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Departments of *Obstetrics and Gynecology and Anesthesiology and Pain Management, University of Texas Southwestern Medical Center, Dallas, Texas
Address correspondence and reprint requests to James M. Alexander, MD, Department of Obstetrics and Gynecology, University of Texas Southwestern Medical Center, 5323 Harry Hines Blvd., Dallas, TX 75235-9032. Address e-mail to jalexa{at}mednet .swmed.edu.
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Abstract |
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Implications: More intense pain during labor, as evidenced by increased self-administered analgesia, is a marker of obstructed labor and the need for cesarean delivery.
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Introduction |
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Since 1989 there has been considerable controversy as to whether epidural analgesia causes dysfunctional labor leading to cesarean delivery for dystocia (2). The belief that cesarean rates increase after labor epidural analgesia is mainly based on results from retrospective studies and, until recently, small prospective investigations (3–6). Retrospective analyses have suffered from inherent selection bias, because it is possible that those women seeking epidural relief had more difficult labors. Women with more intense labor pain, and who therefore requested epidural analgesia, may have had an increased intrinsic risk of cesarean delivery for dystocia because severe pain may be an indication of obstructed labor.
We previously reported a randomized investigation of the effects of epidural analgesia on cesarean delivery rates in which one group of women was randomly allocated to receive patient-controlled IV meperidine analgesia (7). We reasoned that a secondary analysis of this group of women, in which analgesia was controlled by the patient and cesarean rates would not be obscured by epidural issues, might allow us to assess whether the intensity of labor pain was related to dystocia.
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Methods |
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The study protocol was developed by investigators from the Departments of Anesthesiology and Obstetrics and Gynecology and was approved by the IRB of the University of Texas Southwestern Medical Center at Dallas. Healthy parturients with a singleton cephalic gestation at term (
36-wk gestation) in spontaneous active labor were offered the chance to participate in the randomized investigation. Labor was diagnosed by the presence of regular uterine contractions associated with cervical dilation of 3–5 cm. All pregnancies were managed by certified nurse midwives under the direct supervision of obstetric faculty and house officers, according to protocols established by our medical staff.
The study commenced on June 1, 1995 and ended on February 28, 1996. Participation was offered to eligible women by our nurse practitioner staff in the triage area adjacent to the labor and delivery areas. Although they were not encouraged, patients were aware that should the allocated analgesia fail to provide adequate pain relief, switching to an alternative type of analgesia would be allowed. Women giving written consent were then randomly assigned by the nurse practitioner staff in the triage area to receive either epidural analgesia or patient-controlled IV analgesia with meperidine during labor. The randomization sequence was computer derived in blocks of 20 patients; numbered and sealed opaque envelopes were used. After allocation, preprinted admission orders accompanied the patients to the labor and delivery unit. These orders specified either an anesthesia consultation for epidural analgesia or meperidine patient-controlled IV analgesia at the first requirement for pain relief.
All staff followed procedures recorded in a written manual that prescribed the intrapartum management of nulliparous and multiparous women. Routine intrapartum management of all women included IV fluid administration and periodic auscultation with Doppler or continuous electronic fetal heart rate surveillance. Internal electronic fetal heart rate monitoring was used in those women with meconium-stained amnionic fluid, known fetal heart rate decelerations, or inadequate progress of labor. Our labor management approach encouraged amniotomy in active labor when the fetal head was applied to the cervix. Pelvic examinations were performed approximately every 2 h to evaluate the progress of labor.
Cervical change of <1 cm/h coincidental with a hypotonic contraction pattern measured with intrauterine pressure transducers resulted in oxytocin augmentation of labor. Oxytocin was administered per written protocol, which has been previously described (8). Briefly, oxytocin starting at 6 mU/min was increased by 6 mU/min at 40-min intervals, to a maximum of 42 mU/min. Uterine activity of 200–250 Montevideo units for 2–4 h was considered adequate. Dystocia was diagnosed when adequate uterine activity did not result in progressive cervical dilation or descent of the fetal head. Indications for the use of forceps were limited to inadequate voluntary pushing or fetal heart rate abnormalities. Umbilical artery blood was obtained at all births from a doubly-clamped cord segment for the analysis of blood gases.
Women randomized to patient-controlled IV analgesia received 50 mg of meperidine with 25 mg of promethazine hydrochloride IV as an initial bolus, after which an Abbott-Lifecare 4100 patient-controlled pump (Abbott Laboratories, North Chicago, IL) was set to deliver 10 mg of meperidine every 10 min as needed during the first hour and 15 mg every 10 min thereafter as needed until delivery. Additional 25-mg doses of meperidine were given on request, not to exceed 100 mg in 2 h. In the event that pain relief was inadequate despite these measures, epidural analgesia was administered on patient request.
Pain was assessed with a linear 10-cm visual analog scale (0 = no pain, 10 = worst possible pain) before and after the initiation of analgesia. The pain assessment after the initiation of analgesia occurred at complete cervical dilation or, in those women requiring cesarean delivery, just before surgery.
Obstetric data and other additional information were abstracted from maternal and neonatal charts and assessed for completeness and consistency before electronic storage in an online perinatal data system.
Statistical methods included Student’s t-tests, Mann-Whitney U-tests, 
2 tests for contingency tables, and multiple logistic regression. The threshold of meperidine use most associated with cesarean delivery for dystocia was selected by using a receiver operating characteristic curve. We selected the level of meperidine nearest the upper left corner of the curve to maximize sensitivity and specificity. We evaluated the strength of prediction of the meperidine threshold most associated with cesarean delivery for dystocia by using the area under the curve (AUC). An AUC >0.5 suggests a significant association with the threshold, and a value >0.80 is suggestive of strong prediction (9). P values <0.05 were considered significant. Analyses were performed with the SAS statistical package (SAS Institute, Cary, NC).
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Results |
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The 259 women who self-administered IV meperidine with patient-controlled bedside pumps were divided into two groups: those who received 50 mg/h or more meperidine from the time analgesia was initiated until delivery compared with those receiving <50 mg/h. This cutoff was the statistically significant break point with receiver operating characteristic curves (Fig. 1). With this method, the requirement of 50 mg/h was the most sensitive and specific predictor of women who required cesarean delivery for dystocia, with an average AUC of 0.83. The 50 mg/h cutoff was also consistent with previous experience (6) in which meperidine analgesia during labor had to be supplemented by epidural analgesia when total meperidine dosage exceeded 200 mg in 4 h. Multivariate analysis adjusting for parity did not affect this result. Demographic characteristics were similar in the two groups, including age, height, weight, and parity (Table 1).
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50 mg/h compared with those requiring <50 mg/h. Similarly, admission-to-delivery times and augmentation of labor with oxytocin were not significantly different between the two groups. Women receiving 50 mg/h or more of meperidine were in labor longer (9 vs 5 h), but this was not significant (P = 0.09). A power analysis was performed to determine the sample size needed to investigate this difference. Assuming a ß of 0.8 and 
of 0.05, approximately 500 patients would be required to show this difference to be statistically significant.
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50 mg/h had artificial rupture of the membrane, compared with 65% in women given <50 mg/h (P = 0.04). Shown in Figure 2 are visual analog pain scores before and after meperidine analgesia in the two study groups. Pain scores were significantly higher before analgesia in women requiring 50 mg/h of meperidine (mean pain score ± SD, 8.7 ± 1.7 vs 8.0 ± 2.3, P = 0.05, 50 mg/h vs <50 mg/h meperidine dose, respectively). There was no significant difference in mean pain scores between the two study groups after analgesia had been initiated.
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50 mg/h meperidine analgesia, seven were performed for dystocia and two for nonreassuring fetal heart rate tracings. One woman had a cesarean delivery performed for a vaginal stricture related to a previous episiotomy breakdown. The severity of this stricture was not appreciated until the second stage of labor. The group of women who required <50 mg/h of meperidine had fewer cesarean deliveries for dystocia (1.4% vs 14%, P = 0.001) as well as for nonreassuring fetal heart rate tracings (0.5 vs 4%, P = 0.04). Figure 3 shows the probability of cesarean delivery on the basis of the hourly rate of meperidine infusion. The probability of cesarean delivery increased with frequent rates of meperidine use (P = 0.02). Conversely, spontaneous delivery was significantly decreased (78% vs 90%, P = 0.001) in women who required >50 mg/h meperidine. The data were analyzed with multiple logistic regression analysis, adjusting for the difference in nulliparity between the two study groups. Cesarean delivery remained significantly associated with meperidine infusion rates of 50 mg/h (P < 0.001).
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7.0 (1% vs 0%, P = 0.43). Eight percent of infants delivered of women given 50 mg/h of meperidine were given naloxone for respiratory depression in the delivery room, compared with 4% in women given smaller doses of meperidine (P = 0.21). Naloxone administration occurred only after assessment by pediatric personnel and not routinely. |
Discussion |
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It seems intuitive that the intensity of pain would be increased when labor is obstructed. The study of labor pain, however, is made difficult because of the complex nature of pain perception. Multiple factors affect a woman’s perception of labor pain, including anxiety, prior experience, and the heterogeneity inherent in labor, making each woman’s experience unique. These cognitive and emotional factors interact with visceral stimulants of pain, affecting its intensity. Although it has been hypothesized (10), actual reports showing a relationship of labor pain to dystocia are almost nonexistent. In 1989, Wuitchick et al. (11) reported that women who experienced more intense pain in latent labor had longer labors and were more likely to undergo cesarean delivery. However, the authors did not find a relationship between pain in the active phase of labor and dystocia. A variety of anesthetics was used for pain relief, and the amount of anesthetic used was not quantified. Recently, Hess et al. (12) reported that women requiring supplemental epidural boluses were more likely to undergo cesarean or assisted vaginal delivery than were those who did not. Their study did not report direct measurement of patient pain but provides indirect evidence that more intense pain during labor is associated with labor dystocia. There are no other reports in the literature providing evidence that intense pain is related to dystocia.
The type of analgesia used for pain relief during labor has come under intense scrutiny, in particular, labor epidural (2–5). Our data do not establish cause or effect, but they strongly suggest that a woman’s need for labor analgesia is associated with intense pain related to labor dystocia. This relationship should be considered when studying the relationship between the method of labor analgesia and potential effects on the course of labor. Our observation that more intense pain is associated with difficult labor may also alert obstetricians that such pain is not caused by a reduced pain threshold but may also be a marker of intrinsically difficult and ultimately obstructed labor.
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