First-stage arrest of labor is deined as a completed latent phase and contractions exceeding 200 NIontevideo units for more than 2 hours without cervical change. Some have attempted to deine a more accurate duration for active-phase arrest (Spong, 2012). Arulkumaran and coworkers (1987) extended the 2-hour limit to 4 hours and reported a 1.3-percent cesarean delivery rate in women who continued to have adequate con tractions and progressive cervical dilation of at least 1 cm/hr. In another 4 hours of labor, half required cesarean delivery.
Rouse and colleagues (1999) prospectively managed 542 women at term with active-phase arrest and no other complications. Their protocol was to achieve a sustained pattern of at least 200 Montevideo units for a minimum of 4 hours. This time frame was extended to 6 hours if activity of 200 Montevideo units or greater could not be sustained. Almost 92 percent of these women were delivered vaginally. As discussed in Chapter 23 (p. 443), these and other studies support the practice of allowing an active-phase arrest of 4 hours (Rouse, 2001).
Zhang and coworkers (2002) analyzed labor duration from 4 cm to complete dilatation in 1329 nulliparas at term. They found that before dilation of 7 cm was reached, lack of progress for more than 2 hours was not uncommon in those who delivered vaginally. Alexander and associates (2002) reported that epidural analgesia prolonged active labor by 1 hour compared with duration of the active phase as deined by Friedman (1955). Consideration of these changes in the management oflabor, especially in nulliparas, may safely reduce the cesarean delivery rate.
As data have accrued, investigators have increasingly questioned the thresholds for labor arrest disorders established by Friedman and others in the 1960s. In particular, investigators with the Consortium on Sai Labor reported that half of cases of dystocia after labor induction occurred before 6 cm of cervical dilation (Boyle, 2013; Zhang, 2010c). Even for women with spontaneous labor, these researchers found that active-phase labor was more likely to occur at 6 cm, and after slow progress between 4 and 6 cm (Zhang, 2010a). Additionally, they reported that a 2-hour threshold for diagnosing arrest disorders may be too brief when cervical dilation is <6 em (Zhang, 201Ob). This is discussed in detail in Chapter 23 (p. 444). Importantly, however, these studies of data from the Collaborative Perinatal Project included only singleton term gestation with spontaneous onset of labor, vaginal delivery, and a normal perinatal outcome. By excluding abnormal outcomes, cesarean deliveries, and those who were more than 6 cm dilated upon arrival, the above studies that sought to redeine the labor curve
Induction and Augmentation of Labor 511 have been faulted for introducing biases that limit general use of these findings (Cohen, 20 15a, b).
Elective amniotomy with the intention of accelerating labor is often performed. Shown in Table 26-4, amniotomy at approximately 5-cm dilation accelerated spontaneous labor by 1 to 1 Y2 hours. Importantly, neither the need for oxytocin stimulation nor the overall cesarean delivery rate was increased. Although the incidences of mild and moderate cord compression patterns were raised following amniotomy, cesarean delivery rates for fetal distress were not higher. Most importantly, there were no adverse perinatal efects.