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Clinical Confidence • Student Nurse Guide

ABG Practice Examples for Student Nurses

Apply your arterial blood gas knowledge to worked examples covering acidosis, alkalosis, compensation and mixed acid-base disorders.

Use the same routine every time: patient → pH → PaCO₂ → HCO₃⁻ → PaO₂ → clinical context.
Before you begin

Your ABG interpretation routine

The purpose of practice is not to memorise isolated patterns. It is to build a repeatable method that helps you connect results with the patient in front of you.

1

Check pH

Is the overall direction acidic, alkaline or within range?

2

Check PaCO₂

Does the respiratory component explain the pH direction?

3

Check bicarbonate

Does the metabolic component explain the pH direction?

4

Consider compensation

Is the other system moving in a direction that reduces the disturbance?

5

Review oxygenation

Connect PaO₂ with SpO₂, oxygen therapy and respiratory assessment.

6

Return to the patient

Ask whether the result fits the history, ABCDE findings and clinical trend.

Example 1

Respiratory acidosis

ABG result

pH 7.28Reduced
PaCO₂ 7.2 kPaRaised
HCO₃⁻ 25 mmol/LNear reference range
PaO₂ 8.4 kPaInterpret with oxygen therapy

A patient with an acute respiratory illness is becoming increasingly drowsy and appears to be tiring after a period of significant work of breathing.

Interpretation: the pH is acidic and PaCO₂ is raised. The carbon dioxide change explains the pH direction, giving a respiratory acidosis pattern.
Example 2

Respiratory alkalosis

ABG result

pH 7.50Raised
PaCO₂ 3.4 kPaReduced
HCO₃⁻ 23 mmol/LNear reference range
PaO₂ 9.1 kPaAssess oxygenation

A patient with an acute illness has become increasingly tachypnoeic. They remain alert but appear breathless and unwell.

Interpretation: the pH is alkaline and PaCO₂ is reduced. The respiratory change explains the alkaline direction, giving a respiratory alkalosis pattern.
Clinical reminder: do not automatically label rapid breathing as anxiety. Tachypnoea may reflect significant respiratory, metabolic or systemic illness.
Example 3

Metabolic acidosis

ABG result

pH 7.24Reduced
PaCO₂ 3.7 kPaReduced
HCO₃⁻ 15 mmol/LReduced
PaO₂ 10.4 kPaReview clinically

The patient has been vomiting, appears dehydrated and is breathing more rapidly and deeply than earlier in the shift.

Interpretation: the pH is acidic and bicarbonate is reduced, producing a metabolic acidosis pattern. The reduced PaCO₂ may represent respiratory compensation through increased ventilation.
Example 4

Metabolic alkalosis

ABG result

pH 7.49Raised
PaCO₂ 5.8 kPaRaised
HCO₃⁻ 32 mmol/LRaised
PaO₂ 11.0 kPaReview clinically

A patient has experienced persistent vomiting and reduced oral intake. They appear dehydrated and report weakness.

Interpretation: the pH is alkaline and bicarbonate is raised, producing a metabolic alkalosis pattern. The raised PaCO₂ may represent respiratory compensation.
Example 5

Compensated respiratory acidosis

ABG result

pH 7.36Close to reference range
PaCO₂ 7.0 kPaRaised
HCO₃⁻ 30 mmol/LRaised
PaO₂ 9.0 kPaInterpret with oxygen therapy

A patient with a long-standing respiratory condition has a raised PaCO₂. Their bicarbonate is also raised and pH is close to the laboratory reference range.

Interpretation: raised PaCO₂ suggests a respiratory acid-base disturbance while raised bicarbonate may indicate metabolic compensation. The near-normal pH does not mean the ABG is normal.

Do not stop at pH

Compensation can move pH towards the reference range while significant PaCO₂ and bicarbonate abnormalities remain.

Example 6

Mixed respiratory and metabolic acidosis

ABG result

pH 7.18Reduced
PaCO₂ 6.9 kPaRaised
HCO₃⁻ 16 mmol/LReduced
PaO₂ 8.2 kPaAssess urgently

A severely unwell patient is hypotensive, confused and breathing ineffectively.

Interpretation: the raised PaCO₂ pushes pH towards respiratory acidosis while the reduced bicarbonate independently pushes pH towards metabolic acidosis. Both abnormalities contribute to the acidaemia, suggesting a possible mixed disorder.
This is a high-concern pattern. A seriously unwell patient with significant acidaemia and abnormalities in both respiratory and metabolic components requires prompt clinical assessment and escalation.
Example 7

Do not be fooled by a near-normal pH

ABG result

pH 7.40Within reference range
PaCO₂ 3.2 kPaReduced
HCO₃⁻ 15 mmol/LReduced
PaO₂ 10.1 kPaReview clinically

The patient is acutely unwell and tachypnoeic. Although the pH appears normal, both PaCO₂ and bicarbonate are clearly abnormal.

Interpretation: do not label this ABG as normal. Opposing acid-base processes or compensation may be present. The complete clinical picture and formal interpretation are required.
Practice questions

Try these before reading the answers

Practice A

pH ↓ • PaCO₂ ↑ • HCO₃⁻ normal

Which primary acid-base disturbance best fits this pattern?

Practice B

pH ↑ • PaCO₂ ↓ • HCO₃⁻ normal

Which primary acid-base disturbance best fits this pattern?

Practice C

pH ↓ • HCO₃⁻ ↓ • PaCO₂ ↓

Which value is likely primary, and what might the PaCO₂ change represent?

Practice D

pH ↑ • HCO₃⁻ ↑ • PaCO₂ ↑

Which acid-base disturbance and compensatory response might fit?

Practice E

pH ↓ • PaCO₂ ↑ • HCO₃⁻ ↓

Does this look like simple compensation or a possible mixed disorder?

Practice F

pH normal • PaCO₂ abnormal • HCO₃⁻ abnormal

Why should you avoid calling this a normal ABG?

Answers

Check your reasoning

Question Likely interpretation Reason
A Respiratory acidosis Low pH with raised PaCO₂.
B Respiratory alkalosis High pH with reduced PaCO₂.
C Metabolic acidosis with possible respiratory compensation Low bicarbonate explains the acidic pH; reduced PaCO₂ may help oppose it.
D Metabolic alkalosis with possible respiratory compensation Raised bicarbonate explains the alkaline pH; raised PaCO₂ may oppose it.
E Possible mixed respiratory and metabolic acidosis Raised PaCO₂ and low bicarbonate both push pH towards acidity.
F Possible compensated or mixed disorder A normal-looking pH does not erase abnormal PaCO₂ and bicarbonate values.
Clinical Confidence Routine

Interpret the numbers — then return to the patient

Recognise

Identify the ABG pattern

Work through pH, PaCO₂ and bicarbonate systematically.

Assess

Check whether it fits

Relate the pattern to ABCDE, symptoms, observations, oxygen therapy and investigations.

Communicate & escalate

Report the whole picture

Communicate significant ABG abnormalities together with the patient's current condition and clinical trend.

Educational resource: these examples are designed to support student learning. Actual ABGs must be interpreted using the relevant laboratory reference ranges, full clinical context, local guidance, appropriate supervision and professional judgement.
Continue Clinical Confidence

Next: common ABG interpretation mistakes

The next guide will consolidate this cluster by showing the common mistakes students make when interpreting arterial blood gases and how to avoid them.

Explore Clinical Confidence →