Methods
Women presenting for childbirth at a community hospital from May 1992 to April 1994 were invited by a labor and delivery nurse to participate in this prospective observational study. PCBs were regularly used for pain relief, making this an ideal study setting. By using a prospectively designed study, it was possible to make sure that information on likely confounding variables was collected uniformly on all of the patients in the study. Exclusion criteria included scheduled cesarean delivery, multiple gestation, less than37 or more than 42 weeks’ gestation, allergy to lidocaine or other local anesthetic, and suspicion of or known placenta previa or premature separation of placenta. Potential subjects were informed of the study when they arrived at the Labor and Delivery department, and gave their informed consent if they chose to participate. Our study was approved by the Institutional Review Board. Of the 263 women recruited, 2 had cesarean deliveries: 1 for failure to progress, and 1 for fetal distress. Neither of these women received a PCB, but they were eliminated from further analysis since the anesthesia used for cesarean delivery could adversely affect umbilical artery pH values.34,35 This left 261 women in the study.
Labor management was left up to the attending physicians (all of whom were board certified in obstetrics and gynecology or family practice). The attending physician chose the type of analgesia to be used in conjunction with the patient’s needs and desires, as is standard practice. Through special discussions held at the monthly hospital maternal/child health meeting (for all physicians involved in obstetrics and pediatrics), physicians using PCBs agreed to use 1% lidocaine injected in 1 to 2 locations submucosally on each side of the cervix (at the 3 to 4 o’clock and 8 to 9 o’clock positions) for a total dosage of 100 to 200 mg lidocaine. A 10-mg dose of nalbuphine hydrochloride was given intravenously when needed for parenteral pain relief. Mepivacaine 1% was used for pudendal and local blocks. Physicians recorded the types, amounts, and concentrations of all anesthetics used, and the obstetric nurses recorded when they were given and the degree of pain relief they produced.
Standard practice was for the fetal heart rate (FHR) to be recorded by handheld Doppler ultrasonography every 15 minutes during the active phase of labor and every 5 minutes during the second stage. For our study, external continuous FHR monitoring was used for 5 minutes before and for 30 minutes after each PCB to detect any decrease, unless a fetal scalp electrode was already in place. We defined bradycardia using the conservative criteria of a decrease in FHR to less than 100 beats per minute lasting at least 1 minute and occurring within 30 minutes of anesthesia. Decreased FHR was defined as a drop to less than 120 beats per minute lasting at least 1 minute and occurring within 30 minutes of anesthesia.
The obstetric nurse asked the participating women to rate the degree of perceived relief obtained from any analgesics approximately 15 minutes after these medications were given. They used a 4-point scale of “excellent,” “good,” “fair,” and “poor.” “Excellent” indicated complete pain relief and inability to feel contractions; “good” was complete pain relief with ability to feel contractions; “fair” was definite but incomplete relief of pain; and “poor” was little or no relief. Following delivery, the umbilical cord was double-clamped, arterial cord blood was drawn into a heparinized syringe, and the sample was sent on ice to the hospital laboratory for immediate determination of pH, partial pressure of carbon dioxide (pCO2), and partial pressure of oxygen (pO2) using an Instrumentation Laboratory pH/Blood Gas Analyzer Model 1306 (Instrumentation Laboratory, Lexington, Mass).
We extracted data from the woman’s record, including demographics, specialty of delivering physician(obstetrician or family physician), information on events during labor (eg, type and amount of all analgesics used), and neonatal outcomes (such as weight, 1- and 5-minute Apgar scores, umbilical artery blood gas values, and resuscitation with oxygen by mask). Data extracted from the infant’s record included the length of stay. When data was missing, it was because it could not be found in the medical record. We did statistical analyses using the Statistical Package for the Social Sciences 7.5 (SPSS Incorporated, Chicago, Ill). We used descriptive statistics to characterize the distribution of each variable (dependent and independent). We explored bivariate relationships using chi-square tests and used 2-sided t tests to compare means between groups. We used forward stepwise linear regression using umbilical artery pH values at birth to estimate the effect of a PCB while testing for the effect of a number of factors which might affect pH. The covariates we tested were maternal age (years), weight gain during pregnancy (kilograms), tobacco use, preeclampsia, diabetes, parity, induction of labor with artificial rupture of membranes as sole method, induction with gel, oxytocin use, intrauterine pressure catheter use, artificial amniotomy, meconium-stained amniotic fluid, dystocia, first stage length, second stage length, mode of delivery (spontaneous vertex, vaginal breech, midforceps, outlet forceps, vacuum assisted), analgesia used during the first or second stage (nalbuphine, PCB, epidural, caudal, pudendal, local), nuchal cord, knotted cord, specialty of delivering physician (obstetrician or family physician), and infant weight (grams) and sex. We tested the use of PCB and nalbuphine as indicator variables (none, 1 only, 2 or more doses) in the stepwise regression to determine potential dose response. To verify that PCB and nalbuphine use were not associated with umbilical artery pH at birth, we forced them into the final stepwise equation. P values <.05 were considered statistically significant.