Lecture 19 4/4/24

Question 1

Which steps of the nerve conduction pathway can be treated using local anesthetics?

Answer 1

-modulation
-transmission
-transduction

Question 2

What are the characteristics of resting membrane potential?

Answer 2

-involves Na+/K+ pumps
-K+ moves into cells, Na+ moves out
-resting membrane potential stays around -60 to -70 mV

Question 3

What are the characteristics of action potentials?

Answer 3

-start with a stimulation
-Na+ channels are activated
-influx of Na+ into cell
-membrane potential changes to positive

Question 4

What is nociceptive wind up?

Answer 4

painful stimulation leads to more painful stimulation

Question 5

What are the general characteristics of local anesthetics?

Answer 5

-Na+ channel blockers
-goal is to block sensory fibers
-composed of a lipophilic 6 carbon ring and a hydrophilic side chain linked by an intermediate hydrocarbon chain

Question 6

How are esters metabolized?

Answer 6

hydrolyzed by cholinesterase enzymes in the plasma and liver

Question 7

How are amides metabolized?

Answer 7

hepatic microsomal enzymes

Question 8

What are the characteristics of amide metabolism?

Answer 8

-more complex and longer process compared to esters
-greater risk of toxicity

Question 9

What is the regional perfusion technique?

Answer 9

increased pressure from a cuff/tourniquet leads to increased regional concentration of the local anesthetic

Question 10

What are the characteristics of epidural catheters?

Answer 10

-provide regional anesthesia
-can be placed and left in for multiple days for continued access

Question 11

What is a wound soaker catheter?

Answer 11

a catheter/drain that is incorporated into an incision for post-operative local anesthetic infusion

Question 12

What is the pharmacology of local anesthetics?

Answer 12

-can exist in ionized or unionized forms
-unionized fraction is more lipid soluble and can cross nerve membrane
-weak bases; pKa of 8-9
-sufficient fraction of unionized form present at physiologic pH (7.4)
-ionized form more dominant in acidic environments (infection sites); cross into nerve cells more slowly

Question 13

How does high lipophilicity impact how local anesthetics work?

Answer 13

-high potency
-longer onset of action
-longer duration of action

Question 14

How does protein binding impact how local anesthetics work?

Answer 14

-correlates with slower elimination
-longer duration of action

Question 15

What are the characteristics of lidocaine?

Answer 15

-amide LA
-one of the most frequently used
-quick onset and duration of action
-different concentrations and preparations available

Question 16

What are the common uses of lidocaine?

Answer 16

-nerve block and local infiltration
-neuroaxial anesthesia
-topical desensitization
-systemic administration

Question 17

What are the characteristics of mepivicaine?

Answer 17

-amide LA
-clinical applications similar to lidocaine
-longer duration of action than lidocaine
-common in equine practice

Question 18

What are the characteristics of bupivacaine?

Answer 18

-amide LA
-commonly used for local, nerve, and epidural blocks
-slow onset of action
-long duration of action
-multiple available concentrations
-not for IV use; cardiotoxic

Question 19

What are the characteristics of liposomal bupivacaine?

Answer 19

-slow release; 72 hours
-approved for dogs and cats
-used in dogs at the time of incisional closure for cruciate ligament repair
-used in cats prior to onychectomy/declaw

Question 20

What are the characteristics of ropivacaine?

Answer 20

-amide LA
-long duration of action
-structurally related to bupivacaine but less lipophilic
-less CNS and cardiotoxicity risk

Question 21

What happens when local anesthetics are mixed?

Answer 21

-slower onset
-shorter duration

Question 22

What are the characteristics of local anesthetic toxicity?

Answer 22

-due to high concentrations
-more common with amides
-CNS and cardiovascular toxicity; CNS tox. precedes cardiovascular tox. for most LAs

Question 23

What are the characteristics of LA CNS toxicity?

Answer 23

-depression of cortical inhibition
-excitatory response first, followed by depression

Question 24

Which signs of LA CNS toxicity are seen in small ruminants?

Answer 24

-nystagmus and muscle fasciculations
-CNS stimulation

Question 25

What are the characteristics of cardiovascular toxicity?

Answer 25

-occurs at larger doses
-depression of myocardial automaticity and reduction in duration of the refractory period
-depression of contractility and conduction velocity

Question 26

What ECG changes are seen with cardiovascular toxicity?

Answer 26

-widening or inversion of QRS complex
-bradycardia
-ventricular premature complexes
-ventricular tachycardia

Question 27

What is the treatment for local anesthetic toxicity?

Answer 27

-chelation
-O2 supplementation
-intubation and ventilation
-seizure control
-IV fluids
-vasopressors
-inotropes
-anticholinergics
-antiarrhythmics