INTRODUCTION

Arterial blood gas (ABG) analysis is a rapid, point-of-care test essential for assessing oxygenation, ventilation, and acid–base balance. As an allied healthcare professional, your role includes knowing when to sample, interpreting results accurately, and making confident, timely treatment decisions.

ABGs are vital in:
– Respiratory distress
– Sepsis and shock
– Diabetic emergencies
– Cardiac arrest
– Trauma

A structured, repeatable approach helps you work safely under pressure.

TYPES OF BLOOD GASES

– Arterial Blood Gas (ABG): Gold standard for oxygenation and acid–base status
– Venous Blood Gas (VBG): Acceptable for acid–base screening but unreliable for PaO₂

Use VBG for trend monitoring, ABG when oxygenation is critical.

NORMAL RANGES (ADULT)

– pH: 7.35–7.45
– PaCO₂: 4.7–6.0 kPa (35–45 mmHg)
– HCO₃⁻: 22–26 mmol/L
– PaO₂: 10–13.3 kPa (75–100 mmHg)
– Base excess: ±2 mmol/L
– SaO₂: >94% (on air)

SIX-STEP INTERPRETATION FRAMEWORK

Check the pH

– <7.35: acidotic
– >7.45: alkalotic
– 7.35–7.45: may still be compensated

Assess PaCO₂ (respiratory component)

– Raised → respiratory acidosis
– Low → respiratory alkalosis

Assess HCO₃⁻ (metabolic component)

– Raised → metabolic alkalosis
– Low → metabolic acidosis

Determine the primary problem

– Match pH direction to PaCO₂ or HCO₃⁻

Check for compensation

– Opposite system abnormal → compensation
– Normal pH with abnormal parameters → fully compensated
– Abnormal pH with partial change → partial compensation

Review oxygenation

– PaO₂ and SaO₂
– Low oxygen → act promptly

COMMON PATTERNS

– Respiratory acidosis: pH low, PaCO₂ high, HCO₃⁻ normal/raised (e.g., COPD)
– Respiratory alkalosis: pH high, PaCO₂ low, HCO₃⁻ normal/lowered (e.g., anxiety, PE)
– Metabolic acidosis: pH low, HCO₃⁻ low, PaCO₂ normal/low (e.g., DKA, sepsis)
– Metabolic alkalosis: pH high, HCO₃⁻ high, PaCO₂ normal/raised (e.g., vomiting)

WHEN TO ESCALATE — RED FLAGS

– pH <7.2 or >7.55
– Severe hypoxia (PaO₂ critically low)
– Lactate >4 mmol/L with shock
– Broad-complex tachycardia
– New LBBB with chest pain

Escalate promptly to senior support.

CASE STUDIES

Case 1: COPD Exacerbation

– pH 7.28, PaCO₂ 8.0, HCO₃⁻ 28, PaO₂ 7.8
→ Respiratory acidosis with partial metabolic compensation plus hypoxia

Case 2: Sepsis with lactic acidosis

– pH 7.21, PaCO₂ 4.9, HCO₃⁻ 15, Lactate 6.2
→ Metabolic acidosis, no respiratory compensation yet

Case 3: Anxiety hyperventilation

– pH 7.52, PaCO₂ 3.8, HCO₃⁻ 24
→ Respiratory alkalosis

Case 4: Diabetic ketoacidosis

– pH 7.12, PaCO₂ 3.5, HCO₃⁻ 10, Glucose 28
→ Metabolic acidosis with partial respiratory compensation

Case 5: Vomiting

– pH 7.48, PaCO₂ 6.1, HCO₃⁻ 30
→ Metabolic alkalosis with partial respiratory compensation

OSCE STATION FRAMEWORK

1. Introduce yourself and explain ABG purpose
2. Discuss or simulate sampling technique
3. Check for correct oxygen delivery and FiO₂
4. Interpret pH, PaCO₂, HCO₃⁻ systematically
5. Review oxygenation and lactate
6. Explain next steps (e.g., treatment, escalation)
7. Dispose of sharps and wash hands

SUMMARY

– Use a six-step interpretation method
– Always relate to the clinical presentation
– VBG is acceptable for acid–base; ABG is essential for oxygenation
– Know red flags and when to escalate
– Confidence grows with practice

Blood gases are more than a printout. They tell the patient’s story. Interpret them with structure, keep the patient at the centre, and act on what matters.

Keep learning, keep leading. You’ve got this!