To begin with, the scar rupture rate at this hospital is 1.4%, so my first question is, "What are they doing to these poor women?" because we know this rate is three times higher than it should be. Well, actually, we know what they are doing to them because Table 2, p 319 tells us:
- 23% of controls vs. 42% of cases were induced
- 48% of controls vs. 65% of cases were augmented
- 4% of controls vs. 19% had misoprostol AKA Cytotec
- 0% of controls vs. 8% of cases had prostaglandin gel
- 0% of controls vs. 12% of cases had an arrest of labor
- Controls had oxytocin for fewer hours, at lower dosages, or both.
Take home message: Spontaneous labor best avoids scar ruptures, but if induction or labor augmentation is needed, use more physiologic doses of oxytocin, and if induction is needed, wait for a ripe cervix. [Induction has another downside not evaluated in this study: it is consistently found to reduce the VBAC rate. See "Step 6" in "Evidence Basis for the Ten Steps of Mother-Friendly Care," which is downloadable at www.motherfriendly.org.]
One weakness of the study is that it is so small. Small numbers play havoc with the statistical calculations that tell you how likely it is that differences are due to chance. When numbers are small, a different result or characteristic in one or two participants makes a big difference in the percentage of the sample population. An apocryphal story tells of the researchers who announced a new treatment for a disease of chickens by saying, "33.3% were cured, 33.3% died--and the other one got away." For this reason, I didn't pay a lot of attention, which I usually would, to the statistical significance of these characteristics. For example, despite the large differences between groups, the fairly consistent finding in other research, and the association with methods of induction, induced labor was not significantly associated with scar rupture in this study. On the opposite side, use of laminaria (a sponge inserted in the cervix that is used as a mechanical means of softening and opening it) was significantly greater in cases vs. controls despite the fact that this represented one woman vs. no women, and other studies do not find an association between mechanical means of cervical ripening and scar rupture.
Another weakness is that the authors assume that persistent abdominal pain and variable decelerations in the fetal heart rate at "less than 2 hours before acute rupture" are predictors of scar rupture, implying that nothing is currently wrong. I think it far more likely that they are symptoms of a scar rupture in progress that may become a catastrophe if ignored. Given the tragic stories I have heard of VBAC women complaining of unusual pain who were brushed off--"You have to expect labor to hurt, Dear"--I rather wonder if the use of this terminology isn't a kind of weasel wording to avoid malpractice liability.
More useful for your purpose is that the investigators describe the pain as continuous, that is, present in between contractions, and as breaking through an epidural. Persistent abdominal pain in combination with severe variable decels made scar rupture extremely likely.
Take home message: If you are in a VBAC labor and experiencing pain with these qualities, do whatever you need to do to get an urgent cesarean. Do not take “no” for an answer. Not every scar rupture is signaled by pain, but pain like this is a strong sign that something is going wrong. The scar might turn out to be fine, but it isn’t a chance I would recommend taking. Continuous abdominal pain in combination with severely abnormal fetal heart rate is even more ominous.
Finally, under the heading of "they should be ashamed of themselves," the investigators note in their discussion the association between scar rupture and induction/augmentation agents and that it is probably causally related to their finding of an association between uterine hyperstimulation and scar rupture. They conclude that the associations they found may be useful "predictors" of uterine rupture, but not one word do they say about the implications of their study for avoiding practices that they have documented increase risks.
-- Henci