Local anaesthetic agents

Question 1

What is the mechanism of action of local anaesthetic drugs?

Answer 1

Block voltage-gated sodium channels from inside the nerve cell preventing conduction.
The unionised form must cross the cell membrane to be able to exert its effect.

Question 2

What are the two groups of local anaesthetics?

Answer 2

1. Amides e.g. lidocaine, bupivicaine, prilocaine and ropivocaine. Broken down by amidases found in liver. More protein bound than esterases.
2. Esters e.g. amethocaine, cocaine, tetracaine. Broken down by plasma esterases.

Question 3

Describe the following for lidocaine:
1.Properties (pKa, partition co-efficient & protein binding)
2. Presentation
3. Pharmacokinetics
4. Max dose w/without adrenaline
5. Other uses
6.Unwanted effects
Contraindications

Answer 3

1. Weak Base, pKa 7.9. partition co-efficient 3. 65%Portein bound.
2. Water soluble as hydrochloride. Colourless solution 1% or 2%. Comes w/without adrenaline. Also available as 2% gel and 4% spray.
3. Fast onset and medium duration (1-2 hrs). Metabolised in liver by amidases.
4. Max dose without adrenaline 4mg/kg
   Max dose with adrenaline 7 mg/kg
5. Antiarrythmic for ventricular ectopics
6. Circumoral tingling, seizure, coma.
7. Established allergy or hypersensitivity. No adrenaline for digit blocks.

Question 4

Describe the following for Bupivicaine:
1.Properties (pKa, partition co-efficient & protein binding)
2. Presentation
3. Pharmacokinetics
4. Max dose w/without adrenaline
5. Other uses
6.Unwanted effects
Contraindications

Answer 4

1. Two isomeric forms. L form is levobupivicaine. Bupivicaine is a racemic mix. Lipid soluble. Partition co-efficient 28. pKa 8.1, weak base. 95% protein bound.
2. Colourless solution 0.25% and 0.5%. No advantage in using adrenaline due to high protein binding. heavy: 0.5% with glucose 80 mg/ml for subarachnoid block.
3. Slow onset (30 mins) and long duration (2-6h) metabolised in the liver by amidases.
4. Max dose 2 mg/kg
5. Myocardial depression; CNS convulsions, coma. L-isomer less toxic.
6. Established allergy or hypersensitivity. Should not be used for a Biers block.

Question 5

What does EMLA stand for?

Answer 5

Eutetic
Mixture
of
Local
Anaesthetics
comes as 5% Lidocaine and prilocaine.

Question 6

Describe the following features of EMLA cream:
1. Preparation
2. Indication
3. Timing
4. Problems with use
5. Cautions

Answer 6

1. 2.5% Lidocaine and 2.5% prilocaine as a cream. Eutetic describes a gel state that occurs in a mixture of two different agents that would not occur with each alone at a given temperature.
2. Topical anaesthesia. Not for mucus membranes.
3. 40-60 mins to act. Lasts up to 5 hrs
4. Vasoconstriction can make cannulation difficult.
5. If used on a large area in patients methaemoglobinaemia prilocaines metabolites may worsen the condition.

Question 7

Describe the following for Amitop:
1. Prep
2. Indication
3. Timing
4. Problems with use:

Answer 7

1. gel. 4% tetracaine (ester)
2. Topical anaesthesia. Not for mucus membranes.
3. Onset 30 min. shorter acting than emla
4. Ester properties mean it has an increased hypersensitivity profile compared to EMLA. Urticaria and swelling is common.