Calcium Replacement

Calcium replacement treats symptomatic or severe hypocalcaemia and supports membrane stability in hyperkalaemia and calcium channel blocker overdose. Common causes of hypocalcaemia are hypoparathyroidism (often after neck surgery), vitamin D deficiency, kidney disease, pancreatitis, hypomagnesaemia and drugs.

Mechanism. Calcium restores excitability thresholds in nerve and muscle and supports contraction. In hyperkalaemia it stabilises the cardiac membrane within minutes without lowering potassium.

Pharmacokinetics. Oral calcium is absorbed best in divided doses of 500 mg elemental calcium or less with food (carbonate) or any time (citrate). Vitamin D is needed for absorption. Intravenous calcium raises serum calcium for only 2 to 3 hours, so infusion often follows a bolus.

Chronic hypoparathyroidism. Combine oral calcium (1 to 3 g elemental daily) with alfacalcidol or calcitriol. Aim for the low-normal range. Thiazides can reduce urine calcium loss.

Magnesium. Correct low magnesium first; calcium will not stay up without it.

• First-line. Oral calcium and vitamin D for asymptomatic or mild hypocalcaemia.
• Alternative. Calcitriol or alfacalcidol in hypoparathyroidism or kidney disease; parathyroid hormone analogue by specialists.
• Specialist. Post-thyroidectomy tetany, hungry bone syndrome, calcimimetic management, osteoporosis treatment with calcium and vitamin D replete.
• Emergency. Tetany, seizure, laryngospasm, QT prolongation, hyperkalaemia with ECG changes, calcium channel blocker or magnesium overdose.

Selection. Use intravenous gluconate for symptoms or ECG change. Prefer gluconate in peripheral lines, because chloride causes tissue necrosis. Use chloride in arrest.

Contraindications. Hypercalcaemia; severe hyperphosphataemia (risk of calcium phosphate deposits); digoxin toxicity (relative); sarcoidosis (caution with vitamin D).

Serious adverse effects.

• Intravenous: bradycardia and arrhythmia if too fast, tissue necrosis on extravasation, hypotension, a feeling of warmth.
• Oral: constipation, kidney stones, hypercalcaemia, bloating.
• Hypercalcaemia with vitamin D analogues; kidney injury.

Major interactions. Digoxin (arrhythmia with high calcium); ceftriaxone (precipitation in neonates and in the same line); thiazides (reduce calcium loss and may cause hypercalcaemia); quinolones, tetracyclines, levothyroxine and bisphosphonates (absorption falls); phosphate and bicarbonate infusions (precipitate).

Common prescribing errors.

• Giving calcium chloride peripherally.
• Mixing calcium with phosphate or bicarbonate in one line.
• Fast bolus in a person on digoxin.
• Failing to check magnesium and vitamin D.
• Stopping infusion before the oral regimen is established.

Special groups.

• Pregnancy: needs normal calcium; treat symptomatic deficiency.
• Breastfeeding: compatible.
• Children: weight-based; neonatal hypocalcaemia has its own local regimens.
• Older adults: constipation and stone risk; separate from other drugs.
• Kidney: hypocalcaemia is common; use active vitamin D and phosphate binders; avoid excess calcium.
• Liver: vitamin D activation is preserved until severe disease.
• East Africa: vitamin D deficiency and low dietary calcium are plausible contributors; calcium carbonate and alfacalcidol availability varies; gluconate ampoules are usually stocked, but check that chloride is clearly labelled for central use only.

Monitoring. Albumin-adjusted or ionised calcium, magnesium, phosphate, creatinine and parathyroid hormone; ECG with intravenous calcium; urine calcium on long-term therapy.

Referral and escalation. Seizure, laryngospasm, QT prolongation, calcium that will not rise, or suspected hypoparathyroidism.

Comparison with alternative classes. Calcium acts at once and treats symptoms. Vitamin D analogues work over days and keep the level up. Parathyroid hormone replacement is reserved for specialist units.