Key Facts

Acid-base disorders are commonly encountered in acute medicine and critical care. A systematic approach to ABG interpretation is essential for MRCP and clinical practice.

Physiology

• Normal pH maintained at 7.35-7.45 by buffer systems, lungs (CO2 excretion), and kidneys (H+ excretion, HCO3 reabsorption)
• Henderson-Hasselbalch equation: pH = 6.1 + log([HCO3]/[0.03 × PaCO2])
• Lungs respond within minutes; kidneys respond over days
• Body tolerates acidosis better than alkalosis

Anion Gap Calculation

• AG = Na+ - (Cl- + HCO3-)
• Normal: 10-18 mmol/L (varies by lab)
• Elevated AG indicates accumulation of unmeasured anions (lactate, ketones, uraemic toxins, toxins)
• Normal AG metabolic acidosis = hyperchloraemic acidosis (loss of HCO3 or failure to excrete H+)

Compensation Rules

Metabolic Acidosis

• Kussmaul breathing (deep, rapid respirations): respiratory compensation
• Confusion, lethargy, coma in severe cases
• Hypotension: reduced cardiac contractility
• Features of underlying cause: DKA (ketonaemia), lactic acidosis (shock), uraemia (renal failure)

Metabolic Alkalosis

• Muscle cramps, tetany (reduced ionised calcium in alkalosis)
• Arrhythmias: hypokalaemia often coexists
• Shallow breathing: reduced respiratory drive
• Confusion in severe cases

Respiratory Acidosis

• Headache, drowsiness, confusion (CO2 narcosis)
• Tremor, asterixis: CO2 retention
• Peripheral vasodilation: warm, flushed
• Papilloedema: raised intracranial pressure from cerebral vasodilation

Red Flags

• pH <7.0: life-threatening; risk of cardiac arrest
• pH >7.6: life-threatening; arrhythmia risk
• Lactate >4 mmol/L: severe tissue hypoperfusion; high mortality
• Mixed disorder: e.g., salicylate poisoning (respiratory alkalosis + metabolic acidosis)

Essential

• Arterial blood gas (ABG): pH, PaCO2, PaO2, HCO3, base excess, lactate
• Venous blood gas (VBG): adequate for pH and HCO3 (pH ~0.03 lower than arterial); not reliable for PaO2
• U&Es: calculate anion gap; check potassium, renal function
• Lactate: type A (tissue hypoperfusion) vs type B
• Glucose and ketones: DKA
• Blood cultures: if sepsis/lactic acidosis

Specific

• Urinary anion gap: positive (impaired NH4 excretion – RTA) vs negative (appropriate NH4 excretion – GI losses)
• Osmolar gap: elevated in methanol, ethylene glycol poisoning (>10 mOsm/kg is abnormal)
• Toxicology screen: salicylates, paracetamol, methanol, ethylene glycol
• D-lactate: in short bowel syndrome (standard lactate assays miss D-isomer)

Metabolic Acidosis

• Treat underlying cause (fluids for shock, insulin for DKA, dialysis for uraemia, antidotes for toxins)
• Sodium bicarbonate 1.26% IV: consider if pH <7.1 or HCO3 <8 mmol/L (controversial; may worsen intracellular acidosis)
• Haemodialysis: for refractory acidosis or toxic ingestions
• DKA protocol: IV insulin + fluids + potassium replacement (JBDS guidelines)

Metabolic Alkalosis

• Chloride-responsive (urine Cl <20: vomiting, NG suction, diuretics): IV 0.9% NaCl + KCl replacement
• Chloride-resistant (urine Cl >20: Conn syndrome, Cushing): treat underlying cause
• Acetazolamide 250-500mg: promotes bicarbonate excretion (useful in ventilated patients)

Respiratory Acidosis

• Treat underlying cause: bronchodilators (COPD), naloxone (opiates), ventilatory support
• NIV (BiPAP): for type 2 respiratory failure with acidosis (pH 7.25-7.35)
• Intubation and mechanical ventilation: if pH <7.25 or deteriorating

Respiratory Alkalosis

• Treat underlying cause: anxiolytic for panic attacks, anticoagulation for PE
• Rebreathing into paper bag: not recommended (risk of hypoxia)

Referral Criteria

• pH <7.1 or >7.6 → critical care
• Lactic acidosis with haemodynamic instability → ICU
• Suspected toxic ingestion → toxicology/ICU
• Refractory metabolic acidosis → nephrology (dialysis)

• Lactic acidosis >4 mmol/L: in-hospital mortality 30-50%
• DKA: mortality <1% with modern management; higher in children (cerebral oedema) and elderly
• Severe respiratory acidosis (pH <7.2): mortality depends on underlying cause; COPD with NIV has good outcomes
• Metabolic alkalosis: rarely life-threatening; mortality relates to underlying condition
• Osmotic demyelination from rapid correction of hyponatraemia-associated alkalosis is a rare but devastating complication

Causes of Raised Anion Gap Metabolic Acidosis (MUDPILES)

Normal Anion Gap Metabolic Acidosis (HARDUP)

ABG Interpretation Approach

1. Look at pH: acidosis (<7.35) or alkalosis (>7.45)?
2. Look at PaCO2: if matches pH direction → respiratory cause
3. Look at HCO3: if matches pH direction → metabolic cause
4. Calculate anion gap (if metabolic acidosis)
5. Assess compensation: is it appropriate?
6. Check for mixed disorders