Epidural analgesia has proved to be an effective and popular means of pain management during parturition, but some investigators have found women who use epidural analgesia more likely to undergo operative delivery, both abdominal and vaginal.1–3 Furthermore, previous investigators have suggested that this increased propensity toward operative delivery might result from malpositioning of the fetal head in the birth canal resulting from pelvic motor blockade.1,3–7 However, other investigators have failed to identify a causal relationship between epidural analgesia and fetal malpositioning, suggesting that women who have a predisposition to malpositioning of the fetal head may experience greater intrapartum pain and thus may be more likely to request epidural analgesia.8–11 The purpose of this investigation was to determine if the introduction of on-request labor epidural analgesia resulted in an increased frequency of malpositioning of the fetal vertex at vaginal delivery.
Materials and Methods
Tripler Army Medical Center is a large military tertiary care facility located on the island of Oahu, Hawaii. A description of the patient and provider populations has been described previously.11 Before October 1993, epidural analgesia in laboring women was available only to those deemed to have medical indications for the procedure such as severe preeclampsia or severe cardiovascular disease. In October 1993, the Department of Defense made on-request regional analgesia available to laboring women in military hospitals. At our facility, this service became fully functional in January 1994.
All nulliparous women with singleton gestations in vertex presentation and spontaneous onset of labor at 37–42 weeks, who gave birth from October 1, 1992, to September 30, 1993, and from July 1, 1995, to June 30, 1996, were selected for study inclusion and a stepwise exclusion process was then used to select groups of uncomplicated term parturients (Table 1). These two groups will be called “Before” and “After” groups, respectively, hereafter. Information regarding the patient's sociodemographic characteristics, prenatal care, admission assessment, course of labor and delivery, analgesia use, postpartum complications, and neonatal outcome were extracted from each patient's record.
The specific techniques and dosing used for routine labor epidural analgesia were similar between the two study periods, with initiation of epidural analgesia when the patient was at least 4 cm dilated. Following placement, a test dose of 3 mL 1.5% lidocaine with 1:200,000 epinephrine was infused, followed by an 8-mL bolus of 0.125% bupivacaine with 100 μg of fentanyl. The catheters were then infused with either 0.125% or 0.0625% bupivacaine with 2–4 μg/mL of fentanyl begun at 8–10 mL per hour. Thereafter, patients were assessed at hourly intervals and the infusion rate was adjusted to maintain a T10 sensory level with minimal motor block. Infusions were generally continued through the second stage of labor unless the labor or birth attendant requested discontinuation of analgesia because of inadequate pushing.
Recording of cervical assessment was standardized with dilation measured in 0.5-cm increments, effacement in percentage of cervical length, and station of the presenting part divided into thirds above and below the ischial spines (−3 to +3). Fetal head position at delivery was recorded in a standard fashion using eight reference positions at 45-degree increments. For the purposes of analysis, fetal head malpositioning was considered as a position other than occiput anterior, right occiput anterior, or left occiput anterior at vaginal delivery. Operative vaginal deliveries were classified according to the 1988 ACOG criteria.12 Forceps and vacuum procedures were primarily low and outlet procedures with fewer than 1% of procedures done at a midpelvic station in either study period. The choice of delivering instrument was made by the delivering physician. All low operative procedures required a maternal or fetal indication, whereas outlet procedures were occasionally done electively at the discretion of the supervising physician. The diagnosis of nonreassuring fetal testing as an indication for operative vaginal delivery was based on interpretation of fetal heart rate tracings. Fetal scalp pH assessment was rarely used during either study period.
Comparison of categorical variables was performed with χ2 analysis with Yates correction, Fisher exact test, or Wilcoxon rank-sum test as appropriate. Continuous variables were compared with the two-tailed unpaired t test. Multivariable analysis was performed with logistic regression techniques. P < .05 was considered statistically significant for all analyses. Statistical analysis and power calculations were performed with Epi-Info 6.0 (Centers for Disease Control and Prevention, Atlanta, GA) and SAS 6.12 (SAS Institute Inc., Cary, NC).
Data were analyzed for 434 women who fulfilled eligibility criteria in the study period before the availability of on-request labor epidural analgesia and 511 women delivered during the period of on-request labor epidural analgesia. Demographic data, admission assessment, and intrapartum characteristics are compared in Table 2. Labor epidural analgesia was used by four of 434 (0.9%) nulliparas delivered before the availability of on-request epidural analgesia and 423 of 511 (82.9%) nulliparas after epidural analgesia was available on-request (P < .001). The median duration of the second stage of labor was prolonged approximately 24 minutes in the After group compared with the Before group, a statistically significant difference. No women delivered fetuses with either compound or deflexion presentations (brow or face) in either study group. Overall, fetal head malpositioning occurred in 26 of 434 (6.0%) in the Before period compared with 29 of 511 (5.7%) women in the After period [relative risk (RR) 0.95, 95% confidence interval (CI) 0.6, 1.6], a statistically nonsignificant difference. The study sample size provided 85% power with an α of .05 to detect an increase in the incidence of fetal malpositioning from a baseline rate of 6% to 12% (RR 2.0) associated with on-demand epidural use. No statistically significant difference was found in the incidence of fetal head malpositioning between the Before and After periods following stratification of patients according to mode of delivery (Table 3). None of the study participants in either period underwent a rotational forceps delivery of more than 45°. Logistic regression, controlling for maternal age, prepregnancy weight, gravidity, and cervical dilation at admission is presented in Table 4.
During the After period, when epidural analgesia was available on request, malpositioning of the fetal vertex at delivery occurred in 26 of 420 (6.2%) who received epidural analgesia compared with three of 91 (3.3%) women who did not receive epidural analgesia (RR 1.9, 95% CI 0.6, 6.1), a statistically nonsignificant difference. In the population of women who requested labor epidural analgesia, the cervical dilation or fetal station at the initiation of analgesia had no statistically significant effect on the incidence of fetal malpresentation (Table 5). Combining patients from both groups, 28 of 55 (50.9%) women with fetal malpositioning at the time of delivery underwent operative vaginal delivery compared with 222 of 890 (24.9%) women who delivered a fetus in an occiput anterior position (RR 2.0, 95% CI 1.5, 2.7, P < .01); however, this risk was not altered by the study period in which the patient delivered.
One of the primary challenges in interpreting the scientific literature regarding labor epidural analgesia is in examining the potential selection bias that may be present in prospective randomized trials and retrospective cohort studies. High crossover rates and the inability to generalize results to other populations hamper prospective randomized trials.1,13–17 Many parturients have strong opinions either in favor or against labor analgesia, opinions formed before the onset of labor. Ethical and humane patient treatment necessitates the ability of a patient randomized to a nonepidural arm to be able to crossover and receive epidural analgesia on request. All these factors limit the validity of a randomized controlled trial in the study of regional analgesia in labor. Similarly, cohort analysis suffers from the inherent differences in women who request epidural analgesia and those who decline this form of pain management when it is available.
One means of circumventing these difficulties is to use a “natural experiment” in which comparisons are made between populations immediately before a variable is introduced and immediately after the exposure to the variable has reached a steady state. Although a natural experiment may avoid selection bias, such a study is limited by the potential for alterations in practice patterns or other unrecognized factors that could result in differences in the outcome variable of interest between the two study periods.
Under the conditions of our study, no clinically significant increase was found in the incidence of fetal head malpositioning at vaginal delivery associated with the use of on-demand labor epidural analgesia. Whereas women with a malpositioned fetus were more than twice as likely to require operative vaginal delivery as women with an occiput anterior fetus, the liberal use of on-demand labor epidural analgesia did not increase the likelihood of operative or spontaneous vaginal delivery of a malpositioned fetus.
The reported incidence of fetal head malpositioning at the time of delivery in relation to epidural use has varied among previous investigators. High rates of malpositioning, exceeding 15%, tend to be reported from investigations in which malpositioning is defined as an occiput posterior or transverse position at the onset of the second stage, rather than delivery,4 or when instrumental vaginal delivery was performed after an arbitrary duration in the second stage.4,18 Thus, practice patterns may have some impact on the frequency of fetal malpositioning in relation to epidural use. Previous investigations have suggested that incidence of dystocia, fetal malpositioning, and instrumental delivery associated with epidural use may be higher in private practice patient populations, in women who receive higher concentrations of local anesthetic agents in the epidural, and if arbitrary time limits are imposed on second-stage duration.9,19–21 Our study population was exposed primarily to relative low concentrations of local anesthetic agents and expectant management in the second stage of labor as long as progressive descent of the fetal vertex was noted. This management style may have accounted for the relatively low incidence of fetal malpositioning at delivery, the absence of need for rotational forceps procedures, and the lack of a negative effect in regards to fetal malpositioning or instrumental delivery rates associated with on-demand labor epidural analgesia.
How, then, can the previously documented association between epidural analgesia and fetal malpositioning be explained if there is indeed no causal effect? Perhaps the parturient with a predisposition toward fetal malpositioning is also more likely to request epidural analgesia because her labor is more painful or protracted, or her providers are more likely to suggest she receive epidural analgesia because of protracted labor (either as a prelude to cesarean delivery or as a means of allowing more aggressive labor augmentation).22 There are also likely some intrinsic differences in the labor processes that result in a malpositioned fetus relative to the parturient with a fetus presenting in the occiput anterior position.4 As most investigations purporting a link between epidural analgesia and fetal head malposition have been purely observational studies comparing cohorts of parturients who received epidural analgesia with a cohort of women who did not, this self-selection bias appears to be the most plausible explanation. Thus, the parturient who has an increased propensity for fetal malpositioning is more likely to receive epidural analgesia and more likely to require operative vaginal delivery than the parturient with a normally positioned fetal head; however, epidural analgesia should be viewed more as an effect of the underlying factors causing the fetal malpositioning rather than a causative factor. When one views the association between labor epidural analgesia and fetal malpositioning from such a perspective, the seemingly conflicting results of randomized trials, observational cohort studies, and a natural experiment such as ours become congruent.
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