Serum nitrites predict the response to prostaglandin-induced delivery at term

OBJECTIVE: To evaluate whether the clinical response to prostaglandin- induced labor is modulated by nitric oxide (NO) activity. METHODS: Fifty-two cases of nulliparous women at term who delivered vaginally after prostaglandin E (PGE) induction of labor were enrolled. The induction was required mainly for amniotic fluid reduction or late-onset gestational hypertension. Either intracervical (0.5 mg) or vaginal (2.0 mg) PGE was administered every 12 hours, according to the Bishop score. After the third PGE application, in absence of labor onset, intravenous oxytocin was used. Nitrites/nitrates (NO(x)) serum levels were used as a marker of NO activity. They were measured just before the start of induction by using an enzymatic reduction and then a colorimetric evaluation. Time to delivery from the first PGE application was the main outcome variable. RESULTS: Time to delivery ranged from 4 to 62 hours (median: 15.5). Nitrites/nitrates levels were unaffected by both gestational age, Bishop score at entry, indication allowing labor-induction, fetal position, and birth weight. In a multiple regression analysis including the previous factors, NO(x) levels significantly explained 33.9% of the variance of the time to delivery. Indeed, patients delivering within 15 hours (26.4 ± 6.9) showed NO(x) levels significantly lower than in patients delivering after more than 15 hours (39.5 ± 16. 4) from the first PGE application. CONCLUSIONS: A reduced level of NO(x) is associated with a prompt clinical response to PGE-induced labor. Provided we do not know the origin of NO(x) in the general circulation, these data indicate NO(x) levels as predictors of the response to PGE-induced delivery at term and support the hypothesis that labor onset is modulated by the endogenous NO activity.