Arterial Blood Gases (ABGs) give rapid information about:
- Respiratory status (oxygenation and ventilation)
- Acid-base balance (metabolic or respiratory causes)
They are critical for managing sick patients, especially in the ED, ICU, and respiratory wards.
Normal ABG Values (Adults)
| Variable | Normal Range |
| pH | 7.35 – 7.45 |
| PaO₂ | 10 – 14 kPa |
| PaCO₂ | 4.5 – 6.0 kPa |
| HCO₃⁻ | 22 – 26 mmol/L |
| Base Excess | -2 to +2 mmol/L |
Key Concepts You Must Know
To understand how to interpret an ABG you must understand:
- pH
- PaO₂
- PaCO₂
- Bicarbonate
- Base excess
pH
- Reflects acidity or alkalinity (log scale of [H⁺])
- pH < 7.35 = acidosis
- pH > 7.45 = alkalosis
Maintained by:
- Buffers (bicarbonate, proteins, phosphate, haemoglobin)
- Respiratory compensation (CO₂ excretion via lungs)
- Renal compensation (H⁺ excretion, HCO₃⁻ retention)
PaO₂ and PaCO₂
- PaO₂: Measures oxygen in blood (depends on inspired O₂ and gas exchange)
- PaCO₂: Measures carbon dioxide levels (respiratory function marker)
Remember:
- Hypoxaemia = low PaO₂
- Hypercapnia = high PaCO₂
Bicarbonate (HCO₃⁻)
- Major buffer in the blood
- Kidneys regulate HCO₃⁻ to help control pH
- Normal: 22–26 mmol/L
Base Excess (BE)
- Shows the amount of acid or base needed to normalise blood pH
- BE < -2 → metabolic acidosis
- BE > +2 → metabolic alkalosis
Compensation Mechanisms
Respiratory compensation:
- Fast (minutes to hours)
- Changes breathing to adjust CO₂
Metabolic compensation:
- Slower (days)
- Kidneys adjust H⁺ and HCO₃⁻ handling
Respiratory Failure Types
| Type | Definition | Typical ABG |
| Type I (oxygenation failure) | PaO₂ ↓, PaCO₂ normal or ↓ | Low PaO₂, Normal/low PaCO₂ |
| Type II (ventilation failure) | PaO₂ ↓, PaCO₂ ↑ | Low PaO₂, High PaCO₂, often acidotic pH |
Further breakdown of Type II:
- Acute: low pH, high PaCO₂, normal HCO₃⁻
- Chronic: normal pH, high PaCO₂, raised HCO₃⁻
- Acute-on-chronic: low pH, high PaCO₂, high HCO₃⁻
Step-by-Step ABG Interpretation
1. Look at pH
- Normal, acidotic, or alkalotic?
2. Assess PaCO₂
- High = respiratory acidosis
- Low = respiratory alkalosis
3. Assess HCO₃⁻
- Low = metabolic acidosis
- High = metabolic alkalosis
4. Check for Compensation
- Is PaCO₂ or HCO₃⁻ trying to correct the pH?
5. Look at PaO₂
- Is there hypoxaemia?
6. Base Excess
- Confirms metabolic contribution
7. Think Clinical Context
- ABG values mean little without clinical correlation!
Quick Patterns to Recognise
| Pattern | Interpretation |
| Low pH, High PaCO₂ | Respiratory acidosis |
| Low pH, Low HCO₃⁻ | Metabolic acidosis |
| High pH, Low PaCO₂ | Respiratory alkalosis |
| High pH, High HCO₃⁻ | Metabolic alkalosis |
Key Exam Tips
Always start with pH first → then PaCO₂ and HCO₃⁻.
Compensation is never perfect—full correction is rare.
Type I respiratory failure = PaO₂ down, PaCO₂ normal/low.
Type II respiratory failure = PaO₂ down, PaCO₂ up.
Chronic CO₂ retainers (like COPD) usually have high HCO₃⁻.
Clinical context matters — one ABG doesn’t tell you everything.
Header image used on licence from Shutterstock
Thank you to the joint editorial team of MRCEM Exam Prep for this ‘Exam Tips’ blog post.
