Place the patient in the lateral position as for a spinal or lumbar epidural. Note that the posterior superior iliac spines and the sacral hiatus form an equilateral triangle (Figure RA.21). Use the index finger of the non dominant hand to palpate the sacral cornuae either side of the hiatus which normally feels like a small depression between the bony landmarks. With the hiatus located, sterilise and prepare the area and insert the needle at an angle of about 60 degrees to the skin through the subcutaneous tissues. The sacrococcygeal membrane is tough and offers obvious resistance to the needle; once through the membrane, re-angle the needle to 20-30 degrees (Figure RA.22) and carefully advance the needle a few millimeters, ensuring that it remains in free space. If it strikes bone or will not advance freely, withdraw slightly and reposition the needle or begin the whole procedure again. Do not advance the needle more than a few millimetres within the sacral canal, especially in children, because the dural sac extends beyond S2 in some individuals. Aspirate to check for blood and CSF and then slowly inject 3 ml of chosen solution to test for low resistance to injection. If this feels normal and there is no subcutaneous swelling denoting needle misplacement in the superficial tissues, slowly inject the main dose, with frequent aspiration checks.
It is most common to administer a caudal after inducing general anaesthesia. The practical technique is described in Figure RA.20.
Drugs, Doses and Volumes
Paediatric: The linear relationship between age, volume and segmental spread is utilised in a number of formulae. The best known is that of Armitage (1979) and this is shown in Figure RA.23 overpage.
If the volume of bupivacaine used exceeds 20 ml, motor blockade can be minimised by using 0.19% bupivacaine (dilute 3 parts bupivacaine 0.25% with one part normal saline) and use the calculated volume as in Figure RA. 23. Lidocaine 1% in the same volumes gives analgesia
Figure RA.21 Patient position for caudal anaesthesia
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