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Anesth Analg 2003;97:1800-1807 © 2003 International Anesthesia Research Society
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Abstract |
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We enrolled nulliparous women in induced labor in a randomized study to determine whether increasing the concentration of the solution used in a patient-controlled epidural analgesia (PCEA) device was required as labor progressed. Patients were assigned to 6 groups (n = 25 in each group), receiving ropivacaine/fentanyl in concentrations of either 0.1%/0.5 µg/mL or 0.2%/1 µg/mL via a PCEA pump. Three groups received boluses of 12, 16, or 20 mL dilute solution in early labor (uterine contractions every 3 min and 4-cm cervical dilation) then 6, 8, and 10 mL concentrated solution in late labor. Three other groups received boluses of 12, 16, or 20 mL dilute solution during both periods. The lockout interval was 25 min. The primary outcome was time until the first request for staff-administered analgesia supplement. Hourly assessments included pain scores on a visual analog scale (VAS) graded from 0 to 10, satisfaction scores, arterial blood pressure, motor block intensity, and the upper sensory level of epidural anesthesia. Patients, midwives, and the observer were unaware of study solutions and PCEA settings. The maximum pain score was defined as the highest score experienced by each patient during each period. Duration of analgesia was defined as the time from the start of each period to the first injection of rescue analgesia and was compared using a survival analysis. There were no differences among the groups with regard to demographic and obstetric variables, arterial blood pressure, motor block intensity, upper sensory level, or satisfaction scores. At least 75% of the women rated their satisfaction as either good or excellent during each period. During late labor, the maximum pain score was lower in the group receiving 20 mL dilute solution compared with the group receiving 6 mL concentrated solution. Maximum pain score was not significantly different between 20 mL dilute solution and 10 mL concentrated solution (difference between VAS values = -0.4; 95% confidence limits, -1.599 and 0.799; P = 0.5055). During late labor, the duration of analgesia was longer in groups receiving 20 mL dilute solution (99 ± 4 min) (mean ± SD) than in those receiving 12 mL (77 ± 30 min) and 16 mL (80 ± 23 min). Duration of analgesia did not differ between groups receiving 20 mL and 10 mL (92 ± 23 min) or between groups receiving 12 mL and 6 mL (78 ± 30 min) of each respective solution. Duration of analgesia was longer in the groups receiving 8 mL concentrated solution (94 ± 16 min) than in those receiving 16 mL dilute solution. We concluded that 0.1%/0.5 µg/mL ropivacaine/fentanyl was effective throughout labor when 20 mL was injected with each PCEA demand. With 16 mg ropivacaine and 8 µg fentanyl, the duration of analgesia was prolonged by doubling the concentration when labor became active. When 12 mg ropivacaine and 6 µg fentanyl were injected at each demand, analgesia was less satisfactory and doubling the concentration was not clinically effective. These results suggest that the effectiveness of PCEA is dependent on drug mass rather than the volume or concentration administered with each successful pump demand.
IMPLICATIONS: There is no clinical reason for increasing the concentration of the patient-controlled epidural analgesia (PCEA) solution when labor becomes active provided that an effective dose is already being administered with each demand. The quality of PCEA depends on the drug mass given with each demand rather than the concentration of the pump solution.
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Introduction |
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Ropivacaine is an amide local anesthetic used in concentrations of at least 0.2%, with or without opioids, for labor epidural analgesia (1,2). Recently, smaller concentrations of ropivacaine (0.08%–0.125%), administered via a patient-controlled epidural analgesia (PCEA) device (3–7), were found to be effective, providing both a decreased motor block and risk of toxicity.
However, several experimental protocols and clinical studies using PCEA have suggested that as labor progresses, a larger concentration of epidural local anesthetics is required for pain relief. Using an up-down sequential allocation, Capogna et al. (8) showed that the minimum effective concentration of epidural bupivacaine was 2.9-fold larger in late labor (cervical dilation >5 cm) than in early labor. Using a sequential study design, Beilin et al. (9) showed that epidural ropivacaine 0.2% offered adequate analgesia more frequently than either ropivacaine 0.15% or ropivacaine 0.1% in women experiencing uterine contractions at least once every 5 min. PCEA theoretically optimizes drug delivery, but its use in advanced labor may result in a frequent rate of staff interventions to supplement analgesia (10,11).
Thus, the effectiveness of PCEA with a dilute concentration may be inadequate as labor progresses in terms of adequacy of pain relief and the need for staff-administered supplemental analgesic doses. This prospective, randomized, volume-range study was designed to determine whether a solution of ropivacaine 0.1% combined with small-dose fentanyl (0.5 µg/mL) continued to provide adequate analgesia as labor progressed. In addition, the effect of the concentration on the quality of PCEA was evaluated.
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Methods |
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With IRB approval and written informed consent, nulliparous parturients who requested epidural analgesia for labor were recruited. Only parturients agreeing to an oxytocin-induced labor at the prenatal visit were considered for inclusion in the study. Exclusion criteria included ASA physical status III–IV, severe medical or obstetric complications, multiple gestation, contraindications to epidural analgesia, and inability to use the PCEA pump (APM; Abbott Laboratories, North Chicago, IL). Instruction regarding the use of the PCEA device took place during the preanesthe- tic visit. Patients were made aware that staff-administered supplements could be requested at any time if PCEA analgesia was deemed inadequate.
On the day of delivery an IV infusion of 0.9% saline was started, and a 5-orifice 20-gauge epidural catheter (B. Braün, Melsungen, Germany) was inserted 4–5 cm into the epidural space at the L3-4 or L4-5 interspace. All catheters were carefully observed after insertion and then aspirated gently for return of blood or cerebrospinal fluid. All catheters were placed before onset of uterine contractions, with a cervical dilation of 
2 cm, and connected to the PCEA pump before the start of the IV oxytocin infusion. The midwife reinstructed the woman on the use of the PCEA and reminded her that she could receive staff-administered supplements each time she thought that analgesia was inadequate. The midwife then ruptured the amniotic membranes.
By use of a computer-generated random number sequence, participants were randomly assigned to six groups receiving different volumes and concentrations of ropivacaine and fentanyl via a PCEA pump. Two successive periods were defined as labor progressed. The early period was the time from the start of the IV oxytocin infusion until uterine contractions occurred every 3 min and the cervix was 4 cm dilated, defining the beginning of active labor (12). The late period was defined as the time from that point until delivery. Three groups received a solution of 0.1% ropivacaine with 0.5 µg/mL fentanyl. The PCEA pump delivered boluses of 12 mL, 16 mL, or 20 mL with each successful demand, during both the early and late periods (12 mL R 0.1%, 16 mL R 0.1%, 20 mL R 0.1% groups). In the other 3 groups, the patients received the same dose regimen during the early period, then a solution of 0.2% ropivacaine plus 1 µg/mL fentanyl during the late period, but the volume was halved. Boluses of 6 mL, 8 mL, or 10 mL were thus given at each successful demand (6 mL R 0.2%, 8 mL R 0.2%, 10 mL R 0.2% groups) (Fig. 1). The PCEA pump did not deliver a background infusion. There was no dose limit; however, the lockout interval was 25 min in all groups. This time was selected to be compatible with the slowness of the injections and the time for producing epidural effects in each group. Rescue analgesia consisted of 6 mL 0.375% epidural ropivacaine at patient request. This dose was determined based on our previous institutional experience.
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Automated maternal blood pressure, heart rate, arterial hemoglobin oxygen saturation (SpO2), tocodynamometry, and continuous fetal heart rate were monitored throughout labor. The progress of labor was measured at regular intervals, and oxytocin infusion rate was adjusted according to obstetric protocols. Hypotension, defined as systolic blood pressure of <100 mm Hg, was treated with additional left uterine displacement and 10-mg IV doses of ephedrine as needed. Pain intensity was measured using a 10-cm visual analog scale (VAS), and patient satisfaction with analgesia was graded as excellent, good, fair, poor, or unsatisfactory. The motor block intensity was measured using a 0–3 scale (0 = full flexion of feet and knees, 1 = just able to move knees, 2 = able to move feet only, 3 = unable to move feet or knees). The level of sensory block was also obtained as loss of sensation to ice and classified as follows: lower than T12, at T12-11, at T10-8, at T7-5, or higher than T5. Measurements were obtained at baseline (start of oxytocin infusion), every 1 h during the early period, at the change of the pump, and every 1 h during the late period. The duration of the study was the time elapsed between the start of oxytocin infusion and the outcome of labor (natural delivery or cesarean delivery), except in the patients requesting staff interventions to supplement analgesia. In this case, a first request for 0.375% epidural ropivacaine constituted the end of the study for the patient concerned. PCEA demands and delivered demand doses were recorded immediately after the end of the study.
Study solutions were prepared and the pump was programmed by an anesthesiologist (JF) not directly involved in the patient’s care or data collection. In all patients, the pump setting was changed at the beginning of the late period. Data collection, treatments (oxytocin, ephedrine, supplemental ropivacaine), cervical dilation, and tocodynamometry analysis to determine the start of the late period were performed by a midwife unaware of the group allocation. Patients were excluded from the study during the procedure if oxytocin failed to induce labor. Similarly, they were excluded if epidural injection of 0.375% ropivacaine failed to induce sensory block and relief, suggesting a catheter displacement. This decision was made by the anesthesiologist (DL or JMB) who had inserted the epidural catheter and was thus unaware of solution and settings in use. Finally, patient, observer, obstetric team, and the anesthesiologist responsible for patient care were blinded as to the study solutions and PCEA settings.
Sample size was estimated before the study using VAS scores as the main determinant of the effectiveness of PCEA. On the basis of a pilot study and an expected 30% reduction in pain score with increasing volume or concentration, we estimated that a sample size of 25 patients per group was required for a power analysis of 80% at the 5% significance level. Data were presented as mean ± SD, 95% confidence limits, median and ranges, or numbers and percentages of patients. Associations of discrete variables were tested by 
2. Comparisons of nonparametric data were performed by Kruskal-Wallis analysis, followed by the Mann-Whitney U-test for pairwise comparisons. Differences between the groups on continuous data were tested by analysis of variance followed by Student’s t-test with Bonferroni corrections. Analyses were made separately for the early and late periods, then for the whole study when appropriate. Duration of analgesia provided by the different PCEA regimens was defined as the duration from the start of each period to the first injection of rescue analgesia. The duration was analyzed using survival curves; i.e., any patient requesting a staff-administered analgesia supplement was excluded from the analysis at that point. As delivery also ended participation in the study, the survival analysis was limited to a time interval during which all the patients who had not requested rescue analgesia were still in labor. Survival analysis was followed by log-rank tests. Maximum and minimum pain levels were defined as the highest and lowest scores of each patient during the early and late periods. Maximum and minimum satisfaction rates correspond to the highest and lowest scores of each patient during each period. The most intense motor block and upper level of sensory block were defined as the highest score and level experienced by each patient during each period.
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Results |
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One-hundred-fifty-seven patients were enrolled in the study. One patient in the 20 mL R 0.1% group was eliminated from the study because of epidural catheter displacement. Six others were excluded for failure of oxytocin-induced labor (1 patient in the 20 mL R 0.1% group, 1 patient in the 8 mL R 0.2% group, 2 patients in the 12 mL R 0.1% group, and 2 patients in the 10 mL R 0.2% group). The 150 patients who remained in the study were randomly assigned to 6 groups of 25. The demographic and obstetric characteristics of the patients were similar in each group (Table 1).
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During the early period, rescue analgesia was requested by 3 patients in the 6 mL R 0.2% group and 1 patient in the 12 mL R 0.1% group. It was requested by 3 patients in the 16 mL R 0.1% group and 1 patient in the 8 mL R 0.2% group. No rescue analgesia was requested in either the 20 mL R 0.1% group or the 10 mL R 0.2% group. Duration of analgesia did not differ among the groups (log-rank test; P = 0.20).
During the late period, the duration of analgesia differed among the 6 groups (log-rank test; P 0.001). The survival analysis only covered the first 100 min as several women who had not requested rescue analgesia delivered after that time. Figure 2 illustrates the survival curves. Two by two analysis indicated that analgesia was longer with PCEA pump injections of 20 or 10 mL than 12 or 6 mL of each respective solution (99 ± 4 and 92 ± 23 min versus 77 ± 30 and 78 ± 30 min, respectively) (log-rank test; P 0.05). Duration of analgesia was also longer in the groups receiving 8 mL concentrated solution (94 ± 16 min) compared with those receiving 16 mL dilute solution (80 ± 23 min) (log-rank test; P 0.05).
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There were no difference in the minimum and maximum satisfaction scores, irrespective of the group (Fig. 3). There were no significant differences among groups for maximum pain scores during the early period and minimum pain scores during the early and late periods. The maximum pain score during the late period was higher in the 6 mL R 0.2% group than in the 20 mL R 0.1% group (Table 2).
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Fentanyl and ropivacaine doses from PCEA were smaller in the groups receiving 12 mL ropivacaine 0.1% during the early period. At the end of the study (early and late periods), doses from PCEA were smaller in the 12 mL R 0.1% group than in the 8 and 10 mL R 0.2% groups or 16 and 20 mL R 0.1% groups (Table 3). During the late period, the ratio of successful to total PCEA demands differed among groups, but no statistical differences were identified by pairwise comparison.
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There were no intergroup differences for systolic and diastolic arterial blood pressure, ephedrine requirements (from 3 mg [0–30] in the group receiving the least to 8 mg [0–60] in the group receiving the most) (median [range]), and the number of patients receiving at least one ephedrine injection. There were no intergroup differences for the upper level of sensory block (Fig. 4) or motor block intensity. Full flexion of feet and knees was observed in at least 94% of women in each group. No grade 2 or 3 motor block was found.
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Discussion |
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This study provides a comparison between different PCEA regimens in laboring women, using incremental volumes and different concentrations of a ropivacaine and fentanyl mixture. As suggested by previous experimental and clinical studies (8–10), a more concentrated mixture was injected via the PCEA pump, as labor became more active, to improve patient comfort (12). Women’s satisfaction with analgesia, a global indication of the efficiency of the technique, was excellent. The minimum pain score, a direct indication of the efficiency of each self-administered epidural injection, was very low. Together these results suggest that our technique produced effective analgesia, regardless of the volumes and concentrations used.
Some differences, however, were found among the groups, confirming that as labor progressed, a larger volume and hence a larger dose of local anesthetics was needed to provide satisfactory analgesia (13). It was possible to obtain comparable results with a larger volume of dilute solution and a smaller volume of a concentrated solution, suggesting that the most important factor in achieving good analgesia was the drug mass delivered at each demand.
Our study differs considerably from previous reports evaluating PCEA use in obstetrics. First, our approach provided an analgesic titration of different PCEA regimens and addressed how far into the labor these regimens provided adequate analgesia. In contrast to several previously published studies (3–7,14), labor epidural analgesia was not initiated by physician-controlled loading doses in women with intense, frequent uterine contractions. In our study, the epidural catheter was inserted and the PCEA pump was available before labor began. Each parturient was advised to induce analgesia via the pump when she felt the need for it. Each patient was nulliparous and had never experienced labor pain before enrollment in the study. Second, discontinuing the study at the time of the first rescue analgesia eliminated possible bias resulting from staff-administered boluses. Last, and in contrast to several other studies (3–5,15), pain was not related to obstetric variables, which vary among patients and in the individual patient over time. We defined the maximum pain score as the highest score experienced by each patient regardless of the measurement time, thus making it possible to compare the groups regardless of the evolution and outcome of the labor. It might be objected that administering oxytocin and rupturing the amniotic membranes at the start led to more intense pain than spontaneous labor; however, in our case, the same protocol was applied to all parturients. Within the context of this clinical approach, injections of 6 mL concentrated solution and 12 mL dilute solution were associated with the worst results and injections of 10 mL concentrated solution and 20 mL dilute solution were associated with the best.
The patients in the 12 mL R 0.1% group received a smaller overall hourly PCEA dose of ropivacaine and fentanyl than the other groups, suggesting that this dosage had a drug-sparing effect. In fact, this group consumed less analgesic during the early period, whereas consumption was the same in all groups during the late period. Uterine contractions were not very painful during the early period, so efficiency of analgesia was the same in all groups, as indicated by the large ratio of successful to total PCEA demands. According to group allocation, women received a set dose of 12, 16, or 20 mg ropivacaine (with small-dose fentanyl), which resulted in relative overconsumption in the latter two regimens. However, the doses received during the early period probably had an impact on demand in the later period, and thus on overall consumption, as well as on the number of patients who requested supplementation. If the absence of staff-administered supplementation is considered the best criterion for judging the effectiveness of PCEA, the larger dose regimens gave the best results. The drug-sparing effect of small bolus doses may thus be artificial, as this regimen did not meet the patient’s total analgesia needs. It is interesting to note that the overall hourly ropivacaine consumption resulting from the self-administration did not differ among the groups receiving at least 16 mg at each demand. These doses were comparable to those recommended by others using a continuous epidural infusion of ropivacaine for achieving effective labor analgesia, good maternal satisfaction, and acceptable motor block (1,2).
During the late period, hourly drug consumption did not differ among the groups, although the bolus dose was smaller in the groups receiving 6 mL concentrated solution and 12 mL dilute solution. The pump was thus requested more frequently in these two groups than in the other groups, but the number of unsuccessful demands remained large, as indicated by the small ratio of successful to total PCEA demand. This suggests that a shorter lockout interval would be required in the groups receiving 6 mL concentrated solution and 12 mL dilute solution to provide satisfactory analgesia. Few studies have addressed this problem. Gambling et al. (14) compared increasing bolus volumes of 0.125% bupivacaine plus fentanyl with proportionally increasing lockout intervals. No statistical differences were found between the groups, though staff interventions for supplementary analgesia were frequent in the groups receiving the smallest doses with the shortest lockout intervals (14). In a randomized study, we compared the effects of PCEA using a 4-mL bolus dose of 0.125% bupivacaine plus sufentanil with an 8-minute lockout versus a 12-mL bolus dose of the same solution with a 25-minute lockout (11). Parturients in the 4-mL/8-minute group were potentially able to receive as large an amount of bupivacaine and sufentanil because the maximal dose per hour was equivalent in both groups. Despite more pump demands in the 4-mL/8-minute group, this did not happen. Pain scores were greater in this group, suggesting that inadequacy of a PCEA pump setting was mainly dependent on the bolus dose, rather than on the duration of the lockout interval (11).
Both 10 mL concentrated solution and 20 mL dilute solution provided a more prolonged effect and thus reduced manpower requirements as described by Bell et al. (16). A few differences were found between groups when compared two by two. Comparing maximum pain scores in the groups receiving 10 mL concentrated solution and 20 mL dilute solution, power analysis indicated that a sample size of more than 800 patients per group would have been required to reach statistical significance. One of the weaknesses of our study was to have compared six groups in a dose-finding design whereas it would have been more beneficial to compare only two groups receiving the same drug mass either in a dilute or concentrated solution. Furthermore, it is uncertain whether the PCEA technique can be used to qualitatively compare different local anesthetic concentrations. Previous experiences with PCEA indicate that self-administration of epidural analgesia allows most patients to achieve similar levels of comfort despite differences in analgesic solution (5,15,17). Although our study lacks sufficient power to assess analgesic differences, a comparison of the two groups receiving a mass of 16 mg ropivacaine in each bolus showed that more women in the group receiving dilute solution requested supplementary staff-administered injections and needed them earlier in labor than the other group. Under these conditions, it can be stated that concentration had a positive effect on analgesia. One possible explanation is that 16 milligrams was a threshold value. Twenty milligrams improved analgesia and reduced the supplementation rate, irrespective of the concentration. Twelve milligrams necessitated additional staff-administered boluses, irrespective of the concentration administered.
Using a PCEA for the postoperative period, Liu et al. (17) demonstrated that smaller concentrations of ropivacaine plus fentanyl provided comparable analgesia with less motor block despite the use of similar amounts of drugs. Decreasing the degree of motor block in laboring patients may preserve maternal ability to ambulate and may decrease the incidence of instrumental delivery (3). In our study, the intensity of motor block did not differ among the groups. Ambulating, however, was not tested. The practice of elective forceps by our obstetricians may account for the relatively large number of instrumental deliveries in each group, decreasing the beneficial effects of reduced local anesthetic concentration used to helping rotation and descent of the fetal head.
The incidence of hypotension and the ephedrine dose required to maintain systolic arterial blood pressure more than 100 mm Hg were similar in each group, suggesting that, at the small concentrations used in this study, the maximum upper sensory level of the epidural block was unrelated to the different volumes tested. Similar results have been reported in other studies evaluating the effects of different volumes and concentrations injected via a PCEA pump. This supports the opinion that epidural block spread and analgesia are dose-dependent rather than volume-dependent, within limits (18–20).
In the context of this study, the number of patients receiving staff-administered supplementation and the length of time before they requested it were taken as the criteria for effective analgesia. By these standards, the dilute solution (ropivacaine 0.1% and fentanyl 0.5 µg/mL) was effective throughout labor when 20 mL was injected with each PCEA demand. With 16 mg ropivacaine and 8 µg fentanyl, the duration of analgesia was prolonged by doubling the concentration when labor became active. When 12 mg ropivacaine and 6 µg fentanyl were injected at each demand, analgesia was less satisfactory, and doubling the concentration was not clinically effective. These results suggest that the effectiveness of PCEA is dependent on the drug mass rather than the volume or the concentration administered with each successful pump demand.
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Acknowledgments |
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The authors acknowledge the kind assistance provided by the midwife staff in collecting data.
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References |
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