Abnormalities of calcium (Ca2+) and magnesium (Mg2+) metabolism are not infrequent occurrences among infants admitted for neonatal intensive care. Moreover, the disturbances of Ca2+ may be mirrored by Mg2+, or conversely, as in hypocalcemia and hypomagnesemia. Infants of diabetic mothers (IDMs) and infants with fetal growth restriction (FGR) may present with low serum levels of either Ca2+ or Mg2+ or both. Serum values for Ca2+ and Mg2+ above or below accepted normal values are of concern in any infant and warrant further clinical studies. Magnesium disorders are discussed in Chapter 105.
Definition. Hypocalcemia is determined by either total serum calcium (tCa) or ionized calcium (iCa) values. Clinical chemistry values for serum levels vary by units (ie, mEq/L, mmol/L, or mg/dL), by gestational age, and by day of age following the immediate newborn period. Reference textbooks reflect considerable variance of serum values for Ca2+ and Mg2+. Interpretation of serum values for any given patient is dependent on recognition of one’s institution laboratory values and range of acceptable values.
A generally accepted value for hypocalcemia is <2.0 mmol/L (<8.0 mg/dL) for a term infant or <1.75 mmol/L (<7.0 mg/dL) for a preterm infant. A typical range of normal values for a term newborn can be 2.25 to 2.65 mmol/L (9.0–10.6 mg/dL) throughout the first week of life. Preterm infant tCa levels closely parallel those for term infants. Of greater significance is the ionized fraction of Ca2+. It is the active physiologic component and is dependent on the interaction of tCa2+, acid-base status, and serum albumin. Typical iCa2+ values for term infants over the first 72 hours of life are 1.22 to 1.24 mmol/L (4.88–4.96 mg/dL). Preterm infant mean values are similar for 24 to 72 hours: 1.21 to 1.28 mmol/L (4.84–5.12 mg/dL). Thereafter, preterm infants have slightly increased iCa2+ levels, whereas term infants experience a slight decline. iCa levels of <1.2 mmol/dL (4 mg/dL) are considered hypocalcemic.
Incidence. Hypocalcemia is likely the most common disorder of either Ca2+ or Mg2+ in newborn infants, and it affects both preterm and term infants. It occurs in up to 30% of infants with birthweight <1500 g. Late-onset hypocalcemia is more common in developing countries where cow’s milk or formulas with phosphate concentrations are used.
Pathophysiology. iCa2+ is the biologically important form of calcium. The tCa2+ levels have been repeatedly shown to not be predictive of iCa2+ levels. Therefore, tCa2+ levels are unreliable as criteria for true hypocalcemia. In premature infants, it has been shown that tCa2+ levels as low as ≤6 mg/dL correspond to iCa2+ levels >3 mg/dL.
Early-onset neonatal hypocalcemia. During the third trimester of pregnancy, the human fetus receives at least 120 to 150 mg/kg/d of elemental Ca2+ via the umbilical cord. Most of this Ca2+ is readily incorporated into the newly forming bones. After delivery, this massive supply of Ca2+ is suddenly stopped, and Ca2+ must be given enterally.
A full-term infant receiving 100 to 120 mL of normal formula would be receiving 50 ...