Local Anesthetics Flashcards
How do local anesthetics generally work?
Bind to sodium channels in nerves to block nerve transmission
What kind of fibers are easiest to block?
Small myelinated fibers
How does myelination affect blocking by local anesthetic?
Unmyelinated fiberes are more difficult to blokc whereas myelinated fibers only need to block 3 nodes of Ranvier
How is the propagation of block onset?
Nerve block -> Sciatic NErve -> leg will get numb from proximal to distal
What are the 3 fiber types?
A, B ,C
Which fiber types are myelinated?
A and B
Which fiber type is unmyelinated?
C
What are the receptor subtypes of fiber A?
Aa: motor neurons; hardest to block in A fiber group due to large size
Ab: Tactile/Proprioception
Ay: Reflexes;
Adelta: pain cold temperature, easiest to block due to small size
What are B fibers?
Preganglionic sympathetics
Very small size and easiset to block with local anesthetics
What are C fibers?
Visceral pain nerve unmyelinated
Hardest to block -> more dull slow pain
What is the hierearchy of nerve blocks from easiest to hardest?
Sympathectomy (Fiber B) -> Sensory and Pain fibers -> Motor Fibers
What states of the sodium channel do local anesthetics bind to?
Inactivated or Open states rather than resting states
Where does hte local anesthetic bind on the sodium channel?
R site, which is on the intracellular site
What property is important for the local anesthetic to be able to bind to the sodium channel?
Lipophilic
What is frequency dependent blockade?
In very active nerves, sodium channels will be in either activated or inactivated states mostly -> nerves will more quickly be blocked by local anesthetics
Which form of hte local anesthetic can cross the membrane? Which form is active?
Uncharged form can pass membrane
Charged form is the active form
What is the primary determinant of hte potency of a local anesthetic?
Lipid solubility : more potent drug is more lipid soluble
What determines the duraiton of action of a local anesthetic?
Local tissue protien binding
What can be added to the local anesthetic mixture to tspeed hte onset time of a block?
Sodium bicarbonate -> generates more uncharged anesthetics to drive cells through
Why is epinephrine given in adjunct to a local anesthetic?
For vasoconstriction -> keeps anesthetic near the nerve and increases duration of a nerve block
Keeps it out of systemic system : Inject with epi -> if HR inc -> indicate need to move needle
This is not as effective for drugs that are highly protein bound
What are indications for neuraxial anesthesia?
Surgery below the chest
What are the differences between a spinal and epidural?
spinal: inject around cauda equina -> most wil stay there and below (up to T10 blockade and below)
Epidural: site of injection is nerve roots exiting neural foramina -> thus can be at any level
What are hte respiratory effects of neuraxial anesthesia at the thoracic level?
Normal tidal volume
Loss of proprioception -> pts cant feel themselves breathing
Respiratory arrest possible due to blocking B type sympathetic fibers (NOT due to paralyzed phrenic nerves)
What are hte cardiovascular effects of neuraxial anesthesia?
B type fibers get blocked -> arterial vasodilation and venodilation -> dec venous return and BP
What is the Bezold JArisch Reflex?
Unopposed vagal stimulation -> decreased venous return -> hear thinks it needs to slow down to allow filling -> bradycardia
How are local anesthetics eliminated?
Esters: Plasma Cholinesterase
Amides: Liver
What is the max recommended dose of bupivicaine?
3mg/kg for adults
2.5 mg/kg for neonates
What is the max recommended dose for lidocaine?
5 mgkg plain
7 mg/kg with epi
What are some neuro toxicity concerns of local anesthetic?
Inhibit inhibitory neurons -> activate excitatory neurons
Acidoses -> dec protein binding in blood -> inc free fraction of drug to brain
Treat with hyperventilation, succinylcoline, benzo, etc
What are some caridovascular toxicity concerns?
Usually occurs at higher conc than for neurotoxicity Cardiac sodium channel blockade Vasodilation Inhibition of sympathetic effects Slurred QRS, V-fib, Vtach
How is LA toxiicty treated?
Prevention: monitory, use of epi for intravascular marker
Supportive: oxygenation and ventilation, treat seizures and arrhythmias (epi, atropine etc)
What is intralipid?
Specific treatment therapy for local anesthetic toxiciity
Very lipophilic and absorbs LA and sequesters them
What are some complications of neuraxial anesthsia?
Postdural Puncture Headaache (PDPH): leakage of CSF
Transient Neurologic Sx (TNS): sensory disturbances or pain in back of lower extremities; No permanent damage
Backache
What factors increase incidience of PDPH after neuraxial anesthesia?
Younger, female, larger needle, pregnant, multiple punctures
How do we treat PDPH?
Prevention
Fluids
Analgesics
Blood Patch is treatment of choice
What is the blood patch treatment for PDPH?
Take pt’s own blood -> put in epidural sac -> plug dural hole and preven CSF leakage
