Metabolic Alkalosis for Student Nurses
Learn how to recognise metabolic alkalosis on an arterial blood gas and connect a raised bicarbonate with pH, respiratory compensation and the patient's clinical condition.
What is metabolic alkalosis?
Metabolic alkalosis is an acid-base disturbance in which an increase in bicarbonate contributes to a rise in arterial pH. It can develop when hydrogen ions are lost, bicarbonate increases or other metabolic processes disturb normal acid-base balance.
Look at pH
A raised pH shows that the blood is moving towards alkalinity.
Look at HCO₃⁻
A raised bicarbonate provides a metabolic explanation for the alkaline direction of the pH.
Look at PaCO₂
PaCO₂ may rise as ventilation changes in an attempt to reduce the alkalosis.
↑ pH + ↑ HCO₃⁻ = think metabolic alkalosis
How can metabolic alkalosis develop?
Several different processes can increase bicarbonate or reduce the body's acid load. The ABG pattern tells you what is happening to acid-base balance, but not the underlying diagnosis.
Hydrogen ions are lost
Loss of acid from the body can shift acid-base balance towards alkalinity.
Bicarbonate becomes relatively raised
The metabolic component of the ABG shifts in an alkaline direction.
pH rises
If the disturbance is sufficient, arterial pH moves above the normal laboratory reference range.
Respiratory compensation may occur
Ventilation may decrease to retain more carbon dioxide, although compensation is limited by the need to maintain oxygenation.
What may contribute to metabolic alkalosis?
Persistent vomiting
Significant loss of gastric acid through prolonged vomiting may contribute to metabolic alkalosis.
Nasogastric losses
Prolonged gastric aspiration or drainage can result in substantial acid and fluid losses.
Diuretic therapy
Some diuretics can contribute to fluid and electrolyte changes that promote metabolic alkalosis.
Fluid depletion
Significant extracellular fluid depletion can contribute to the persistence of an alkalotic state.
Potassium abnormalities
Metabolic alkalosis may occur alongside important electrolyte disturbances requiring clinical assessment.
Look for the cause
The acid-base pattern should prompt assessment of fluid balance, medications, gastrointestinal losses and relevant laboratory results.
What might you notice in the patient?
- A history of repeated or prolonged vomiting.
- Significant nasogastric losses.
- Reduced oral fluid intake.
- Clinical signs of dehydration.
- Postural dizziness or weakness.
- Changes in pulse or blood pressure.
- Reduced urine output where fluid depletion is significant.
- Muscle weakness, cramps or other symptoms associated with electrolyte disturbance.
- A relevant medication history, particularly diuretic use.
Interpret metabolic alkalosis systematically
| Question | Finding | Interpretation |
|---|---|---|
| What is the pH? | Raised | The blood is moving in an alkaline direction. |
| What is the HCO₃⁻? | Raised | The metabolic component explains the alkaline direction. |
| What is the PaCO₂? | May be raised | A rise may reflect respiratory compensation. |
| What are the electrolytes? | Review results | Potassium, chloride and other abnormalities may provide important context. |
| What is the fluid balance? | Assess losses and intake | Vomiting, gastric drainage or diuresis may be clinically important. |
| How is the patient? | ABCDE assessment | Determine the significance and urgency from the whole clinical picture. |
Use the direction test
If pH is moving up and bicarbonate is also moving up, the bicarbonate change is consistent with a metabolic process driving the alkalosis.
Why might PaCO₂ rise?
The respiratory system may reduce ventilation slightly in response to a metabolic alkalosis. Retaining more carbon dioxide pushes acid-base balance back towards acidity.
Bicarbonate rises
The metabolic disturbance contributes to the alkaline pH.
Ventilation may reduce
The body may retain additional carbon dioxide as part of compensation.
PaCO₂ may rise
Increased PaCO₂ can move pH towards the normal range without correcting the underlying metabolic problem.
Why are fluid balance and electrolytes important?
Check the balance chart
Look for vomiting, gastric drainage, high urine output or other significant fluid losses.
Review laboratory results
Potassium abnormalities may coexist with metabolic alkalosis and may have important clinical consequences.
Part of the metabolic picture
Chloride abnormalities may help the clinical team understand the underlying physiological process.
ABG + U&Es + fluid balance
For a patient with metabolic alkalosis, the arterial blood gas often makes more sense when reviewed alongside electrolytes, renal function, fluid intake and fluid losses.
Return from the ABG to the patient
Airway
Confirm airway patency and identify any immediate concerns.
Breathing
Assess respiratory rate, depth, oxygen saturation and any change in breathing pattern.
Circulation
Assess pulse, blood pressure, perfusion and signs of volume depletion.
Disability
Review consciousness, weakness and neurological symptoms where electrolyte disturbance may be relevant.
Exposure
Consider vomiting, gastrointestinal losses, medications and other relevant clinical findings.
Escalate
Significant alkalosis, electrolyte disturbance or associated clinical deterioration requires appropriate senior review.
Putting metabolic alkalosis into context
Example
A patient has experienced repeated vomiting over the previous 24 hours and has struggled to maintain oral fluid intake.
They now appear clinically dehydrated and complain of weakness and dizziness when sitting upright.
Their arterial blood gas shows a raised pH and raised bicarbonate.
The important pattern is alkaline pH + raised bicarbonate + significant gastric fluid loss + dehydration + weakness.
As a student nurse, recognise the metabolic alkalosis pattern, assess the patient within ABCDE, review relevant fluid balance and promptly communicate the clinical findings and ABG result to the registered and medical team.
Metabolic alkalosis errors to avoid
- Looking at bicarbonate without checking the pH.
- Assuming raised PaCO₂ automatically means respiratory acidosis. Consider compensation.
- Ignoring vomiting or gastric losses.
- Failing to assess hydration and fluid balance.
- Ignoring electrolyte results.
- Missing relevant medication history.
- Treating the ABG pattern as the final diagnosis.
- Delaying escalation when the patient is clinically deteriorating.
Report the blood gas in clinical context
Example escalation
“I'm concerned about Mrs Khan. She has had repeated vomiting and now appears dehydrated and weak. Her latest ABG shows an alkaline pH with a raised bicarbonate, consistent with a metabolic alkalosis pattern.”
This communicates the acid-base disturbance together with the likely relevant clinical history and current patient condition.
Recognise → assess → communicate → escalate
Spot the pattern
Connect an alkaline pH with a raised bicarbonate and consider a metabolic process.
Find the clinical cause
Review vomiting, gastric losses, medications, fluid balance, electrolytes and the wider ABCDE assessment.
Report the whole picture
Communicate the ABG together with symptoms, observations, fluid losses, medication history and relevant investigations.
Next: understanding compensation on an ABG
Now that you can recognise the four core acid-base patterns, the next step is understanding how respiratory and metabolic compensation can change the appearance of an arterial blood gas.
Explore Clinical Confidence →