Q&A
- What is the role of infiltration with local anaesthetic at the beginning of a case?
- Infiltration with local anaesthetic prevents the activation of nociceptors during surgery and substantially lessens the need for analgesic medication.
- Local anaesthetic molecules are composed of a benzene ring and a quaternary amine separated by an intermediate chain. Which part of the molecule determines the metabolic pathway of the drug?
- The intermediate chain
Reference of common local anaesthetic agents
Agent | Max dose without epinephrine | Max dose with epinephrine | Duration of action | Notes |
Lidocaine | 5 mg/kg | 7 mg/kg | 30–90 minutes | 1% = 10 mg/mL; 2% = 20 mg/mL |
Bupivicaine | 2.5 mg/kg | 3 mg/kg | 6–8 hours | 0.5% = 5 mg/mL |
Mepivicaine | 7 mg/kg | 8 mg/kg | ㅤ | ㅤ |
Ropivacaine | 3 mg/kg | ㅤ | ㅤ | ㅤ |
Local anesthetics
- Drug classes
- Esters: benzocaine, chloroprocaine, cocaine, tetracaine.
- Amides: bupivacaine, lidocaine, mepivacaine, prilocaine, ropivacaine (amides have two “i”s in their names)
- Mechanism
- Local anesthetics block neurotransmission by binding to the inner portion of voltage‑gated sodium channels along nerve fibres. They work best in rapidly firing neurons.
- Tertiary amine local anesthetics cross the membrane in uncharged form, then bind to the channel as charged form
- Co‑administration with vasoconstrictors such as epinephrine prolongs block by reduces systemic absorption.
- In acidic (infected) tissue, the drugs are more ionised and penetrate poorly, so higher doses are required
- The order of functional loss is pain first, then temperature, touch, and finally pressure.
- Clinical use
- Used for minor surgical procedures and spinal anaesthesia; patients allergic to ester local anesthetics can generally receive amide agents instead, as cross‑reactivity is rare.
- Adverse effects
- Potential adverse effects include CNS excitation, severe cardiovascular toxicity (especially with bupivacaine), hypertension or hypotension, arrhythmias (notably with cocaine), and methaemoglobinaemia with benzocaine or prilocaine.
- Racemic bupivacaine is an equimolar mixture of dextro and levobupivacaine.
- Levobupivacaine has a lower affinity for cardiac sodium channels and greater plasma protein binding affinity compared with the dextro isomer; thus, reducing the risk of cardio-toxicity
Deciphering drug concentrations and dilutions
- Concentrations
- Drug concentration is expressed as a percentage (for example, bupivacaine 0.25%, lidocaine 1%).
- Percentage is measured in grams per 100 mL.
- 1% means 1 g per 100 mL, which is 1000 mg/100 mL, or 10 mg/mL.
- To convert percentage to mg/mL quickly, move the decimal point one place to the right
- Bupivacaine 0.25% = 2.5 mg/mL
- Tetracaine 0.5% = 5 mg/mL
- Lidocaine 1% = 10 mg/mL
- Viscous lidocaine 2% = 20 mg/mL
- Benzocaine 20% = 200 mg/mL
- Dilutions
- When epinephrine is combined in an anesthetic solution, the result is expressed as a dilution (eg, 1:100,000).
- 1:1,000 means 1 mg per 1 mL (0.1%).
- 1:10,000 means 1 mg per 10 mL (0.01%).
- 1:2,000 means 1 mg per 2 mL (0.05%).
- 1:20,000 means 1 mg per 20 mL (0.005%).
- If 0.1 mL of 1:1,000 epinephrine is added to 10 mL of anesthetic solution, the final concentration is 1:100,000, which equals 0.01 mg/mL.
AAGBI safety guidelines
- Treatment
- Lipid Sink Theory
- Highly lipid-soluble local anesthetics, such as bupivacaine, are absorbed into the lipid emulsion of the plasma, removing them from tissues affected by toxicity.
- This theory is supported by studies demonstrating enhanced removal of bupivacaine from cardiac tissues in isolated rat hearts treated with lipid emulsion (Intralipid®)
- Enhanced Redistribution (Lipid Shuttle)
- Lipid emulsion may facilitate the redistribution of local anesthetics from tissues to the bloodstream, allowing for their removal from toxic sites and reducing their concentration in affected tissues.
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