Local Anaesthetic Agents

Question 1

Local Anaesthetics(MOA)

Answer 1

Blocks the intracellular end of voltage-gated Na+ channels -> blocks Na+ influx and nerve depolarisation

-INCREASES nerve’s excitability threshold and refractory period
-REDUCES the amplitude and SLOWS the conduction of pain sensory signals

Question 2

Amino esters(e.g. procaine, cocaine) VS Amino Amides(e.g. lignocaine, bupivacaine, prilocaine, articaine)

Answer 2

AE
-UNSTABLE in solution
-LOW TISSUE PENETRATION
-Onset of action SLOWER
-LESS LIKELY to cause systemic toxicity

AA
-VERY STABLE in solution
-BETTER TISSUE PENETRATION
-Onset of action is moderate to FAST
-MORE LIKELY to cause systemic toxicity

Question 3

What happens during the co-administration of adrenaline or noradrenaline or Felypressin?

Answer 3

Vasocontriction of BVs at the gums -> delay absorption of LA from site of injection into general circulation

Question 4

Pros n Cons of co-admn. of A/NA/Felypressin?

Answer 4

P
-PROLONG duration of action
-REDUCE systemic toxicity
-BLOODLESS FIELD for surgery

C
-REDUCE O2 SUPPLY and enhance O2 consumption in the affected area (delay wound healing, cause local tissue edema, necrosis)
-May RAISE BP and promote ARRYTHMIA in susceptible individuals.

Question 5

Lignocaine

Answer 5

-Most widely used LA
-Good for surface
application(relieve itching, burning and pain due to skin inflammation) and injection(dental anaesthesia and minor surgery)

-Used for surface application, infiltration, nerve block, epidural, spinal and iv regional block

-Overdose cause muscle twitching, convulsions, cardiac arrhythmias, fall in BP, coma and respiratory arrest

Question 6

Articaine

Answer 6

-hydrolysed very quickly in the blood(can inject more times, for more reckless clinicians)
-lower risk of systemic toxicity
-used for infiltration injections> other LA in penetrating dense cortical bone(mandible)
-OD may cause allergy and hemoglobinopathy

Question 7

Mepivacaine

Answer 7

• less intrinsic vasodilation than lignocaine
  -diffused slower into the bloodstream when injected locally
  -Not formulated together with adrenaline, “safer” lidocaine alternative to be used in:
• children(lacks fully functional metabolic enzymes)
  -elderly
  -cardiovascular disease patients
  -patients who are allergic to sulphites(sodium metabisulphite is added to the LA-vasoconstrictor formulation to prevent oxidation of the vasoconstrictor)

Question 8

Prilocaine

Answer 8

• X vasodilatation at the site of infiltration
• lower CNS toxicity
• cause methaemoglobinemia at high dose(not used for obstetric analgesia)

Question 9

Bupivacaine

Answer 9

-long duration of action
-greater cardiotoxicity(not used for iv regional anaesthesia)
-Ropivacaine(bupivacaine congener)-equally long acting but less cardiotoxic

Question 10

Methods of LA administration(2)

Answer 10

Surface anaesthesia(lignocaine, prilocaine, lignocaine + prilocaine)
-application of LA spray, solution or cream to the skin or mucous membrane

-short lasting effect

Infiltration anaesthesia
-injection of LA directly into the tissue to reach nerve branches/sensory nerve terminals

-almost immediate onset

-duration of action is shorter than that of nerve block

-used for minor operation(oral tissue incisions, excisions etc.)

Question 11

Conduction block(2)

Answer 11

Field Block
-Subcutaneous injection of a LA into the area bordering the field to be anaesthetized

-block all nerves supplying to the targeted field

Nerve Block (intercostal, ulnar, sciatic, femoral, brachial plexus, facial etc.)
-LA is injected close to(NOT INTO) the nerve trunk

-Block nerve signals transmission to the area where the nerve trunk supplied