The duration of action of bupivacaine, levobupivacaine, ropivacaine and pethidine in peripheral nerve block in the rat

Background : There is a current interest in local anaesthetic drugs/formulations exhibiting long durations of sensory block and minor motor–blocking effects.
Objectives : To compare the duration of sensory and motor blockade in peripheral nerve blocks induced by the new agents ropivacaine and levobupivacaine, with that of racemic bupivacaine and pethidine.
Methods : Groups of 8 male Sprague–Dawley rats were subjected to infraorbital (IONB) or sciatic nerve block (SNB) employing 0.2 ml of differently concentrated solutions of bupivacaine, levobupivacaine, ropivacaine or pethidine. The sensory blocking effect in IONB is expressed as (i) the time to elicitation of an abdominal jerk by electrical stimulation at arbitrarily chosen (threshold) intensities (IONB degree 3, 5, 8 and 10), and as (ii) the area under the curve (AUC, threshold intensities vs time). The duration of motor block in SNB is given as the time from injection to regained ability to walk and grip normally with the toes. Comparisons of the dose–effect relationships for the investigated agents were made by analysis of covariance.
Results : In IONB the log (dose)–log (effect) lines for bupivacaine, levobupivacaine and ropivacaine did not deviate from parallelism. The duration of sensory block induced by equimolar doses of these agents was similar, although bupivacaine exerted more pronounced effects than levobupivacaine (AUC by 25%, P=0.001; IONB degree 3 by 14%, P=0.03). In SNB only the log (dose)–log (duration) lines for bupivacaine vs levobupivacaine were found not to deviate from parallelism, both agents exerting similar durations of action. The motor–blocking effects of ropivacaine showed an inverse dose–duration relationship (P=0.019).
Conclusions : Equimolar doses of the investigated local anaesthetics exerted similar durations of sensory blockade in a peripheral nerve block model in the rat.     

Initial placement and secondary displacement of a new suture‐method catheter for sciatic nerve block in healthy volunteers: a randomised,double‐blind pilot study

We performed a randomised double‐blind pilot study in 16 healthy volunteers to investigate the success rate for placing a new suture‐method catheter for sciatic nerve block. A catheter was inserted into both legs of volunteers and each was randomly allocated to receive 15 ml lidocaine 2% through the catheter in one leg and 15 ml saline in the other leg. Successful placement of the catheter was defined as a 20% decrease in maximum voluntary isometric contraction for dorsiflexion of the ankle. Secondary outcomes were maximum voluntary isometric contraction for plantar flexion at the ankle, surface electromyography and cold sensation. After return of motor and sensory function, volunteers performed standardised physical exercises; injection of the same study medication was repeated in the same leg and followed by motor and sensory assessments. Fifteen of 16 (94%; 95%CI 72–99%) initial catheter placements were successful. The reduction in maximum voluntary isometric contraction and surface electromyography affected the peroneal nerve more often than the tibial nerve. Eleven of 15 (73%; 95%CI 54–96%) catheters remained functional with motor and sensory block after physical exercise, and the maximal displacement was 5 mm. Catheters with secondary block failure were displaced between 6 and 10 mm. One catheter was displaced 1.8 mm that resulted in a decrease in maximum voluntary isometric contraction of less than 20%. After repeat test injection, 14 of the 16 volunteers had loss of cold sensation. Neither motor nor sensory functions were affected in the legs injected with placebo. We conclude that the suture‐method catheter can be placed with a high success rate, but that physical exercise may cause displacement.     

Epinephrine as an adjuvant to amino-amide local anesthetics does not prolong their duration of action in infraorbital nerve block in the rat

The effects of epinephrine as an adjuvant to local anesthetics were studied in the rat infraorbital nerve block (IONB) model, using solutions of 0.5% prilocaine, 0.5% mepivacaine, 0.125% bupivacaine or 0.125% ropivacaine in 50 mmol/l tris-hydroxymethylaminomethane (THAM) tested both without and with epinephrine (EPI) added at 2, 4, 8 or 16 micrograms/ml. Solutions of THAM and EPI in normal saline did not induce IONB. THAM-buffered solutions of bupivacaine induced IONB of longer duration than bicarbonate-buffered solutions. Intensity of block during onset was increased only when EPI at 2 and 16 micrograms/ml was included in bupivacaine solutions. The duration of block induced by prilocaine, bupivacaine and ropivacaine was not significantly prolonged by addition of EPI at any of the concentrations tested. Only at a concentration of 16 micrograms/ml did EPI significantly prolong the duration of mepivacaine-induced block (+48%). Low concentrations of EPI in solutions of bupivacaine and ropivacaine significantly reduced their duration of action by up to 22% and 57%, respectively. It is concluded that the duration of action of local anesthetics in buffered solutions is only moderately affected by the inclusion of EPI, the effects differing only slightly from one to another. The efficacy of EPI as an adjuvant would seem to be governed by factors affecting the local disposition of the main drugs, such as non-specific binding, buffering of solutions and tissue pH.