SummaryThe results of studies on the effect of volume, concentration or total dose of local anaesthetic on the spread of spinal anaesthesia are inconclusive. Most support the assumption that the total dosage is more important than the volume. We compared low-dose bupivacaine (6 mg) in 0.5% and 0.18% solutions as sole anaesthetic to achieve predominantly unilateral spinal anaesthesia for knee arthroscopy. Sixty patients were randomly allocated to two groups to receive either 1.2 ml 0.5% bupivacaine (6 mg) (n ¼ 30) or 3.4 ml 0.18% hypobaric bupivacaine (6.1 mg) (n ¼ 30). Drugs were administered at the L 3-4 interspace with the patient in the lateral position. Patients remained in this position for 20 min before being turned supine for the operation. Spinal block was assessed by pinprick and modified Bromage scale and compared between the operated and nonoperated sides. No significant changes were found in the spread or duration of sensory or motor block (p > 0.05). The haemodynamic changes were also similar between the groups. The same pinprick level of analgesia, degree of motor block and duration of spinal anaesthesia was obtained with bupivacaine (6 mg) in low (1.2 ml) or high (3.4 ml) volumes. The ideal spinal anaesthesia for ambulatory surgery should provide good surgical anaesthesia with rapid recovery from sensory and motor block [1]. Lignocaine has been widely advocated for ambulatory anaesthesia but there are concerns about possible neurotoxicity. Recent editorials have questioned the use of hyperbaric 5% lignocaine for spinal anaesthesia [2][3][4]. In contrast, the frequency of radiating backache after bupivacaine has been reported as less than 1% [5,6]. There is increasing interest in the use of small doses of bupivacaine for spinal anaesthesia.There has been controversy concerning the relationship between volume, concentration and total dose of spinally administered drugs. Most of the studies suggest that the total dosage is more important than the volume [7][8][9]. In these studies, the doses of bupivacaine were fairly large. Sheskey et al. used 10, 15 and 20 mg doses [7] and Bengtsson et al. used bupivacaine 22.5 mg [8]. In the study of Cherng et al., plain bupivacaine 15 mg was diluted with cerebrospinal fluid (CSF) [9]. The effect of volume on spinal anaesthesia when a small dose (5-10 mg) of hypobaric bupivacaine is used has not been investigated.In a previous study, we achieved predominantly unilateral spinal anaesthesia with minimal sensory and motor block on the nonoperated side using a low dose of hypobaric 0.18% bupivacaine [10]. We have now compared low-dose bupivacaine (6 mg) in two volumes for predominantly unilateral spinal anaesthesia for day-case knee arthroscopy.
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