Local anaesthesia
- Local anaesthetic agents act by reducing membrane permeability to sodium
- Act on small unmyelinated C fibres before large A fibres
- Reduce pain and temperature sensation before touch and power
Lignocaine
- Lignocaine is a weak base (pKa=7.8)
- At physiological pHs mainly ionised
- Has a duration of action of about one hour
- With addition of adrenaline duration of action can be increased to 2 hours
- Main toxicity is on central nervous and cardiovascular systems
- Plain lignocaine should be used for local anaesthesia in digits and appendages
- Adrenaline containing solutions can cause tissue ischaemia.
Maximum doses of local anaesthetic (average adult)
|
Plain solution |
With adrenaline |
| Lignocaine |
200 mg (20 ml of 1%) |
500 mg (50 ml of 1%) |
| Bupivicaine |
150 mg (30 ml of 0.5%) |
200 mg (40 ml of 0.5%) |
| Prilocaine |
400 mg (80 ml of 1.5%) |
600 mg (120 ml of 1.5%) |
Spinal and epidural anaesthesia
- Spinal anaesthesia - local anaesthetic or opiate into CSF below termination of cord at L1
- Epidural anaesthesia - local anaesthetic or opiate into fatty epidural space
- Both can produce good anaesthesia for up to 2 hours
- The quality of the block is often better with a spinal
- Epidural anaesthesia is technically more demanding
Contraindications
- Pre-existing neurological disease
- Coagulopathy
Complications
| Timing |
Characteristic |
Spinal |
Epidural |
| Immediate |
Hypotension |
Common |
Less common |
|
LA toxicity |
Rare |
Occasional |
|
High Blockade |
Occasional |
Occasional |
| Early |
Urinary retention |
Common |
Less common |
|
Headache |
1-5% |
Never unless dural puncture |
|
Local infection |
Almost never |
Uncommon |
|
Meningism |
Uncommon |
Very rare |
|
Epidural haematoma |
Almost never |
Very rare |
|
Backache |
Common |
Common |
Hypotension
- Sympathetic outflow form spinal cord occurs between T1 and L2
- Blocked to varying degrees in both spinal and epidural anaesthesia
- The higher the block the greater the degree of blockade
- In hypovolaemic patients there is a greater risk of hypotension
- Hypotension during spinal and epidural anaesthesia usually requires fluid resuscitation
Post spinal headache
- Seen following between 1 - 5% of spinal anaesthetics
- Usually due to CSF leak
- In most patients is settles after about 3 days
- Headache is characteristically occipital
- Worse on standing and relieved by lying down
- Initial treatment is with bed rest, simple analgesia and fluids
- If persists consider 'blood-patch'
- Patients own blood injected into epidural space
Postoperative epidural infusions
- Attenuates postoperative stress response
- Improves postoperative pain control
- Reduces incidence of postoperative pulmonary complications
- Allows more rapid return of gastrointestinal function
- Reduces duration of hospital stay
Opioid alone
- Allows opioid analgesia without sedation
- No motor or sympathetic blockade
- Quality of analgesia can however be variable
- Itch is common
- Serious respiratory depression can occurs after stopping infusion
Local anaesthetic alone
- Potential for complete anaesthesia
- No sedative effects or respiratory depression
- Sympathetic and motor blockade are common
- Cardiovascular side effects can occur
- Block occasionally patch or unilateral
Combination of LA and opioid
- Synergy between sites of action
- Reduced doses of both drugs
- Optimal analgesia possible
Bibliography
Buggy D J, Smith G. Epidural anaesthesia and analgesia: better outcome after major
surgery? Br Med J 1999; 319: 530-531.
Fotiadis R J, Badvie S, Weston M D et al. Epidural anaesthesia in gastrointestinal
surgery. Br J Surg 2004; 91: 828-841.
Rigg J R A, Jamrozik K, Myles P S et al. Epidural anaesthesia and analgesia and
outcome of major surgery: a randomised trial. Lancet 2002; 359: 1276-1282.
Turnbull D K, Sheppard D B. Post-dural puncture headache: pathogenesis, prevention and
treatment. Br J Anaesth 2003; 91: 718-729.
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