Local Anaestetics

Question 1

Analgesia vs anaesthesia

Answer 1

Analgesia is the loss of sense of pain without loss of sensation while anaesthesia is loss of both pain and sensation

Question 2

Local anaesthesia induces absence of sensation in specific part of the body leading ot

Answer 2

Analgesia and paralysis

Question 3

How does local anesthesia cause absence of sensation

Answer 3

Block afferent activity in peripheral and central nervous system.

Question 4

What technique is used to deliver local anesthesia to the maxilla and mandible

Answer 4

Infiltration. Small terminal nerve endings are flooded with LA solution

Question 5

What s a plexus block

Answer 5

A plexus block is when you deposit LA solution in close proximity to the main nerve trunk, preventing nerve impulses from travelling beyond that point

Question 6

How is spinal local anesthesia administer

Answer 6

Subarachnoid block. Administered directly into fluid sac

Question 7

What are the localised adverse effects of prolonged local anesthesia

Answer 7

There may be severing of nerves and support tissue due to infection, hematoma, or excessive fluid pressure in the confined cavity

Question 8

What are some systemic adverse effects of local anesthesia

Answer 8

Local anesthesia may enter systemic circulation due to prolonged delivery or because LA was accidentally injected directly into blood

This can lead to systemic toxicity
- Depressed CNS: sleepiness, lightheadedness, metallic taste, tongue numbness, seizure, coma

• Cardiovascular effect. Depress myocardial contractility, systemic hypotension, arrthymia, arteriolar dilatation
• Hematology. Methaglobinema due to accumulation of prilocaine metabolite
• Allergic reaction due to metabolites of ester LAs

Question 9

What are the amide local anesthesics

Answer 9

Lidocaine 
Mepivacaine
Prilocaine (methemoglobinema)
Endocrine 
Bupivacaine

Question 10

What is lidocaine

Answer 10

Medium acting amide local anesthesic

Question 11

What is bupivacaine

Answer 11

Long acting amide local anesthesic

Question 12

What is procaine

Answer 12

Short acting ester local anesthesic

Question 13

What are some ester local anesthesics

Answer 13

Cocaine, procaine, chloroprocaine, tetracaine

Question 14

What is the structure of local anesthesics

Answer 14

Lipophilic group connected via ester/amide to ionisable group

Question 15

Ester vs amide which has shorter duration of action and why

Answer 15

Esters have a shorter duration of action as they are more prone to hydrolysis (by butyrylcholinesterases)

Question 16

Are local anesthesics acidic or basic? Charged or uncharged?

Answer 16

Weak bases. Active when charged as cannot readily exit ion channels. Uncharged state allows for rapid penetration of lipid membranes

Question 17

Why are local anesthesics less effective when injected into infected/ischemic tissues

Answer 17

Infected/ischemic tissues are acidic. LAs are weakly basic. Hence lower proportion is non ionised ie more ionised form, hence less available for diffusion into nerves

Question 18

Name some factors that affect the systemic absorption of local anesthetics

Answer 18

Dosage, site of action eg is it highly vascularised, local blood flow, use of vasoconstrictors to prolong action

Question 19

How are amide local anesthesics eliminated

Answer 19

Metabolised in the liver by microsomal cytochrome P450 isoenzymes

Hence decreased metabolism in patient with liver disease, in patient with decreased hepatic blood flow, if patient is taking cytochrome p450 inhibitor drugs

Question 20

How are ester local anesthesics eliminated

Answer 20

In the blood (vs liver for amides) by butyrlylcholinesterases

Question 21

What is the mechanism of action of local anesthetic

Answer 21

1. LA enters the cell through the phospholipid bilayer (uncharged form)
2. Binds to receptor near intracellular end of the sodium channel to increase threshold for excitation (charged form)
3. Causes closing of gate at intracellular end, inactivated state, na+ unable to pass through the channel, block conduction of impulses

Reduce depolarisation and prolong depolarisation

Question 22

What fibres are more susceptible to effects of local anesthetics

Answer 22

Rapidly firing fibres are more likely to become hyperpolarised. Use dependent block. Drug has better access to binding site when the channel opens frequently

Smaller fibre diameter. Unmyelinated and passively propagate electrical impulses over short distance. LA only need to block short segment vs large fibres where LA need to block at least 2-3 nodes of ranvier

Fibres located circumferentially in nerve trunk is first to be exposed to LA. Proximal sensory fibres first, proximal before distal sensory loss

Question 23

What kind of LA technique do i use to numb the upper limb

Answer 23

Nerve block of the brachial plexus

Question 24

What does epidural spinal blockage cause

Answer 24

Numb feeling in lower half of the body

Question 25

What is a bier block

Answer 25

Intravenous administration of LA. Inject into distal vein isolated by proximal tourniquet. Inject LA into venous system of extremity exsanguinated by compression of gravity, isolated from central circulation

Question 26

How to prolong effect of local anesthetic

Answer 26

Increase dose, use vasoconstrictor eg adrenaline

Question 27

How to accelerate action of local anesthetic

Answer 27

Add sodium bicarbonate into the solution. More alkaline, more likely to be unionised so that can penetrate lipid membrane and diffuse into nerves

Question 28

Patient reports history of allergic reaction to procaine. What LA should you use?

Answer 28

Clarify with patient what she means by allergic reaction —.difficulty breathing, anaphylaxis, swelling

Procaine is ester type. Use lidocaine (amide, allergic reaction unlikely)

Question 29

Patient given bupivacaine. Complains of light headed ness, loses consciousness and develops tonic clinic movement of extremities. How to respond?

Answer 29

Systemic toxicity. Stop injection of local anesthetic to limit toxicity.

Check breathing and maintain airway

Administer small dose of benzodiazepine