Oxytocin-Induced Labor: Effects on Fetal Oxygen Saturation and Heart Rate Patterns

In the context of fetal well-being, less is known about assessment of uterine activity than about fetal heart rate (FHR) monitoring. Researchers conducted a retrospective study in 56 healthy nulliparous women admitted for elective labor induction to evaluate effects of oxytocin-induced uterine hyperstimulation in labor on fetal oxygen saturation (FSpO₂) and FHR patterns. Electronic fetal monitoring and FSpO₂ sensors were used.

All participants had singleton, vertex presentations with reassuring FHR patterns. Uterine hyperstimulation was identified based on two definitions: five or more but fewer than six contractions in 10 minutes (group 1) or six or more contractions in 10 minutes (group 2). The preceding 30 minutes of normal uterine activity were used as a comparator (group 3). Differences in FSpO₂ and FHR during the 5 minutes before and the last 5 minutes of each 30-minute evaluation period were analyzed. Interventions to treat hyperstimulation included decreasing or discontinuing oxytocin, lateral repositioning, and intravenous bolus administration of lactated Ringer’s solution. Hyperstimulation was considered to be resolved when fewer than five contractions per 10 minutes occurred for at least 20 minutes.

Approximately one hundred 30-minute periods for group 1 and 56 for group 2 were evaluable, which represented 15% of the total oxytocin-exposure time. Hyperstimulation was identified in 41 patients. FSpO₂ was significantly lower at the end than at the beginning of the 30-minute intervals for both group 1 and group 2, but the absolute decrease in FSpO₂ was greater in group 2 than in group 1 (P<0.001). As contraction frequency increased, the effect on FSpO₂ became more pronounced, with progressive oxygen desaturation during the 30 minutes of hyperstimulation. For groups 1 and 2, compared with group 3, no differences were observed in baseline FHR, but more periods of absent and minimal FHR variability, fewer accelerations, and more late and recurrent decelerations occurred. Changes in FHR variability first appeared after 24 and 22 minutes of excessive uterine activity for groups 1 and 2, respectively, but fetal O₂ desaturation occurred within the first 5 minutes and decreased during the entire 30-minute period. Use of all three interventions resolved hyperstimulation episodes more quickly than did one or two interventions.