Abnormalities of calcium (Ca+2) and magnesium (Mg+2) metabolism are not infrequent occurrences among infants admitted for neonatal intensive care. Moreover, the disturbances of Ca+2 may be mirrored by Mg+2, or conversely, as in hypocalcemia and hypomagnesemia. Infants of diabetic mothers (IDMs) and infants with intrauterine growth restriction (IUGR) may present with low serum levels of either Ca+2 or Mg+2, or both. Serum values for Ca+2 and Mg+2 above or below accepted normal values are of concern in any infant and warrant further clinical studies. (See also Chapter 107.)
Definition. Hypocalcemia is likely the most common disorder of either Ca+2 or Mg+2 in newborn infants and affects both preterm and term infants. Hypocalcemia is determined as either total serum calcium (tCa) or ionized calcium (iCa). 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 Ca+2 and Mg+2. 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 2.25–2.65 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 Ca+2. It is the active physiological component and is dependent on the interaction of tCa+2, normal acid-base status, and normal serum albumin. Typical iCa+2 values for term infants over the first 72 hours of life are 1.24 (1.13–1.35) mmol/L to 1.22 (1.08–1.36) mmol/L (4.88–4.96 mg/dL). Preterm infant mean values are similar for 24 and 72 hours: 1.21–1.28 mmol/L (4.84–5.12 mg/dL). Interestingly, preterm infants slightly increase their iCa+2 levels, whereas term infants experience a slight decline. Ionized calcium levels of <4 mg/dL are considered hypocalcemic.
Incidence. Hypocalcemia is likely the most common disorder of either Ca+2 or Mg+2 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. Ionized Ca+2 is the biologically important form of calcium. The tCa+2 levels have been repeatedly shown to not be predictive of iCa+2 levels. Therefore, tCa+2 levels are unreliable as criteria for true hypocalcemia. In premature infants, it has been shown that tCa+2 levels as low as ≤6 mg/dL correspond to iCa+2 levels >3 mg/dL.
Early-onset neonatal hypocalcemia. During the third trimester of pregnancy, the human fetus receives at least 120–150 mg/kg/d of elemental Ca+2 via the umbilical cord. Most of this Ca+2 is readily incorporated into the newly forming bones. After delivery, this massive supply of Ca+2 is suddenly stopped, and Ca+2 must be given enterally.
A full-term infant receiving 100–120 mL of normal formula would be receiving 50–60 mg/kg/d of Ca+2 orally. Despite this drop in supply, full-term infants tolerate the change well and do not become hypocalcemic....