Locals Flashcards
What do local anesthetics do?
Produce reversible conduction blockade of impulses along the central and peripheral nerve pathways
What are the lipophilic and hydrophilic portion separated by?
hydrocarbon
True/False: Lipophilic portion is the Benzene ring and is necessary for activity
True
Ester
-CO-
Amide
-NHC-
S enantiomers
Left = sinister
less neuro- and cardio-toxic
R enantiomers
Right = rectus
How do they work?
Inhibit Na+ ions passage through ion-selective Na+ channels
Slows rate of depolarization
Threshold potential not reached
No action potential propogated
What does locals not alter?
Resting membrane potential
Threshold potential
What is Cm?
minimum concentration to produce conduction blockade
Analogous to MAC
Fibers that are more easily blocked have a low Cm
Factors affecting Cm?
Increases: larger diameter
Decreases: higher frequency stimulation, higher pH
Nodes of Ranvier
Must block at least 2, preferably 3
What is conducting velocity increased by?
myelination and a wider axon terminal
Order of peripheral nerve block speed
B fibers, C fibers, small A fibers (delta, gamma), large A fibers (alpha, beta)
regression occurs in opposite fashion
What is differential blockade?
Differential spinal blockade is the clinical phenomenon referring to the temporal blockade of autonomic, sensory, and motor nerve fibers when using neuraxial local anesthetics. Autonomic fibers are blocked first, followed by sensory loss to touch/pinprick, followed by loss of proprioception, and lastly motor loss.
What is an example of a differential blockade?
epidural bupivacaine. In lower doses is provides analgesia but spares motor function. In higher doses, it blocks motor function.
locals bind to the voltage gated sodium channel when it is in the _____ and _____ states
active and inactive
What form to cross lipid bilayer?
Un-ionized
Distribution
1st large uptake to lungs
2nd distribution to high perfused tissue
(heart, brain, kidneys)
3rd distribution to low perfused tissue
(muscle and fat)
Amides are more widely distributed
Why is placental transfer important?
ion trapping
Why do we worry about which vasopressor can cause fetal acidosis?
Once un-ionized local crosses placenta and hits low fetal pH more drug is ionized and can’t cross back
Build up of trapped local in fetal circulation leads to toxicity in fetus
Not good for new baby to have local anesthetic toxicity
Onset determined by _____
State of Ionization – most important
Lipid Solubility
Potency determined by ____
lipid solubility
duration determined by____
protein binding
Metabolism
amides: hepatic
esthers: pseudocholinesterase, Rapid hydrolysis
Fastest local
Prilocaine
One exception to hydrolysis?
Cocaine – significant metabolism in liver
What common local injection site contains little to no cholinesterase enzyme?
CSF
Must wait until drug goes into systemic circulation for hydrolysis
Which 2 local anesthetics have no vasodilator activity?
Cocaine
Ropivacaine (only parenteral with vasoconstrictive activity)
Epi
1:200,000 or
5 mcg/ml
Limits systemic absorption
Maintains drug concentration around nerves
Can prolong Lidocaine by 1/3
No effect to onset
Helps to decrease toxicity
Mixing Locals
Combos
Faster onset
Longer Duration
Lido/Bupivicaine
Cholorprocaine/Bupivicaine
Effects additive and not synergistic
What is systemic toxicity most common from?
Most common from IV injection
Less common from absorption
Fastest to Slowest blood flow of tissues
IV
Tracheal
Intercostal
Caudal
Paracervical
Epidural
Brachial Plexus
Subarachnoid
Subcutaneous
In Time I Can Please Everyone But Suzi and Sally
What do you avoid when treating LAST?
AVOID vasopressin, calcium channel blockers, betablockers, or local anesthetic
