The maximum change in systolic blood pressure in women with and without sensory blocks also did not differ, as determined by a t-test, with both groups experiencing an average decrease in SBP of 23 +/- 2 mm Hg. Decreases in blood pressure did not result in any emergent cesarean sections or low Apgar scores. In six cases, ephedrine was given based on the clinical judgment of the anesthesiologist. In all cases, the treated SBP was less than 100 mm Hg, and it was at least 30% less than the baseline SBP. There was one fetal bradycardia, unrelated to hypotension (SBP was always greater than 110 mm Hg), which resolved without intervention. Despite these events, maternal and neonatal outcomes were excellent in all groups (Table 1). Spontaneous vaginal delivery occurred in 37 women (84%) with four cesarean deliveries and four instrumented vaginal deliveries. No subject delivered or entered Stage 2 of labor before receiving local anesthetics via the catheter. Postdelivery interviews revealed a high degree of satisfaction with the analgesia during the first stage of labor; 85% of patients rated their satisfaction as 8 or greater on a scale of 0-10.
Pruritus occurred in 55% of patients and was the most significant side effect mentioned the day after delivery. However, only four women required treatment for itching during labor; two patients received diphenhydramine, one received nalbuphine, and one received both drugs. Eight patients received metoclopramide for nausea and vomiting, although only four mentioned this the following day as an adverse effect. One postdural puncture headache occurred, but the patient did not require an epidural blood patch.
ITS generally provided excellent analgesia. Failure to obtain adequate analgesia because of lack of CSF flow in the spinal needle (four cases), or lack of response to ITS (three cases) was a problem in 14% of patients. Some of these failures may be due to improper placement of the epidural needle or to the spinal needle being too short relative to the epidural needle .
As in a previous report , we found a high incidence of sensory changes and decreased blood pressure after ITS administration. These findings suggest that ITS may have some local anesthetic effect on nerve conduction. In support of this hypothesis, Power et al.  and Gissen et al.  have shown that fentanyl and sufentanil decreased conduction in isolated nerve fibers in vitro. However, in both of these studies, high opioid concentrations (50 and 100 micro g/mL) were needed to significantly depress nerve conduction. In the current study, administered sufentanil concentrations were only 3 to 10 micro g/mL, with further dilution occurring in the CSF. Despite the increased sensitivity to opioids found in pregnancy  based on the results from animal studies, these concentrations are probably too low for sufentanil to function clinically as a local anesthetic.
Findings from the current study that demonstrate the separation of sensory changes and analgesia are inconsistent with a clinically important local anesthetic mechanism of ITS analgesia. With ITS, neither the presence nor the extent of sensory changes were predictive of the quality or duration of analgesia or of the degree of blood pressure changes. If ITS does not have a clinically significant local anesthetic action on the spinal cord or nerve root, why does it cause decreased blood pressure and sensory changes? We believe that the sensory changes may be caused by the direct effect of sufentanil on micro receptors in the spinal cord. Alfentanil, another lipophilic opioid, has been shown by Wang et al.  to depress A partial and C-fiber afferent pathways to the spinal cord when given intrathecally to dogs. This effect was naloxone reversible. Altered sensation, therefore, may be an expected finding after intrathecal administration of opioids.
Reduction of afferent input to the spinal cord does not however, explain the decreases in blood pressure that follow ITS administration. In Wang et al.'s study , clinical doses of alfentanil depressed C-fiber-mediated responses in dogs but did not affect blood pressure. Thus, there does not appear to be a direct micro-receptor-mediated sympatholytic effect. This hypothesis is supported by studies in humans. Moses et al.  found no decrease in blood pressure when nonlaboring women having elective cesarean sections were prehydrated and then given intrathecal fentanyl 25 micro g. Similarly, in another study in which we gave male volunteers ITS, there was no significant change in blood pressure . Perhaps the relief of preexisting pain during labor leads to the observed decreases in blood pressure. In the current study, all the subjects were experiencing severe labor pain. Intrathecal administration of sufentanil probably blocked the C-fiber-mediated pain stimuli to the spinal cord and either inhibited the sympathetic reflex arc and/or decreased circulating catecholamines that had been causing relative hypertension. In support of this explanation, Nagasaka and Yaksh  recently demonstrated that intrathecal administration of micro-agonists to unstimulated halothane-anesthetized rats had no effect on blood pressure or heart rate. In contrast, pretreatment with the same micro-agonists produced dose-dependent inhibition of pain-induced blood pressure and heart rate increases resulting from a painful stimulus applied to the tail. In addition, Cascio et al.  found that serum epinephrine levels decreased after administration of intrathecal fentanyl to laboring women. A preliminary report by Newman et al.  confirms our results. They found that blood pressure decreased when laboring women received ITS when, as in the current study, the baseline blood pressure was measured immediately before the block. However, when the baseline blood pressure was the last blood pressure recorded in the prenatal chart, blood pressure was actually higher after ITS. This suggests that laboring women are relatively hypertensive during painful labor compared with the prenatal period prior to labor.
Although the sensory changes and blood pressure decreases are probably not mediated by a local anesthetic or sympatholytic mechanism, they still may be clinically important. For example, high levels of sensory blockade involving the cranial nerves have been reported after ITS administration in labor and can be quite distressing to the patient . In the current study, several patients required ephedrine for treatment of hypotension, with systolic blood pressures as low as 80 mm Hg in some instances.
Another area of concern with ITS use during labor is its possible effect on the fetus. A recent report  documents several cases of severe fetal bradycardia after the administration of intrathecal fentanyl and suggests that this may result from increased uterine tone after removal of sympathetic inhibition as maternal catecholamines decrease with the onset of analgesia. One case of fetal bradycardia occurred in the current study. In a previous study in which 73 fetal heart rate tracings were examined after maternal administration of ITS, Cohen et al.  found a 15% incidence of fetal heart rate changes, including one bradycardia. As in the current study, fetal heart rate changes were not correlated with maternal hypotension. An effect of ITS on the spinal cord, nerve roots, or the sympathetic nervous system or the rapid onset of analgesia with a sudden decrease in circulating catecholamines could affect uterine blood flow or uterine tone with resultant slowing of the fetal heart rate. Alternatively, such fetal bradycardias may merely be coincidental with ITS administration. Women often request analgesia as the intensity of their pain increases. This may be a sign that labor is progressing into an accelerated phase. Fetal bradycardia from head or cord compression may occur at this point in labor even when women have not received regional analgesia.
We also found that diluent volume did not affect analgesia or the incidence of sensory changes or hypotension. Considering the relatively large volume of CSF compared with injectate volume and the narrow range of diluent volumes tested, these findings are not surprising. In support of our results, Van Zundert et al.  found that diluent volume did not affect the duration or the intensity of lidocaine spinal anesthesia.
In summary, ITS provides rapid onset of intense analgesia to laboring women. It produces sensory and blood pressure changes in some patients, which are most consistent with opioid-mediated spinal cord effects. Within the ranges tested, the volume of diluent was unimportant with respect to analgesia or side effects.
We acknowledge Beemeth T. Robles, MD, and Jayshree B. Desai, MD, for helping to collect the data.
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© 1997 International Anesthesia Research Society
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