Effects of infusion of the beta-adrenoceptor agonist terbutaline on serum magnesium in pregnant women.

Seventeen third trimester pregnant women received a 30-min intravenous infusion of terbutaline 20 micrograms/min. The drug caused a prompt (30 min) drop in serum potassium (p less than 0.001) and a decrease after 120 min in serum magnesium concentrations (p less than 0.002). No significant changes in calcium or sodium levels were observed. The altered serum magnesium concentrations were significantly (p less than 0.05) correlated to increases in plasma free fatty acid levels but not to elevated plasma glucose concentrations. Whatever the mechanism for the lowering of serum magnesium may be, the potential danger of administering a beta-adrenoceptor agonist to pregnant women with low serum potassium and magnesium levels is underlined.