Caudal analgesia is produced by injection of local anaesthetic into the caudal canal. This produces block of the sacral and lumbar nerve roots. It is useful as a supplement to general anaesthesia and for provision of postoperative analgesia. This technique is popular in paediatric patients. Catheter insertion may be performed for continuous caudal block.
The sacrum is a triangular bone that articulates with the fifth lumbar vertebra, the coccyx and the ilia. The dorsal roof consists of the fused laminae of the five sacral vertebrae and is convex dorsally. In the midline is a median crest which represents the sacral spinous processes. Lateral to this is the intermediate sacral crest with a row of four tubercles which represent the articular processes. The S5 processes are remnants and form the cornua, which provide the main landmarks for indentifying the sacral hiatus. The hiatus is covered by the sacro-coccygeal membrane. The canal contains areolar connective tissue, fat, sacral nerves, lymphatics, the filum terminale and a rich venous plexus.
The patient is usually in the left lateral position with the knees drawn up to the chest. The sacral hiatus lies at the third point of an equilateral triangle formed with the two posterior superior iliac spines (look for the dimples in the skin). The cornua are palpable on either side of the hiatus. Adopt an aseptic technique. A needle (or 22/20 g cannula) is introduced in a slightly cranial direction through the hiatus. A click is felt as the needle pierces the sacrococcygeal membrane. The needle/cannula is then directed cranially. The dura ends at S2, but may extend further. Aspirate to confirm the absence of blood/cerebrospinal fluid and inject local anaesthetic while feeling for inadvertent subcutaneous injection with the other hand. There should be very little resistance to injection. In children, the block should be performed after general anaesthesia has been induced and before surgery has commenced.
Choice of local anaesthetic
0.5 ml/kg, 0.25% bupivacaine (sacro-lumbar block)
1 ml/kg, 0.25% bupivacaine (upper abdominal block)
1.2 ml/kg,0.25% bupivacaine (mid-thoracic block)
(Doses described by Armitage).
In this age group, epidural analgesia is accompanied by very little change in blood pressure or cardiac output. Continuous caudal catheters have been used intraoperatively for more prolonged surgery. The maximum safe dose range for epidural bupivacaine is 0.15-0.2 mg/kg/h in neonates and 0.2-0.3 mg/kg/h in infants. The addition of clonidine (1-2 mcg/kg) to bupivacaine extends its duration of action significantly. Preservative-free Ketamine (0.5 mg/kg) also prolongs the duration of analgesia.
20-30 ml 0.25-0.5% bupivacaine. Average volume of the sacral canal is 30-35 ml.
Epidural fat in children has a loose and wide-meshed texture, whereas in adults it becomes more densely packed and fibrous. Hence, local anaesthetic spread is greater in children.
i] Caudal bupivacaine and s(+)-ketamine for postoperative analgesia in children.
Weber F, Wulf H.
Paediatr Anaesth 2003; 13(3): 244-8.
ii] The 'swoosh' test--an evaluation of a modified 'whoosh' test in children.
Orme RM, Berg SJ.
Br J Anaesth 2003; 90(1): 62-5.