Local Anesthetics

Question 1

Amides

Answer 1

Plasma protein binding
Longer lasting
Greater risk for systemic toxicity

Question 2

Esters

Answer 2

Hydrolysis by plasma esterases
Shorter acting
Broken down in blood so lower risk of systemic toxicity

Question 3

Mechanism of Action

Answer 3

Block sodium channels from the cytoplasmic mouth

Stabilize inactivated state of the channel

Question 4

Factors that influence Effectiveness

Answer 4

Diameter: Greater diameter less effective/longer before working
Firing Rate, Action potential duration: Rapid firing rate makes them more sensitive. The nerve has to depolarize before anesthetized
Position of Fiber in Nerve Bundle: Deeper fibers are less effected
Local Blood Flow: Blood takes anesthetic away from the area
Drug size and hydrophobicity: Needs to be hydrophobic and small to pass the membrane
Liver Function: Limited liver function causes slow breakdown
Tissue pH: Acidic tissues keep the LA in the ionized form where it can’t pass membrane.

Question 5

Toxicity

Answer 5

CNS: Initially drowsiness and excitation (up to seizures) followed by general depression coma
Cardiac (especially Bupivacaine): Depressed pacemaker activity, decreased conduction, arrhythmias, potentiated by hyperkalemia, synergize with anti-arrhythmics
Peripheral Blood Vessels: Vasodilation leading to hypotension
Allergies: Esters: PABA derivatives, amides: Preservatives

Question 6

Lidocaine

Answer 6

Most broadly used local anesthetic
Intermediate duration AMIDE
30-60 minutes for topical or infiltration
Higher systemic toxicity than esters
Metabolized by liver
Uses- spinal, epidural, local, topical
Careful with use with amiodarone

Question 7

Procaine

Answer 7

Introduced in 1905
Short acting ester (40s plasma half-life)
Low toxicity
Main use - infiltration and regional anesthesia
Interaction: PABA blocks sulfonamide action

Question 8

Tetracaine

Answer 8

Intermediate duration of action Ester
2hr spinal, 30-60 minute topical
Higher systemic toxicity than other esters
Tends to cause mucous membrane irritation, urticaria, burning
Main uses- spinal and topical anesthesia of nose and throat

Question 9

Bupivacaine

Answer 9

Widely used amide local anesthetic
Long duration amide (3-9 hr regional, plasma half-life 3.5 hours adult, >8 hours neonate)
Concurrent epidural use with opiates in labor
More cardiac effects than lidocaine due to slow dissociation from cardiac Na channels
Synergistic with anti-arrhythmics

Question 10

Selection

Answer 10

Short duration (20-45 minutes): Procaine, Benzocaine
Medium Duration (1-2 hours): Lidocaine
Medium to Long (3-9 hours): Tetracaine, bupivacaine
Topical: Benzocaine, proparacaine
Local: Lidocaine, procaine
Regional: Tetracaine, Benzocaine