Although the caudal route was the first widely used approach to the epidural space for continuous block in labour (Hingson and Edwards, 1942), and remained in frequent use in a few centres until the late 1970's (McCaul), its present role is very limited. Apart from providing good analgesia for low instrumental deliveries and perineal repair, the use of caudal block is usually restricted to a very few enthusiasts and to those cases in which a lumbar epidural is anatomically impossible or contraindicated by local infection (Chapter 70). (However, local sepsis may also preclude the use of caudal block) (Chapter 2).

The patient should be prepared according to the guidelines shown in Chapter 44. Following the provision of secure venous access and preloading with crystalloid, the patient is either turned into the left lateral position with both knees flexed and forward tilt of the upper hip, or (occasionally) the knee-elbow position according to patient or operator preference. The skin of the lumbosacral area is prepared with a bactericidal solution and sterile drapes are applied. A bleb of local anaesthetic is raised in the skin overlying the sacral hiatus, between the sacral cornua, at a site deep to the proximal extremity of the natal cleft. A 21- gauge needle may then used to infiltrate the deeper tissues, but this is not essential.

A variety of needles and cannulae may be used for the block itself, including a:
1. 7.6 cm 19-gauge caudal needle,
2. disposable 3.8 cm 19-23g hypodermic needle,
3. 20-24-g intravenous catheter for a "single-shot" technique, or
4. Tuohy needle (17 or 18g) and catheter for a continuous technique.

Whichever needle is used, it is gently inserted at an angle of 70-80degrees to the skin and slowly advanced until the bony resistance of the sacrum is detected (Figure 22.1). The needle is withdrawn slightly, the entry angle reduced by about half, and then reinserted until the resistance of the sacrococcygeal ligament is felt at a depth of 1.5-3.8 cm. The ligament is then penetrated and the needle or catheter inserted to a depth of 2-3 cm, bearing in mind that the dural sac terminates at the S2 level, and that dural puncture should be avoided (Figure 16.1).

Test aspiration for blood or CSF is performed. If the test is negative, this is followed by the injection of a 3ml test dose of local anaesthetic, with a hand positioned over the sacrum to detect any tissue swelling, resulting from malposition of the needle or catheter either subperiosteally or along the dorsal surface of the sacrum. Following a negative test dose and in the absence of pain on injection, the definitive dose may be injected slowly in small, repeated increments (Chapter 91). If CSF is aspirated or if blood continues to be aspirated after repositioning of the needle or catheter the block should be abandoned.

Single-shot needles or cannulae are withdrawn following injection. Catheters are affixed to their connectors and filters and strapped in position.

Lignocaine is preferred to bupivacaine in view of the potential for large doses of bupivacaine to produce cardiovascular collapse and intractable ventricular dysrhythmias (Table 36.6) should intravascular injection occur into one of the abundant epidural veins (1, 2). The dose and concentration of lignocaine depends on the indication for the block. A recommended dose for outlet forceps delivery would be 15 to 20mls of 1.5% lignocaine with adrenaline. For perineal analgesia, a dose of 10 to 15mls of 1.0% lignocaine with adrenaline would suffice (Chapter 2).

References:
1. Albright GA Cardiac arrest following regional anesthesia with etidocaine or bupivacaine Anesthesiology 1979; 51:285-287