2. A appropriate vasopressor, such as ephedrine, should be close at hand (Chapter 79). An aseptic technique should be adhered to. A midline or paramedian approach may be used.
3. For the midline approach, the skin and underlying tissues are infiltrated with local anaesthetic in the midline at the mid-point of either the L2 - L3 or L3 - L4 interspace. (The spinal cord ends at L3 or above in at least 97% of the population (Figure 16.1)). The bony and ligamentous structures of the region are shown in Figure E.2. An epidural (Tuohy) needle is inserted in the sagittal plane, with its bevel pointing cephalad. After traversing the supraspinous ligament, the needle is then passed into the interspinous ligament which grips it tightly. The stylet is removed from the Tuohy needle and the loss-of-resistance syringe attached. The syringe may contain saline, air or both according to preference. The Tuohy needle may then be advanced slowly using either a continuous or intermittent movement. Loss-of-resistance is detected by applying gentle pressure to the plunger of the syringe (Figure E.3). When the needle is in the ligamentum flavum, increased resistance is transmitted to the plunger, only to disappear when the epidural space has been entered (Chapter 32).
4. For the paramedian approach, local anaesthetic is infiltrated 15mm lateral to the cephalad end (upper border) of the vertebral spinous process below the selected interspace. The Tuohy needle is inserted at 900 to all skin planes. The paravertebral muscles are penetrated until the bone of the vertebral lamina is encountered. The depth of needle insertion is noted. The needle is partially withdrawn and redirected cephalad and towards the midline until the previous depth is attained. Once the ligament is entered, the syringe is attached and the needle is slowly advanced while searching for loss-of-resistance.
5. Whichever approach has been used, the depth to which the needle has been inserted is noted. Following negative aspiration for blood or CSF, a small volume of local anaesthetic (3-4 ml) may be injected into the epidural space to aid the subsequent catheter insertion. The catheter is advanced 3-4cms (Figure E.5)beyond the previously-noted distance between the skin and epidural space. If too much catheter is threaded into the epidural space, it may emerge through an intervertebral foramen causing failure of the epidural.
6. The Tuohy needle is gently removed and the catheter is fixed to the skin under a transparent adhesive dressing. A test dose (Chapter 91) of local anaesthetic is given through the catheter before the main dose is administered in increments (Chapter 60).
7. Use of a continuous catheter is generally preferable to a single-shot technique to cover most obstetric eventualities.
The Australian and New Zealand College of Anaesthetists has outlined the requirements for the safe conduct of epidural analgesia (P14).