Fluid, Electrolyte & Acid–Base Disturbances

Forfar & Arneil 7th Edition · Sodium, potassium, calcium, magnesium disorders · Water balance · Acid-base (metabolic/respiratory acidosis/alkalosis) · Dehydration · Renal regulation · Anion gap
📌 Key principles: Maintenance fluid calculation (4-2-1 rule). Dehydration: isonatraemic, hyponatraemic, hypernatraemic. Sodium disorders reflect water balance. Potassium disorders affect cardiac conduction. Acid-base: use Henderson-Hasselbalch, Winter's formula for compensation, anion gap for metabolic acidosis.

📖 Fluid, Electrolyte & Acid–Base Disturbances

💧 Body water compartments
Total body water (TBW): 75% of weight in infants, 60% in adults. Intracellular (ICF) 2/3, extracellular (ECF) 1/3 (interstitial + plasma). Maintenance fluid: Holliday-Segar (4-2-1 rule).
🧂 Sodium disorders
Hyponatraemia (<135): water excess or sodium loss. Hypernatraemia (>145): water deficit (common) or sodium excess. Serum Na reflects water balance. Urine Na helps differentiate causes.
⚡ Potassium disorders
Hypokalaemia (<3.5): vomiting, diarrhoea, diuretics, alkalosis. Hyperkalaemia (>6.0): renal failure, CAH, cell lysis. ECG changes: peaked T waves (hyperK), U waves (hypoK).
📊 Acid-base physiology
pH = pKa + log [HCO3-]/(0.03×PCO2). Respiratory acidosis: ↑PCO2 (hypoventilation). Metabolic acidosis: ↓HCO3 (diarrhoea, DKA, renal failure). Anion gap = Na - (Cl + HCO3). Normal 8-12.
🩺 Dehydration assessment
Mild (<5% loss), moderate (5-10%), severe (>10%). Clinical signs: mucous membranes, skin turgor, capillary refill, heart rate, urine output, fontanelle, sunken eyes, shock.
📊 Key formulas: Maintenance fluids (4-2-1): 4 ml/kg for first 10 kg, 2 ml/kg for next 10 kg, 1 ml/kg thereafter. Anion gap = Na - (Cl + HCO3). Winter's formula: expected PCO2 = 1.5×HCO3 + 8 ± 2 (for metabolic acidosis).

🔍 Stepwise approach to fluid, electrolyte & acid-base disturbances

1
Assess hydration status – Mild: weight loss 3-5%, normal vitals. Moderate: 6-10%, tachycardia, dry mucous membranes, decreased skin turgor, sunken eyes, reduced urine output. Severe: >10%, shock (tachycardia, hypotension, poor perfusion, anuria).
2
Calculate maintenance and deficit fluids – Maintenance (4-2-1 rule). Deficit: weight loss (kg) × % dehydration × 1000. Replace deficit over 24h (half in first 8h). Add ongoing losses (vomiting, diarrhoea).
3
Interpret serum sodium – Hyponatraemia: water excess or sodium loss. Check urine sodium (<20 mmol/L → extrarenal loss; >40 → renal loss). Hypernatraemia: water deficit. Urine osmolality helps distinguish diabetes insipidus vs osmotic diuresis.
4
Evaluate acid-base status (pH, PCO2, HCO3) – Primary disorder: metabolic acidosis (low HCO3), metabolic alkalosis (high HCO3), respiratory acidosis (high PCO2), respiratory alkalosis (low PCO2). Check compensation: Winter's formula for metabolic acidosis (expected PCO2 = 1.5×HCO3 + 8 ± 2).
5
Calculate anion gap (AG) – AG = Na - (Cl + HCO3). Normal 8-12. Elevated AG metabolic acidosis: MUDPILES (methanol, uraemia, DKA, propylene glycol, iron/isoniazid, lactic acidosis, ethylene glycol, salicylates). Normal AG (hyperchloraemic) metabolic acidosis: diarrhoea, RTA, pancreatic fistula.
6
Potassium emergencies – ECG changes: peaked T waves, wide QRS (hyperkalaemia); U waves (hypokalaemia). Hyperkalaemia >7.0 with ECG changes: IV calcium gluconate (cardioprotection), insulin+glucose, salbutamol, calcium resonium.
📌 Clinical pearl: In a child with metabolic acidosis, calculate the anion gap. If elevated, think of DKA, lactic acidosis, uraemia, or toxins. If normal gap (hyperchloraemic), think diarrhoea or renal tubular acidosis.

📋 Stepwise management of common fluid, electrolyte & acid-base disorders

1
Dehydration – rehydration protocol – IV 0.9% saline bolus (20 ml/kg) for shock/severe dehydration. Then calculate maintenance + deficit (replace over 24h). For isonatraemic dehydration: 0.9% saline or 0.45% saline with 5% dextrose. Monitor electrolytes.
2
Hyponatraemia – management – Symptomatic (seizures, coma): 3% NaCl 2 ml/kg bolus over 10-15 min (max 100 ml). Asymptomatic: treat underlying cause, fluid restrict if SIADH. Correct slowly (avoid osmotic demyelination, limit rise to 8-10 mmol/L/24h).
3
Hypernatraemia – management – Replace water deficit slowly over 48h to avoid cerebral oedema. Deficit = TBW × (Serum Na/140 - 1). Use hypotonic fluids (0.45% saline or 0.2% saline with 5% dextrose). Correct at rate 0.5-1 mmol/L/h.
4
Hyperkalaemia (emergency) – ECG monitoring. IV calcium gluconate 10% (0.5 ml/kg over 2-5 min) for cardioprotection. Shift K intracellular: insulin (0.1 U/kg) + glucose (0.5 g/kg) IV, nebulised salbutamol (2.5-5 mg). Remove K: calcium resonium enema/oral, furosemide, dialysis if renal failure.
5
Metabolic acidosis (DKA, diarrhoea) – Treat underlying cause. Correct dehydration, restore perfusion. Bicarbonate NOT routinely indicated (risk of worsening intracellular acidosis, shifting oxygen dissociation curve). Indicated only if pH <6.9 (rare).
6
Metabolic alkalosis (pyloric stenosis, vomiting) – Correct volume deficit (0.9% saline). Hypokalaemia and hypochloraemia common. Potassium replacement helps correct alkalosis (intracellular shift). Acetazolamide if severe.
⚠️ Maintenance fluid caution: Isotonic fluids (0.9% saline with 5% dextrose) recommended for maintenance in children to avoid hyponatraemia. Avoid hypotonic fluids (0.18% saline) in post-op or sick children.

🧠 Reflex prompts: fluid, electrolyte & acid-base

💧 A 10 kg child with moderate dehydration (10% deficit). Calculate 24h maintenance + deficit fluid?
Maintenance: 4×10 = 40 ml/h = 960 ml/24h. Deficit: 10% of 10kg = 1000 ml. Total 1960 ml/24h (80 ml/h). Half deficit (500 ml) in first 8h, then remainder.
🧂 A 5-year-old with vomiting, Na 128 mmol/L, urine Na 5 mmol/L. Most likely diagnosis?
Hyponatraemia with low urine Na suggests extrarenal loss (vomiting, diarrhoea, sweating). Treat with 0.9% saline.
⚡ A child with renal failure, K 7.2 mmol/L, ECG shows peaked T waves. Next step?
Immediate IV calcium gluconate (cardioprotection), then insulin+glucose, salbutamol, calcium resonium. Dialysis if severe.
📊 A child with diarrhoea: pH 7.20, PCO2 30 mmHg, HCO3 12 mmol/L. Primary disorder?
Metabolic acidosis (low HCO3). Winter's formula: expected PCO2 = 1.5×12 + 8 = 26 ± 2 (actual 30) → appropriate respiratory compensation.
🧪 Anion gap = 25 (normal 12). What causes elevated AG metabolic acidosis?
MUDPILES: Methanol, Uraemia, DKA, Paraldehyde/Iron, Isoniazid, Lactic acidosis, Ethylene glycol, Salicylates.
💊 A child on spironolactone develops hyperkalaemia. Mechanism?
Spironolactone is aldosterone antagonist → reduces potassium excretion. Other causes: renal failure, ACE inhibitors, NSAIDs, K-sparing diuretics.
🩺 A 2-month-old with projectile vomiting, metabolic alkalosis (HCO3 35, pH 7.55), hypochloraemia, hypokalaemia. Most likely?
Pyloric stenosis. Loss of gastric acid (HCl) causes metabolic alkalosis. Correct with 0.9% saline and potassium.
🌊 A child with polyuria, hypernatraemia, urine osmolality 100 mOsm/kg, no response to DDAVP. Diagnosis?
Nephrogenic diabetes insipidus (renal resistance). Treat with thiazides, low-sodium diet, NSAIDs.
📉 A child with DKA has HCO3 8 mmol/L, pH 7.10. Is sodium bicarbonate indicated?
Generally NOT recommended (risk of cerebral oedema, worsening intracellular acidosis). Use only if pH <6.9 with shock.
🩸 A child with SIADH (hyponatraemia, high urine osmolality, euvolemia). First-line treatment?
Fluid restriction (50-70% maintenance). Demeclocycline or tolvaptan for chronic refractory cases.