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 length20,23 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 series21,25 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