Risk:benefit considerations for the use of isoxsuprine in the treatment of premature labor

Seventy patients treated with isoxsuprine for premature labor were studied. In patients with intact membranes prolongation of pregnancy for more than 7 days occurred in 77% of women with 50% cervical effacement or less and 3 cm dilatation or less at the initiation of therapy, and in none with more than 50% effacement and more than 3 cm dilatation. Cervical effacement was the primary factor in determining success. Cord isoxsuprine concentrations averaged 90% of maternal concentrations at delivery. Maternal and cord isoxsuprine concentrations at delivery were inversely correlated with the drug-free interval before delivery. An interval of more than 5 hours was necessary to attain a cord concentration of less than 2 ng/ml, a level not associated with neonatal problems. Drug-free intervals of 2 hours or less usually resulted in cord isoxsuprine values of more than 10 ng/ml, levels that are associated with severe neonatal problems. Seventy-seven percent of infants with cord isoxsuprine concentrations of more than 2 ng/ml and 91% with values of more than 10 ng/ml were delivered of mothers with more than 3 cm dilatation or more than 50% effacement at the initiation or reinstitution of intravenous therapy. Most severe neonatal problems are preventable if patients are selected carefully.