LAST: PREVENTION AND TREATMENT

PART A

PART A: OVERVIEW

• What are local anesthetics• Classification• MOA• Anesthetic potency• Clearance• Uses• Prolongation of action

PART B: ADVERSE EFFECTS&SYSTEMIC TOXICITY

• Allergic reaction• Local toxicity• Systemic toxicity

What are LA

• Weak bases which produce a transient and reversible loss of sensation (analgesia) in a circumscribed region of the body without loss of consciousness.

• Normally, the process is completely reversible.

Classification

• Local anesthetics - esters or amides.

• Major difference is their potential for producing adverse effects and the mechanisms of their metabolism.

Esters

• Eg: Procaine, Cocaine, tetracaine• The ester linkage is cleaved by plasma

cholinesterase. Short half life( abt 1min).• Amides:• Eg: lidocaine, Marcaine(Bupivacaine),

ropivacaine. Amide linkage is cleaved in the liver. Half life is about 2-3 hrs

MECHANISM OF ACTION

• LA block nerve conduction by impairing propagation of action potential in axons. Interact directly with Na+ channels and stop Na+ ion influx. May also act on K+ & Ca+ channels.

• LA need to diffuse passively in uncharged state (lipophilic) to reach target-axoplasmic side of Na+ channel.

HOW MUCH IS TOO MUCH?

ESTERS MAX DOSE(mg/kg)

DURATION (HOURS)

Chloroprocaine 12 0.5 – 1

Procaine 12 0.5 – 1

Cocaine 3 0.5 – 1

Tetracaine 3 1.5 – 6

Amides Max Dose (mg/kg) Duration (h)

Lidocaine 4.5/(7 with epi) 0.75 – 1.5

Mepivacaine 4.5/(7 with epi) 1– 2

Prilocaine 8 0.5 – 1

Bupivacaine 3 1.5 – 8

Ropivacaine 3 1.5 – 8

ANESTHETIC POTENCY• The more LA is lipophilic the more potent it is

( increased rate of diffusion).• More protein binding prolongs the effect.

Bupivacaine is approximately 95% protein-bound. • Intermediate-duration LAs (lidocaine and

mepivacaine) have a smaller protein-bound fraction (60-70%).

• pH; higher pH speeds action (keeps LA uncharged).

• pKa: lower pKa faster onset.• Concentration; higher conc. =rapid onset (mass

effect).

CLEARANCE

• ESTERShydrolysis via tissue cholinesterase

• AMIDESmetabolism via hepatic enzymes

USES

•Surface anesthesia

•Infiltration anesthesia

•Regional anesthesia

SURFACE ANESTHESIA

• On intact skin –

• eutectic mixture of

Local anesthetics

• (EMLA)

• Slow absorption,

Action up to ½ Hr

INFILTRATION ANESTHESIA

• LA infiltrates locally into tissues to numb the area.

• Purpose of pain free procedures-before injections with large needles, lumbar puncture

REGIONAL ANESTHESIA

• Nerve block

• Intravenous

• Epidural/Extradural

• Intrathecal block/ spinal anaesthesia

NERVE BLOCKS

• Inject a drug around the nerve

• Anaesthetise a region

INTRAVENOUS ANESTHESIA

• 0.5-1% lidocaine without adrenaline

• Bier’s Block

EPIDURAL

• Thoracic, lumbar, sacral

• Act on nerve roots• Less hypotention

SPINAL BLOCK

• Sympathetic nerve block

• hypotension