Despite the theoretical benefits of ITN and a growing body of clinical experience with its use during labor, only limited analytic data are available to judge their effects on important variables, such as the duration of labor, the need for oxytocin augmentation, and labor outcomes.16,17 The limited published results of the length of labor with ITN use are conflicting: The duration of first-stage labor is reportedly unaffected20 or even shortened,22,23 while the second stage has been variously observed to be unaffected20,23 or prolonged.15,24 The American Society of Anesthesiologists Task Force on Obstetrical Anesthesia concluded in an evidence-based review that the current medical literature is insufficient to compare the effects of ITN on labor with those of parenteral opiods.16
The purpose of our study was to investigate differences in duration of labor, oxytocin use, and type of delivery among 2 cohorts of women: those who had received ITN according to customary practices at our hospital and those who received either parenteral narcotics or no pain medication at all. We also sought to compare maternal side effects and neonatal outcomes in the 2 groups.
Methods
We included women with singleton pregnancies who presented in spontaneous labor to the BirthPlace, Fairview University Medical Center, between July 1, 1996, and December 31, 1996. A total of 1915 women gave birth during that 6-month period. To select our sample of 100 women who had received ITN and 100 women who had received no spinal analgesic, we used a computer-generated list of patients, sorted into 4 subgroups by parity (P=0 or P Ž1) and the presence or absence of the ITN procedure code (03.91) on the medical record face sheet. We then overselected 75 women from each of the 4 lists, using a random number table. After reviewing the actual records, the first 50 that met our inclusion and exclusion criteria were entered into the study. Reasons for exclusion were: induced labor (49), cervical dilation exceeding 7 cm on admission (24), misclassification (13 women who had received ITN, although the diagnosis was missing from the face sheet), gestation less than 36 weeks (2), epidural use (1), and other reasons (9). Reasons for excluding charts were similarly distributed among groups, except that presentation with advanced cervical dilation was limited almost exclusively to parous women who had not received ITN. In all, we reviewed 298 medical records to select 100 women who had received ITN and 100 who had not, with 50 nulliparous and 50 parous patients in each group.
We collected data from the selected records using a form specifically designed for this study. To increase reliability, the data were crosschecked in multiple sites within the chart whenever possible. The completed data extraction forms were verified for the first 30 charts and for any forms with missing data.
The primary outcomes for this investigation were the lengths of active phase and second-stage labor. Secondary outcomes included the rates of oxytocin use, delivery type (spontaneous vaginal, instrument-assisted vaginal, or cesarean section), and the frequencies of ITN side effects, specifically pruritus, nausea, urinary retention requiring catheterization, and maternal hypotension (systolic blood pressure <90, diastolic blood pressure <50). Neonatal outcomes included Apgar scores, need for resuscitation after delivery, and neonatal intensive care unit admissions.
We calculated our sample size to detect a difference of 1 hour in length of the active phase of labor with 80% power and an a of 0.05. The active phase was defined as the time from 4 cm to complete cervical dilation. We chose to evaluate the active phase rather than the entire first stage of labor, because pinpointing the onset of labor is difficult and likely to be imprecise. In addition, ITN are generally administered at the beginning of the active phase, and any effect they would have on labor progress would be noted subsequently. If the time when a subject was 4 cm dilated was not recorded in the chart (54 women in the ITN group, 52 in the comparison group) we obtained a reliable estimate by plotting all known cervical examinations versus time. Women who presented to the hospital with cervical dilation of 6 cm or greater were not included in this portion of the analysis.
Our univariate analysis compared the primary and secondary outcome variables for women who received ITN with those who did not, using chi-square tests for proportions and Student t tests for means. When appropriate, we used multivariate and logistic regression to adjust for potential confounders. Data were analyzed using the Statistical Package for the Social Sciences for Windows, Version 8.0 (SPSS, Inc, Chicago, Ill).