Unreliability of plasma magnesium values in asphyxiated neonates

The severely asphyxiated neonate usually shows a decreased blood pH and, if measured, a normal plasma Mg value. This is a retrospective review of the changes in these two parameters measured 2 or 3 times in 16 asphyxiated neonates as homeostasis was reestablished. The group was comprised of all asphyxiated neonates with 2 or 3 blood pH determinations, each with a plasma Mg determination within 6 h. Repeating these two tests had been prompted by poor clinical courses that suggested acid-base imbalance and Mg deficiency. The blood pH found on first sampling (day 0.42, with 0 the first day of life) was 7.24 +/- 0.02 (normal 7.35-7.45). This increased to 7.40 +/- 0.02 a mean of 2 days later (p less than 0.001 compared to first value by t test) and was still normal 4 days later (NS, compared to second value). The plasma Mg showed an opposing trend, with the first value 1.76 +/- 0.1 mEq/l (hospital normal 1.6-2.2 mEq/l); the second value 1.38 +/- 0.06 (p less than 0.004, compared to the first value), and the third, 1.30 +/- 0.11 (NS compared with the second value). It is concluded that plasma Mg determined during acidosis may give a falsely elevated value and may mask a true Mg deficiency which can be diagnosed only after homeostasis is reestablished, as when the blood pH is normal. While some asphyxiated infants were found to be hypomagnesemic, this study does not implicate Mg deficiency in meconium aspiration or any other cause of asphyxia.