Local anaesthetic
"reversibly prevent transmission of nerve impulses, without affecting consciousness."
- Most local anaesthetics contain an aromatic ring, and are basic and lipid soluble
- Inactivates voltage-gated sodium channels so no further depolarisation can occur, and nerve impulses cannot propagate proximally.
- Progresses sequentially though loss of pain, temperature, touch, proprioception, skeletal muscle tone.
- Promotes vasoconstriction at low concentrations, and vasodilation at high
Dosages
- 1% solutions contain 10mg/mL, 2% contains 20mg/mL, etc
- Easiest just to use 3mg/kg for both rupivicaine and lignocaine
- All these agents are cleared by the liver
| Drug | Dose (plain) | Dose (adren) | Onset (m) | Duration (h) | Comments |
| Lignocaine | 2-4mg/kg | 7-9mg/kg | 5-10
'fast' |
1-2 (p)
2-3 (a) |
Rapid onset, good for short procedures |
| Bupivicaine | 2.5 | 2.5 | 10-15
'fastish' |
3-4 (p)
3-5 (a) |
Slower onset. Can precipitate arrhythmias |
| Prilocaine | 5 | 5 | 5-10
'fast' |
1-2 (p)
2-3 (a) |
Can be used IV (Bier's block) |
| Ropivicaine | 3mg/kg / 200mg total | N/A | 1-15
'slow' |
2-6 |
Contraindications
- Hypersensitivity
- Bupivicaine for regional anaesthetic
- Prilocaine in anaemia/methaemoglobinaemia
- Adrenaline in end-arteries… sometimes
- CVS instability
Toxicity
- Neurological early: mouth and tongue tingling numbness, fingertip numbness, tinnitus, anxiety, drowsiness/confusion, tremor
- Neurological late - progresses to fitting, coma, resp distress, death
- CVS
- Respiratory
- Allergic
- If suspected - cease drug, give oxygen via facemask, call MET, give diazepam if convulsion
Treatment of toxicity
- High-flow oxygen
- Treat seizure with midazolam
- Monitoring - may need IVF and other haemodynamic support
Reduced effectiveness
- Acidic environments
- Inflammation
- Sepsis
- Ischaemia
- Increased vascularity leading to higher rate of washout
EMLA
- Eutectic Mixture of Local Anaesthetics
- Mix of lignocaine and prilocaine