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Neonatal hypocalcemia

INTRODUCTION

Hypocalcemia is a common metabolic problem in newborns. The diagnosis, clinical manifestations, and treatment of neonatal hypocalcemia are reviewed here. Calcium (Ca) requirements and the etiology of hypocalcemia after the neonatal period are discussed elsewhere. (See "Calcium and phosphorus requirements of newborn infants" and "Etiology of hypocalcemia in infants and children".)

PERINATAL METABOLISM

During pregnancy, calcium is transferred actively from the maternal circulation to the fetus by a transplacental Ca pump regulated by parathyroid hormone-related peptide (PTHrP) [1]. The majority of fetal Ca accretion occurs in the third trimester. This process results in higher plasma Ca concentrations in the fetus than in the mother and leads to fetal hypercalcemia, with total and ionized Ca concentrations of 10 to 11 mg/dL (2.5 to 2.75 mmol/L) and 6 mg/dL (1.5 mmol/L), respectively, in umbilical cord blood at term [2].

After the abrupt cessation of placental transfer of Ca at birth, total serum Ca concentration falls to 8 to 9 mg/dL (2 to 2.25 mmol/L) and ionized Ca to as low as 4.4 to 5.4 mg/dL (1.1 to 1.35 mmol/L) at 24 hours [3,4]. Serum Ca concentration subsequently rises, reaching levels seen in older children and adults by two weeks of age [5].

MEASUREMENT

Within the plasma, Ca circulates in different forms. Approximately 40 percent is bound to serum proteins, principally albumin; 10 percent is complexed with citrate, bicarbonate, sulfate, or phosphate; and 50 percent exists as the physiologically important ionized (or free) calcium [6]. The ionized Ca concentration is tightly regulated by parathyroid hormone and vitamin D.

Measurement of the total plasma Ca concentration alone can be misleading because the relationship between total and ionized Ca is not always linear. (See "Relation between total and ionized plasma calcium concentration".) Correlation is poor when the serum albumin concentration is low or, to a lesser degree, with disturbances in acid-base status, both of which occur frequently in premature or ill infants. With hypoalbuminemia, the total Ca concentration will be low while the ionized fraction will be normal unless some other factor is affecting Ca metabolism. In general, the plasma calcium concentration falls by 0.8 mg/dL (0.2 mmol/L) for every 1.0 g/dL (10 g/L) fall in the plasma albumin concentration.
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