Caudal analgesia remains the most popular and commonly used regional block in paediatric anaesthesia. Bupivacaine is the most widely used local anaesthetic for this technique and provides postoperative analgesia lasting 4-8 h. More recently, ropivacaine has been shown to be suitable for caudal anaesthesia in children. Its duration of action is similar to that of bupivacaine (in equivalent doses), but the motor block is slower in onset, less intense and shorter in duration for a given level of sensory block. Furthermore, ropivacaine appears to cause less cardiac and CNS toxicity than bupivacaine. Levobupivacaine is a new aminoamide local anaesthetic with a similar duration of action to bupivacaine, but with less potential for cardiotoxicity. These newer agents need to be studied further but seem promising prospects, particularly when extending the use of caudal analgesia into day-case surgery. It is clear that the extended duration of analgesia that can be achieved by using caudal additives is significant. What is not so clear is whether the perceived benefits justify the potential risks, and which is the ideal agent. Epinephrine prolongs the analgesic effects of lidocaine, but seems to have little effect on bupivacaine. The addition of opioids significantly prolongs the duration of caudal analgesia but carries with it a number of unpleasant side-effects, as well as the risk of late respiratory depression. The use of caudal opioids seems to have been superseded by clonidine and ketamine. Clonidine 1-2 μg kg-1 and ketamine 0.5 mg kg-1 offer the potential to significantly prolong the duration of 'single-shot' caudal injections with minimal risk of side-effects. The addition of clonidine 1 μg kg-1 to plain bupivacaine 0.25% can extend the duration of postoperative analgesia by 4 h, with mild sedation as the only side-effect. A combination of ketamine 0.5 mg kg-1 and bupivacaine is even more effective, providing analgesia for up to 12 h. At this dose, behavioural problems do not appear to be significant. However, further study and the introduction of ketamine into mainstream clinical practice is limited by the difficulties in obtaining preservative-free ketamine and ongoing concerns about potential neurotoxicity. The use of caudal additives for day-care anaesthesia is controversial and at present their routine use cannot be recommended. It has become something of a holy grail in recent years to achieve longer and longer duration of analgesia from a single caudal injection. Perhaps it is time to reappraise what we hope to achieve by this, and whether the same or similar clinical effects can be achieved by the use of standard single-agent caudals in combination with regular systemic analgesics. However, in the absence of new longer acting local anaesthetic drugs, and until such time as alternative methods of prolonging caudal analgesia are validated, the use of caudal additives is likely to continue. The use of the caudal route has a long and impressive track record but, like any old friendship, it is important that we nurture and foster it appropriately. Rather than continually testing its limits, perhaps it is time to re-explore its strengths.
