Arterial Blood Gas Interpretation
Ampath Chat No. 94-May 2025
Importance of ABG Analysis
Arterial blood gas (ABG) analysis is essential for:
- Evaluating oxygenation and ventilation status
- Monitoring response to treatments (e.g., oxygen therapy, mechanical ventilation, insulin in DKA)
- Assessing acid-base disturbances
- Detecting abnormal haemoglobins (e.g., carboxyhaemoglobin, methaemoglobin)
Modern ABG analysers also measure electrolytes, haemoglobin, glucose, lactate, bilirubin, ionized calcium, and magnesium.
Key Parameters
- pH: Indicates blood acidity or alkalinity
- pCO₂: Reflects respiratory component
- HCO₃⁻: Reflects metabolic component
- Base excess: Assesses metabolic contribution to pH
- pO₂: Indicates oxygenation ability of lungs
- SaO₂: Oxygen saturation of haemoglobin
- Acidaemia/Alkalaemia: pH <7.35 or >7.45
- Hypoxaemia: pO₂ <60 mmHg
- Acidosis/Alkalosis: Processes causing pH changes
- Buffer: Minimises pH changes
- Single vs Mixed Disorders: One or multiple processes affecting pH
Physiology of Acid-Base Balance
Metabolism produces volatile (CO₂) and non-volatile acids (lactate, ketones, etc.). Acid-base balance is maintained by:
- Respiratory elimination of CO₂
- Renal excretion of non-volatile acids
- Buffering by bicarbonate, proteins, phosphate, and haemoglobin
The bicarbonate/carbonic acid system is the most important buffer. At pH 7.4, the HCO₃⁻:H₂CO₃ ratio is 20:1.
Specimens for ABG Analysis
- Gold standard: Arterial blood
- Venous samples: Only suitable when oxygenation assessment is not required
- Collection tips:
- Use heparinised syringes
- Avoid air exposure
- Mix well to prevent clots
- Transport within 30 minutes or on ice if delayed
Interpreting ABG Results
- pH: Normal range 7.35–7.45
- pCO₂: Respiratory component
- ↑ pCO₂ = respiratory acidosis
- ↓ pCO₂ = respiratory alkalosis
- HCO₃⁻: Metabolic component
- ↓ HCO₃⁻ = metabolic acidosis
- ↑ HCO₃⁻ = metabolic alkalosis
Compensation
The body compensates to restore pH:
- Respiratory compensation: Rapid (within 24 hours)
- Metabolic compensation: Slower (2–5 days)
- Compensation may be complete (normal pH) or incomplete (abnormal pH)
Step-by-Step ABG Analysis
- Check pH: Determine acidosis or alkalosis
- Evaluate pCO₂ and HCO₃⁻: Identify primary disorder
- Respiratory disorders: pH and pCO₂ change in opposite directions
- Metabolic disorders: pH and HCO₃⁻ change in same direction
- Assess Compensation: Are changes in pCO₂ and HCO₃⁻ appropriate?
- Check Oxygenation: Evaluate pO₂ and SaO₂
- Use Anion Gap, Delta Ratio, Delta Gap: Further clarify metabolic acidosis and mixed disorders
Delta Ratio & Gap
Used in high anion gap metabolic acidosis (HAGMA):
- Delta Ratio = ΔAG / ΔHCO₃⁻
- Delta Gap = Difference between ΔAG and ΔHCO₃⁻
- Helps identify mixed acid-base disorders
Acid-Base Nomogram
A graphical tool to classify acid-base status using pH, HCO₃⁻, and pCO₂. If the patient’s values fall outside the shaded area, a mixed disorder is likely.
Summary
ABG analysis is vital for diagnosing acid-base disorders and assessing ventilation and oxygenation. Accurate specimen handling and a systematic approach make interpretation manageable.