Metabolic Acidosis for Student Nurses
Learn how to recognise metabolic acidosis on an arterial blood gas and connect a low bicarbonate with pH, respiratory compensation and the patient's wider clinical condition.
What is metabolic acidosis?
Metabolic acidosis is an acid-base disturbance in which reduced bicarbonate contributes to a fall in arterial pH. It can occur when acids accumulate, bicarbonate is lost or the body is unable to maintain normal acid-base balance.
Look at pH
A reduced pH shows that the blood is moving towards acidity.
Look at HCO₃⁻
A reduced bicarbonate provides a metabolic explanation for the acidic direction of the pH.
Look at PaCO₂
PaCO₂ may fall if the respiratory system is attempting to compensate by increasing ventilation.
↓ pH + ↓ HCO₃⁻ = think metabolic acidosis
How does metabolic acidosis develop?
Metabolic acidosis can develop through several different physiological pathways. Recognising the ABG pattern is only the first step.
Acid accumulates
Increased acid production or reduced acid removal can disturb acid-base balance.
Bicarbonate is consumed or lost
Bicarbonate may fall as it buffers excess acid or because it is lost from the body.
pH falls
When metabolic compensation is insufficient, arterial pH moves towards acidity.
Breathing may increase
The respiratory system may compensate by increasing ventilation and reducing PaCO₂.
What may cause metabolic acidosis?
The ABG identifies a metabolic acid-base disturbance. The clinical team then needs to establish the underlying cause.
Lactate accumulation
Significant circulatory compromise can impair tissue perfusion and contribute to lactic acidosis.
Ketone production
Diabetic ketoacidosis can cause accumulation of ketone acids and a significant metabolic acidosis.
Reduced acid excretion
Significant renal dysfunction can impair acid removal and bicarbonate regulation.
Bicarbonate loss
Severe gastrointestinal losses can contribute to metabolic acidosis in some clinical situations.
Some ingestions
Certain toxicological conditions can produce significant acid-base disturbances.
Find the underlying process
Metabolic acidosis is not the final diagnosis. Ask what is causing the acid-base disturbance.
What might you notice in the patient?
- An increased respiratory rate or deeper breathing pattern.
- Signs of dehydration or significant fluid loss.
- Tachycardia.
- Hypotension or evidence of poor perfusion.
- Reduced urine output.
- Vomiting or abdominal symptoms.
- Evidence of infection or sepsis.
- Hyperglycaemia or ketones where clinically relevant.
- Increasing weakness, confusion or reduced consciousness.
Interpret metabolic acidosis systematically
| Question | Finding | Interpretation |
|---|---|---|
| What is the pH? | Reduced | The blood is moving in an acidic direction. |
| What is the HCO₃⁻? | Reduced | The metabolic component explains the acidic direction. |
| What is the PaCO₂? | May be reduced | Reduced PaCO₂ may represent respiratory compensation. |
| What is the lactate? | Review if available | An elevated lactate may help identify an important underlying process. |
| What is the anion gap? | Consider where appropriate | It can help clinicians investigate the cause of metabolic acidosis. |
| How is the patient? | ABCDE assessment | The clinical picture determines urgency and significance. |
Use the direction test
If pH is moving down and bicarbonate is also moving down, the bicarbonate change is consistent with a metabolic process driving the acidosis.
Why might PaCO₂ be low?
The lungs can respond to metabolic acidosis by increasing ventilation. Removing more carbon dioxide helps reduce the acidifying effect of CO₂.
Bicarbonate falls
The metabolic disturbance contributes to the reduction in pH.
Ventilation increases
The patient may increase respiratory rate or depth in an attempt to compensate.
PaCO₂ may fall
Reduced PaCO₂ can move the pH back towards the reference range, although the underlying metabolic problem remains.
How does the anion gap fit into metabolic acidosis?
Once metabolic acidosis has been identified, clinicians may use the anion gap alongside other investigations to help understand the underlying cause.
Additional acids may be present
Some metabolic acidoses are associated with accumulation of unmeasured acids, such as lactate or ketones.
Different mechanism
Other metabolic acidoses may occur without a raised anion gap, including some forms of bicarbonate loss.
One part of investigation
The anion gap should be interpreted alongside history, observations, laboratory results and clinical assessment.
Build on what you already know
First recognise metabolic acidosis from pH and bicarbonate. Then use the anion gap and other investigations to help understand why the disturbance is present.
Return from the ABG to the patient
Airway
Confirm airway patency and identify immediate airway concerns.
Breathing
Assess respiratory rate, depth, effort and oxygenation. Increased ventilation may represent compensation.
Circulation
Assess pulse, blood pressure, perfusion and fluid status. Look carefully for evidence of circulatory compromise.
Disability
Review consciousness, blood glucose where appropriate and any neurological change.
Exposure
Consider temperature, infection, fluid losses and other signs of underlying illness.
Escalate
Significant metabolic acidosis or associated clinical deterioration requires prompt senior clinical assessment.
Putting metabolic acidosis into context
Example
A patient is becoming increasingly unwell with vomiting, dehydration and worsening weakness.
Their respiratory rate has increased and their breathing appears deeper than earlier in the shift.
An arterial blood gas shows a reduced pH and reduced bicarbonate. PaCO₂ is also reduced.
The important pattern is acidic pH + low bicarbonate + increased ventilation + reduced PaCO₂ + a clinically deteriorating patient.
As a student nurse, recognise the metabolic acidosis pattern, assess the patient systematically and promptly communicate the ABG findings and clinical deterioration to the registered and medical team.
Metabolic acidosis errors to avoid
- Looking at bicarbonate without first checking pH.
- Assuming low PaCO₂ means respiratory alkalosis without considering compensation.
- Ignoring perfusion and lactate.
- Missing glucose and ketones when clinically relevant.
- Ignoring renal function and fluid balance.
- Assuming tachypnoea is simply anxiety.
- Focusing on the ABG while missing worsening shock or sepsis.
- Trying to determine the entire diagnosis before escalating a deteriorating patient.
Communicate the ABG and the patient together
Example escalation
“I'm concerned about Mr Patel. He is increasingly unwell and tachypnoeic, with signs of dehydration. His ABG shows an acidic pH with a low bicarbonate, consistent with a metabolic acidosis pattern. His respiratory rate has also increased.”
This communicates the acid-base disturbance, physiological response and bedside deterioration, rather than reporting an isolated laboratory value.
Recognise → assess → communicate → escalate
Spot the pattern
Connect an acidic pH with a reduced bicarbonate and consider a metabolic cause.
Find the clinical problem
Review perfusion, respiration, hydration, glucose, renal function and the wider ABCDE picture.
Report the pattern
Communicate the ABG alongside observations, symptoms, trends and relevant investigations.
Next pattern: metabolic alkalosis
Continue developing your ABG interpretation skills by learning how an alkaline pH and raised bicarbonate form the core metabolic alkalosis pattern.
Explore Clinical Confidence →