Key Facts

Body fluid is distributed between intracellular (ICF, ~28L) and extracellular (ECF, ~14L) compartments. The ECF is further divided into interstitial fluid (~11L) and plasma (~3L). Osmolarity is the key determinant of water distribution between compartments.

Epidemiology

Electrolyte disturbances are among the most common abnormalities encountered in hospital medicine. Hyponatraemia affects 15-30% of hospitalised patients. Hyperkalaemia occurs in approximately 8% of hospitalised patients and is the most common electrolyte cause of cardiac arrest. Hypokalaemia affects approximately 20% of hospitalised patients.

Aetiology

Electrolyte disturbances arise from:

• Altered intake: IV fluids, diet, TPN
• Altered losses: GI (vomiting, diarrhoea, fistulae), renal (diuretics, RTA), insensible (fever, burns)
• Redistribution: Insulin shifts K⁺ intracellularly; metabolic alkalosis drives K⁺ into cells
• Hormonal: Aldosterone (Na⁺ retention, K⁺ excretion), ADH (water retention), PTH (Ca²⁺ regulation)

Pathophysiology

• Sodium determines ECF osmolality and volume - hyponatraemia reflects relative water excess
• Potassium determines resting membrane potential - disturbances cause cardiac arrhythmias
• Calcium is essential for muscle contraction, nerve conduction, and coagulation - 40% protein-bound
• Magnesium is a cofactor for >300 enzymatic reactions - deficiency causes refractory hypokalaemia and hypocalcaemia
• Starling forces govern fluid movement across capillaries: hydrostatic vs oncotic pressure gradients

Hyponatraemia

• Mild (130-134): Often asymptomatic
• Moderate (125-129): Nausea, headache, confusion
• Severe (<125): Seizures, coma, respiratory arrest

Hyperkalaemia

• Muscle weakness, paraesthesia, palpitations
• ECG changes: tall tented T waves → loss of P waves → broad QRS → sine wave → cardiac arrest

Hypokalaemia

• Muscle weakness, cramps, constipation, ileus
• ECG: Flattened T waves, ST depression, prominent U waves, prolonged QT

Hypercalcaemia

• "Bones, stones, groans, and psychic moans": Bone pain, renal stones, abdominal pain/constipation, confusion/depression

Hypocalcaemia

• Perioral paraesthesia, carpopedal spasm, Chvostek's sign, Trousseau's sign, prolonged QT

Red Flags

• K⁺ >6.5 mmol/L or ECG changes - immediate treatment
• Na⁺ <120 mmol/L with seizures - 100mL hypertonic (3%) saline bolus
• Corrected Ca²⁺ >3.5 mmol/L - urgent hydration and bisphosphonate

Bedside

• ECG: Essential in hyper/hypokalaemia and hypocalcaemia
• Fluid balance chart: Input/output monitoring
• Weight: Daily - 1kg change ≈ 1L fluid
• Urine dipstick: Specific gravity, osmolality estimate

Bloods

• U&Es: Na⁺, K⁺, urea, creatinine, eGFR
• Bone profile: Corrected Ca²⁺, phosphate, ALP
• Magnesium: Always check in refractory hypokalaemia
• Serum osmolality: Normal 280-295 mOsm/kg
• Thyroid function, cortisol: Exclude hypothyroidism and adrenal insufficiency in hyponatraemia

Imaging

• CXR: Fluid overload assessment
• Renal USS: Obstructive uropathy, renal size

Special Tests

• Paired urine and serum osmolality: Diagnose SIADH
• Urine Na⁺: <20 mmol/L suggests pre-renal/hypovolaemic; >40 mmol/L suggests renal loss or SIADH
• Short Synacthen test: Exclude adrenal insufficiency
• PTH level: Differentiate causes of hypercalcaemia

Non-pharmacological

• Fluid restriction (750-1000 mL/day): First-line for SIADH
• IV fluid therapy per NICE CG174: Assess volume status before prescribing
• Dietary potassium restriction: In CKD with hyperkalaemia

Pharmacological

Hyperkalaemia emergency treatment:

1. Calcium gluconate 10% 10mL IV over 2 min (cardioprotection) - does NOT lower K⁺
2. Insulin 10 units + glucose 50mL 50% IV - shifts K⁺ intracellularly (onset 15-30 min)
3. Salbutamol 10-20mg nebulised - additional K⁺ shifting
4. Calcium resonium 15g TDS PO or sodium zirconium cyclosilicate 10g TDS - K⁺ binders

Hyponatraemia:

• Acute symptomatic: Hypertonic saline (3%) 100-150mL over 20 min
• Chronic: Fluid restriction, demeclocycline 300mg BD, tolvaptan 15mg OD (specialist only)

Hypocalcaemia:

• Acute symptomatic: 10mL calcium gluconate 10% IV over 10 min, then infusion
• Chronic: Alfacalcidol 0.25-1mcg OD + calcium carbonate 1.25g BD

Referral Criteria

• Refractory electrolyte disturbances - endocrinology/nephrology
• Sodium <120 mmol/L or >155 mmol/L - critical care
• Suspected endocrine cause - endocrinology

• Severe hyponatraemia (<120 mmol/L): mortality up to 50% if untreated; osmotic demyelination syndrome if corrected too rapidly (>10-12 mmol/L/day)
• Hyperkalaemia-related cardiac arrest: poor outcomes; preventable with early recognition
• SIADH: Prognosis depends on underlying cause; drug-induced SIADH resolves on drug withdrawal
• Hypercalcaemic crisis (Ca²⁺ >3.5): mortality 50% without treatment
• Chronic electrolyte disorders: increase falls risk, hospital stay, and mortality in elderly patients

IV Fluid Composition

Hyponatraemia Classification