A loading model for ritodrine administration in preterm labour.

Abstract

OBJECTIVE: To develop a ritodrine infusion scheme for preterm labour that avoids plasma levels above those needed for tocolysis, requires only one rate adjustment, and is easy to apply in practice. DESIGN: Prospective study of tocolytic effect and plasma ritodrine concentrations during application of the infusion scheme. SETTING: High risk labour ward. SUBJECTS: Consecutive series of 31 women in labour at less than 36 weeks' gestation. INTERVENTION: Loading dose ritodrine infusion followed, as soon as tocolysis is reached, by a decrease in the infusion rate calculated on the basis of the interval between start of treatment and tocolysis. RESULTS: Overall, steady state ritodrine levels were nearly identical to those at the time of tocolysis and correlated well with levels anticipated on the basis of our calculation (n = 30; r = 0.91; P < 0.001). Adjustments during steady state were made in 12 women (40%), but in only two of them within 12 h after tocolysis had been reached. Delivery was postponed for more than 48 h in 29 women (93.5%) and beyond 37 weeks' gestation in 19 (61.3%). CONCLUSION: The loading model is easy to apply, avoids relative overdoses, requires few adjustments, is well tolerated, uses smaller quantities of ritodrine, and results in lower plasma ritodrine concentrations than the conventional infusion scheme.