Low volume, high concentration block of the sciatic nerve

Abstract

SummarySciatic nerve block was performed in two groups of patients using a low power peripheral nerve stimulator to aid nerve location. The demonstration that greater accuracy of needle placement can be achieved by use of a low power peripheral nerve stimulator during sciatic blockade, should permit the use of smaller volumes and, therefore, higher concentrations of local anaesthetic solutions. Accurate needle placement and the use of concentrated solutions should result both in a reduction of latency and in greater duration of the block. This study was undertaken to test this hypothesis in the clinical setting. Patients and methodsForty adult patients scheduled for either elective or emergency surgery of the lower limb were studied. Patients with arterial hypertension or known hypersensitivity to local anaesthetic agents were excluded from the study. Formal consent was obtained in all cases. All patients were premedicated with diazepam 10 mg orally approximately 2 hours pre-operatively. Patients were allocated according to a random number sequence to one of two groups. In group A sciatic block was performed using 1% prilocaine with felypressin 0.01 IU/ml using a volume of 0.25 ml/kg body weight. In group B the block was performed using 3% prilocaine with felypressin 0.03 IU/ml at a volume of 0.08 ml/kg body weight. Sciatic block was performed via the posterior approach described by Raj et aL2 using a 22-gauge uninsulated needle. A low power peripheral nerve stimulator3 was used, as described by Smith and Allison.' Onset of block was specified as loss of sensation to pinprick in both the tibial and common peroneal nerve territories. The degree of motor block was assessed using 10-cm visual analogue scores based on both the power and degree of plantar and dorsiflexion of the foot, that ranged from power normal to power absent. Testing was performed a t 3-minute intervals for pinprick and 10-minute intervals for motor block by an observer blind to the patient grouping. Any block not effective a t 45 minutes was deemed to have failed and alternative anaesthetic techniques were employed. The duration of block was assessed on the basis of analgesia to pinprick as above and assessments were done at approximately 2-hourly intervals, again by an observer blind to the patient grouping. Recovery of sensation in either the tibial or peroneal nerve territory was taken as regression of the block. Similarly, any complaint of pain in the sciatic distribution was noted and this point taken as the time to recovery of the block. Arterial blood pressure was monitored at 3-5-minute intervals throughout the period of anaesthesia and surgery, using automated oscillotonometry (Dinamap), and the electrocardiograph monitored continuously. The data were analysed using Wilcoxon rank sum testing or Chi-square testing (with Yates' correction) as appropriate. ResultsThere were no significant differences between the two groups in respect of age, weight, sex or the duration and nature of surgery undertaken. The results for latency of B.E. Smit...

Transient Adverse Neurologic Effects of Spinal Pain Blocks

“…Little is known about the relative importance of local anesthetic volume versus concentration for peripheral nerve blockade24,25,31,33) or for spinal pain blocks. The optimum dose of local anesthetics in spinal pain blocks is adjusted to prevent pain, minimize motor block, and prolong block duration.…”

“…This report suggested that when a large volume of local anesthetics (ropivacaine) was administered, the placement accuracy was less critical and motor block was associated with the local anesthetic concentration. In studies of sciatic nerve blocks, the authors concluded that administering a low volume and a high concentration of local anesthetic was associated with a shorter time to onset of complete sensory and motor block33) and a longer duration of analgesia31) than a larger volume and a lower concentration of local anesthetic. Individual patients may have different responses to the same dose of the same drug delivered in the same location.…”

Unanticipated motor block with brachial plexus analgesia after increasing dilute ropivacaine infusion

“…A number of recommendations have been made in an effort to reduce the risk of this complication [6,7].…”

“…The relationship of local anesthetic concentration and the volume needed to achieve analgesia without a motor block varies with the anatomical location where it is given [5][6][7][8][9][10]. For epidurally administered local anesthetics, concentration is the primary determinant of the depth of motor block whereas the total mass of local anesthetic determines spread [5].…”

Student Anesthetist Learning Curve Perspectives on Sciatic Nerve Localization Proficiency - A Pilot Study

“…Smith and Siggins (1988) anesthetized. This is the usual situation in which regional anesthesia is performed in children (Bosenberg, 1995).…”