2. Skin infiltration with local anaesthetic (Figure 16.1) should be performed at the L2-3 or L3-L4 interspace.
3. A wide variety of needles are available (Figure E.1) but, for obstetric use, a non-cutting, dural-fibre-splitting needle such as a 24G Sprotte or a 26G Whitacre is preferred because of the lower incidence of post dural puncture headache (Chapter 97). If a cutting needle has to be used (eg 26G Quincke) it should be inserted with the bevel pointing laterally.
4. A spinal needle introducer (eg. Sise) may facilitate insertion of the spinal needle. If an introducer is not available, an 18G hypodermic needle may suffice.
5. The needle traverses the skin, subcutaneous tissue, supraspinous and interspinous ligaments and ligamentum flavum before crossing the epidural space and perforating the dura with a characteristic "pop" (Figure E.3).
6. Withdrawal of the stylet should allow CSF to appear at the hub of the needle within a few seconds. Failing this, gentle aspiration with a syringe and, sometimes, rotation of the needle may confirm that the needle tip is located within the subarachnoid space. It should be remembered that the flashback time for very fine needles may be quite long. For example, for a patient lying in the lateral position, with a typical CSF pressure of 5cm of water, the time taken for CSF to appear in the hub of a 90mm, 0.32mm internal diameter Pajunk needle is quoted as 12 seconds.
7. The syringe containing the local anaesthetic or opioid solution is now attached to the needle and, following further aspiration, the drug is injected slowly over 10 to 15secs. The needle is now removed and a sterile dressing applied.
8. Patients who were in the lateral position are now turned onto the opposite side. Seated patients are placed in a supine, lateral position.
9. Close monitoring of the extent of block is commenced (Chapter 10). If hyper- or hypo-baric solutions are used, posture can be used to control the spread of anaesthesia (Figure 30.1) (Chapter 30).