local anaesthetics

Question 1

what is the action of local anesthetic

Answer 1

• local anesthetics work by blocking Na+ channels (on inside of the cell) on cell membrane so Na+ cant enter the cell and depolarization the membrane

Question 2

structure-activity relationship

Answer 2

ester or amide bonds determine side of metabolism and potential to produce allergic reactions

esters are more rapidly metabolised (& shorter acting) due to weaker bonds

Question 3

pH of anaesthetics and its importance

Answer 3

all anaesthetics are weak bases

• only free bases (non-ionized form) can cross membranes and enter cells
• the amount of free base present at physiological pH depends on the pKa of the drug
• more free base present = faster onset of action (local anaesthetics with pKa closest to physiological pH have fastest onset of action)

Question 4

rank drugs from longest duration of action to shortest

Answer 4

1. longest = bupivacaine
2. ropivacaine
3. lignocaine (fast onset, short acting)
4. prilocaine

Question 5

lignocaine

Answer 5

• amide
• low pKa = fast onset
• low lipid solubility
• low protein binding = short duration of action

ideal to cover short surgical procedures e.g. dental or mole removal

Question 6

bupivacaine

Answer 6

• amide
• high lipid solubility
• high pKa = slow onset
• high protein binding = longer duration of action than lignocaine

ideal for nerve blocks for analgesia (last a long time)

Question 7

ropivacaine

Answer 7

amide
slow onset
long acting like bupuvacaine
but less cardiac toxicity

Question 8

prilocaine

Answer 8

amide
safest agent
used in IV regional anaesthesia

Question 9

local anaesthetic toxicity

Answer 9

• cause allergic reactions (rare)
• dose dependent CNS toxicity e.g. seizure
• dose dependent cardiac toxicity e.g. cardiac arrest

Question 10

CC:CNS ratio

Answer 10

the ratio of dose that you need to produce cardiac arrest compared to a seizure (bigger the better)

• caused by putting the LA into a vein by accident

Question 11

administration and uses

Answer 11

topical

• to the skin
• to mucus membranes

soft tissue infiltrations e.g. mole removal

nerve blocks

Question 12

peripheral nerve block

Answer 12

LA is infiltrated around a specific nerve (can often be guided with ultrasound)

Question 13

spinal anaesthetic

Answer 13

LA injected into the intrathecal space (only below L2 where spinal cord terminates)

• profound distal motor and sensory blockage
• allows major surgery in wake patients e.g. C sections, hip replacements)

Question 14

epidural anaesthesia

Answer 14

• small catheter inserted into epidural space and LA infused through catheter. Affects spinal nerves passing through space (can be done at any level)
• can stay there and keep injecting LA for 3-4 days

Question 15

local anaesthetics work by

Answer 15

blocking influx of Na+ across neuronal membranes

Question 16

you wish to achieve rapid onset of local anaesthesia for insertion of large IV catheter. What LA would you choose to infiltrate the skin with

Answer 16

lignocaine

Question 17

what does a drugs pKa determine

Answer 17

it is the most important factor for determining speed of onset for a local anaesthetic drug injected into healthy subcutaneous tissue