LOCAL ANAESTHETIC AGENTS (see page 19.23 for toxicity)
Lignocaine : Low lipid solubility and potency (use 0.5-2%); low pKa 7.9 onset 5-10 min.);
low protein binding 70% (duration short 30-60 min); maximum safe dose (MSD) 3 mg/kg. Vasodilates, so use with adrenaline to increase duration to 2-3h, and MSD to 7 mg/kg.
Bupivacaine : High lipid solubility (use 0.1-0.5%); high pKa 8.1 (onset 10-15min): high protein binding 95% (duration 2-4h). No vasodilatation. MSD 2mg/kg.
Ropivacaine : Pure 'S' enantiomer of bupivacaine, with less lipophylicity, toxicity, and motor block, and therefore most useful for nerve blocks, labour and post operative pain relief. Use 0.2%. Onset 10 min. Duration 2-6h. MSD 3-4mg/kg.
PRE-OPERATIVE ASSESSMENT
Similar to that of general anaesthesia but should specifically include: 1. An examination of the spine (kyphoscoliosis, skin lesions, local sepsis and fat
deposition which renders landmark identification difficult) 2. Evaluation of coagulopathy and anticoagulant therapy (see page 5.11) 3. Informed consent with explanation of technique, advantages and risks
TECHNIQUE
1. Check anaesthetic machine, resuscitation equipment and drugs (vasopressors). 2. Set up a rapid intravenous infusion of saline (0.9%NaCl).
3. Monitor pulse, NIBP, heart rate and Spo,
Precautions
1. Do not place antiseptic solutions on the equipment tray as antiseptics and talc in the sub-arachnoid space may cause chemical meningitis. 2. Ensure asepsis with cap, mask, scrub up, sterile gown and gloves. Clean skin with
iodine and 70% alcohol.
3. Antiseptics must dry on the skin before the needle is inserted.
4. If there is shooting pain or paraesthesia after insertion of the needle, withdraw the needle slightly and ensure that there is no pain before proceeding.
Never inject the LA if the patient complains of pain (to avoid neurological sequelae).
Positions
. The seated position is the easiest. Sit patient on the operating table side-ways with the legs on a stool, and arms crossed on a pillow placed on the lap. Ask the patient to flex the neck and arch the back to open up the spinal interspaces.
Lateral decubitus (foetal) position with the hips and knees maximally flexed and the chest and neck flexed towards the knees (Fig. 6.1). Identify landmarks
Select the lowest lumbar space L3/4 or LA/5 (safest) to avoid the conus medullaris. Midline approach : just below the mid point between two spines Paramedian approach: 1-2 finger breadths lateral (to bypass calcified ligaments)