Local Anaesthetics

Question 1

Classification of LA?

Answer 1

1. Ester: O-C=O

2. Amide: NH-C=O

Question 2

Example of Amide LA?

Answer 2

1. Bupivacaine
2. LIdocaine
3. Levobupivacaine
4. Ropivacaine
5. Mepivacaine
6. Prilocaine
7. Etidocaine
8. Articaine (Dental)

Question 3

Example of Ester LA?

Answer 3

1. Chloroprocaine
2. Procaine
3. Tetracaine
4. Cocaine

Question 4

Potency of LA ?

Answer 4

Determined by size of the alkyl substituents on or near the tertiary amine and on the aromatic ring = The bigger the more potent

Bupivacaine > Ropivacaine > Mepivacaine

Question 5

Properties of Local anaesthetics?

Answer 5

1. Weak bases
2. Does not dissociate completely in water
3. Crosses cell membrane in the unionized form
4. The ionized form intracellular blocks sodium channels

Question 6

Properties of Ester LA?

Answer 6

1. Short duration of action
2. Lower toxicity
3. Hydrolysed by plasma cholinesterase
4. Allergic potential due to para-aminobenzoic acid origin

Question 7

Properties Amide LA ?

Answer 7

1. Intermediate or long duration of action
2. High toxicity
3. Hepatic metabolism
4. Methemoglobinaemia - Prilocaine metabolite in high doses.
5. pKa correlates to onset of action of LA
6. Lipid solubility correlates to potency
7. Lipid solubility + Protein binding correlates to duration of action

Question 8

Speed of onset ?

Answer 8

1. Physiochemical properties

2. concentration (Higher=fatser)

Question 9

Potency and duration ?

Answer 9

1. Extent of protein binding
2. Lipid solubility
3. More hydrophobic LA more potent
4. Ropivacaine has vasoconstrictor properties
5. Levobupivacaine less potent than racemic bupivacaine

Question 10

Absorption ?

Answer 10

1. Site of injection determines amount of absoroption; Intercostal > Caudal > Lumbar > Brachial > S/C
2. Dose / plasma concentration = Linear relationship
3. Lipophilic drugs - slower absorption

Question 11

Distribution ?

Answer 11

1. Described in two compartment model
2. Rapid phase - highly perfused organ
3. Slow phase - Function of particular drug
4. Heart and brain - Higher concentration
5. Dependent on plasma protein binding

Question 12

Metabolism?

Answer 12

1. Esters - Plasam pseudocholinesterase
2. Amids - Degradation in liver but also in lungs
3. Excretion by the kidney
4. Half-life = Increased in elderly, neonates, liver disease and pregnancy (Decreased clearance)

Question 13

LAST ( Local anaesthetic systemic toxicity) ?

Answer 13

1. Light-headedness
2. Tinnitus
3. Numbness of tongue
4. Seizure
5. Unconsciousness
6. Coma
7. Respiratory arrest

8 Cardiovascular depression

Question 14

LAST ( Local anaesthetic systemic toxicity) ? Increased by?

Answer 14

1. Age
2. Dose
3. Type of block
4. Blind block
5. Hypoproteinaemia
6. Pregnancy
7. Acidosis
8. Hypercarbia

Question 15

Management of LAST?

Answer 15

1. CNS - Benzodiazepines preferred
2. CPR - Small doses of epinephrine, avoid vasopressin, CCB and Beta-blockers
3. Amiodarone is best used
4. IV 20% lipid emulsion
5. Adult: 100ml (2-3mins) STAT/ infusion 250ml over 20 mins
6. < 70kg: 1.5ml/kg (2-3mins) STAT/ Infusion 0.25 ml/kg/min
7. Maximum initial dose not greater than 12ml/kg