Epidurals and cesarean deliveries: a new look at an old problem.

Abstract

The influence of epidural analgesia on the course of labor continues to generate debate both within the anesthesia community and, even more so, outside of it. The issue is far from trivial. The literature is divided on the phenomenology itself (Does epidural analgesia in fact adversely affect the course of labor?) and nearly devoid of unblemished studies that attempt to explain possible mechanisms of any putative effects (Why might epidural analgesia affect the course of labor?). Nonetheless, obstetricians, midwives, nurses, lay labor personnel, patients, hospital administrators, insurance executives, and health care policymakers are all participating in this debate. Despite a lack of evidence to support their claims, epidural analgesia continues to be indicated by a vocal minority as an important cause of cesarean delivery as a result of dystocia. In this issue, Hess et al. (1) add a new twist to this old controversy. In a retrospective evaluation of 4493 patients receiving epidural analgesia, they concluded that more pain in labor (as measured by number of requests for additional doses of local anesthetic after initiation of epidural analgesia) was independently associated with dystocia. Because pain is the reason why parturients request epidural labor analgesia, it may be that it is the pain itself, and not the initiation of an epidural, that might be the cause of cesarean deliveries in women receiving epidurals. This proposition could be of astounding importance in understanding the suggested association between epidural analgesia and cesarean delivery for dystocia. To understand how this finding may contribute to our understanding of this relationship, some review of the present state of the debate is necessary. In recent years, several randomized, prospective trials of epidural analgesia compared with an alternative analgesia method, usually parenteral meperidine, have been reported by both anesthesiologists and obstetricians (2–7). Statistical meta-analyses have confirmed the lack of impact of epidural on the rate of cesarean delivery (8–9). These trials have commonly, but not always, concluded that there is no increased risk of cesarean delivery associated with epidural analgesia. However, many of the trials that have reported an association between epidural analgesia and cesarean delivery have been plagued by methodological problems. Crossover between treatment groups, inadequate sample size, and inappropriate statistical analysis have led many clinicians and academicians to question the results of these trials. Moreover, randomized and blinded trials cannot be easily performed, because it is ethically unacceptable to assign women to either receive or not receive labor analgesia against their wishes, and the obstetrician who must make the clinical decisions about mode of delivery is always aware of the patients’ treatment group. In addition, there is a question of external validity. Women agreeing to a 50% (random) chance of receiving epidural analgesia in labor may represent an uncharacteristically ambivalent subset of the labor and delivery population, making it difficult to generalize from the results of these trials. Retrospective comparisons of women who choose epidural analgesia to those who do not are much easier to perform and eliminate the issues of external validity, crossover, and blinding, but they suffer from an even greater problem: selection bias. Women who select epidural analgesia in labor are different from women who do not in many ways that may make them more likely to have a longer, slower labor and an operative delivery. They are more frequently nulliparous, tend to come to the hospital earlier in labor and with higher fetal station, have slower cervical dilation before analgesia, more often are already receiving oxytocin for the induction or augmentation of labor, deliver larger babies, and may have received epidural analgesia because of other perceived risk factors for operative delivery, such as poor fetal status or maternal systemic disease (10–11). Women requesting epidural analgesia also have smaller pelvic dimensions, another risk factor for dystocia (12). The article by Hess et al. (1) is another retrospective attempt to resolve this controversy. For many women, labor is the most painful experience of their lives. The idea that more pain in labor could reflect dysfunctional labor (dystocia) is a dramatic one. Perhaps a relative disproportion between the dimensions of the fetus and the pelvis causes more stimulation of pelvic nociceptors. Uncoordinated uterine contractions or contractions in the absence of cervical dilation may cause enhanced pain. Perhaps pain either causes or is a marker for an enhanced stress response, including sympathetic activation, which may cause reduced intensity or effectiveness of uterine activity (13). Regardless of the mechanism, the observation itself means that any retrospective comparison between epidurals and other forms of analgesia must account for pain or be subject to selection bias. To our knowledge, this has never been accomplished. Although an excellent beginning, the study by Hess et al. (1) has not definitively proven the relationship between labor pain and cesarean delivery as a result of dystocia. Several methodological limitations require a cautious interpretation of their results. First, the authors did not measure pain in labor per se, but rather the surrogate measure of the number of supplemental doses of local anesthetic required. The relationship between pain and request for additional epidural medication is logical, but unproven. Patients might ask for additional local anesthetic for other reasons besides pain: they may desire a greater feeling of numbness, want more attention from their nurses or physicians, be less sensitive to local anesthetic, or may not have an optimally functioning epidural catheter. Second, the actual dose of local anesthetic given to each patient was not standardized or reported, and the meaning of a “bolus” of local anesthetic may vary from patient to patient. Of note, the authors found that the association between number of boluses and dystocia persisted, even after accounting for the variable concentration of bupivacaine used during the study period. Nonetheless, the volume of drug given may have varied, and this makes the number of boluses an even more indirect surrogate measure of pain. Third, the authors analyzed bolus number rather than bolus frequency (boluses divided by time). Women with dysfunctional labor tend to have longer labors, and therefore, it might not be surprising that such women required more boluses of anesthetic. Indeed, after correcting for the longer labors in women requiring cesarean delivery, the number of boluses per hour was identical in women who delivered vaginally and abdominally (1). Further work will be required to elucidate the meaning of different patterns of requests for additional analgesia. That the association between pain and dystocia is not confounded by other factors must be demonstrated. For example, if the increased pain of labor in patients with dystocia was the result of nulliparity (which is a commonly observed association), then demonstrating an association between pain and dystocia would be trivial. We already know that nulliparity is associated with dystocia. Hess et al. (1) controlled for several potential confounders by multiple logistic regression (including parity, maternal age, body mass index, local anesthetic concentration, neonatal birthweight, and the induction of labor). However, it is certainly possible that another factor could contribute to both the risks of dystocia and of increased pain. An important example (raised by the authors themselves) is obstetrical practice style: obstetricians may have treated pain as an indication for cesarean delivery. Is a prospective study of this phenomenon possible? In reality, a more applicable study would involve prospectively separating out a group that is more likely to have dystocia and compare it with a group with no reason for dystocia. The groups would include only nulliparous patients, and the study would control for maternal age, body mass index, fetal position, induction of labor, and fetal station, and the study would measure pain scores at different dilations before the epidural was administered. Given the limitations imposed by their methodology, the article by Hess et al. (1) is an important, but nondefinitive step in evaluating a possible association between epidural analgesia and cesarean delivery for dystocia. Because most women in the United States now receive epidural analgesia in labor (14), it is unlikely, however, that a clinical researcher will be able to convince a parturient to forego the epidural experience while assessing her degree of labor pain for any proposed study, no matter how important. This study by Hess et al. (1) is yet another look at the old question of whether epidurals cause cesarean deliveries. However, it is of significant general interest and importance (15) and a meaningful step to what might forever change the landscape of this debate and the practice of obstetric anesthesiology.