🦴 Chapter 96: Disorders of Calcium, Magnesium & Phosphorus

Hypocalcaemia (ionised Ca <1 mmol/L) · Chvostek/Trousseau signs · QT prolongation · Calcium gluconate vs chloride · Hypomagnesaemia (refractory hypokalaemia/hypocalcaemia) · Hypermagnesaemia · Hypophosphataemia (refeeding syndrome, respiratory muscle weakness) · Hyperphosphataemia

🔍 Core Concepts: Ca, Mg, PO₄ Disorders

📌 Hypocalcaemia (iCa <1 mmol/L)
Causes: sepsis, pancreatitis, post-transfusion (citrate), CKD, hypoparathyroidism. ECG: prolonged QT. Symptoms: tetany, seizures, Chvostek/Trousseau signs. Treatment: IV calcium gluconate (0.3-0.5 mL/kg).
⚠️ Calcium Chloride vs Gluconate
10% calcium chloride: 27 mg/mL elemental Ca (3x more) — use via central line (extravasation necrosis). 10% calcium gluconate: 9 mg/mL — peripheral line safe.
🧪 Hypomagnesaemia (Mg <1.5 mg/dL)
Causes: diuretics, aminoglycosides, diarrhoea. Mg deficiency causes refractory hypokalaemia and hypocalcaemia (impaired PTH release/action). Treat with IV Mg 25-50 mg/kg over 30-60 min.
🩸 Hypophosphataemia (PO₄ <3 mg/dL)
Severe (<1.5 mg/dL) → respiratory muscle weakness, inability to wean, rhabdomyolysis, haemolysis. Causes: refeeding syndrome, DKA, burns. Replace IV 0.15-0.33 mmol/kg over 6h.
📈 Hyperphosphataemia (>4.5 mg/dL)
Causes: renal failure, tumour lysis, rhabdomyolysis. Leads to hypocalcaemia (precipitation). Treat: low-phosphate diet, phosphate binders, dialysis.
💊 Refeeding Syndrome
Starved patient refed → insulin surge → intracellular shift of phosphate, K, Mg → severe hypophosphataemia. Prevent: start feeds slowly (25-50% goal), replete electrolytes.

🩺 Stepwise Approach: Electrolyte Disorders

1
Hypocalcaemia (symptomatic)
Confirm ionised Ca <0.8-1.0 mmol/L. ECG: prolonged QT. Give IV calcium gluconate 0.3-0.5 mL/kg (10%) over 10-20 min with ECG monitoring. For severe/life-threatening (cardiac arrest): calcium chloride (central line).
2
Hypomagnesaemia (refractory K/Ca)
Check Mg if hypokalaemia or hypocalcaemia not correcting. Give IV MgSO4 25-50 mg/kg over 30-60 min (max 2 g). Rapid infusion causes hypotension, flushing.
3
Hypophosphataemia (severe <1.5 mg/dL)
IV phosphate replacement: 0.15-0.33 mmol/kg (5-10 mg/kg) over 6h. Use potassium phosphate (KPhos) or sodium phosphate. Monitor Ca (risk hypocalcaemia).
4
Refeeding syndrome prevention
At-risk: starvation >5 days, malnutrition, anorexia. Start feeds at 25-50% goal calories. Check phosphate, K, Mg daily for first 3-5 days. Replete aggressively.
5
Hypermagnesaemia (Mg >2.6 mg/dL)
Stop Mg-containing medications. Give calcium gluconate 0.5 mL/kg to antagonise. Diuresis (furosemide) if renal function ok. Dialysis if severe.
6
Hyperphosphataemia (tumour lysis, renal failure)
Low-phosphate diet, phosphate binders (calcium acetate, sevelamer). IV fluids (NS) to promote excretion. Dialysis if severe + renal failure.