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.