Pharmacology of tocolytic agents.

Various drugs have been used to inhibit preterm labour. Selective beta-receptor agonists are the drugs of choice and have been extensively studied for more than ten years. Several controlled studies have demonstrated a significant prolongation of the pregnancy after treatment with beta-receptor agonists and some have demonstrated improved fetal outcome. There is apparently sufficient evidence to conclude that in selected patients, treated with these drugs for inhibition of preterm labour, there is a beneficial effect on the fetal outcome. These drugs are often given in high doses and therefore there is a potential risk of severe side effects. Careful monitoring of mother and infant is essential. There has been doubt as to the efficacy of beta-receptor agonist therapy since the rate of preterm deliveries (less than 37 weeks) is unchanged in most countries despite frequent use of these drugs. However, only 15 to 17 per cent of the preterm population constitutes deliveries before week 32, which is of greater interest than the overall preterm rate. Since the introduction in Sweden of beta-receptor agonist therapy there has been a significant decrease of deliveries prior to week 32 and of infants with a birthweight of less than 1500 g. In the same population the perinatal mortality rate has decreased from 60 per cent in 1973 to 40 per cent in 1980. Among the various factors influencing these figures the inhibition of preterm labour with beta-receptor agonists may have contributed to the successful results.