1Department of Anaesthetics, The Ulster Hospital, Dundonald, Belfast BT16 1RH, UK. 2Department of Anaesthesia, Queens University, Belfast, UK*Corresponding author
Accepted for publication: June 12, 2001
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Abstract |
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Br J Anaesth 2001; 87: 7337
Keywords: analgesia, obstetric; anaesthetic techniques, combined spinal-epidural; anaesthetics local, bupivacaine; anaesthetics local, ropivacaine
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Introduction |
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Efforts to improve epidural analgesia led to Collis and colleagues3 popularizing the combined spinal-epidural technique (CSE) for analgesia in labour. This technique involved an initial intrathecal injection of opioid (fentanyl) and bupivacaine to establish analgesia, and subsequent epidural injections to restore and maintain analgesia. The doses of drugs involved were such that ambulation in labour was possible.
However, after the initial intrathecal injection, motor blockade may be present for up to 20 min. Recent research has shown that, in the absence of motor weakness, there is no functional impairment of balance after CSE analgesia in labouring women4 and therefore ambulation is safe.
Ropivacaine is a newer local anaesthetic which has been shown to cause less motor weakness by the epidural route while still providing effective analgesia. Ropivacaine has been used for spinal analgesia in general surgical patients, in whom doses of 15 and 22.5 mg were shown to be effective without major side-effects.5 6 More recently, intrathecal ropivacaine has also been used to provide analgesia in labour as part of a CSE technique.7 In that study the opioid used was sufentanil.
The aim of this study was to compare the routine intrathecal injection used in our unit, fentanyl 0.025 mg and bupivacaine 2.5 mg, with an intrathecal injection of fentanyl 0.025 mg and ropivacaine 2.5 mg. Efficacy, sensory and motor blockade and fetal effects were studied in detail.
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Methods |
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The women were given an intrathecal injection of one of two anaesthetics by random allocation. Those in group B received bupivacaine 2.5 mg with fentanyl 0.025 mg made up to a total volume of 2 ml. Those in group R received ropivacaine 2.5 mg with fentanyl 0.025 mg, also made up to 2 ml with normal saline. Both the patient and the observer were blinded to the contents of the intrathecal injection, the same two observers being involved in all cases. The study period commenced after the intrathecal injection and finished at request for further analgesia by epidural bolus.
Maternal heart rate and arterial blood pressure (non-invasive) were recorded at 2-min intervals for the first 20 min and at 5-min intervals thereafter. Hypotension, defined as a drop of systolic blood pressure of greater than 20% from baseline or to a systolic pressure of less than 90 mm Hg, was documented and treated with i.v. boluses of ephedrine 6 mg as required. The cardiotocogram (CTG) was recorded continuously throughout the study period. CTG recordings were analysed by one of three obstetric registrars blinded to the treatment groups. The occurrence of late or variable decelerations or fetal bradycardia of less than 110 beats min1 was recorded as significant.
The time to first painless contraction was taken as the onset of analgesia. The duration of analgesia was defined as the interval between intrathecal injection and request for epidural top-up. Quality of analgesia was assessed using a four-point verbal rating score (0 = no pain, pressure or tightening; 1 = aware of tightening or pressure but not painful; 2 = tolerable pain, not distressing; 3 = distressing pain or pressure).
Analgesia assessments were carried out at 5-min intervals throughout the study period. The numbers of recordings of each score were summed from all women in each study group to produce a total for each of the four pain scores. This was then expressed as a percentage of the total number of pain assessments for each study group [cumulative analgesia score (%)].8 Sensory testing to cold and pinprick was performed at 2-min intervals for the first 20 min and every 5 min thereafter. Vibration sense was assessed with a tuning fork tested at both the knee and the ankle. Proprioception was assessed by testing the joint position sense of the metatarsophalangeal joint of both big toes. Both these functions were tested with the womens eyes closed, at 2-min intervals for the first 20 min and every 5 min thereafter. Motor blockade was assessed using a modified Bromage scale at 5-min intervals (0 = able to straight leg raise against resistance, i.e. no detectable motor block; 1 = unable to straight leg raise but able to flex knee; 2 = unable to flex knee but able to flex ankle; 3 = unable to move hip, knee or ankle). The incidences of itch, nausea and sedation were assessed by direct questioning at 5-min intervals.
Parametric data were analysed using Students t-test and non-parametric data using the MannWhitney U-test or the 2 test as appropriate. For clarity, some data are expressed as a percentage. A P value of <0.05 was considered statistically significant.
In a large series of labouring women given analgesia in the form of a CSE, it was demonstrated that 49% of women receiving this walking epidural were unable to straight leg raise against resistance9 and thus were unable to walk unaided. This figure was confirmed by an assessment of 30 consecutive women receiving a standard bupivacaine CSE in our unit before commencing this study. We found that, in the period immediately immediate after the subarachnoid injection, 43% of the women (13/30) developed demonstrable motor weakness, limiting full ambulation. We felt that it would be clinically significant if the incidence of motor block was reduced to below 10% in the ropivacaine group. The group size of 20 patients in our study gave 85% power to detect this difference.
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Results |
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No statistical differences were detected between the groups with respect to age, weight, height, gestation, parity or cervical dilatation before analgesia (Table 1). Onset and duration of analgesia are shown in Table 2. The extent of sensory block was similar in the two groups (Table 2). Proprioception was intact in all patients and vibration sense was impaired in one patient in each group. This loss of vibration sense lasted for 35 min in the patient who received ropivacaine and 15 min in the patient who received bupivacaine.
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Discussion |
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Early studies comparing ropivacaine with bupivacaine for epidural blockade assumed that the drugs were equipotent and compared 0.25% concentrations of both. They demonstrated a tendency towards less leg weakness with ropivacaine, which they attributed to a greater differential block.10 Alternatively, this could be explained on the basis of reduced potency of ropivacaine compared with bupivacaine. The potency issue has recently been addressed by Polley and colleagues11 using an updown sequential allocation design. They have shown that, with regard to analgesia, ropivacaine has a potency 0.6 times that of bupivacaine when given by the epidural route.
Experience of the use of intrathecal ropivacaine is limited. In one volunteer study, McDonald and colleagues,12 using transcutaneous electrical stimulation (TES) as a measure of block intensity, found that ropivacaine 12 mg, given by the intrathecal route, produced tolerance of TES at the ankle similar to that given by bupivacaine 8 mg. Ropivacaine 8 mg did not produce any tolerance to TES and the authors concluded that ropivacaine was less potent than bupivacaine to such a degree that its use for providing spinal anaesthesia was questionable.
In contrast, two clinical studies investigating the use of intrathecal ropivacaine found effective surgical anaesthesia with 15 and 22.5 mg5 6 without major adverse effects. Although these studies are reassuring with respect to the safety of ropivacaine by intrathecal injection, it is impossible to extrapolate from their results to intrathecal labour analgesia, in which much smaller doses of drug are used.
Little has been published on the use of ropivacaine for intrathecal labour analgesia. Levin and colleagues7 compared two doses of ropivacaine (2 and 4 mg) with bupivacaine 2.5 mg in a CSE technique and found no difference in duration or quality of analgesia. They also reported no motor block in any of the women, including those receiving intrathecal bupivacaine. This is in contrast to the findings of other researchers9 and may possibly be explained by the smaller numbers involved (10 per group). The opioid used in conjunction with the local anaesthetic was sufentanil and the ropivacaine dose was chosen on the basis of ease of administration (1 and 2 ml of 0.2% solution respectively) rather than on the basis of potency. We compared equal doses of local anaesthetic (2.5 mg in both groups) and combined this with fentanyl 0.025 mg, as is current practice in our unit. We chose motor power retention as a major end-point and considered a reduction in the number of women with detectable motor blockade to less than 10% as clinically relevant.
We found that both bupivacaine 2.5 mg and ropivacaine 2.5 mg with fentanyl 0.025 mg by intrathecal injection (2 ml) provided effective labour analgesia with rapid onset and duration, similar to the findings of other workers.3 7 The nature and frequency of side-effects seen in both groups were similar to those reported elsewhere.3 7 13
Our most significant finding was the difference in detectable motor block between the two groups. Only one patient in the ropivacaine group developed leg weakness compared with eight in the bupivacaine group (P=0.019). As mobility has been a major consideration in the development and popularity of CSE analgesia in labour, this finding may have important clinical implications. It is not clear if this is due to an intrinsic property of ropivacaine or to a difference in potency between ropivacaine and bupivacaine. Further dose-finding studies would be required to answer this question.
In the past, the safety of ambulation in labour without intact dorsal column function and proprioception was questioned,14 with implications for allowing women to mobilize freely after CSE analgesia. Recently, however, minimal change in balance function has been demonstrated following CSE analgesia, even in the presence of clinical dorsal column sensory loss [4]. In that study, the only women unable to complete posturography testing were those exhibiting overt motor weakness. This is reassuring in terms of allowing women to mobilize after intrathecal analgesia in labour, but also further emphasizes the importance of minimizing motor block. In this context, our finding of reduced leg weakness using ropivacaine may have significant clinical importance.
Quality of analgesia in this study was assessed on a verbal rating scale by the women themselves at 5-min intervals throughout the study period. Cumulative analgesia scores have been used in other published work to assess analgesic efficacy with both epidural8 and intrathecal15 techniques. The benefit of this scoring method is that any periods of inadequate analgesia or breakthrough pain documented are related to the duration of the labour or study period. More pain assessments in the bupivacaine group were grade 0 (unaware of contractions), while more assessments in the ropivacaine group were grade 1 (awareness of tightening or pressure but not pain). This is hardly unexpected, given the difference in potency between the drugs involved. However, when those experiencing effective analgesia (grade 0 or 1) were compared with those experiencing inadequate analgesia (grade 2 or 3) there was no significant difference between the ropivacaine and bupivacaine groups. This is reflected by the fact that all women in both groups reported satisfaction with their analgesia for labour when questioned 24 h after delivery. Indeed, several women in the ropivacaine group commented positively on their ability to feel the contractions without any pain.
We found a similar duration of analgesia in the two groups. Duration is influenced by a number of factors distinct from potency, including systemic uptake and elimination, and in this respect the two drugs appear to be quite well matched. The fentanyl used in the intrathecal injection will also have an effect on the duration of analgesia. However, as the same dose (0.025 mg) was used in the two groups we would still expect any difference in duration due to intrinsic properties of the local anaesthetic to be demonstrated.
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References |
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