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OBSTETRIC ANESTHESIA

An Association Between Severe Labor Pain and Cesarean Delivery

Department of Anesthesia and Critical Care, Beth Israel Deaconess Medical Center, Harvard Medical School, Boston, Massachusetts

	Abstract

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The relationship between epidural analgesia and cesarean delivery remains controversial. Several studies have documented an association, although others have not. This inconsistency may result from an association between severe labor pain and dystocia. We hypothesized that dystocia causes severe labor pain, such that more epidural medication is required to maintain comfort. We examined the relationship between labor outcome and severe pain, defined by the number of supplemental epidural boluses. We retrospectively reviewed the anesthesia records of 4493 parturients who received small-dose labor epidural analgesia. An independent association was found between operative delivery and maternal age, body mass index, nulliparity, fetal weight, induction of labor, and the number of boluses required during labor. By using multivariate analysis, the odds ratio of cesarean delivery among women who required at least three boluses was 2.3 compared with those who required two boluses or less. No association was found between the concentration of bupivacaine in the epidural infusion and operative delivery. Because women with cesarean deliveries appeared to have more pain, degree of labor pain may be a confounding factor in studies examining epidural analgesia and outcome.

Implications: This is a retrospective observational study demonstrating an association between labor pain and cesarean delivery. Our results provide an alternative explanation of why epidural analgesia is associated with cesarean delivery.

	Introduction

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Aparturient with dysfunctional labor may suffer more severe pain, leading to a request for epidural analgesia. Several studies have demonstrated higher rates of cesarean delivery among patients who receive epidurals (1–6). Others have been unable to support this (7–9). One prospective study (10) comparing parturients given epidurals to those given IV narcotic via patient-controlled analgesia found no difference in rates of cesarean delivery between groups. Selection bias is one explanation for this inconsistency. Severity of pain during the latent phase of labor has been associated with labor outcome (11). Women with severe pain early in labor may be more likely to request an epidural, and this may result in selection bias.

Small-dose epidurals (0.125% bupivacaine or less) were designed to minimize loss of strength and sensation, while maintaining comparable pain relief and maternal satisfaction. Small-dose epidurals have been proven effective in most women, but some parturients require supplemental doses of epidural medication to treat breakthrough pain (12). These women may have been suffering from pain so severe that they require more epidural medication for effective pain relief. Our hypothesis was that dystocia causes more severe labor pain, leading to increased requirements for epidural medication. We conducted this study to evaluate whether there was an association between the requirements for supplemental medications during small-dose labor epidural analgesia and cesarean delivery.

	Methods

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This study was developed from an analysis of a database examining the effectiveness of small-dose epidural analgesia in labor at Beth Israel Deaconess Medical Center, Boston, MA. After approval by the hospital committee on clinical investigations, we retrospectively reviewed the anesthetic records of patients who received epidural analgesia for labor during two periods: September 1, 1989 to August 31, 1990, and September 1, 1993 to August 31, 1994. During the first period, the most common concentrations of bupivacaine used during infusion were 0.125% or 0.0625%, combined with fentanyl. During the second period, the center had fully adopted a very small dose solution (0.04% bupivacaine with fentanyl). All women had multiorifice epidural catheters placed by a resident under the supervision of an attending anesthesiologist or by an attending anesthesiologist. Exclusion criteria were 1) use of a technique other than epidural infusion; 2) use of medications other than bupivacaine as the primary local anesthetic; 3) use of combined spinal epidural techniques, 4) epidural catheter replacement; or 5) incomplete information obtained from chart review. We excluded women who received a combined spinal epidural because this was not a standard technique during the study period. The intrathecal dosage was variable and most women did not have an infusion started immediately after spinal placement. Women in whom the epidural catheter was replaced were excluded to limit the influence of technical problems as a cause of supplementation.

We recorded maternal age, height, weight, duration of epidural analgesia, mode of delivery, the bupivacaine concentration of the epidural infusion, and number of supplemental epidural boluses. Epidural medications given for cesarean anesthesia, for assisted vaginal delivery, or after delivery were excluded. Additional data were available during the second period, including maternal parity, whether labor was induced, and fetal birth weight. We analyzed parturients by body mass index (BMI), as this value may be a better indicator of physical size and cesarean risk (13,14). We defined abnormal labor pain as that which required more than two supplemental boluses and referred to this as "recurrent breakthrough pain" (RBP). Women who required zero to two boluses were defined as having "minimal breakthrough pain" (MBP). We chose two boluses as our cutoff for two reasons. First, in an observational pilot study, this was more than one standard deviation greater than the mean number of boluses. Second, on analysis, women who required more than two boluses appeared to have characteristic outcomes.

We performed a univariate analysis of the factors associated with cesarean delivery, including previously described factors of age and BMI, and the number of supplemental epidural boluses received. Then, multiple logistic regression was used to evaluate the influence of covariates on the risk of both cesarean delivery and assisted vaginal delivery compared with normal vaginal delivery. By using the combined study periods, initial analysis compared maternal age, BMI, and the number of boluses. Because the concentration of bupivacaine used for infusion may be a confounding variable, we forced this into the analysis based on clinical importance. We performed an additional multivariate analysis of the subgroup from the second study period to determine the influence of maternal parity, induction of labor, and fetal weight on cesarean delivery. Because the center had converted to a single concentration of local anesthetic, we were unable to evaluate the effect of bupivacaine concentration in the second analysis.

Continuous data were compared by using the two-tailed t-test; however, the number of boluses did not resemble a normal distribution, so the Mann-Whitney U-test was used for this variable. Frequency data were compared by using the ² test. Odds ratios of cesarean delivery versus individual bolus numbers were calculated and compared by using the Mantel-Haenzel ². We determined significance at P < 0.05.

	Results

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Data were collected from 5061 women. We excluded 25 women (0.5%) because of incomplete records, 171 women (3.4%) who did not receive bupivacaine as the primary medication or did not receive an infusion, and 372 (7.4%) women who received intrathecal analgesia. Of the remaining 4493 women, 2019 were from the first study period and 2474, from the second. Of these, 701 parturients received 0.125% bupivacaine as an infusion, 1285 received 0.0625%, and 2507 received 0.04%. The results of univariate analysis of demographic and labor data appear in Table 1. Women who delivered by cesarean were older (P < 0.001), had a larger BMI (P < 0.001), and required a greater number of boluses (P < 0.001) than women who delivered vaginally. We also found that they were twice as likely to suffer from RBP (RBP = 22.3%, MBP = 11.1%, P < 0.001). Odds ratios for cesarean delivery among women requiring zero, one, and two boluses were 0.75, 0.85, and 0.88, respectively. The odds ratios for three, four, five, six, and seven-plus boluses were 1.9, 2.2, 2.2, 2.2, 2.3, respectively. Odds ratios for cesarean delivery among women receiving between zero and two boluses were similar and could be combined with validity, as well as those between three and seven. However, no bolus number greater than two could be combined with any of those between zero and two (P < 0.001). Because greater than seven boluses was rarely required (<0.1% of data), these boluses were analyzed as part of the seven-bolus group.

View larger version (17K): [in this window] [in a new window]   Figure 1. Cesarean delivery rate by number of boluses received.

	Discussion

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Epidural analgesia is associated with cesarean delivery; however, whether this association is because of a cause-and-effect relationship remains unresolved. Retrospective studies have shown that women who received epidural analgesia had higher rates of cesarean delivery when compared with control subjects (1–3). These studies, however, cannot ensure matched cohorts, because factors that may play a significant role, such as the reason the women received an epidural, may have been lost in review; therefore, their results must be interpreted with caution. Prospective studies have shown mixed results. In the study by Thorpe et al. (4), women who received an epidural had a 25% cesarean rate versus only a 2.2% rate in the control group. This study has been criticized for several methodological problems, including lack of blinding, lack of a strict protocol for obstetric management, and early termination of the study (15). Philipsen and Jensen (7) demonstrated a difference in cesarean rates between the epidural (17.5%) and nonepidural (11.1%) groups that was not statistically significant. In retrospectively reviewing the data from a nonrandomized trial evaluating the efficacy of active management of labor, Lieberman et al. (6) found a nearly 4-fold increase in the risk of cesarean delivery among women who received an epidural. This study stratified parturients according to their cesarean risk factors to determine whether an epidural would affect specific subgroups of women. Women in the higher risk groups were more likely to receive an epidural; however, the cesarean rate was similar among women who received an epidural regardless of the preexisting risk factors. As mentioned in the discussion of that study, there are two interpretations of these data: first, epidural analgesia may lead to a significant increase in the rate of cesarean delivery; second, women who will ultimately deliver by cesarean may have painful labors and request epidural analgesia. Because the epidural rate was 60%, Lieberman et al. (6) rejected the second explanation as not plausible. Our data, however, support this second explanation. After controlling for cesarean risk factors of age, BMI, fetal weight, nulliparity, and induction of labor, women delivering by cesarean were more likely to suffer from severe pain requiring epidural supplementation. In other words, the cause of the dystocia (e.g., cephalo-pelvic disproportion) may have been the cause of the severe pain. Unfortunately, because we were not able to compare our data with those parturients who did not receive labor analgesia, we cannot make a definitive statement regarding the full spectrum of maternal pain and cesarean risk. Logic would indicate that as the pain and difficulty of labor increase, so would the cesarean rate and requests for analgesia. In a study by Sharma et al. (10) comparing epidural analgesia and unlimited narcotic in the control group, no difference in the cesarean rate was found. Thus, among women who request labor analgesia, the method of pain relief (epidural versus narcotic) does not appear to alter the cesarean delivery rate.

Our explanation for the association between breakthrough pain and cesarean delivery is that dysfunctional labor, or dystocia, causes both significant maternal pain and the need for cesarean delivery. This maternal pain leads to a request for epidural analgesia; however, because of the severity of the pain, multiple supplemental boluses are required to maintain comfort. Parallel to this, dysfunctional labor leads to the need for a cesarean delivery. Thus, the association between breakthrough pain and cesarean delivery is not causative, but related via the original source of both. We assume that not all of the parturients who required multiple boluses were suffering from severe pain; despite eliminating catheters which needed to be replaced, many of these women must have had technically inadequate epidurals that required increased supplementation. Previous research has demonstrated that some women may have poor spread of epidural medications (16), and this may cause the need for supplementation. However, Le Coq et al. (17) found that epidural analgesic failure (characterized by higher visual analog pain scores and increased epidural supplementation) was associated with maternal weight, fetal weight, and abnormal fetal presentations. If technical issues were the cause of breakthrough pain, then characteristics, such as fetal weight, and abnormal presentation, which cannot be altered, should be equally distributed among women with normal or dysfunctional labors. That these characteristics are also risk factors for dystocia suggests that maternal pain is the cause of increased epidural requirements. Our results, demonstrating an association between breakthrough pain and labor outcome, complete the link between maternal risk factors, epidural analgesic failure, and dysfunctional labor.

Two alternative explanations for the association between increased epidural supplementation and cesarean delivery are possible: First, breakthrough pain may cause dystocia. Possible mechanisms include increased sympathetic activity or increased tension of the pelvic musculature. Epinephrine is known to decrease uterine activity (18), and epidural analgesia is known to decrease plasma levels of this hormone (19). It is possible that breakthrough pain causes an increase in maternal sympathetic activity, resulting in less effective uterine contractions. However, this explanation would logically lead one to conclude that women who undergo natural childbirth would have higher cesarean rates than those with epidural analgesia. A second explanation would be that the increased dosage of medication in women who received supplemental boluses resulted in poor labor progression. Three observations argue against this: 1) by multivariate analysis we could not detect an influence of bupivacaine infusion concentration on cesarean delivery rate; 2) women receiving 0.04% bupivacaine who had multiple boluses received less total epidural medication than women receiving 0.125% bupivacaine with no boluses, yet they had a higher cesarean rate; and 3) the incidence of cesarean delivery increased dramatically once parturients required three epidural boluses, but did not increase significantly thereafter.

There are several limitations to our study. This was a retrospective examination, incurring the risk that unobserved or unrecorded information may have confounded the results. For example, obstetric practice patterns may have changed between 1989 and 1994; also, obstetric decision for cesarean delivery may have included maternal discomfort. There was, however, no change in obstetric staffing or policies during the study period. Additionally, our use of epidural supplementation as an indirect measure of severity of labor pain is imprecise. We did not have a direct measure of maternal pain, such as visual analog scores with which to compare. Thus, we cannot guarantee that all supplemental boluses were used to treat pain. Women may request supplemental analgesia for many reasons, including individual variations in pain thresholds, desired sensory level, or a technically inadequate catheter. It appears unlikely that these factors would be significantly associated with operative delivery. Because parturients receiving supplementation for causes other than dystocia-related pain would have the outcomes and characteristics of those with less breakthrough pain, our results may underestimate the significance of maternal pain. Likewise, some parturients may have dysfunctional labor without severe pain or requests for supplementation. Response to painful stimulation is modulated by many factors, such as cutaneous stimulation, medications, and neural inhibition (20–23). These women could have the outcomes and characteristics of those with significant breakthrough pain but be included in the MBP group, resulting in a decrease in the association between pain and dysfunctional labor. Finally, during the first study period, data were not collected regarding some of the other factors that may have influenced our results; for example, the induction of labor, parity, and fetal weight. We were able to use these variables for analysis in only one-half of our data; however, this still represents more than 2400 women. Other data, which may be of value but were not available, included the use of oxytocin, obstetrician, insurance, length of labor, and cervical dilation rates. We acknowledge this as a limitation.

In conclusion, our data demonstrate that women who proceed to cesarean delivery require more epidural medication than those who have a vaginal delivery. Thus, the need for excessive supplemental epidural medication, and perhaps even the request for an epidural, may be markers for severe pain caused by dysfunctional labor. Because of this evidence, a reevaluation is required of previous studies that have compared outcomes between women who chose epidural analgesia and those who do not.

	Acknowledgments

 
Supported, in part, by the Beth Israel Anesthesia Foundation.

The authors are grateful to Karen Eckstein, MS, Department of Biostatistics, Harvard School of Public Health, Boston, MA, for assistance with statistical analysis.

	Footnotes

 
Accepted for publication December 7, 1999.

Address for correspondence and reprints to Philip E. Hess, MD, Dept. of Anesthesia and Critical Care, St. 308, 330 Brookline Ave., Boston, MA 02215.

	References

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Anesthesia & Analgesia® is published for the International Anesthesia Research Society® by Lippincott Williams & Wilkins with the assistance of Stanford University Libraries' HighWire Press®. Copyright 2006 by the International Anesthesia Research Society. Online ISSN: 1526-7598   Print ISSN: 0003-2999