Patricia Fontaine, MD

Pain management during labor and delivery is an important issue affecting both the quality of the experience and its outcome. Early attempts at anesthesia with barbiturates, intravenous narcotics, or the combination of ether, morphine, and scopolamine (known as “twilight sleep”) were all associated with major side effects for mothers and infants.¹ Contemporary emphasis has been on safer, more natural methods of pain management, including childbirth training, relaxation, and support from a labor companion. Though helpful, these approaches are not sufficiently effective for the majority of patients in labor.² Recently, postpartum depression and posttraumatic stress disorder have been linked to difficult or painful deliveries,^3,4 and good analgesia has been identified as a major determinant of women’s overall satisfaction with their birthing experiences.⁵ The need for safe, effective relief for labor pain was acknowledged directly by the medical community in 1992 in a joint statement issued by the American College of Obstetricians and Gynecologists and the American Society of Anesthesiologists: “Labor results in severe pain for many women. There is no other circumstance where it is considered acceptable for a person to experience severe pain amenable to safe intervention, while under a physician’s care.”⁶

In response to the need for effective labor pain management, neuraxial (spinal) analgesia has become increasingly popular in the United States. Three techniques are currently in use: epidural anesthesia (continuous infusion of local anesthetic with or without an adjuvant narcotic), intrathecally administered narcotics (ITN), and the combined spinal-epidural approach.^7-10 Of the 3, the epidural is the most established and best studied method. According to a national survey published in 1986, epidural anesthesia was used for pain management for 16% of deliveries, while there was no reported use of ITN.¹¹ Today, epidural analgesia is used for more than 60% of nulliparous labors in large urban hospitals in the United States and Canada.^12,13

ITN were adapted to obstetric practice in the early 1980s, when it was demonstrated that a single subarachnoid injection of morphine sulfate could eliminate the pain of contractions in the first stage of labor.^14,15 Since the initial reports, use of ITN has achieved increasing acceptance as a simple and effective technique for labor pain management. Its potential advantages include better pain control than parenteral narcotics¹⁶ and more rapid onset of analgesia with less intervention than an epidural.¹⁷ Because ITN do not involve a local anesthetic, they do not cause the sympathetic or motor blockade commonly observed with epidural anesthesia.^10,17 Women receiving ITN retain sensation and motor control of pelvic musculature and lower extremities; thus, they are able to ambulate and change positions during labor and have essentially no blunting of the urge to push in the second stage.^17,18 Although recent meta-analyses have found that epidurals are associated with a prolonged second stage of labor¹⁹ and a qualified decreased incidence in spontaneous vaginal deliveries,^16,19 ITN have been characterized as not interfering with the natural progress of labor.^8,20

ITN have been a particularly successful addition to labor services in community or military hospitals where anesthesiology services and access to epidurals are limited.^19,21,22 Our hospital’s 15-year experience with ITN also supports their role in a busy, urban teaching hospital where health maintenance organizations influence the anesthesia standard toward a simple, low cost method. Typically, more than 4000 women deliver at our hospital each year, and approximately half receive ITN for their labor pain. Fewer than 5% receive epidurals; the rest deliver with parenteral narcotics or natural childbirth.

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 unaffected^20,23 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).

Results

Baseline characteristics for the 2 groups are shown in Table 1. Women receiving ITN were similar to the comparison group in mean age, marital status, and insurance type but were significantly more likely to be white. Women in the ITN group had less advanced cervical dilation on admission and were somewhat more likely to present with spontaneous rupture of membranes.

The technique of ITN administration was consistent. All procedures were performed by one of approximately 20 anesthesiologists in a private-practice group. The anesthesiologists used 25-gauge conical-tipped Whitacre spinal needles to puncture the dura at the L3-L4 interspace. Eighty-six percent of these women received both morphine and fentanyl, typically 0.25-mg and 25-mg doses, respectively. Fourteen percent received fentanyl only, a choice generally reserved for multiparous patients expected to have rapid labor progress. The mean cervical dilation at the time of ITN administration was 4.7 cm ± 1.4 cm. Six women required a second ITN procedure, and another 5 women received intravenous nalbuphine for pain after ITN (usually more than 4 hours after ITN administration).

Intravenous drugs administered for pain in the comparison group also followed a consistent pattern, with nalbuphine being the parenteral analgesic of choice. Forty-one percent of these women received one dose of intravenous nalbuphine ranging from 5 to 20 mg; an additional 3% required 2 doses. Overall, women receiving ITN were significantly less likely to receive nalbuphine than women in the comparison group (P=.000). A total of 17 women in the ITN group received nalbuphine (including both those who had it before and those after ITN) versus 41 in the comparison group.

Data on lengths of labor, oxytocin use, and delivery outcomes for the 2 groups are shown in Table 2. The mean length of active-phase labor was similar: approximately 4 to 5 hours in each group. However, women with ITN experienced significantly longer second stages. This difference remained significant after controlling with multivariate regression for the following confounders: ethnicity, parity, maternal age, cervical dilation on admission, nalbuphine use, oxytocin use, and infant weight (b coefficient=26.965; t=3.261; P=.001). When labor lengths were further analyzed by parity Table 3, the prolonged second stage in women receiving ITN was statistically significant among nulliparas and approached significance among parous women as well. Of note, the magnitude of prolongation was similar for both nulliparous and parous women; that is, the length of the second stage was increased by more than 50% in both groups receiving ITN.

Women in the ITN group were twice as likely to receive oxytocin augmentation during labor Table 2. This difference persisted in a logistic regression model taking into account ethnicity, parity, maternal age, cervical dilation on admission, duration of active-phase labor, nalbuphine use, and infant weight (b coefficient=1.55; P=.001). After controlling for confounders, the relationship between ITN and oxytocin use was strengthened, with women who received ITN more than 4 times as likely to receive oxytocin (odds ratio [OR]=4.69).

With regard to delivery outcomes Table 2, 8 women from both groups (4% of the total) required a cesarean delivery. Seven of these were in the ITN group, and 1 was in the comparison group. Six of the cesarean sections were performed for dystocia, 1 for fetal distress, and 1 for other reasons. Nine women (4.5%) had an instrumental vaginal delivery (6 with ITN, 3 without). Among instrumental vaginal deliveries, half were for maternal exhaustion, a diagnosis seen exclusively in nulliparous women. Differences between the groups in the rates of cesarean section versus combined spontaneous and instrumental vaginal delivery did not reach statistical significance.

Table 4 shows the frequency of ITN side effects gathered from the chart review. Among women receiving ITN, 2 side effects-urinary retention requiring urethral catheterization and pruritus-were recorded with significantly higher frequency than in the comparison group. Nausea or vomiting, generally viewed as a side effect of ITN, was equally common in both groups, as was maternal hypotension. To counter side effects, essentially all women who received ITN were routinely given 6.25 mg naltrexone sublingually immediately postpartum. Other than the naltrexone, medications to treat side effects were infrequently used. Diphenhydramine, hydroxyzine (for pruritus), and prochlorperazine (for nausea) were administered to 11 women in the ITN group. An additional complication seen only in the ITN group was the occurrence of a postdural puncture ("spinal") headache. Four spinal headaches were diagnosed, and 3 of them were treated with a blood patch. None of the women experienced respiratory depression following ITN.

Neonatal outcomes were similar in both groups. In the ITN group, the mean infant birth weight was 3491 ± 552 g compared with 3506 ± 454 g for the comparison group (P=.82). Four infants in the ITN group and none in the comparison group had 5-minute Apgar scores less than 7 (P=.12). There were no significant differences in the number of infants in each group requiring resuscitation beyond simple stimulation and blow-by oxygen (17 in the ITN group, 14 in the comparison group, P=.25) or admission to the neonatal intensive care unit (5 infants vs 3, P=.72). However, infants whose mothers were in the comparison group were more likely to be given naloxone postdelivery, presumably for prophylaxis against respiratory depression (10 infants vs 3 in the ITN group, P=.08).

Discussion

In our retrospective study of spontaneous labors from a large urban teaching hospital, ITNs were associated with prolonged second-stage labor in both nulliparous and parous women, with the greatest impact (both statistically and clinically) on the nulliparous women. This could have important consequences for a compromised fetus that is ill-suited to handle periods of repetitive hypoxia associated with prolonged pushing. Although we also found a trend toward more cesarean deliveries in the ITN group, the sample size was too small and the event rate too low to draw definitive conclusions about differences in delivery outcomes between groups.

Results from previous studies of ITN use are contradictory, indicating either no difference in second-stage labor length^20,23 or prolongation.^15,24 It is possible that the ITN’s advantages (maternal mobility and full sensation for pushing in the second stage) can be offset by pain. ITN do not provide relief from the pain of perineal distention,^14,18 and women at our hospital received perineal local anesthesia only when episiotomy was performed (29% of cases). Likewise, pudendal nerve blocks, used routinely as an adjuvant method of pain management in early reports of ITN’s clinical successes,²⁵ were rarely used during our study.

We found no significant difference in the mean duration of active labor in the 2 study groups. This is consistent with a previous finding²⁰ but in contrast to the assertion of another²³ that ITN use shortens the first stage significantly in both nulliparous and primiparas. The latter study, like ours, involved retrospective methodology. However, it investigated 4 groups: combined spinal-epidural, epidural, ITN, and no spinal analgesia. Women were allowed to convert from ITN to a combined spinal-epidural analgesic, presumably if ITN alone were insufficient. The more favorable outcome for women with ITN may be due to selection bias, with longer labors becoming ITN failures and crossing over to the spinal-epidural group.

Our finding that ITN use was associated with a 2-fold increase in oxytocin augmentation is different from 2 previous studies showing similar rates of oxytocin use for women with ITN and those without.^20,24 Nevertheless, the strength of the association we discovered (OR=4.69 correcting for confounders) warrants further investigation. If replicated, our finding has implications for informed consent. Women in labor who choose ITN use for its reputation as a simple, more natural alternative to epidural analgesia deserve to know whether it carries increased risk of an intervention with oxytocin. Animal studies have suggested that morphine inhibits uterine contractions through direct action on uterine opioid receptors.²⁶ Baraka and colleagues²⁷ noted a significant increase in oxytocin augmentation in laboring women receiving 2.0 mg intrathecal morphine versus those receiving only 1.0 mg. The effects of ITN on uterine contractions are likely modified by complex interactions. Various hypotheses have been proposed, including a spinal cord site of action for morphine resulting in a depressant effect analogous to its effects on micturition²² and a hypothalamic-pituitary level interaction of narcotics with oxytocin.²⁸ Although our study shows an association between ITN and oxytocin augmentation, no causal connection can be drawn. It is possible that certain practice patterns-such as care providers’ desires to “take advantage of the ITN” by starting oxytocin during the 2- to 3-hour period during which it is most effective-account for the high percentage of women in our study whose labors were augmented.

Maternal side effects, such as itching and urinary retention, were clearly associated with ITN use despite routine postpartum administration of naltrexone to reverse narcotic effects. Spinal headache occurred in 4% of cases despite the routine use of atraumatic spinal needles. Fetal effects of ITN were minimal, presumably because absorption of narcotics from cerebral spinal fluid into the maternal and fetal circulation is limited²⁹ but also possibly because our sample size was too small to detect differences. In our hospital, where ITN is the primary alternative to parenteral narcotics, it had the beneficial effect of sparing newborns from treatment with naloxone.

Strengths and Limitations

The strengths of our study are the random sampling of medical records, the stratification for parity, and an adequate sample size for detecting differences in the primary outcomes. Previous reports of the effects of ITN use have been limited to case series^21,25 or studies using convenience samples with concurrent controls.^20,23,24

The primary limitations of our study are an inherent selection bias and the retrospective nature of the data. Our conclusions regarding the length and progress of labor are limited by the fact that the self-selected treatment groups were not necessarily unbiased and comparable. At our medical center, epidurals are rarely used, and half of all women in labor receive ITN. A significant number of births are attended by midwives, and many women enter labor hoping to achieve “natural childbirth.” These women choose ITN as a fallback alternative when relaxation techniques, tubs, massage, and intravenous narcotics are insufficient. Under these circumstances, a request for ITN may be a marker for a difficult labor, one where oxytocin augmentation or a prolonged second stage would be more likely, irrespective of the analgesic received. This phenomenon has also been noted when women requesting epidural analgesia are compared with those not requesting spinal analgesia.¹⁹

Our retrospective study design also limited our ability to assess the effectiveness of pain control in the 2 groups. Since objective pain measurements are not part of routine labor monitoring at our hospital, we could only infer the effectiveness of ITN from subjective chart notations or indirect measures of “method failure,” such as additional narcotic doses or second procedures required. Also, since the comparison group is defined by selecting out a certain percentage of “method failures” (ie, women who entered labor intending to use intravenous or no medication but changed their minds and requested ITN), effectiveness of pain control in the 2 groups cannot be directly compared.

Conclusions

The combined findings of a prolongation of the second stage of labor and increased oxytocin augmentation raise concerns that ITN alters the natural course of labor. However, caution is warranted, because selection bias in this retrospective study prohibits any conclusions regarding cause and effect. Prospective randomized trials comparing intrathecal narcotics with epidural analgesia and no spinal analgesia could substantially improve our understanding of the effects of spinal analgesia on the course and outcomes of labor. Objective measures of the effective duration of ITN are also needed, and strategies to enhance effectiveness (such as addition of local anesthetic to the ITN or routine use of a pudendal block in the second stage) also deserve experimental trials. Finally, women’s voices must be heard in evaluating the tradeoffs between analgesic efficacy and side effects. Such tradeoffs may greatly influence their analgesic choices and have an impact on their overall satisfaction with the birthing experience.

Acknowledgments

Our research was funded through the Department of Family Practice and Community Health, University of Minnesota. We acknowledge Deborah Finstad for database management, Dr Bruce Center for assistance with statistical analysis, Dr James Pacala for manuscript review, and Dr Anne Marie Weber-Main for editorial assistance.

Pain management during labor and delivery is an important issue affecting both the quality of the experience and its outcome. Early attempts at anesthesia with barbiturates, intravenous narcotics, or the combination of ether, morphine, and scopolamine (known as “twilight sleep”) were all associated with major side effects for mothers and infants.¹ Contemporary emphasis has been on safer, more natural methods of pain management, including childbirth training, relaxation, and support from a labor companion. Though helpful, these approaches are not sufficiently effective for the majority of patients in labor.² Recently, postpartum depression and posttraumatic stress disorder have been linked to difficult or painful deliveries,^3,4 and good analgesia has been identified as a major determinant of women’s overall satisfaction with their birthing experiences.⁵ The need for safe, effective relief for labor pain was acknowledged directly by the medical community in 1992 in a joint statement issued by the American College of Obstetricians and Gynecologists and the American Society of Anesthesiologists: “Labor results in severe pain for many women. There is no other circumstance where it is considered acceptable for a person to experience severe pain amenable to safe intervention, while under a physician’s care.”⁶

In response to the need for effective labor pain management, neuraxial (spinal) analgesia has become increasingly popular in the United States. Three techniques are currently in use: epidural anesthesia (continuous infusion of local anesthetic with or without an adjuvant narcotic), intrathecally administered narcotics (ITN), and the combined spinal-epidural approach.^7-10 Of the 3, the epidural is the most established and best studied method. According to a national survey published in 1986, epidural anesthesia was used for pain management for 16% of deliveries, while there was no reported use of ITN.¹¹ Today, epidural analgesia is used for more than 60% of nulliparous labors in large urban hospitals in the United States and Canada.^12,13

ITN were adapted to obstetric practice in the early 1980s, when it was demonstrated that a single subarachnoid injection of morphine sulfate could eliminate the pain of contractions in the first stage of labor.^14,15 Since the initial reports, use of ITN has achieved increasing acceptance as a simple and effective technique for labor pain management. Its potential advantages include better pain control than parenteral narcotics¹⁶ and more rapid onset of analgesia with less intervention than an epidural.¹⁷ Because ITN do not involve a local anesthetic, they do not cause the sympathetic or motor blockade commonly observed with epidural anesthesia.^10,17 Women receiving ITN retain sensation and motor control of pelvic musculature and lower extremities; thus, they are able to ambulate and change positions during labor and have essentially no blunting of the urge to push in the second stage.^17,18 Although recent meta-analyses have found that epidurals are associated with a prolonged second stage of labor¹⁹ and a qualified decreased incidence in spontaneous vaginal deliveries,^16,19 ITN have been characterized as not interfering with the natural progress of labor.^8,20

ITN have been a particularly successful addition to labor services in community or military hospitals where anesthesiology services and access to epidurals are limited.^19,21,22 Our hospital’s 15-year experience with ITN also supports their role in a busy, urban teaching hospital where health maintenance organizations influence the anesthesia standard toward a simple, low cost method. Typically, more than 4000 women deliver at our hospital each year, and approximately half receive ITN for their labor pain. Fewer than 5% receive epidurals; the rest deliver with parenteral narcotics or natural childbirth.

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 unaffected^20,23 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).

Results

Baseline characteristics for the 2 groups are shown in Table 1. Women receiving ITN were similar to the comparison group in mean age, marital status, and insurance type but were significantly more likely to be white. Women in the ITN group had less advanced cervical dilation on admission and were somewhat more likely to present with spontaneous rupture of membranes.

The technique of ITN administration was consistent. All procedures were performed by one of approximately 20 anesthesiologists in a private-practice group. The anesthesiologists used 25-gauge conical-tipped Whitacre spinal needles to puncture the dura at the L3-L4 interspace. Eighty-six percent of these women received both morphine and fentanyl, typically 0.25-mg and 25-mg doses, respectively. Fourteen percent received fentanyl only, a choice generally reserved for multiparous patients expected to have rapid labor progress. The mean cervical dilation at the time of ITN administration was 4.7 cm ± 1.4 cm. Six women required a second ITN procedure, and another 5 women received intravenous nalbuphine for pain after ITN (usually more than 4 hours after ITN administration).

Intravenous drugs administered for pain in the comparison group also followed a consistent pattern, with nalbuphine being the parenteral analgesic of choice. Forty-one percent of these women received one dose of intravenous nalbuphine ranging from 5 to 20 mg; an additional 3% required 2 doses. Overall, women receiving ITN were significantly less likely to receive nalbuphine than women in the comparison group (P=.000). A total of 17 women in the ITN group received nalbuphine (including both those who had it before and those after ITN) versus 41 in the comparison group.

Data on lengths of labor, oxytocin use, and delivery outcomes for the 2 groups are shown in Table 2. The mean length of active-phase labor was similar: approximately 4 to 5 hours in each group. However, women with ITN experienced significantly longer second stages. This difference remained significant after controlling with multivariate regression for the following confounders: ethnicity, parity, maternal age, cervical dilation on admission, nalbuphine use, oxytocin use, and infant weight (b coefficient=26.965; t=3.261; P=.001). When labor lengths were further analyzed by parity Table 3, the prolonged second stage in women receiving ITN was statistically significant among nulliparas and approached significance among parous women as well. Of note, the magnitude of prolongation was similar for both nulliparous and parous women; that is, the length of the second stage was increased by more than 50% in both groups receiving ITN.

Women in the ITN group were twice as likely to receive oxytocin augmentation during labor Table 2. This difference persisted in a logistic regression model taking into account ethnicity, parity, maternal age, cervical dilation on admission, duration of active-phase labor, nalbuphine use, and infant weight (b coefficient=1.55; P=.001). After controlling for confounders, the relationship between ITN and oxytocin use was strengthened, with women who received ITN more than 4 times as likely to receive oxytocin (odds ratio [OR]=4.69).

With regard to delivery outcomes Table 2, 8 women from both groups (4% of the total) required a cesarean delivery. Seven of these were in the ITN group, and 1 was in the comparison group. Six of the cesarean sections were performed for dystocia, 1 for fetal distress, and 1 for other reasons. Nine women (4.5%) had an instrumental vaginal delivery (6 with ITN, 3 without). Among instrumental vaginal deliveries, half were for maternal exhaustion, a diagnosis seen exclusively in nulliparous women. Differences between the groups in the rates of cesarean section versus combined spontaneous and instrumental vaginal delivery did not reach statistical significance.

Table 4 shows the frequency of ITN side effects gathered from the chart review. Among women receiving ITN, 2 side effects-urinary retention requiring urethral catheterization and pruritus-were recorded with significantly higher frequency than in the comparison group. Nausea or vomiting, generally viewed as a side effect of ITN, was equally common in both groups, as was maternal hypotension. To counter side effects, essentially all women who received ITN were routinely given 6.25 mg naltrexone sublingually immediately postpartum. Other than the naltrexone, medications to treat side effects were infrequently used. Diphenhydramine, hydroxyzine (for pruritus), and prochlorperazine (for nausea) were administered to 11 women in the ITN group. An additional complication seen only in the ITN group was the occurrence of a postdural puncture ("spinal") headache. Four spinal headaches were diagnosed, and 3 of them were treated with a blood patch. None of the women experienced respiratory depression following ITN.

Neonatal outcomes were similar in both groups. In the ITN group, the mean infant birth weight was 3491 ± 552 g compared with 3506 ± 454 g for the comparison group (P=.82). Four infants in the ITN group and none in the comparison group had 5-minute Apgar scores less than 7 (P=.12). There were no significant differences in the number of infants in each group requiring resuscitation beyond simple stimulation and blow-by oxygen (17 in the ITN group, 14 in the comparison group, P=.25) or admission to the neonatal intensive care unit (5 infants vs 3, P=.72). However, infants whose mothers were in the comparison group were more likely to be given naloxone postdelivery, presumably for prophylaxis against respiratory depression (10 infants vs 3 in the ITN group, P=.08).

Discussion

In our retrospective study of spontaneous labors from a large urban teaching hospital, ITNs were associated with prolonged second-stage labor in both nulliparous and parous women, with the greatest impact (both statistically and clinically) on the nulliparous women. This could have important consequences for a compromised fetus that is ill-suited to handle periods of repetitive hypoxia associated with prolonged pushing. Although we also found a trend toward more cesarean deliveries in the ITN group, the sample size was too small and the event rate too low to draw definitive conclusions about differences in delivery outcomes between groups.

Results from previous studies of ITN use are contradictory, indicating either no difference in second-stage labor length^20,23 or prolongation.^15,24 It is possible that the ITN’s advantages (maternal mobility and full sensation for pushing in the second stage) can be offset by pain. ITN do not provide relief from the pain of perineal distention,^14,18 and women at our hospital received perineal local anesthesia only when episiotomy was performed (29% of cases). Likewise, pudendal nerve blocks, used routinely as an adjuvant method of pain management in early reports of ITN’s clinical successes,²⁵ were rarely used during our study.

We found no significant difference in the mean duration of active labor in the 2 study groups. This is consistent with a previous finding²⁰ but in contrast to the assertion of another²³ that ITN use shortens the first stage significantly in both nulliparous and primiparas. The latter study, like ours, involved retrospective methodology. However, it investigated 4 groups: combined spinal-epidural, epidural, ITN, and no spinal analgesia. Women were allowed to convert from ITN to a combined spinal-epidural analgesic, presumably if ITN alone were insufficient. The more favorable outcome for women with ITN may be due to selection bias, with longer labors becoming ITN failures and crossing over to the spinal-epidural group.

Our finding that ITN use was associated with a 2-fold increase in oxytocin augmentation is different from 2 previous studies showing similar rates of oxytocin use for women with ITN and those without.^20,24 Nevertheless, the strength of the association we discovered (OR=4.69 correcting for confounders) warrants further investigation. If replicated, our finding has implications for informed consent. Women in labor who choose ITN use for its reputation as a simple, more natural alternative to epidural analgesia deserve to know whether it carries increased risk of an intervention with oxytocin. Animal studies have suggested that morphine inhibits uterine contractions through direct action on uterine opioid receptors.²⁶ Baraka and colleagues²⁷ noted a significant increase in oxytocin augmentation in laboring women receiving 2.0 mg intrathecal morphine versus those receiving only 1.0 mg. The effects of ITN on uterine contractions are likely modified by complex interactions. Various hypotheses have been proposed, including a spinal cord site of action for morphine resulting in a depressant effect analogous to its effects on micturition²² and a hypothalamic-pituitary level interaction of narcotics with oxytocin.²⁸ Although our study shows an association between ITN and oxytocin augmentation, no causal connection can be drawn. It is possible that certain practice patterns-such as care providers’ desires to “take advantage of the ITN” by starting oxytocin during the 2- to 3-hour period during which it is most effective-account for the high percentage of women in our study whose labors were augmented.

Maternal side effects, such as itching and urinary retention, were clearly associated with ITN use despite routine postpartum administration of naltrexone to reverse narcotic effects. Spinal headache occurred in 4% of cases despite the routine use of atraumatic spinal needles. Fetal effects of ITN were minimal, presumably because absorption of narcotics from cerebral spinal fluid into the maternal and fetal circulation is limited²⁹ but also possibly because our sample size was too small to detect differences. In our hospital, where ITN is the primary alternative to parenteral narcotics, it had the beneficial effect of sparing newborns from treatment with naloxone.

Strengths and Limitations

The strengths of our study are the random sampling of medical records, the stratification for parity, and an adequate sample size for detecting differences in the primary outcomes. Previous reports of the effects of ITN use have been limited to case series^21,25 or studies using convenience samples with concurrent controls.^20,23,24

The primary limitations of our study are an inherent selection bias and the retrospective nature of the data. Our conclusions regarding the length and progress of labor are limited by the fact that the self-selected treatment groups were not necessarily unbiased and comparable. At our medical center, epidurals are rarely used, and half of all women in labor receive ITN. A significant number of births are attended by midwives, and many women enter labor hoping to achieve “natural childbirth.” These women choose ITN as a fallback alternative when relaxation techniques, tubs, massage, and intravenous narcotics are insufficient. Under these circumstances, a request for ITN may be a marker for a difficult labor, one where oxytocin augmentation or a prolonged second stage would be more likely, irrespective of the analgesic received. This phenomenon has also been noted when women requesting epidural analgesia are compared with those not requesting spinal analgesia.¹⁹

Our retrospective study design also limited our ability to assess the effectiveness of pain control in the 2 groups. Since objective pain measurements are not part of routine labor monitoring at our hospital, we could only infer the effectiveness of ITN from subjective chart notations or indirect measures of “method failure,” such as additional narcotic doses or second procedures required. Also, since the comparison group is defined by selecting out a certain percentage of “method failures” (ie, women who entered labor intending to use intravenous or no medication but changed their minds and requested ITN), effectiveness of pain control in the 2 groups cannot be directly compared.

Conclusions

The combined findings of a prolongation of the second stage of labor and increased oxytocin augmentation raise concerns that ITN alters the natural course of labor. However, caution is warranted, because selection bias in this retrospective study prohibits any conclusions regarding cause and effect. Prospective randomized trials comparing intrathecal narcotics with epidural analgesia and no spinal analgesia could substantially improve our understanding of the effects of spinal analgesia on the course and outcomes of labor. Objective measures of the effective duration of ITN are also needed, and strategies to enhance effectiveness (such as addition of local anesthetic to the ITN or routine use of a pudendal block in the second stage) also deserve experimental trials. Finally, women’s voices must be heard in evaluating the tradeoffs between analgesic efficacy and side effects. Such tradeoffs may greatly influence their analgesic choices and have an impact on their overall satisfaction with the birthing experience.

Acknowledgments

Our research was funded through the Department of Family Practice and Community Health, University of Minnesota. We acknowledge Deborah Finstad for database management, Dr Bruce Center for assistance with statistical analysis, Dr James Pacala for manuscript review, and Dr Anne Marie Weber-Main for editorial assistance.

Pain management during labor and delivery is an important issue affecting both the quality of the experience and its outcome. Early attempts at anesthesia with barbiturates, intravenous narcotics, or the combination of ether, morphine, and scopolamine (known as “twilight sleep”) were all associated with major side effects for mothers and infants.¹ Contemporary emphasis has been on safer, more natural methods of pain management, including childbirth training, relaxation, and support from a labor companion. Though helpful, these approaches are not sufficiently effective for the majority of patients in labor.² Recently, postpartum depression and posttraumatic stress disorder have been linked to difficult or painful deliveries,^3,4 and good analgesia has been identified as a major determinant of women’s overall satisfaction with their birthing experiences.⁵ The need for safe, effective relief for labor pain was acknowledged directly by the medical community in 1992 in a joint statement issued by the American College of Obstetricians and Gynecologists and the American Society of Anesthesiologists: “Labor results in severe pain for many women. There is no other circumstance where it is considered acceptable for a person to experience severe pain amenable to safe intervention, while under a physician’s care.”⁶

In response to the need for effective labor pain management, neuraxial (spinal) analgesia has become increasingly popular in the United States. Three techniques are currently in use: epidural anesthesia (continuous infusion of local anesthetic with or without an adjuvant narcotic), intrathecally administered narcotics (ITN), and the combined spinal-epidural approach.^7-10 Of the 3, the epidural is the most established and best studied method. According to a national survey published in 1986, epidural anesthesia was used for pain management for 16% of deliveries, while there was no reported use of ITN.¹¹ Today, epidural analgesia is used for more than 60% of nulliparous labors in large urban hospitals in the United States and Canada.^12,13

ITN were adapted to obstetric practice in the early 1980s, when it was demonstrated that a single subarachnoid injection of morphine sulfate could eliminate the pain of contractions in the first stage of labor.^14,15 Since the initial reports, use of ITN has achieved increasing acceptance as a simple and effective technique for labor pain management. Its potential advantages include better pain control than parenteral narcotics¹⁶ and more rapid onset of analgesia with less intervention than an epidural.¹⁷ Because ITN do not involve a local anesthetic, they do not cause the sympathetic or motor blockade commonly observed with epidural anesthesia.^10,17 Women receiving ITN retain sensation and motor control of pelvic musculature and lower extremities; thus, they are able to ambulate and change positions during labor and have essentially no blunting of the urge to push in the second stage.^17,18 Although recent meta-analyses have found that epidurals are associated with a prolonged second stage of labor¹⁹ and a qualified decreased incidence in spontaneous vaginal deliveries,^16,19 ITN have been characterized as not interfering with the natural progress of labor.^8,20

ITN have been a particularly successful addition to labor services in community or military hospitals where anesthesiology services and access to epidurals are limited.^19,21,22 Our hospital’s 15-year experience with ITN also supports their role in a busy, urban teaching hospital where health maintenance organizations influence the anesthesia standard toward a simple, low cost method. Typically, more than 4000 women deliver at our hospital each year, and approximately half receive ITN for their labor pain. Fewer than 5% receive epidurals; the rest deliver with parenteral narcotics or natural childbirth.

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 unaffected^20,23 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).

Results

Baseline characteristics for the 2 groups are shown in Table 1. Women receiving ITN were similar to the comparison group in mean age, marital status, and insurance type but were significantly more likely to be white. Women in the ITN group had less advanced cervical dilation on admission and were somewhat more likely to present with spontaneous rupture of membranes.

The technique of ITN administration was consistent. All procedures were performed by one of approximately 20 anesthesiologists in a private-practice group. The anesthesiologists used 25-gauge conical-tipped Whitacre spinal needles to puncture the dura at the L3-L4 interspace. Eighty-six percent of these women received both morphine and fentanyl, typically 0.25-mg and 25-mg doses, respectively. Fourteen percent received fentanyl only, a choice generally reserved for multiparous patients expected to have rapid labor progress. The mean cervical dilation at the time of ITN administration was 4.7 cm ± 1.4 cm. Six women required a second ITN procedure, and another 5 women received intravenous nalbuphine for pain after ITN (usually more than 4 hours after ITN administration).

Intravenous drugs administered for pain in the comparison group also followed a consistent pattern, with nalbuphine being the parenteral analgesic of choice. Forty-one percent of these women received one dose of intravenous nalbuphine ranging from 5 to 20 mg; an additional 3% required 2 doses. Overall, women receiving ITN were significantly less likely to receive nalbuphine than women in the comparison group (P=.000). A total of 17 women in the ITN group received nalbuphine (including both those who had it before and those after ITN) versus 41 in the comparison group.

Data on lengths of labor, oxytocin use, and delivery outcomes for the 2 groups are shown in Table 2. The mean length of active-phase labor was similar: approximately 4 to 5 hours in each group. However, women with ITN experienced significantly longer second stages. This difference remained significant after controlling with multivariate regression for the following confounders: ethnicity, parity, maternal age, cervical dilation on admission, nalbuphine use, oxytocin use, and infant weight (b coefficient=26.965; t=3.261; P=.001). When labor lengths were further analyzed by parity Table 3, the prolonged second stage in women receiving ITN was statistically significant among nulliparas and approached significance among parous women as well. Of note, the magnitude of prolongation was similar for both nulliparous and parous women; that is, the length of the second stage was increased by more than 50% in both groups receiving ITN.

Women in the ITN group were twice as likely to receive oxytocin augmentation during labor Table 2. This difference persisted in a logistic regression model taking into account ethnicity, parity, maternal age, cervical dilation on admission, duration of active-phase labor, nalbuphine use, and infant weight (b coefficient=1.55; P=.001). After controlling for confounders, the relationship between ITN and oxytocin use was strengthened, with women who received ITN more than 4 times as likely to receive oxytocin (odds ratio [OR]=4.69).

With regard to delivery outcomes Table 2, 8 women from both groups (4% of the total) required a cesarean delivery. Seven of these were in the ITN group, and 1 was in the comparison group. Six of the cesarean sections were performed for dystocia, 1 for fetal distress, and 1 for other reasons. Nine women (4.5%) had an instrumental vaginal delivery (6 with ITN, 3 without). Among instrumental vaginal deliveries, half were for maternal exhaustion, a diagnosis seen exclusively in nulliparous women. Differences between the groups in the rates of cesarean section versus combined spontaneous and instrumental vaginal delivery did not reach statistical significance.

Table 4 shows the frequency of ITN side effects gathered from the chart review. Among women receiving ITN, 2 side effects-urinary retention requiring urethral catheterization and pruritus-were recorded with significantly higher frequency than in the comparison group. Nausea or vomiting, generally viewed as a side effect of ITN, was equally common in both groups, as was maternal hypotension. To counter side effects, essentially all women who received ITN were routinely given 6.25 mg naltrexone sublingually immediately postpartum. Other than the naltrexone, medications to treat side effects were infrequently used. Diphenhydramine, hydroxyzine (for pruritus), and prochlorperazine (for nausea) were administered to 11 women in the ITN group. An additional complication seen only in the ITN group was the occurrence of a postdural puncture ("spinal") headache. Four spinal headaches were diagnosed, and 3 of them were treated with a blood patch. None of the women experienced respiratory depression following ITN.

Neonatal outcomes were similar in both groups. In the ITN group, the mean infant birth weight was 3491 ± 552 g compared with 3506 ± 454 g for the comparison group (P=.82). Four infants in the ITN group and none in the comparison group had 5-minute Apgar scores less than 7 (P=.12). There were no significant differences in the number of infants in each group requiring resuscitation beyond simple stimulation and blow-by oxygen (17 in the ITN group, 14 in the comparison group, P=.25) or admission to the neonatal intensive care unit (5 infants vs 3, P=.72). However, infants whose mothers were in the comparison group were more likely to be given naloxone postdelivery, presumably for prophylaxis against respiratory depression (10 infants vs 3 in the ITN group, P=.08).

Discussion

In our retrospective study of spontaneous labors from a large urban teaching hospital, ITNs were associated with prolonged second-stage labor in both nulliparous and parous women, with the greatest impact (both statistically and clinically) on the nulliparous women. This could have important consequences for a compromised fetus that is ill-suited to handle periods of repetitive hypoxia associated with prolonged pushing. Although we also found a trend toward more cesarean deliveries in the ITN group, the sample size was too small and the event rate too low to draw definitive conclusions about differences in delivery outcomes between groups.

Results from previous studies of ITN use are contradictory, indicating either no difference in second-stage labor length^20,23 or prolongation.^15,24 It is possible that the ITN’s advantages (maternal mobility and full sensation for pushing in the second stage) can be offset by pain. ITN do not provide relief from the pain of perineal distention,^14,18 and women at our hospital received perineal local anesthesia only when episiotomy was performed (29% of cases). Likewise, pudendal nerve blocks, used routinely as an adjuvant method of pain management in early reports of ITN’s clinical successes,²⁵ were rarely used during our study.

We found no significant difference in the mean duration of active labor in the 2 study groups. This is consistent with a previous finding²⁰ but in contrast to the assertion of another²³ that ITN use shortens the first stage significantly in both nulliparous and primiparas. The latter study, like ours, involved retrospective methodology. However, it investigated 4 groups: combined spinal-epidural, epidural, ITN, and no spinal analgesia. Women were allowed to convert from ITN to a combined spinal-epidural analgesic, presumably if ITN alone were insufficient. The more favorable outcome for women with ITN may be due to selection bias, with longer labors becoming ITN failures and crossing over to the spinal-epidural group.

Our finding that ITN use was associated with a 2-fold increase in oxytocin augmentation is different from 2 previous studies showing similar rates of oxytocin use for women with ITN and those without.^20,24 Nevertheless, the strength of the association we discovered (OR=4.69 correcting for confounders) warrants further investigation. If replicated, our finding has implications for informed consent. Women in labor who choose ITN use for its reputation as a simple, more natural alternative to epidural analgesia deserve to know whether it carries increased risk of an intervention with oxytocin. Animal studies have suggested that morphine inhibits uterine contractions through direct action on uterine opioid receptors.²⁶ Baraka and colleagues²⁷ noted a significant increase in oxytocin augmentation in laboring women receiving 2.0 mg intrathecal morphine versus those receiving only 1.0 mg. The effects of ITN on uterine contractions are likely modified by complex interactions. Various hypotheses have been proposed, including a spinal cord site of action for morphine resulting in a depressant effect analogous to its effects on micturition²² and a hypothalamic-pituitary level interaction of narcotics with oxytocin.²⁸ Although our study shows an association between ITN and oxytocin augmentation, no causal connection can be drawn. It is possible that certain practice patterns-such as care providers’ desires to “take advantage of the ITN” by starting oxytocin during the 2- to 3-hour period during which it is most effective-account for the high percentage of women in our study whose labors were augmented.

Maternal side effects, such as itching and urinary retention, were clearly associated with ITN use despite routine postpartum administration of naltrexone to reverse narcotic effects. Spinal headache occurred in 4% of cases despite the routine use of atraumatic spinal needles. Fetal effects of ITN were minimal, presumably because absorption of narcotics from cerebral spinal fluid into the maternal and fetal circulation is limited²⁹ but also possibly because our sample size was too small to detect differences. In our hospital, where ITN is the primary alternative to parenteral narcotics, it had the beneficial effect of sparing newborns from treatment with naloxone.

Strengths and Limitations

The strengths of our study are the random sampling of medical records, the stratification for parity, and an adequate sample size for detecting differences in the primary outcomes. Previous reports of the effects of ITN use have been limited to case series^21,25 or studies using convenience samples with concurrent controls.^20,23,24

The primary limitations of our study are an inherent selection bias and the retrospective nature of the data. Our conclusions regarding the length and progress of labor are limited by the fact that the self-selected treatment groups were not necessarily unbiased and comparable. At our medical center, epidurals are rarely used, and half of all women in labor receive ITN. A significant number of births are attended by midwives, and many women enter labor hoping to achieve “natural childbirth.” These women choose ITN as a fallback alternative when relaxation techniques, tubs, massage, and intravenous narcotics are insufficient. Under these circumstances, a request for ITN may be a marker for a difficult labor, one where oxytocin augmentation or a prolonged second stage would be more likely, irrespective of the analgesic received. This phenomenon has also been noted when women requesting epidural analgesia are compared with those not requesting spinal analgesia.¹⁹

Our retrospective study design also limited our ability to assess the effectiveness of pain control in the 2 groups. Since objective pain measurements are not part of routine labor monitoring at our hospital, we could only infer the effectiveness of ITN from subjective chart notations or indirect measures of “method failure,” such as additional narcotic doses or second procedures required. Also, since the comparison group is defined by selecting out a certain percentage of “method failures” (ie, women who entered labor intending to use intravenous or no medication but changed their minds and requested ITN), effectiveness of pain control in the 2 groups cannot be directly compared.

Conclusions

The combined findings of a prolongation of the second stage of labor and increased oxytocin augmentation raise concerns that ITN alters the natural course of labor. However, caution is warranted, because selection bias in this retrospective study prohibits any conclusions regarding cause and effect. Prospective randomized trials comparing intrathecal narcotics with epidural analgesia and no spinal analgesia could substantially improve our understanding of the effects of spinal analgesia on the course and outcomes of labor. Objective measures of the effective duration of ITN are also needed, and strategies to enhance effectiveness (such as addition of local anesthetic to the ITN or routine use of a pudendal block in the second stage) also deserve experimental trials. Finally, women’s voices must be heard in evaluating the tradeoffs between analgesic efficacy and side effects. Such tradeoffs may greatly influence their analgesic choices and have an impact on their overall satisfaction with the birthing experience.

Acknowledgments

Our research was funded through the Department of Family Practice and Community Health, University of Minnesota. We acknowledge Deborah Finstad for database management, Dr Bruce Center for assistance with statistical analysis, Dr James Pacala for manuscript review, and Dr Anne Marie Weber-Main for editorial assistance.