Question 22

College Answer

The amide group oflocal anaesthetics consist of lignocaine, prilocaine, ropivacaine and bupivacaine. The systemic toxicity primarily relates to toxic plasma levels and the factors that influence this. The main factors expected can be categorized under drug factors (including kinetics), patient factors, site of injection and external factors. Many candidates omitted important details such as pKA, lipid solubility and addition of vasoconstrictors. For  example, absorption is affected by drug pKA, (thecloser to physiological pH themore rapid the absorption), use of vasoconstrictors and the drugs own vasoactive properties, site of injection (intercostal>epidural>brachial plexus>subcutaneous infiltration). Distribution is dependent on physicochemical properties of the amide. The rate of metabolism, mechanism of action (bupivacaine, in comparison to lignocaine has stronger binding to inactivated 
resting sodium channels and a slower rate of dissociation) and external factors (e.g. systemic acidosis) are other factors that should have been mentioned, and expanded upon with relevant detail.

Discussion

Drug factors:

• Choice of agent matters; bupivacaine is thought to have greater potential for cardiotoxicity. CNS toxicity usually requires about three times less local anaesthetic than cardiovascular toxicity. This empirically derived ratio is as high as 7.1 for lignocaine, and as low as 2.4 for bupivacaine (Garg et al, 2020).
• Dose of the drug:  overdose could happen accidentally if dose calculations to an actual body weight are performed in obese or pregnant patients
• Site of administration: 
  • Increased risk of direct intravascular injection:
    • Interscalene block
    • Brachial plexus block
    • Stellate ganglion block
    • Intercostal nerve block
  • Increased risk of rapid absorption:
    • Scalp
    • Bronchial mucosa
    • Interpleural cavity
    • Epidural
• Drug interactions can increase or decrease the risk of local anaesthetic toxicity:
  • Adrenaline: decreased systemic absorption
  • GTN infusion: increase systemic absoprtion
  • Compete for protein binding with local anaesthetics: eg. phenytoin, quinidine and desipramine can displace bupivacaine from plasma proteins.
  • Interference with hepatic metabolism of amide anaesthetics (eg. by cimetidine) or with hydrolysis of esters (eg. ecothiopate) could result in delayed clearance.
  • Pharmacodynamic interactions (eg. where flumazenil lowers the seizure threshold and potentiates the CNS toxicity)

Patient factors:

• Acidosis: changes the protein binding and makes more of the agent available
• Old age: the elderly have slower clearance
• Young age: lower α₁-acid glycoprotein level, thus greater free fraction
• Pregnant patients also have a lower α₁-acid glycoprotein level
• Hyperkalemia potentiates the cardiotoxic effects of local anaesthetics

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