Lecture 19 4/4/24 Flashcards
Which steps of the nerve conduction pathway can be treated using local anesthetics?
-modulation
-transmission
-transduction
What are the characteristics of resting membrane potential?
-involves Na+/K+ pumps
-K+ moves into cells, Na+ moves out
-resting membrane potential stays around -60 to -70 mV
What are the characteristics of action potentials?
-start with a stimulation
-Na+ channels are activated
-influx of Na+ into cell
-membrane potential changes to positive
What is nociceptive wind up?
painful stimulation leads to more painful stimulation
What are the general characteristics of local anesthetics?
-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
How are esters metabolized?
hydrolyzed by cholinesterase enzymes in the plasma and liver
How are amides metabolized?
hepatic microsomal enzymes
What are the characteristics of amide metabolism?
-more complex and longer process compared to esters
-greater risk of toxicity
What is the regional perfusion technique?
increased pressure from a cuff/tourniquet leads to increased regional concentration of the local anesthetic
What are the characteristics of epidural catheters?
-provide regional anesthesia
-can be placed and left in for multiple days for continued access
What is a wound soaker catheter?
a catheter/drain that is incorporated into an incision for post-operative local anesthetic infusion
What is the pharmacology of local anesthetics?
-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
How does high lipophilicity impact how local anesthetics work?
-high potency
-longer onset of action
-longer duration of action
How does protein binding impact how local anesthetics work?
-correlates with slower elimination
-longer duration of action
What are the characteristics of lidocaine?
-amide LA
-one of the most frequently used
-quick onset and duration of action
-different concentrations and preparations available
What are the common uses of lidocaine?
-nerve block and local infiltration
-neuroaxial anesthesia
-topical desensitization
-systemic administration
What are the characteristics of mepivicaine?
-amide LA
-clinical applications similar to lidocaine
-longer duration of action than lidocaine
-common in equine practice
What are the characteristics of bupivacaine?
-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
What are the characteristics of liposomal bupivacaine?
-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
What are the characteristics of ropivacaine?
-amide LA
-long duration of action
-structurally related to bupivacaine but less lipophilic
-less CNS and cardiotoxicity risk
What happens when local anesthetics are mixed?
-slower onset
-shorter duration
What are the characteristics of local anesthetic toxicity?
-due to high concentrations
-more common with amides
-CNS and cardiovascular toxicity; CNS tox. precedes cardiovascular tox. for most LAs
What are the characteristics of LA CNS toxicity?
-depression of cortical inhibition
-excitatory response first, followed by depression
Which signs of LA CNS toxicity are seen in small ruminants?
-nystagmus and muscle fasciculations
-CNS stimulation
What are the characteristics of cardiovascular toxicity?
-occurs at larger doses
-depression of myocardial automaticity and reduction in duration of the refractory period
-depression of contractility and conduction velocity
What ECG changes are seen with cardiovascular toxicity?
-widening or inversion of QRS complex
-bradycardia
-ventricular premature complexes
-ventricular tachycardia
What is the treatment for local anesthetic toxicity?
-chelation
-O2 supplementation
-intubation and ventilation
-seizure control
-IV fluids
-vasopressors
-inotropes
-anticholinergics
-antiarrhythmics
