Calcium

From Ganfyd

With an average of 1 kg of calcium in the body, calcium is one of the most abundant cations in the body. It has several functions:

• mineral component of bone
• clotting factor in clotting cascade (Factor IV)
• second messenger in intra-cellular signalling
• required in excitable tissues, particularly nerve and muscle
• enzymatic co-factor

Calcium Distribution

99% of calcium is stored in a mineral form as hydroxyapatite in bone. The remaining 1% is found mainly in plasma, with small amounts in extra- and intra-cellular fluid.

Within the plasma, calcium exists in equilibrium between the 3 pools:

1. ionised, 'free' calcium (~50%)
2. calcium bound to albumin (~40%)
3. unionised calcium salts, e.g. bicarbonate, citrate and phosphate (~10%)

Hydrogen ions compete with calcium for the binding sites on albumin. Any decrease in pH (i.e. acidosis/rise in hydrogen ion concentration) will cause calcium to become displaced. This will increase the unbound fraction and in extreme cases cause hypercalcaemia.

Calcium Metabolism

The main organs involved in calcium homeostasis are:

• bone
• parathyroid gland
• kidney
• GI tract

The main hormones involved in calcium homeostasis are:

• Parathyroid hormone
• activated Vitamin D
• Calcitonin

Disorders

• Hypocalcaemia
• Hypercalcaemia

Correction

It is the 'free' portion of calcium that has its effects physiologically although this is very difficult to measure in the laboratory. Total calcium is reported and thus an adjustment must be applied to this figure. This is calculated by adding (or subtracting) 0.02 mmol of calcium for each g/l that albumin is below (or above, respectively) 40g/l.

Treatment

It is used as a calcium salt (see calcium gluconate, calcium chloride, calcium lactate, calcium carbonate, calcium glubionate in various indications

• Hypocalcaemia
  • About 5% transient incidence after parathyroidectomy
  • In acute hypocalcaemia (causing tetany) a soluble preparation such as calcium gluconate 10% is given slowly: typically for an adult 10ml but see local protocols as there is likely to be a need for continuous infusion of up to 9mmol/day until other measures kick in.
• Osteoporosis with calcium deficiency^[1]
  • Currently the issue of cardiovascular risk with calcium supplementation needs further study as it may be protective in younger elderly^[2] and increase risk in the very old^[3]
• Other calcium deficiency states
• Hyperkalaemia as a temporary cardiac stabilising measure iv at similar doses to acute hypocalcaemia

References