ↑ HCO₃⁻
What if one process causes acidosis and another alkalosis?
Mixed disorders can also oppose each other. This can make the pH appear deceptively close to the reference range.
Metabolic acidosis
One disease process may reduce bicarbonate and push pH towards acidity.
Metabolic alkalosis
Another process may raise bicarbonate or otherwise push acid-base balance towards alkalinity.
pH can look less abnormal
Opposing processes may partially offset each other, making the overall pH less dramatic than the patient's illness.
Six steps when the ABG looks complicated
Check pH
Identify the overall acidic or alkaline direction.
Check PaCO₂
Decide whether the respiratory component is acidifying or alkalinising.
Check HCO₃⁻
Decide whether the metabolic component is acidifying or alkalinising.
Ask what is primary
Determine which value best explains the pH direction.
Ask whether compensation fits
Does the other value oppose the primary disturbance as expected, or is it worsening it?
Return to the patient
Use history, ABCDE, observations and investigations to determine what clinical processes could explain the pattern.
Other information can help explain the ABG
Think perfusion and illness severity
Lactate may provide important information when metabolic acidosis is present.
Look beyond the blood gas
Sodium, potassium, chloride and bicarbonate may help explain the metabolic component.
Consider metabolic causes
These may be particularly relevant when diabetic ketoacidosis is clinically suspected.
Review kidney function
Renal impairment can contribute to significant acid-base disturbance.
Check losses and output
Vomiting, diarrhoea, gastric drainage and urine output can provide important clues.
Look for trends
Comparing earlier ABGs and blood results can help distinguish new deterioration from longer-standing abnormalities.
Do not let a complex ABG distract from deterioration
Airway
Confirm airway patency and address immediate concerns.
Breathing
Assess respiratory rate, depth, work of breathing, SpO₂ and prescribed oxygen therapy.
Circulation
Assess pulse, blood pressure, perfusion, fluid balance and urine output.
Disability
Review consciousness, confusion and blood glucose where appropriate.
Exposure
Look for infection, fluid losses, medications and other clues to the underlying processes.
Escalate
A clinically deteriorating patient with a complex ABG needs prompt senior assessment.
Putting a mixed disorder into context
Example
A patient with severe systemic illness becomes increasingly drowsy and hypotensive.
Their ABG shows a reduced pH, raised PaCO₂ and reduced bicarbonate.
Raised PaCO₂ is pushing the pH towards respiratory acidosis, while reduced bicarbonate is independently pushing the pH towards metabolic acidosis.
Rather than representing compensation, both abnormalities are contributing to the acidaemia.
This may indicate a mixed acid-base disturbance in a seriously unwell patient and requires prompt clinical assessment and escalation.
Mixed disorder errors to avoid
- Assuming every ABG contains only one primary disorder.
- Calling any two abnormal values compensation without checking their direction.
- Using pH alone to judge the severity of the acid-base disturbance.
- Ignoring a normal-looking pH when PaCO₂ and bicarbonate are abnormal.
- Forgetting to review lactate, electrolytes, glucose and renal function.
- Ignoring previous ABGs and clinical trends.
- Trying to solve a complex ABG before assessing the patient.
- Delaying escalation because you cannot completely classify the disorder.
Communicate what you know clearly
Example escalation
“I'm concerned about Mr Williams. He is increasingly drowsy and hypotensive. His ABG shows significant acidaemia with both a raised PaCO₂ and reduced bicarbonate. The values do not appear to fit a straightforward compensation pattern.”
You do not need to provide a complete specialist acid-base diagnosis before escalating. Communicate the abnormal pattern and the patient's deterioration.
Recognise → assess → communicate → escalate
Spot the mismatch
Recognise when PaCO₂ and bicarbonate do not fit a straightforward single-disorder compensation pattern.
Look for multiple causes
Use ABCDE, history, observations and investigations to understand what may be happening clinically.
Do not wait for certainty
Report the abnormal ABG pattern and patient deterioration promptly to the appropriate clinical team.
Next: ABG practice examples for student nurses
You now have the core framework: pH, PaCO₂, bicarbonate, the four primary disorders, compensation and mixed patterns. The next step is applying that knowledge to worked clinical examples.
Explore Clinical Confidence →