NTA Level 5 • Semester 1 • PST05104

Pharmacodynamics of Local Anaesthetics

Pharmacology and Therapeutics • Source Session/Topic 37

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Session 37: Pharmacodynamics of Local Anaesthetics

Total Session Time: 120 minutes

Prerequisites

None

Learning Tasks

By the end of this session students are expected to be able to:

Describe mechanism of action of Local Anaesthetics

Describe drug interactions associated with Local Anaesthetics

Describe side effects of Local Anaesthetics

Describe contraindications of Local Anaesthetics

Resources Needed:

Flip charts, marker pens, and masking tape

Black/white board and chalk/whiteboard markers

Computer and LCD projector

SESSION OVERVIEW

Step

Time

Activity/

Content

Step

Time

Activity/

Content

Step

Time

Method

Content

Method

05 minutes

Presentation

Introduction, Learning Tasks

45 minutes

Presentation

Mechanism of Action of Local Anaesthetics

20 minutes

Presentation/

Drug Interactions Associated With Local

PST 05104 Pharmacology & Therapeutics

293

NTA Level 5 Semester 1 Facilitator Guide

brainstorming

Anaesthetics

20 minutes

Presentation

Side Effects of Local Anaesthetics

20 minutes

Presentation/

Contraindications of Local Anaesthetics

20 minutes

Brainstorming

Contraindications of Local Anaesthetics

Brainstorming

05 minutes

Presentation

Key Points

05 minutes

Presentation

Evaluation

PST 05104 Pharmacology & Therapeutics 294 NTA Level 5 Semester 1 Facilitator Guide

SESSION CONTENTS

STEP 1: Presentation of Session Title and Learning Tasks (5 minutes)

READ or ASK students to read the learning tasks and clarify

ASK students if they have any questions before continuing.

STEP 2: Mechanism of Action of Local Anaesthetics (45 minutes)

Activity: Buzzing (5 minutes)

ASK students to pair up and buzz on the following question for 2 minutes

How do local anaesthetics produce their pharmacological effects?

ALLOW few pairs to respond and let other pairs add on points not mentioned

WRITE their response on the flip chart/board

CLARIFY and SUMMARIZE by using the content below

Local anaesthetics are drugs which upon topical application or local injection cause reversible loss of sensory perception, especially of pain in a localized area of the body.

They block generation and conduction of nerve impulses at a localized site of contact without structural damage to neurons.

Clinically they are used to block pain sensation from or sympathetic vasoconstrictor impulses to specific areas of the body

Local Anaesthetics are classified as follows; o Injectable anaesthetic:

Low potency, short duration – Procaine and Chlorprocaine

Intermediate potency – Lidocaine (Lignocaine) and Prilocaine

High potency and long duration – Tetracaine, Bupivacaine, Ropivacaine,

Surface anaesthetic:

Soluble – Cocaine, Lidocaine, Tetracaine and Benoxinate

Insoluble – Benzocaine, Butylaminobenzoate and Oxethazine o Miscellaneous drugs:

Clove oil, phenol, chlorpromazine and diphenhydramine etc.

The mechanism of action of Local Anaesthetics is as follows;

All local anesthetics are membrane stabilizing drugs

They slows down speed of Action Potential (AP) ultimately stoping AP generation

PST 05104 Pharmacology & Therapeutics 295 NTA Level 5 Semester 1 Facilitator Guide

They reversibly decrease the rate of depolarization and repolarization of excitable membranes

They inhibiting sodium influx through sodium-specific ion channels in the neuronal cell – voltage-gated sodium channels

When the influx of sodium is interrupted – action potential cannot rise and signal conduction is inhibited

Local anesthetic s bind (located at inner surface) more readily to sodium channels in activated state – and slows its reversion to the resting state – refractory period is increased – ―state dependent blockade‖ – no action on resting nerve.

STEP 3: Drug Interactions Associated with Local Anaesthetics (20 minutes)

Activity: Brainstorming (5 minutes)

Ask students to brainstorm on the following question:

What are drug interactions associated with local anaesthetics drugs?

ALLOW few students to respond

WRITE their responses on the flip chart/ board

CLARIFY and SUMMARISE by using the content below

Anti-arrhythmics: increased myocardial depression when anti-arrhythmics given with Bupivacaine, Levobupivacaine, Prilocaine or Ropivacaine

Beta-Blockers: Increased risk of bupivacaine toxicity when given with Propranolol

STEP 4: Side Effects of Local Anaesthetics (20 minutes)

CNS Stimulation:

It is dose-related spectrum of effects and All effects are due to depression of neurons

First an apparent CNS stimulation (convulsions most serious) followed by CNS depression (death due to respiratory depression)

Premonitory signs include: ringing in ears, metallic taste, and numbness around lips

Cocaine causes euphoria (unique in its ability to stimulate CNS)

Lidocaine causes sedation even at non-toxic doses.

Cardiocascular system

Arrhythmias:

o The adverse cardiovascular effects of local anaesthetics are due mainly to myocardial depression, conduction block and vasodilatation.

PST 05104 Pharmacology & Therapeutics 296 NTA Level 5 Semester 1 Facilitator Guide

Reduction of myocardial contractility probably results indirectly from an inhibition of the Na+ current in cardiac muscle

The resulting decrease of [Na+]i in turn reduces intracellular Ca2+ stores and this reduces

the force of contraction.

Interference with atrioventricular conduction can result in partial or complete heart block, as well as other types of dysrhythmia

Hypotension

Vasodilatation, mainly affecting arterioles, is due partly to a direct effect on vascular

smooth muscle, and partly to inhibition of the sympathetic nervous system.

This leads to a fall in blood pressure, which may be sudden and life-threatening.

Cocaine is an exception in respect of its cardiovascular effects, because of its ability to inhibit noradrenaline reuptake

STEP 5: Contraindications of Local Anaesthetics (20 minutes)

Activity: Brainstorming (5 minutes)

Ask students to brainstorm on the following question:

What are the contraindications of Local Anaesthetics? ALLOW few students to respond?

WRITE their responses on the flip chart/ board

CLARIFY and SUMMARISE by using the content below

Local anaesthetics should not be injected into inflamed or infected tissues nor should they be applied to damaged skin.

o Increased absorption into the blood increases the possibility of systemic side-effects, and the local anaesthetic effect may also be reduced by altered local pH.

Patients with Heart block, second or third degree (without pacemaker)

Patients with Severe sinoatrial block (without pacemaker).

Concurrent treatment with quinidine, flecainide, disopyramide, procainamide (class 1 antiarrhythmic agents) should be avoided.

PST 05104 Pharmacology & Therapeutics 297 NTA Level 5 Semester 1 Facilitator Guide

STEP 6: Key Points (5 minutes)

Local anaesthetic drugs act by causing a reversible block to conduction along nerve fibres.

They are administered by injection or by application to mucous membranes to produce local analgesia.

They vary widely in their potency, toxicity, duration of action, stability, solubility in water, and ability to penetrate mucous membranes

STEP 7: Evaluation (5 minutes)

What are local Anaesthetics?

What is the mechanism of action of local anaesthetics?

What are the side effects of local anaesthetics?

PST 05104 Pharmacology & Therapeutics 298 NTA Level 5 Semester 1 Facilitator Guide

References

Katzung, B. G. (2018). Basic and clinical pharmacology. New York: Mcgraw Hill Education.

Santos, R. R., Rang, H. P., Dale, M. M., Ritter, J. M., & Flower, R. J. (2007). Rang & Dale Farmacologia. Rio de Janeiro: Elsevier.

Tripathi, K. (2018). Essentials of Medical Pharmacology. Place of publication not identified:

Jaypee Brothers Medical P.

Ministry of Health and Social Welfare. (2013). Standard Treatment Guidelines & National Essential Medicines List Tanzania Mainland (4th ed.). Dar es salaam, Tanzania government printers.

Robert L. Talbert, Gary C. Yee, Gary R. Matzke, Barbara G. Wells, L. Michael. (2014). Pharmacotherapy: A Pathophysiologic Approach (9th ed.). New York, McGraw-Hill Education.

Sally S.R, Jeanne C.S. (2000). Introductory Clinical Pharmacology (6th ed) New York, Lippincott Williams and Wilkins.

School of Pharmaceutical sciences. (2011).Tanzania Pharmaceutical Handbook (2nd ed.).

Dar es Salaam, ARDHI University press.

The Royal Pharmaceutical Society of Great Britain. (2007). Martindale, the Extra Pharmacopoeia (5TH ed). London, pharmaceutical press.

The Royal Pharmaceutical Society of Great Britain. 2009. British National Formulary (59th ed). London, BMJ Group and RPS Publishing.

PST 05104 Pharmacology & Therapeutics 299 NTA Level 5 Semester 1 Facilitator Guide

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