Calcium, magnesium and phosphate disorders — formative SAQs

SAQ 1 (10)

A 10-day-old exclusively formula-fed neonate presents with a generalised seizure. Total calcium is 1.6 mmol/L with an albumin of 35 g/L, phosphate is 3.6 mmol/L, and the ionised calcium is 0.85 mmol/L. The neonate is jittery but maintains the airway. [1][3]

1. Interpret the total and ionised calcium, define hypocalcaemia, and explain why the phosphate is high. (3) [1]
2. Give the emergency management, naming the drug, dose, route, rate and monitoring, and state the key peripheral-safety point. (4) [1][3]
3. If the calcium fails to rise after two adequate doses, name the single most likely cause and its treatment, and explain the mechanism. (3) [7]

Model answer

Interpretation and definition. About half of extracellular calcium is bound to albumin and the other half is ionised and active, so a laboratory corrected calcium (with albumin) and an ionised calcium are both needed. Hypocalcaemia is a total calcium below 2.2 mmol/L or an ionised calcium below 1.0 mmol/L, and an ionised of 0.85 is severe. The phosphate is high because this is late neonatal hypocalcaemia driven by a phosphate load from cow-milk formula; the high phosphate complexes calcium and depresses the ionised fraction. [1]

Emergency management. Give intravenous 10 percent calcium gluconate at 0.5 mL per kilogram (about 0.1 mmol per kilogram), to a maximum of about 20 mL, slowly over 10 to 60 minutes with continuous cardiac monitoring. Calcium gluconate rather than calcium chloride is preferred peripherally because it is less vesicant if it extravasates, and a good intravenous line is essential because extravasation causes tissue necrosis. Stop the infusion if the heart rate drops, because rapid calcium itself causes bradycardia and arrhythmia. [1][3]

Refractory cause. If the calcium fails to rise, the single most likely cause is hypomagnesaemia. Low magnesium suppresses parathyroid hormone release, producing a functional hypoparathyroidism that no amount of calcium will correct, and it causes concurrent renal potassium wasting. The treatment is intravenous magnesium 0.1 to 0.2 mmol per kilogram, up to 0.4 mmol per kilogram (maximum 8 mmol), over 2 to 4 hours with monitoring. [7]