Studies have revealed a wide range in the incidence and severity of uterine hyperstimulation. Crane and colleagues related the incidence and timing of excessive uterine activity to the routes and forms of misoprostol compared with spontaneous labor and other forms of induced labor.
The authors studied all deliveries between 1994 and 1997 at a Canadian university hospital. Women in three randomized controlled trials evaluating misoprostol for induction of labor and a cohort of women who presented with spontaneous labor were included. Each trial included singleton pregnancies in cephalic presentation at more than 37 weeks' gestation, with fetal heart rates and uterine activity graphs available for review. Other inclusion requirements were maternal age of at least 19 years, no contraindication to vaginal birth, and any prior uterine surgery or use of misoprostol. The study compared the fetal heart rate recordings and uterine activity graphs for the 519 women who were induced and for 86 women with spontaneous labor. Tachysystole was defined as more than five contractions within 10 minutes for two consecutive 10-minute periods. Hyperstimulation was defined as exaggerated uterine response with late fetal heart rate decelerations or fetal tachycardia of more than 160 beats per minute or other worrisome fetal heart rate changes.
|Group||Tachysystole n/N (%)||Hyperstimulation n/N (%)|
|Misoprostol vaginal tablet (50 mcg)||51/105 (48.6)||8/105 (7.6)|
|Misoprostol vaginal gel (50 mcg)||31/101 (30.7)||8/101 (7.9)|
|Misoprostol oral tablet (50 mcg)||12/54 (22.2)||1/54 (1.9)|
|Misoprostol oral crushed form (50 mcg)||16/103 (15.5)||1/103 (1.0)|
|Dinoprostone intravaginal gel (1 or 2 mg) or intracervical gel (0.5 mg)||34/103 (33.0)||17/103 (16.5)|
|Induction with oxytocin||16/53 (30.2)||1/53 (1.9)|
|Spontaneous onset of labor (all)||20/86 (23.3)||2/86 (2.3)|
|No augmentation||9/43 (20.9)||0/43 (0.0)|
|Augmentation (oxytocin)||11/43 (25.6)||2/43 (4.7)|
Tachysystole occurred most frequently when misoprostol was given vaginally, with the highest incidence (48.6 percent) occurring when the vaginal tablet was used (see accompanying table). Induction with oral misoprostol was associated with lower rates of tachysystole than oxytocin induction. The lowest rates of tachysystole, associated with crushed oral misoprostol (15.5 percent), were comparable to those of nonaugmented spontaneous labor (20.9 percent). A similar pattern was observed for hyperstimulation, with the highest rates occurring with intravaginal forms.
The authors conclude that the incidence of tachysystole hyperstimulation and time to tachysystole vary with the type of misoprostol used and the route of administration.