Local Anaesthesia
Written by Rhiana Devadasan
Local Anaesthesia: a technique to induce the absence of sensation (i.e. numbness) in a specific small part of the body, during short, minor procedures/day cases. You usually stay awake during the procedure.
How they work:
- Reversibly block nerve conduction, predominantly by inactivating Na⁺ channels & prevent electrical depolarisation.
- Smaller nerve fibres are more sensitive to touch, pressure, temperature, pain, proprioception, motor impulses, etc. With increasing dose of local anaesthetic, comes first the loss of pain and temperature, then subsequent loss of gentler stimuli.
This phenomenon is known as DIFFERENTIAL CONDUCTION BLOCKADE, and explains why the patient can feel pressure but no pain. And why a nerve block therefore should be tested by checking for sharp pain or temperature, rather than merely slight pressure. Or rather, why the patient cannot feel pain, but has intact motor fuction.
Types:
- Ester Group LAs
- Benzocaine
- Cocaine
- Procaine (Novocaine)
- Chloroprocaine
- Cyclomethycaine
- Amide Group LAs
- Lidocaine
- Articaine
- Bupivacaine
- Prilocaine
- Naturally Derived
- Cocaine
- Menthol
- Saxitoxin
- Tetrodotoxin
- Spilanthol
Examples
Lidocaine
- Most commonly & widely used
- Rapid onset, short duration
- Antiarrhythmic
- Comes in 3 strengths 0.5% (5 mg/mL), 1% (10 mg/mL), 2% (20 mg/mL)
- The upper dose limit for lidocaine is 3 mg/kg or 7 mg/kg when used with adrenaline
Bupivacaine
- Slower onset, longer duration
- About 4 times more potent than Lidocaine
- Has selective cardiac toxicity, particularly in pregnancy, often causing life-threatening ventricular arrhythmias
- Comes in two strengths: 0.25% (2.5 mg/mL), 0.5% (5 mg/mL).
- Upper dose limit is 2 mg/kg
- Adrenaline, does not increase the safe dose of bupivacaine and is only used with it to reduce bleeding.
Prilocaine
- Rapid onset, shorter duration than Lidocaine
- Has a wider margin of safety
- Comes as 1% solution(10 mg/mL)
- Upper dose limit is 6 mg/kg
Cocaine
- Most local anaesthetics are derivatives of cocaine, which is also a vasoconstrictor. This makes it the drug of choice in ENT surgery.
- Its vasoconstrictive effects can cause myocardial ischemia, tachycardia and cerebrovascular accident (AKA stroke).
Agents
Local anaesthetics may be mixed with adrenaline:
- Adrenaline is a vasoconstrictor which means it diminishes local blood flow, slows the rate of absorption of the local anaesthetic and prolongs its local effect. And permits a higher dose of drug to be used, as it is more slowly absorbed into the circulation.
- It should be used in low concentrations, e.g. 1 in 200 000 (5 mg/mL). The total dose of adrenaline should not exceed 500 mg