Acute cord occlusion increases blood ionized magnesium concentration in preterm fetal sheep during maternal magnesium sulfate exposure

This study tested the hypothesis that a pathophysiologic insult to the fetu s that decreases pH (umbilical cord occlusion) produces an increase in phys iologically active (i.e., ionized) magnesium concentration. Preterm pregnan t sheep (n = 7) were instrumented with maternal and fetal catheters and an inflatable vascular occluder was placed around the umbilical cord. After a 2-day recovery period, each ewe received a 4-g loading dose, followed by co ntinuous intravenous infusion of 1 g magnesium sulfate/h. After 48 h, an ep isode of acute fetal distress was produced by inflation of the umbilical oc cluder for 10 min. Maternal and fetal arterial blood samples were collected at regular intervals to quantitate ionized magnesium concentration and mon itor physiologic status. Magnesium sulfate infusion increased maternal and fetal blood ionized magnesium concentration. In vitro blood analysis demons trated that there was a linear inverse correlation (r(2) = 0.99) between fe tal sheep blood pH and ionized magnesium concentration. In vivo, 10 min of umbilical cord occlusion produced an increase in fetal blood ionized magnes ium concentration in all animals (P = 0.02) that was temporally related to the decrease in fetal blood pH. Whether this increase in physiologically ac tive magnesium concentration is beneficial (via neuroprotection) or deleter ious (via suppression of stress response) to the distressed fetus remains t o be determined.