Local anaesthetics

Question 1

What are the mechanism of action of local anaesthetics?

Answer 1

• blinds to receptors near the intracellular end of sodium channels
• Reducation of depolarisationa nd prolongation of repolarisation
• brings conduation to a halth by preventing sodium entry
  Note: effect increases in rapidly firing fibers as 1) LA molecules gain access to the channel more readily when channel is open; 2) they have a higher affinity for inactivated channels

Question 2

What are the factors affecting LA action?

Answer 2

1) lipid soluble drugs are more potent and act longer
2) Size: Smaller nerve>bigger nerve
3) Frequency of firing: High (sensory) >low (motor)
4) myelinated > non-myelinated
5) pH: alkaline pH > acidic pH (due to LA being a weak base, not ionised in akalaine pH) –> lower effect in infection and hypoxia

Question 3

What are LAs from the Ester group?

Answer 3

Cocaine, Procaine, Tetracaine, Benzocaine (2 or 3 syllabus, no 4 syllabus)

Question 4

What are LAs from the amides group?

Answer 4

Lidocaine, Mepivacaine, Bupivacaine, Etidocaine, Prilocaine (theres always a i in the prefix)

Question 5

How are ester type and amide type LA metabolised? Hence, what is the clinical significance?

Answer 5

Ester: plasma/tissue non-specific esterase –> metabolised in blood which goes to kidney –> not suitable for pts w severe kidney diseases

Amide: hepatic enecyme (CYP450) –> not suitable in pts with liver failure

Question 6

How can the systemic toxicity be prevented in large dose of LA?

Answer 6

Combine with ephinephrine –> vasoconstrict surface vessel –> reduce spread of LA in the system

Question 7

Which LA is the most cardiotoxic?

Answer 7

Bupivacaine

Question 8

What are some adverse effect of LAs?

Answer 8

Depressed CNS : sleepiness visual and auditory disturbances,restlessness,seizures and coma
despressed CVS: decreased cardiac contration, arteriolar dilation, hypotension –> leading to cardioveascular collapse

Question 9

Which LA block NA reuptake, causing vasocontriction and hypeprtension through increased NA?

Answer 9

Cocaine

Question 10

Which LA are used as epidural anaesthetics?

Answer 10

Lidocaine and bupivacaine

Question 11

What is the propensity of amide and ester LA causing an allergic reaction?

Answer 11

Ester: occasionally
Amide: rarely