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Maternal Posture Influences the Extent of Sensory Block Produced by Intrathecal Dextrose-Free Bupivacaine with Fentanyl for Labor Analgesia

Richardson, Michael G. MD; Thakur, Rajbala MD; Abramowicz, Jacques S. MD; Wissler, Richard N. MD, PhD

Obstetric Anesthesia

The cephalad extent of sensory block produced by intrathecal (IT) dextrose-free local anesthetics and opioids has been reported to be quite variable. Most reports describing the effects of IT analgesics do not control for patient posture. Because these medications are hypobaric relative to cerebrospinal fluid (CSF), parturients in a sitting position may develop greater cephalad extents of sensory block than those in a lateral position during IT injection. Parturients in labor were randomized to sitting or lateral position during IT administration of dextrose-free bupivacaine 0.25% with fentanyl 0.005%. Extent of sensory block was evaluated at intervals thereafter. Free flow of CSF was obtained in 20 parturients. Those in a sitting position during IT injection had significantly higher maximal cephalad extent of block than those in a lateral position (mean +/- SD, T-3.1 +/- 2.9 vs T-6.3 +/- 3.4, P = 0.036). Mean cephalad extent of block was greater in the sitting group at 20 and 30 min. When sensory block asymmetry was observed, the extent of block was greater on the nondependent side. Posture during IT injection of this dextrose-free analgesic combination affects extent of sensory block in laboring parturients.

(Anesth Analg 1996;83:1229-33)

Departments of (Richardson, Thakur, Wissler) Anesthesiology and (Abramowicz, Wissler) Obstetrics and Gynecology, University of Rochester School of Medicine and Dentistry, Rochester, New York.

Presented in part at the 1995 Annual Meeting of the American Society of Anesthesiologists, Atlanta, GA, October 24, 1995.

Accepted for publication August 8, 1996.

Address correspondence to Michael G. Richardson, MD, Department of Anesthesiology, University of Rochester School of Medicine and Dentistry, Box 604, Strong Memorial Hospital, 601 Elmwood Ave., Rochester, NY 14642. Address e-mail to

The effects of intrathecal (IT) analgesics on sensory blockade, onset and duration of analgesia, and side effects in parturients have been reported [1-5]. Small doses of IT opioids alone or in combination with dextrose-free local anesthetics produce variable, yet high levels of sensory blockade [1-3,5]. Possible explanations for such high levels have been proposed, including nondependent, cephalad spread of hypobaric IT injectate [1,3,5]. Most of the commonly used dextrose-free IT analgesic solutions are hypobaric with respect to human cerebrospinal fluid (CSF) [6,7]. Therefore, patient position during IT injection should affect extent of sensory blockade, and possibly onset and duration of analgesia, as well as associated side effects. Most studies examining the effects of IT analgesia in laboring parturients do not provide details of objective sensory changes [5,8-10]. Of the studies that do describe sensory changes [1-5], some do not state patient position during IT injection [1,2]. Those that do, describe parturients positioned exclusively in either sitting [3,5] or lateral [4] positions during IT injection. We undertook this study to determine if parturient position during IT injection of bupivacaine with fentanyl affects extent of sensory block, onset or duration of analgesia, characteristics of block onset and regression, or side effects.

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After human subjects' review board approval and written, informed consent, 24 parturients in labor were enrolled in this study. Inclusion criteria included ASA physical status I or II, active labor, cervical dilation between 2 and 8 cm, and age more than 17 yr. Women were excluded if they had severe preeclampsia, diabetes mellitus, prematurity, multiple gestation, fetal complications, infection, or epilepsy. Parturients were prospectively randomized to sitting or full lateral position. All women received 500 mL of lactated Ringer's solution intravenously. Monitors included maternal oscillometric blood pressure cuff, pulse oximeter, tocodynamometer, and an external Doppler fetal heart rate monitor. An 18-gauge Tuohy needle (Durasafe[TM], Becton Dickinson, Franklin Lakes, NJ) was inserted into the epidural space at the L3-4 interspace using the loss-of-resistance-to-air technique. A 27-gauge Whitacre needle was inserted through the Tuohy into the subarachnoid space, confirmed by free flow of CSF. CSF, 0.1 mL, was aspirated immediately prior to IT injection. One mL of 0.25% bupivacaine mixed with 0.5 mL of 0.005% fentanyl was injected IT over 5 s with the Whitacre needle side port directed cephalad. No CSF aspiration was attempted after IT injection. The 27-gauge needle was then removed and a 20-gauge epidural catheter was passed through the Tuohy needle into the epidural space. No medication was injected through the catheter at that time. The parturient was placed recumbent with left uterine displacement, and the time from IT injection to repositioning was noted. An observer blinded to patient position during IT injection then entered the room to collect all outcome data. The time of the first painless contraction was noted by the observer, unless it occurred before repositioning, in which case the time was noted by the anesthesiologist.

The following outcomes were determined 20 and 30 min after IT injection, then every 15 min until the time of first request for further analgesia: bilateral upper and lower extent of sensory analgesia to pinprick, nausea scores, and pruritus scores. When sensory block asymmetry occurred, the mean sensory level was used for comparisons of sensory block extent. Pain scores were recorded immediately prior to and 20 min after IT injection using a 10-cm visual analog scale. The time of first request for further analgesia was recorded. Vasopressor, antiemetic, and antipruritic therapies were administered at the discretion of the anesthesiologist and were noted. Pairs of fetal heart rate (FHR) tracings from each parturient were evaluated by a perinatologist. One tracing was recorded prior to and one after IT injection. Each recording was 20 min long. The perinatologist was unaware of the chronologic order of the tracings, parturient position, or anesthetic procedure performed. FHR tracings were assigned a score according to the system described by Lyons et al. [11], which considers baseline rate, accelerations, decelerations, and FHR oscillation amplitude and frequency, with 10 the best possible score and 0 the worst. FHR tracings were then assigned to high- (>7), intermediate- [5-7], and low- (<5) score groups [11]. Additionally, FHR was qualitatively judged as reactive if two or more accelerations each lasting 15 s occurred during a 20-min period. Frequency of uterine contractions averaged over 10 min was determined. All complications and modes of delivery were noted. All patients were evaluated on the first and second postpartum days for the presence of postdural puncture headache.

Continuous data are expressed as mean +/- SD. Normally distributed continuous data were analyzed using an unpaired, two-tailed t-test. The Mann-Whitney rank sum test was used to analyze nonnormal distributions of continuous data and ordinal data. Nominal data were analyzed using the chi squared test. Linear regression was used to evaluate the effect of the length of time sitting after IT injection on extent of block in the sitting group. A value of P < 0.05 was considered statistically significant.

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Twenty-four parturients were studied. CSF was not obtained in four women (two in each group), and each of these had successful epidural catheter placement and analgesia and were eliminated from further analysis. The 20 parturients receiving IT analgesia (10 in each group) did not differ with regard to demographic data (Table 1). Those positioned sitting during IT injection had significantly higher sensory block compared to those positioned laterally (Table 2 and Figure 1 and Figure 2). Seven of 10 women in the sitting group had maximum sensory levels of T-4 or higher, whereas 7 of 10 in the lateral group had levels of T-6 or lower. Three of the women in the sitting group had cervical sensory levels, compared with none in the lateral group. Sensory block asymmetry (the number of dermatomes blocked on the right minus those blocked on the left) was plotted for each parturient at 20, 30, and 45 min (Figure 3). When asymmetry was observed in the lateral group, the greater extent of sensory block occurred exclusively on the side that was nondependent during IT injection. In those seated during IT injection, asymmetric blocks were usually more extensive on the right side. Block asymmetry diminished with time. Extent of block did not correlate with the length of time parturients in the sitting group remained seated after IT injection (P = 0.09).

There were no significant differences between groups in times to first painless contraction, onset of one-segment regression of sensory block, or first request for analgesia (Table 2). Baseline and 20-min pain visual analog scale scores were not different between groups (Table 2). Pruritus occurred in 8 of 10 women (80%) in each group (Table 3). No parturient experienced nausea or postdural puncture headache. There were no significant FHR or uterine contraction frequency changes in either group. FHR reactivity remained unchanged in 13 of the 20 cases (six reactive, seven nonreactive). Four nonreactive FHRs became reactive. Two changed from reactive to nonreactive (both were in the high-score group before and after). One pair of FHR tracings could not be evaluated for reactivity due to discontinuous monitoring; however, this pair remained in the intermediate-score group before and after. In 15 of the pairs, there was no change in FHR score group (10 high, 5 intermediate). Four FHR tracings improved (two from intermediate to high, and two from low to intermediate), whereas one worsened (intermediate to low). There was a single cesarean section in each group, both due to arrest of descent (Table 2).

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Small doses of IT bupivacaine consistently produce sensory blockade in laboring women [1,4]. Intrathecal lipid-soluble opioids alone often produce sensory blockade as well [1-3,5]. The cephalad extent of sensory blockade using dextrose-free IT opioids, local anesthetics, or combinations has been reported to be surprisingly high in laboring women. High thoracic and cervical sensory levels are commonly observed [1-5,12], and cranial sensory levels have been reported [3,5]. Although some investigators do not describe patient position, high sensory levels have been reported to occur rapidly in sitting parturients after IT injection of sufentanil (10 micro g) [3,5]. Many of the women in this study also developed high levels of sensory blockade detected by pinprick testing. Furthermore, our results demonstrated a significantly higher upper extent of sensory block in women positioned sitting compared with those positioned laterally during IT injection.

Density differences between IT injectate and CSF may explain the observed postural differences in extent of sensory block. Stienstra et al. [13] demonstrated that very small density differences significantly influence distribution patterns of local anesthetic solutions injected into a spinal canal model. Both fentanyl 0.005% and dextrose-free bupivacaine 0.25% are hypobaric relative to human CSF at 37 degrees C [6,7]. The combination of both is hypobaric as well [7]. This less-dense IT injectate would be expected to rise rostrally to a greater extent in seated parturients than in those positioned laterally. If hypobaricity of IT injectate accounts for the differences in cephalad extent of block between sitting and lateral groups, a greater extent of block would also be expected on the nondependent side in the lateral group. Half of the parturients positioned laterally during IT injection did, in fact, have varying degrees of asymmetric block, always greater on the nondependent side. Those seated during IT injection tended to have a slightly greater extent of block on the right side, which is nondependent when positioned recumbent with left uterine tilt. Consistent with this mechanism is our common clinical observation that, in parturients positioned laterally during IT injection, sensory block in the nondependent leg is almost immediate and precedes measurable sensory changes in the dependent leg. Similar observations have been reported with IT dextrose-free bupivacaine in women undergoing cesarean section [14,15]. Russell [14] infused dextrose-free bupivacaine (0.5%, 0.25%, 0.125%, and 0.083%) IT over 30 minutes while the parturients remained in full right lateral decubitus position. Most women developed extensive unilateral sensory block on the nondependent side during the infusion period. Carrie and O'Sullivan [15] administered 0.5% plain bupivacaine intrathecally to parturients in the left lateral position and demonstrated unilateral block on the nondependent side. They observed symmetric blocks in those who were turned to the right lateral position immediately after IT injection.

Although density differences between IT injectate and CSF may account for the observed differences in extent of sensory block in this study, there are other possible mechanisms. Changes in posture may acutely alter the distribution of CSF and IT injectate within the IT space. Moving from the sitting to recumbent positions causes gravity-related rostral displacement of CSF in nonpregnant individuals, although the volume displaced is only several milliliters [16]. Physical straining and increased intrabadominal pressure during postural changes [17,18] as well as position alone [17,19] may contribute to epidural venous engorgement, thereby compressing the thecal sac and displacing CSF rostrally [4,14,17].

The observed range within each group in the extent of sensory block was striking. These results are similar to other studies involving the use of dextrose-free opioids [1-3], local anesthetics [1,4], and combinations [1] in parturients. Likewise, in nonobstetric patients, plain 0.5% and 0.75% bupivacaine produce highly variable and somewhat unpredictable spread of IT sensory block [20-22]. Although hypobaric, the densities of commonly used dextrose-free IT analgesics are very close to that of CSF [6,7]. Small interpatient differences in CSF density [6] may be enough to result in widely differing IT analgesic distribution patterns [13,23] and may account for the wide ranges in extent of block observed by us and others. Density differences between different lots of bupivacaine 0.25% are negligible [7] and therefore less likely to account for variability in extent of block.

Despite differences in upper extent of sensory block, we did not detect differences in other important clinical effects. There were no differences between groups with regard to onset or duration of analgesia, which were comparable to observations in another series using the same IT drugs [8]. None of our patients experienced nausea. Pruritus occurred in 80% of the parturients and required treatment in a single patient (5%). Most described pruritus as mild and not bothersome. Collis et al. [8] report a much lower pruritus incidence of 17.3% after IT bupivacaine and fentanyl, likely due to their retrospective questioning of parturients about side effects. Campbell et al. [1] prospectively questioned parturients receiving IT bupivacaine with sufentanil and, like us, found an 80% incidence of pruritus [1]. Pruritus has been reported to occur in 80% to 94% after IT opioids alone [1-3,9,10,24]. Some investigators have suggested that IT local anesthetics may block opioid-induced pruritus [9,25]. Our results and those of Campbell et al. [1] suggest this is not so.

In summary, patient position does affect the extent of sensory block in laboring parturients after IT injection of a mixture of 0.25% bupivacaine and fentanyl. Parturient position did not affect other clinical variables in this series. Studies examining the effects of non-dextrose-containing IT solutions should specify patient position at the time of and after IT injection.

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© 1996 International Anesthesia Research Society