Hypocalcemia is the the presence of serum calcium less than 2.1 mmol/L or 9 mg/dl or an ionized calcium level of less than 1.1 mmol/L or 4.5 mg/dL. In the blood, about half of all calcium is bound to proteins such as serum albumin, but it is the unbound, or ionized, calcium that the body regulates. Plasma calcium may be inaccurate for patients with abnormal blood proteins.
For patients with low serum albumin the formula for corrected calcium is:
Corrected calcium (mg/dL) = measured total Ca (mg/dL) + 0.8 (4.0 – serum albumin [g/dL]), where 4.0 represents the average albumin level in g/dL.
SI units: Corrected calcium (mmol/L) = measured total Ca (mmol/L) + 0.02 (40 – serum albumin [g/L]), where 40 represents the average albumin level in g/L.
Causes of Hypocalcemia
- Major cause is hypoparathyroidism due to underproduction of PTH.
- Severe vitamin D deficiency.
- Precipitation of calcium from extensive burns, fat emboli, rhabdomyolysis and pancreatitis.
- Drugs such as Furosemide and antiepileptic drugs. In the operating room,
- Chelation of calcium through massive transfusion of red blood or albumin.
Clinical Signs of Hypocalcemia
- Chronic hypocalcemia causes lethargy, muscle cramps, a prolonged QT interval, renal failure, cataracts, dementia, and personality changes.
- Acute hypocalcemia produces neuromuscular irritability with muscle cramps and hand, foot, and circumoral paresthesias. Patients may demonstrate facial nerve irritability to percussion (Chvostek sign) or carpal spasm with tourniquet ischemia for 3 minutes (Trousseau sign).
- Severe hypocalcemia results in stridor, laryngospasm, tetany, apnea, coagulopathy, hypotension with catecholamine resistance, psychosis/confusion, and seizures unresponsive to conventional treatment.
Treatment of Hypocalcemia
- Severe or symptomatic hypocalcemia should be treated with IV calcium. A 10 mL ampule of calcium gluconate contains 93 mg of elemental calcium; a 10 mL ampule of calcium chloride contains 273 mg of calcium.
- Mild to moderate can be treated with oral calcium and vitamin D.
- Hypocalcemia should be corrected preoperatively.
- Patients may have hypotension with insensitivity to β-adrenergic agonists, a prolonged QT interval, advanced atrioventricular block, and digitalis insensitivity.
- Responses to NMBA’s are unpredictable and require close monitoring with a nerve stimulator.