Local anaesthetics in postoperative pain relief — what is their role?

Abstract

Spinal Local anaesthetics have been injected into both the intrathecal and epidural spaces for surgical anaesthesia since the early 1900s. Although the duration of anaesthesia with the longer-acting agents, tetracaine or bupivacaine is no more than three to four hours, for some procedures such as endoscopic urological surgery, single injections of 5% lidocaine (0.5 m g ' k g -l) can produce analgesia for an average of 11 hr. l For more major procedures, however, the duration of analgesia is more limited. The duration of spinal local anaesthetic analgesia can be extended with the use of indwelling catheters. Until recently they have not been used in the postoperative period for fear of postdural puncture headache and/or infection. With the introduction of 26-32 gauge (G) microcatheters, and the subsequent low incidence of headache, continuous spinal anaesthesia has now been used for postoperative analgesia. In one study of ten patients undergoing vascular procedures, lidocaine, in concentrations of 0.5-2.0%, was infused at 2-10 mlhr -~ for up to 48 hr. 2 This provided excellent analgesia, and only two patients required supplemental morphine. In this study half the patients had 28 G catheters and half had 18 G catheters. There was no evidence of haemodynamic instability or motor block, however 2/5 patients in whom a 28 G catheter was used experienced pump occlusion difficulty. A follow-up study showed no increased risk of infection in 19 similar patients. 3 In another study the addition of morphine was found to improve analgesia further and allow lower lidocaine concentrations of 0.1-0.2%. 4 However, subsequent to these studies, two case reports were published describing the development of cauda equina syndrome in six patients in whom microcatheters had been used intraoperatively. The presumed aetiology was maldistribution of large amounts of 5% hyperbaric lidocaine which can be neurotoxic. These reports lead to the withdrawal of these microcatheters from use by the FDA. Attempts are being made to reintroduce modified microcatheters which will avoid this problem. If the attempts are successful, further studies will be needed to determine the long-term safety of this technique, or whether the use of larger 18 G catheters is acceptable.