Local anesthetics

Question 1

Local Anesthesia

Answer 1

Loss of sensation in a limited area of the body

Question 2

MOA of local anesthetics

Answer 2

administered in Non-ionic form –> diffuse into the cells –> become Ionized –> block Na channel from the inside

Question 3

Loss of sensation (chronologically)

Answer 3

1. Pain
2. Cold/warmth
3. Touch
4. Pressure
5. Motor (could be affected)

**Effects faster n small fibers

Question 4

What are the major uses of local anesthetics

Answer 4

• Minor & quick procedure
• Adjunct to anesthesia
• Adjunct to treatment of acute pain –ANALGESIA **
• adjuvant to sedation
• anesthesia of fish/amphibians

Question 5

Intravenous block (local anesthetic)

Answer 5

Large volumes of diluted local anesthetics injected into an extremity ISOLATED from remainder of the circulation by TOURNIQUET

Question 6

What parameters determine by the structure of local anesthetic?

Answer 6

1. Onset
2. Duration
3. Potency

Question 7

What are the 3 general segments of the local anesthetic drugs?

Answer 7

Weak Base

1. Hydrophobic (lipophilic) domain
2. Intermediate ester OR Amide linkage
3. Hydrophilic (lipophobic) domain

Question 8

What determines the pKa value of local anesthetic?

Answer 8

Lipophobic group
• Tertiary amine
• Determines the ONSET of ACTION!!

Question 9

What does the lipophilc group determine of local anesthetic?

Answer 9

Binding to hydrophobic
• affects Duration (protein binding) & Potency!
• also ↑ toxicity

Question 10

What determines the “fate”/metabolism of local anesthetic?

Answer 10

** Intermediate chain

• Aminoamines = liver metabolism
• Aminoesters = Hydrolyzed in plasma (less toxic)

Question 11

Which local anesthetic have low potency, short duration?

Answer 11

Procaine**

chlorprocaine

Question 12

Which local anesthetic have intermediate potency/duration

Answer 12

Lidocaine*
Mepivacaine**
prilocaine

Question 13

Which local anesthetic have High potency, long duration

Answer 13

Tetracaine*
Bupivacaine*
Roprivacaine**

Question 14

Why does damaged tissue diminish efficacy of local anesthetics?

Answer 14

bc environment becomes acidic –> becomes ionized before entering cells

Question 15

What is important to know about administration of local anesthetic?

Answer 15

• Rapid & voluminous vascular absorption is NOT desired –> CNS / Cardiotoxic

• Vascular absorption ↓ by Epi (vasoconstrictor)
– also delays healing

Question 16

If accidentally injected in the blood, which will be distributed more widely? (amino amides OR amino esters?

Answer 16

Aminoamines

Question 17

What are systemic toxic effects of local anesthetics?

Answer 17

CNS tox
  • sedation to excitation (nausea)
  • muscle twitch 
  • convulsions
  ••• POSSIBLE DEATH -- Respiratory & Cardiac arrest

Cadiovascular system
• bradycardia
• arrhythmia
• hypotension

Allergic rxn – Esp AminoESTERS

Question 18

Lidocaine

Answer 18

Injectable local anesthetic
• AminoAMIDE
• 4X more potent than procaine
• DOG/CATTLE
• Overdose --> ventricular fibrillation / cardiac arrest
    --> death

Question 19

Mepivacaine

Answer 19

Injectable local anesthetic
• AminoAMIDE
• longer duration than lidocaine
• HORSE – less likely to cause tissue rxn

Question 20

Bupivacaine

Answer 20

Injectable local anesthetic
• AminoAMIDE
• More potent than lidocaine
• LONGER duration

Question 21

Ropivacaine

Answer 21

Injectable local anesthetic
• longer lasting
• HIGH affinity for pain fibers (< motor)
• Epidural for Horse surgery

Question 22

Topical local anesthetics

Answer 22

AminoESTERS

1. Tetracaine – eye, nose & throat
2. Proparacaine – cornea

** NOT if corneal ulcers

Question 23

What is used for lameness dx in horse?

Answer 23

Mepivacaine

• DO NOT use Epi

Question 24

Epidural local anesthetic

Answer 24

• inject in space btwn dura mater & bone/ligamentous wall of spinal canal
• lose of SENSORY & MOTOR innervation

• LG animals – general anesthesia more used
- Xylazine + local anesthetic

Question 25

Risks assoc’d w/ Epidural use of local anesthetics?

Answer 25

1. Hypotension (Symp paralysis)
2. Motor Paralysis (hind limb)
3. Ventilation impairment (intercostal paralysis if too far front)

Question 26

Examples of uses of local anesthetics in dogs

Answer 26

1. Punch biopsy
   - - around lesion (2-3 mls of lidocaine)
2. Needle cor biopsy of LN
   - - around lesion & under skin (overlying the LN)
3. Bone Marrow
   - - in & around site where the marrow needle will be introduced (skin to bone)