Local Anesthesia Flashcards
(41 cards)
What is the mechanism of action of local anesthetics? What does this result in?
lipophilic. unionized local anesthetic crosses the phospholipid membrane and dissociates into the ionized form, binding to and blocking voltage-gated Na+ channels = initial opening followed by stabilization
suppression of action potential in excitable tissues, increased firing threshold, lengthened refractory period = prevented conduction and propagation of nerve impulses as concentration increases
What amino ester is used as a local anesthetic? What is onset, potency, and duration like
Procaine - 8.9 pKa
- 10-20 min
- low potency
- short duration (30-60 mins)
What are 2 short-acting amino amides used as local anesthetics?
LIDOCAINE (7.9) - 5-10 min onset, intermediate potency and duration of action (60-120 mins) —> good for laceration repair
MEPIVACAINE (7.7) - 5-10 min onset, intermediate potency and duration of action (90-180 mins)
What are 2 longer acting amino amides used as local anesthetics?
BUPIVACAINE (8.1) - 10-20 min onset, high potency, long duration of action (180-360 min)
ROPIVACAINE (8.1) - 10-20 min onset, high potency, long duration of action (120-240 min)
should be onboard well in advance
What fibers are affected by lidocaine, bupivacaine, and ropivacaine?
LIDOCAINE - non-specific, blocks all fiber types
B/R - sensory > motor
How does the location of nerve bundles affect onset of action of local anesthetics?
- superficial nerve bundles are penetrated first
- proximal areas blocked sooner than distal areas
(femoral-sciatic = knee before tarsus)
How does myelination affect onset of action of local anesthetics?
small myelinated and unmyelinated fibers blocked before large myelinated fibers
What affects the discharge rate of local anesthetics?
local anesthetics’ higher binding affinity for open and inactivated channels compared to activated channels (phasic or use-dependent blockade)
What is the order of the first 5 neuron types in the blockade? What do they result in?
- B - increased skin temperature
- A delta, C - pain relief, loss of temperature sensation
- A gamma - loss of proprioception
- A beta - loss of sensation to touch and pressure
- A alpha - loss of motor function
pain relief before blocked motor function
What are the recommended doses of lidocaine in dogs, cats, horses, and cattle?
- DOGS: 2-6 mg/kg
- CATS: 0.5-1 mg/kg
- HORSES/CATTLE: max 6 mg/kg
What are the recommended doses of mepivacaine in dogs, cats, and horses?
- DOGS: 5 mg/kg
- CATS: 2-3 mg/kg
- HORSES: max 5-6 mg/kg
What are the recommended doses of bupivacaine in dogs, cats, and horses?
- DOGS and CATS: 2 mg/kg
- HORSES: 2 mg/kg (toxic at 4 mg/kg)
What are the recommended doses of ropivacaine in dogs and cats?
- DOGS: 2 mg/kg
- CATS: 1.5 mg/kg
What toxicities are associated with the use of IV lidocaine?
- CNS: seizures, nystagmus, muscle twitching, hypoventilation, respiratory arrest
- CV signs: depressed myocardial automaticity, reduced refractory period duration, reduced contractility and conduction velocity
What CV are associated with lidocaine and bupivacaine/ropivacaine? What ECG changes are expected?
- LIDOCAINE: hypotension
- B/R: profound arrythmias
wide or inverted QRS, bradycardia, VPCs, V-tach
What is the best treatment to avoid local anesthesia toxicity? What supportive therapy is recommended?
avoid IV administration
- oxygen
- anti-seizure medication
- IV fluids
- vasopressors, inotropes
- anticholinergics
- antiarrhythmics (bretylium magnesium)
What is lipid rescue?
Intralipid - 20% IV lipid emulsion to extract the lipid soluble local anesthetic from aqueous plasma phase and make it unavailable to tissues
- bolus 4 mL/kg followed by 0.5 mL/kg/min for 10 mins
What formation of local anesthetics should be used for epidurals? What technique is recommended?
preservative-free
- sterile technique
- aspirate before injection and anytime the needle is repositioned
- if there is resistance to injection, STOP - most likely in the nerve bundle
How can resistance to injection be detected for local anesthetics?
use a 3 mL syringe and have an air bubble to watch for compression
- note: dental blocks are done with 1 mL syringe, so detection is difficult
What vasoconstrictor is used as an adjunct? Alpha-2 agonist? Opioid? What do they do?
epinephrine - increases intensity and duration of block
dexmedetomidine - enhances local anesthetic duration of action
buprenorphine - enhances peripheral nerve blcoks cia sodium channel mechanism
How is sodium bicarbonate used as an adjunct?
decreases pain on injection, but not likely to speed up onset of action
What risk is associated with epinephrine usage as adjuncts?
ischemic damage to nerve and surrounding tissues (vasoconstrictor!)
What are the most common adjuncts used as local anesthetics? Why are they commonly diluted?
opioids and dexmedetomidine (variable use among clinicians due to variable effectiveness in different species)
when max dose is reached, it allows for large areas of distribution without increasing dose any more
What is sarapin?
biologic derived from pitcher plant used as a local anesthetic, usually for chronic pain - limited data available
