Sunday, November 02, 2008

Should we redefine labor arrest?

A new study, just published in the November issue of Obstetrics and Gynecology, raises the possibility of redefining "arrest of labor."

The classic definition comes from the original work on the Friedman curve. Arrest of labor can only be diagnosed when specific conditions are met. First, a woman must be in active labor, which generally means that the cervical exam must be 5-6 cm or more. Second, there must be adequate contractions for at least 2 hours with no progress. In practice, that typically means 2 hours of labor with no progress followed by 2 hours of labor augmented with pitocin with no progress. The authors of the new paper asked the question: "What happens when we ignore that definition and women continue to labor?"

The paper is Perinatal Outcomes in the Setting of Active Phase Arrest of Labor by Henry et al. The study is retrospective and observational. The authors had no control over which women continued to labor and which women underwent C-sections or at what point they were operated on. That means that the study has some serious limitations. It is not a randomized control trial; the women in the group allowed to labor beyond a diagnosis of arrest might have differed in important ways from the other women; it is also quite underpowered because it contains only 1,014 women. The authors are quite forthcoming about those limitations. Nonetheless, the study has value in suggesting the design of future prospective, randomized trials structured to answer the question of whether we should redefine labor arrest.

What did the study show? Of 1,014 women who met the criteria for the diagnosis of arrest of labor, 335 (33%) went on to deliver vaginally. The women who ultimately had a vaginal delivery differed in important ways from the women who had C-sections:
Compared with women who had cesarean delivery, women who delivered vaginally had a lower BMI (mean BMI 23.4 compared with 25.3 kg/m,2 P less than .001), and delivered slightly smaller infants (mean birth weight +/- standard deviation 3,533 g (+/-658) compared with 3,700 g (+/-493), P less than .001
Maternal outcomes differed for the two groups, but neonatal outcomes did not:
When the frequencies of adverse outcomes were compared using a multivariable logistic regression model to control for potential confounders, we found that cesarean delivery was associated with an increased risk of chorioamnionitis (adjusted odds ratio [aOR] 3.37, 95% confidence interval [CI] 2.21–5.15), endomyometritis (aOR 48.41, 95% CI 6.61–354), postpartum hemorrhage (aOR 5.18, 95% CI 3.42–7.85) ... Adverse neonatal outcomes, however, were not statistically significantly associated with cesarean delivery.
The women who had vaginal deliveries after a diagnosis of active phase arrest were compared to a larger group of women who had vaginal deliveries without arrest of labor. There were important differences between the two groups.
Among women with active phase arrest, there was an increased rate of operative vaginal delivery (28% compared with 17%, P less than .001) as well as increased rates of several adverse maternal outcomes, including, chorioamnionitis (18% compared with 8%, p less than .001), third- or fourth-degree perineal lacerations (16% compared with 9%, P less than .001), and postpartum hemorrhage (26% compared with 17%, P less than .001), compared with other women having a vaginal delivery without active phase arrest... Examination of neonatal outcomes revealed increased rates of shoulder dystocia (4% compared with 2%, P less than .01) and 5 minute Apgar less than 7 (5% compared with 2%, P less than .001) among women with active phase arrest compared with those without.
The authors summarize their results:
To systematically evaluate the rates of adverse perinatal outcomes among women with active phase arrest, we made two comparisons. First, we looked only at women with active phase arrest and compared the outcomes by mode of delivery: vaginal delivery to cesarean delivery. In women with active phase arrest, cesarean delivery was associated with an increased risk of chorioamnionitis, endomyometritis, and postpartum hemorrhage. However, cesarean delivery was not associated with adverse neonatal outcomes in women with active phase arrest. These findings suggest that efforts to achieve vaginal delivery in the setting of active phase arrest may reduce the maternal risks associated with cesarean delivery without additional risk to the neonate.
Is it time to change the definition of active phase arrest of labor? Not yet. There are two critical questions that have not been answered by this observational study. First, though the authors tell us that 33% of women diagnosed with active phase arrest went on to have a vaginal delivery, they don't tell us how the decision for C-section was made for the other 66%. What proportion of those women continued to labor and then had a C-section after 3 or 4 hours of arrest? What proportion of those women proceeded to immediate C-section because there were other factors (such as chorioamnionitis) that led to a decision to operate? We want to know what happens when women are allowed to labor beyond 2 hours of arrested progress, and this study cannot tell us that. Second, what happens in larger groups. This study is underpowered, and would not be expected to demonstrate a difference in neonatal mortality rates. A much larger study is needed to explore whether extending labor beyond the definition of arrest has an increase in neonatal mortality or severe morbidity.

This study lays the groundwork for a prospective, randomized controlled trial to answer the question of whether the definition of arrest of labor needs to be changed.

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