Department of Anaesthesia, Ninewells Hospital and Medical School, Dundee DD1 9SY, UK*Corresponding author
Accepted for publication: January 3, 2001
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Abstract |
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Br J Anaesth 2001; 86: 6747
Keywords: anaesthetic techniques, regional, sciatic; anaesthetics local, ropivacaine; anaesthetics local, bupivacaine; pharmacokinetics, bupivacaine
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Introduction |
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Patients and methods |
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On arrival in the anaesthetic room, a 20 g cannula was placed in a peripheral vein for the administration of sedative and other drugs, and a 14 g cannula was inserted in a vein of the antecubital fossa of the other arm for blood sampling. In addition to routine clinical monitoring devices, a separate temperature probe (Hewlett Packard M1029A) was applied to the dorsum of each foot, over the space between the first and second metatarsal bones. These were covered and insulated with gauze swabs and the skin temperature of both feet was measured continuously, and recorded every minute.
Midazolam 12 mg was administered i.v. before the block procedure, which was performed aseptically. After infiltration of the skin and subcutaneous tissues with 1% lidocaine, the sciatic nerve injection was performed using the posterior approach of Labat. All blocks were performed by the same investigator, and neither patient or block administrator knew which solution was used. The nerve was located using a 120 mm, 21 swg, short-bevelled insulated needle attached to a Stimuplex peripheral nerve stimulator (B. Braun medical). Needle placement was considered optimal when maximal gastrocnemius contraction and plantar flexion of the foot was obtained at a current of 0.5 mA. After careful aspiration, the test solution was injected over a 2-min period. The end of the injection was defined as time zero.
Development of nerve block was assessed in three ways. The sensory response to pin-prick on the dorsal and plantar aspects of the foot was assessed using a short-bevelled 27 swg dental needle before, and 5, 10, 15, 20, 25, and 30 min after injection. Sensation was categorized as sharp (same as the contra-lateral foot), dull (pin-prick perceived as pressure), or absent (complete loss of awareness of pinprick). Onset of block was defined as the time taken to achieve complete loss of sensation on both dorsal and plantar aspects of the foot.
Motor block was assessed using a simple 3 point score:
0=normal muscle power;
1=reduced power (plantar or dorsiflexion); and
2=complete motor block (plantar and dorsiflexion).
The time of onset of a progressive increase in the temperature of the blocked foot was used to indicate the onset of sympathetic nerve block.
Once the sensory block was complete, an arterial tourniquet was applied to the calf, one hands breadth below the tibial tuberosity to avoid proximal compression of the peroneal nerve. The patient was then taken into the operating theatre and positioned for surgery. The foot was exanguinated using an Esmarch bandage, and the arterial tourniquet inflated. During the surgical procedure the patient listened to music by themselves through headphones and was able to communicate with an investigator at all times.
The clinical efficacy of the block was assessed in two ways. Patients were asked to provide a verbal rating of the quality (excellent, good, fair, poor) of anaesthesia 10 min after the start of surgery, and again immediately after its end. The time that elapsed until the patient first requested post-operative analgesia was recorded as an indicator of the duration of sensory block.
Peripheral venous blood samples were taken before, and 10, 20, 30, 45, 60, and 120 min after the end of the injection. The blood was centrifuged immediately and the supernatant plasma frozen at 20°C for subsequent batch analysis using a gas chromatographic technique with a coefficient of variation of 6%.
Data are presented as median (range) and statistical comparisons were performed using the MannWhitney U test for the characteristics of the nerve block. Peak concentrations (Cmax) of the two drugs were compared by t-test after log transformation, and times to peak concentration (Tmax) by Wilcoxon signed rank test.
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Results |
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There was some variability in the time to the onset of both a sustained increase in skin temperature and complete anaesthesia of the foot, but there were no differences between the two groups (Table 2). Eight patients in the ropivacaine, and seven in the bupivacaine group, developed complete motor block of the foot. The remaining patients developed only some degree of motor weakness, but this did not interfere with surgery. Ten min after surgery began, 10 patients in the ropivacaine group rated anaesthesia as excellent and two as good. In the bupivacaine group, 10 rated anaesthesia as excellent and two as fair.
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Mean peak plasma concentration of ropivacaine was significantly higher than that of bupivacaine, but in no patient did the maximum figure exceed the accepted threshold for systemic effects (Table 3). The time taken to reach the peak plasma concentration was the same in both groups.
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Discussion |
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Although similar doses of local anaesthetic are required, there has been far less study of systemic drug disposition after sciatic than epidural block. The peak plasma concentrations reported here for bupivacaine are of the same order as seen previously. Coventry and Todd compared standard and alkalinized formulations of bupivacaine with epinephrine in a slightly greater dose (2 mg kg1) than used here.10 Peak plasma concentrations, 0.4 and 0.6 mg litre1, respectively, were somewhat lower than reported here, but epinephrine was not added in the present study. Others have studied combinations of lower limb nerve blocks with bupivacaine. Moore and colleagues observed a peak plasma concentration of 1.6 mg litre1 after 400 mg bupivacaine with epinephrine for combined sciatic, femoral, and lateral cutaneous nerve of thigh block.11 Misra and colleagues compared plasma concentrations after bupivacaine 3 mg kg1, plain or with epinephrine, after combined sciatic and femoral 3-in-1 block, and noted peak concentrations of 0.75 and 0.7 mg litre1, respectively.12
The peak concentrations of ropivacaine were somewhat higher than those for bupivacaine (Table 3). Primarily this would reflect the greater dose administered, although slightly higher concentrations of ropivacaine have been reported than after injection of equal doses of bupivacaine epidurally.13 Lee and colleagues did report that the threshold for early signs of systemic toxicity with ropivacaine was in the range 12 mg litre1, but that study involved direct i.v. infusion of the drug in volunteers.14 Many subsequent clinical studies have found plasma concentrations of ropivacaine to be in that range, but, as here, this has been without any indication of systemic toxicity.13 Much higher concentrations have been reported after accidental i.v. administration of ropivacaine, but again without any real concern for patient safety.15
This study was designed to assess also the clinical efficacy of sciatic nerve block as the primary anaesthetic for foot surgery and to compare the effects of the two commercially available long acting agents, both of which performed well. Sciatic block is a useful technique for unilateral lower limb surgery, having several advantages over central nerve block in elderly patients in whom a regional technique is preferred. This is especially so in terms of avoidance of the complications to which the elderly are particularly disposed such as hypotension and urinary retention. It may also be used more readily in the presence of a minor degree of coagulopathy or after head injury when central block is relatively contraindicated. The more selective block distribution may improve post-operative mobility, and longer acting agents provide prolonged post-operative analgesia.
Adverse effects of sciatic block are rare, but would include intravascular injection.9 Because a significant total dose of local anaesthetic is deposited at a very specific site to achieve accurate block, it is possible that a significant proportion of the total dose could be injected i.v. The length of needle used may also conceal intravascular placement particularly if over-vigorous aspiration is applied. The total dose should, therefore, be given slowly in increments and the reduction in cardiotoxicity afforded by ropivacaine should be seen as a significant advantage in these circumstances. The addition of epinephrine to either of the study agents does not appear to offer any clinical advantage in terms of duration of block, but may be useful in determining early intravascular placement.
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Acknowledgements |
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References |
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