Blood Gas Interpretation
Arterial blood gas analysis provides critical information on oxygenation, ventilation, acid-base status, and metabolic parameters, essential for managing acutely unwell patients in anaesthesia and intensive care.
Key Facts
Normal ABG values: pH 7.35-7.45, PaCO2 4.7-6.0 kPa, PaO2 >10 kPa, HCO3⁻ 22-26 mmol/L, BE ±2, lactate <2 mmol/L A 5-step systematic approach ensures no abnormality is missed: oxygenation → pH → primary disorder → compensation → anion gap Anion gap = Na⁺ − (Cl⁻ + HCO3⁻); normal 10-18; raised in MUDPILES causes of metabolic acidosis Type 1 respiratory failure: PaO2 <8 kPa with normal/low PaCO2 (ventilation-perfusion mismatch, shunt) Type 2 respiratory failure: PaO2 <8 kPa with PaCO2 >6 kPa (hypoventilation) A-a gradient = PAO2 − PaO2; normal <2 kPa (increases with age); raised in VQ mismatch, shunt, diffusion impairment Venous blood gas (VBG) is adequate for screening pH (~0.03 lower than arterial) and HCO3⁻ but NOT for PaO2 assessment P/F ratio (PaO2/FiO2) classifies ARDS severity and guides management — normal >400; <300 defines ARDS
Overview
Key Facts
Blood gas analysis is the most frequently performed point-of-care test in acute and critical care medicine. It provides immediate information on respiratory function, acid-base balance, and metabolic status, enabling rapid clinical decision-making.
Epidemiology
ABG analysis is performed millions of times annually in UK hospitals. It is essential in emergency departments, theatres, ICU, and acute medical units. The ability to interpret blood gases accurately is a core competency for all acute care clinicians.
Aetiology
ABG abnormalities arise from primary respiratory or metabolic disturbances, with compensatory responses from the other system. Four primary disorders exist:
- Respiratory acidosis: High PaCO2 (hypoventilation)
- Respiratory alkalosis: Low PaCO2 (hyperventilation)
- Metabolic acidosis: Low HCO3⁻ (acid gain or base loss)
- Metabolic alkalosis: High HCO3⁻ (acid loss or base gain)
Pathophysiology
Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3⁻] / [0.03 × PaCO2])
Buffer systems: Bicarbonate (extracellular, most important), haemoglobin (intracellular), phosphate, proteins.
Compensation:
- Respiratory compensation for metabolic disorders: Minutes to hours (hyperventilation or hypoventilation)
- Renal compensation for respiratory disorders: Days (3-5 days for full renal compensation — HCO3⁻ retention or excretion)
- Compensation moves pH towards normal but NEVER fully corrects it (if pH is normal with opposing disturbances, consider a mixed disorder).
Clinical Presentation
Metabolic Acidosis (pH ↓, HCO3⁻ ↓)
- Raised anion gap (MUDPILES): Methanol, Uraemia, DKA, Propylene glycol, Isoniazid/Iron, Lactic acidosis, Ethylene glycol, Salicylates
- Normal anion gap: Diarrhoea, RTA, Addison's, pancreatic fistula, saline excess
Metabolic Alkalosis (pH ↑, HCO3⁻ ↑)
- Vomiting, NG suction, diuretics, Conn's syndrome, Cushing's
Respiratory Acidosis (pH ↓, PaCO2 ↑)
- COPD, neuromuscular disease, sedative overdose, chest wall deformity, obesity hypoventilation
Respiratory Alkalosis (pH ↑, PaCO2 ↓)
- Anxiety/hyperventilation, PE, salicylate poisoning (early), high altitude, pregnancy
Red Flags
- pH <7.1 or >7.6 — life-threatening
- Lactate >4 mmol/L — tissue hypoperfusion, sepsis
- PaO2 <8 kPa on room air — respiratory failure
- K⁺ >6.5 or <2.5 mmol/L on ABG — immediate treatment needed
Differential Diagnosis
| ABG Pattern | pH | PaCO2 | HCO3⁻ | Interpretation |
|---|---|---|---|---|
| ↓ pH, ↑ PaCO2, normal HCO3⁻ | <7.35 | >6 kPa | 22-26 | Acute respiratory acidosis |
| ↓ pH, ↑ PaCO2, ↑ HCO3⁻ | <7.35 | >6 kPa | >26 | Chronic respiratory acidosis (with renal compensation) |
| ↓ pH, normal PaCO2, ↓ HCO3⁻ | <7.35 | 4.7-6 | <22 | Metabolic acidosis (uncompensated) |
| ↓ pH, ↓ PaCO2, ↓ HCO3⁻ | <7.35 | <4.7 | <22 | Metabolic acidosis (with respiratory compensation) |
| ↑ pH, ↓ PaCO2, normal HCO3⁻ | >7.45 | <4.7 | 22-26 | Respiratory alkalosis |
| ↑ pH, normal PaCO2, ↑ HCO3⁻ | >7.45 | 4.7-6 | >26 | Metabolic alkalosis |
Diagnosis / Investigation
Bedside
- ABG sampling: Radial artery (most common), femoral artery, brachial artery
- VBG: Adequate for pH, HCO3⁻, K⁺, lactate screening (not PaO2)
- Co-oximetry: Measures MetHb, COHb (important in poisoning)
- Point-of-care analyser: Results within 1-2 minutes
Bloods
- U&Es: Calculate anion gap, assess renal function
- Lactate: Serial levels for monitoring response to resuscitation
- Glucose, ketones: If DKA suspected
- Toxicology: Salicylate, ethanol, methanol levels if poisoning suspected
Imaging
- CXR: If respiratory cause suspected
- CT: Guided by clinical context (CTPA for PE, CT head for neurological cause)
Special Tests
- Urine anion gap: Differentiates renal from GI cause of normal AG metabolic acidosis
- Osmolar gap: Measured − calculated osmolality >10 suggests toxic alcohol ingestion
- A-a gradient: Calculate to differentiate causes of hypoxaemia
Management
Non-pharmacological
- Treat the underlying cause: This is always the priority
- Supplemental oxygen: For hypoxaemia — titrate to SpO2 94-98% (88-92% in COPD)
- Ventilatory support: NIV or invasive ventilation for respiratory failure
Pharmacological
- DKA: IV insulin 0.1 units/kg/hr, IV 0.9% saline, potassium replacement
- Lactic acidosis: Treat cause (fluids, vasopressors for sepsis)
- Sodium bicarbonate: Only in severe acidosis (pH <6.9 in DKA; renal failure); 50mmol IV over 30-60 min
- Metabolic alkalosis: 0.9% saline, potassium replacement, acetazolamide 250mg BD if resistant
- Type 2 respiratory failure in COPD: NIV (BiPAP) with inspiratory/expiratory pressures titrated to response
Surgical/Interventional
- RRT: For refractory metabolic acidosis in renal failure or toxic ingestion (methanol, ethylene glycol)
Referral Criteria
- pH <7.1 or >7.6 — critical care
- Type 2 respiratory failure failing NIV — ICU for intubation
- Suspected toxic ingestion — toxicology/NPIS advice
Prognosis
- Lactic acidosis >10 mmol/L: Mortality 50-80%
- DKA: Mortality <1% in specialist centres with protocol-driven care
- Acute respiratory acidosis requiring NIV: Hospital mortality ~15% for COPD exacerbation
- pH: Values outside 6.8-7.8 are generally incompatible with life
- Lactate clearance: >10% reduction at 6 hours predicts improved survival in sepsis
Other Relevant Information
5-Step ABG Interpretation
| Step | Action |
|---|---|
| 1 | Assess oxygenation: PaO2, A-a gradient, P/F ratio |
| 2 | Assess pH: Acidaemia (<7.35) or alkalaemia (>7.45)? |
| 3 | Identify primary disorder: PaCO2 (respiratory) or HCO3⁻ (metabolic)? |
| 4 | Assess compensation: Is it appropriate? |
| 5 | Calculate anion gap if metabolic acidosis |
Expected Compensation Rules
| Primary Disorder | Expected Compensation |
|---|---|
| Metabolic acidosis | PaCO2 = (1.5 × HCO3⁻) + 8 ± 2 (Winter's formula) |
| Metabolic alkalosis | PaCO2 rises 0.7 kPa per 10 mmol/L rise in HCO3⁻ |
| Acute respiratory acidosis | HCO3⁻ rises 1 mmol/L per 1 kPa rise in PaCO2 |
| Chronic respiratory acidosis | HCO3⁻ rises 3.5 mmol/L per 1 kPa rise in PaCO2 |
| Acute respiratory alkalosis | HCO3⁻ falls 2 mmol/L per 1 kPa fall in PaCO2 |
| Chronic respiratory alkalosis | HCO3⁻ falls 5 mmol/L per 1 kPa fall in PaCO2 |
Causes of Raised Anion Gap Metabolic Acidosis (MUDPILES)
| Letter | Cause |
|---|---|
| M | Methanol |
| U | Uraemia |
| D | Diabetic ketoacidosis |
| P | Propylene glycol / Paraldehyde |
| I | Isoniazid / Iron |
| L | Lactic acidosis |
| E | Ethylene glycol |
| S | Salicylates |