Calcium salts

Why this drug is interesting

Calcium is the only drug in the hyperkalaemia algorithm that does nothing to the potassium. It does not shift it, it does not remove it, and it does not alter the serum concentration at all. What it does is raise the threshold potential of the cardiac myocyte, restoring the gap between resting and threshold that hyperkalaemia has closed — buying thirty to sixty minutes in which the treatments that actually work can be given.

It is also the drug most likely to be given in the wrong amount, for a reason that has nothing to do with clinical judgement: the UK stocks three different intravenous calcium preparations, two of them labelled 10%, containing 2.23 mmol, 6.8 mmol and 10 mmol of calcium respectively. Reaching for the familiar volume of the unfamiliar ampoule is a three-fold error in either direction.

The preparations — read the ampoule

The reason for the difference is molecular weight and stoichiometry. Calcium gluconate is a large molecule of which calcium is a small fraction; calcium chloride is a small one of which calcium is a large fraction. Nothing about the naming conveys this, which is why the mmol figure, not the percentage, is the number worth carrying.

Hyperkalaemia

What the calcium does and does not do

• Onset within about three minutes; duration 30–60 minutes
• Repeat doses are often needed if hyperkalaemia remains uncontrolled and ECG changes persist
• It does not lower serum potassium. Insulin-glucose, salbutamol and potassium removal are what treat the hyperkalaemia — calcium buys the time to give them
• Give calcium in severe hyperkalaemia with ECG changes even when emergency dialysis is already planned²

The calcium gluconate SmPC gives 30 mL over 10 minutes for acute severe hyperkalaemia (K⁺ above 6.5 mmol/L) with or without ECG changes, with further doses considered after 5 minutes until ECG improvement is achieved.¹ In cardiac arrest due to hyperkalaemia it gives 30 mL as a rapid intravenous injection, repeated if there is no return of spontaneous circulation within 5–10 minutes.

Children

• Neonates and children: 0.5 mL/kg of 10% calcium gluconate (0.11 mmol/kg) by slow IV injection over 5–10 minutes, repeated if ECG changes persist after 5–10 minutes
• Give centrally where possible; if not, dilute to five times the volume with 0.9% sodium chloride
• In children of 20 kg or more, a maximum of 20 mL of 10% calcium gluconate is recommended
• Maximum 1 mL/kg of 10% calcium gluconate across all paediatric ages, because of aluminium exposure from the glass ampoule

Other indications

Symptomatic hypocalcaemia

• Adults: 10–20 mL of 10% calcium gluconate (2.23–4.46 mmol) over 10 minutes, undiluted or in 50–100 mL of 5% glucose or 0.9% sodium chloride, with plasma calcium and ECG monitoring¹
• Repeat according to clinical condition and serum calcium
• Calcium chloride is an acidifying salt and is described in its own label as "usually undesirable in the treatment of hypocalcaemia of renal insufficiency" — gluconate is the better choice for straightforward replacement

Calcium channel blocker poisoning

Calcium is one of the three first-line interventions in the expert consensus recommendations for calcium channel blocker poisoning, alongside high-dose insulin and a vasopressor.³ The pharmacology is straightforward competition — raising extracellular calcium increases flux through the residual unblocked L-type channels. Doses used are considerably higher and more frequently repeated than for hypocalcaemia, and ionised calcium should be monitored. Discuss with NPIS.

Hydrofluoric acid and fluoride poisoning

Fluoride chelates calcium and magnesium, producing profound hypocalcaemia and refractory arrhythmia. Calcium gluconate is licensed for fluoride- or lead-induced hypocalcaemia at 10–30 mL of 10% (2.23–6.69 mmol) intravenously, and the SmPC directs that NPIS be consulted for specific advice.¹ Topical calcium gluconate gel and local infiltration have separate roles in dermal exposure.

Magnesium toxicity

Calcium is the antidote to magnesium sulphate toxicity — relevant in eclampsia management, where loss of deep tendon reflexes is the first sign and precedes respiratory depression.

Massive transfusion

Citrate in stored blood chelates ionised calcium. Hypocalcaemia in major haemorrhage is common, is associated with worse outcomes, and is easily missed because total calcium is unhelpful — measure ionised calcium on the gas analyser and replace it. See tranexamic acid for the rest of that pathway.

Contraindications and hazards

Other contraindications

• Hypercalcaemia and hypercalciuria — malignancy with bone decalcification, hyperparathyroidism, hypervitaminosis D, sarcoidosis, milk-alkali syndrome, immobilisation osteoporosis
• Renal calculi, or a history of them
• Ceftriaxone in neonates — calcium-ceftriaxone precipitation has caused fatal reactions in premature and term newborns under 1 month, including where the two were given at different times through different lines. The neonatal contraindication is age-specific, but the rule against simultaneous administration is not: "in patients of any age ceftriaxone must not be mixed or administered simultaneously with any calcium-containing IV solutions, even via different infusion lines or at different infusion sites" — over 28 days they may be given sequentially through different sites, or through lines flushed between infusions
• In patients over 28 days, ceftriaxone and calcium may be given sequentially through different sites, or with lines replaced or thoroughly flushed between
• Repeated or prolonged calcium gluconate treatment in children and in renal impairment — because of aluminium leached from the glass ampoule. It is licensed for acute symptomatic use only
• Both calcium chloride labels additionally contraindicate the treatment of asystole and electromechanical dissociation. The 10% pre-filled syringe label alone adds that "in cardiac resuscitation, the use of calcium is contraindicated in the presence of ventricular fibrillation" — the 10 millimoles in 10 mL label carries no such clause, and no cardiac-glycoside contraindication either

Administration hazards

• Extravasation of calcium chloride causes severe tissue necrosis and sloughing. The label instructs that intravenous administration be stopped at once if perivascular infiltration occurs. Use a large, well-sited, well-tested vein — or central access
• A moderate fall in blood pressure due to vasodilatation may accompany the injection, which is one reason for the specified rates
• Rapid administration can cause bradycardia, arrhythmia and a sensation of heat, a chalky taste and flushing
• Discontinue if plasma calcium exceeds 2.75 mmol/L or 24-hour urinary calcium excretion exceeds 5 mg/kg — arrhythmias may occur at these levels¹

Practical use in the ED

1. Decide the salt before the volume. Arrest or peri-arrest — chloride, 10 mL of 10%, over 5 minutes. Everything else — gluconate, 30 mL of 10%, over 10 minutes. (In a genuine arrest the rate ceases to be the binding concern, but peri-arrest is still 5 minutes, and the chloride label says "for slow intravenous infusion only".)
2. Look at the ampoule and find the millimoles. If the label says millimoles rather than a percentage, you have the 14.7% chloride and 10 mL is 10 mmol, not 6.8.
3. Get a big vein for chloride. The consequence of extravasation is disproportionate to the convenience of the cannula you already have.
4. Then treat the potassium. Calcium is Step 1 of five, and the commonest failure in hyperkalaemia is a patient who receives excellent calcium and inadequate everything else.
5. Re-dose at 30–60 minutes if the ECG deteriorates again. The duration of effect is short and the underlying potassium is unchanged.

Critical appraisal

1. The evidence for calcium in hyperkalaemia is graded 1C — a strong recommendation on low-quality evidence. That grading is honest: the physiological rationale is compelling and the clinical trial evidence is close to absent. Nobody is going to randomise a sine-wave ECG to placebo, and the recommendation is right, but the certainty implied by an algorithm box is not the certainty the data support.
2. The "gluconate needs the liver to release its calcium" teaching is wrong, and the label says so. It is widely repeated as a reason to prefer chloride in shock or hepatic failure, and the gluconate SmPC refutes it directly: "Hepatic function does not impact the availability of ionized calcium after intravenous administration of calcium gluconate. Dose adjustments in hepatically impaired patients may not be necessary." Bioavailability after intravenous administration is 100%, and the calcium simply joins the intravascular pool. The real reasons to prefer chloride in an arrest are the smaller volume and the faster permitted rate — not a metabolic step that does not exist.
3. The digoxin contraindication is stronger in the labels than in the data. The stone-heart phenomenon is poorly substantiated in modern human series. The practical resolution is that Fab makes the question rarely necessary to answer, but the confidence with which the prohibition is taught outstrips its evidence.
4. The three-preparation problem is a system fault being managed by individual vigilance, which is the least reliable control available. Departments that stock one salt for one indication in one place have fewer of these errors than departments that rely on people reading carefully at four in the morning.

References

1. 1
   Calcium Gluconate 10% solution for injection/infusion BP — Summary of Product Characteristics, hameln pharma. electronic Medicines Compendium. Sections 2, 4.1–4.4. Source of the mmol contents, the non-equivalence warning, the hyperkalaemia and paediatric doses and the aluminium restriction. Verified 23 Aug 2026.
2. 2
   Alfonzo A, Harrison A, Baines R, Chu A, Mann S, MacRury M. Treatment of Acute Hyperkalaemia in Adults — UK Kidney Association Clinical Practice Guideline, October 2023. ukkidney.org. Guidelines 16.2a and 16.2b, Table 12. Verified 23 Aug 2026.
3. 3
   St-Onge M, Anseeuw K, Cantrell FL, et al. Experts consensus recommendations for the management of calcium channel blocker poisoning in adults. Crit Care Med 2017;45(3):e306–15. PubMed
4. 4
   Calcium Chloride 10% w/v Intravenous Infusion, pre-filled syringe — Summary of Product Characteristics, Martindale Pharma. electronic Medicines Compendium. Source of the CPR indication and the asystole/VF/cardiac glycoside contraindications. Verified 23 Aug 2026.
5. 5
   Calcium Chloride Injection 10 millimoles in 10 ml — Summary of Product Characteristics, Martindale Pharma. electronic Medicines Compendium. The 14.7% preparation. Verified 23 Aug 2026.
6. 6
   TOXBASE — calcium channel blockers; hydrofluoric acid. National Poisons Information Service. toxbase.org (NHS login required. NPIS: 0344 892 0111.)