Local Anesthetics Flashcards
(47 cards)
Local anesthetic uses?
Infiltrated around the nerve
Applied topically to the skin
Injected into blood vessels that are first exsanguinate
Injected into the subarachnoid or epidural spaces
What is the general difference between A, B, and C fibers? (which are myelinated, speed, and size)
A fibers: myelinated, size 1-22 micrometers, FAST
B fibers: myelinated, 1-3 micrometers, slower than A, but faster than C
C fibers: unmyelinated fibers, 0.1-2.5 micrometers, SLOW
List each fiber from fastest to slowest and list what it senses.
(A: alpha, beta, delta, gamma; B; C)
A-alpha: motor and proprioception
A-beta: motor, touch, pressure
A-gamma: motor/muscle tone (fine motor modulation)
A-delta: pain, temp, touch
B: PREganglionic autonomic
C: dull pain, temp, touch, POSTganglionic autonomic
Large fibers have the _____ conduction velocity, _____ capacitance, and _____ threshold for excitability
(highest/lowest)
Highest conduction velocity
Highest capacitance
Lowest threshold for excitability
Between sensory, motor, and autonomic, which is blocked first when given an LA?
Autonomic blocked first, more on the outside/mantle of the nerve (C fibers)
Sensory blocked next, then motor blocked last because it is at the core/inner surface and hardest for the LA to reach
More specifically, what is the clinical sequence of local anesthesia blockade (5 steps), what is blocked?
1- sympathetic block (vasodilation, warm skin) 2- loss of pain and temp 3- loss of proprioception 4- loss of touch and pressure 5- motor blockade
Which ion establishes and maintains resting membrane potential?
K
Local anesthetics MOA?
LAs bind to the Na channel alpha subunit when it is in the inactivated closed state
Impulse conduction blockade is caused by inhibition of the influx of Na ions (during depolarization phase of action potential)
Do locals change the threshold potential, the ability to reach threshold, or both?
ONLY the ability to reach threshold
Describe the typical structure of a LA
Lipophilic head (aromatic ring) Intermediate chain containing an amide (NH) or an ester (COO-) Hydrophilic tail (tertiary amine)
How can the name of a LA identify it as an amide vs ester?
One eyed ester (procaine)
Two i’s amide (lidocaine)
How are esters vs. amides metabolized?
Esters: hydrolyzed by non-specific esterase’s in the plasma and tissues (mostly liver), metabolite = PABA (para-aminobenzoic acid), except for cocaine
Amides: metabolized in the liver, CP450, slower than esters, aromatic hydroxylation, N-dealkylation, amide hydrolysis
Highly lipid soluble anesthetics are ___ (more/less) potent and have a ____ (shorter/longer) DOA than water soluble anesthetics
More potent
Longer
Increase in the length of an intermediate chain, more carbons, _____ (inc/dec) potency and toxicity
Increases
Also increased with length of the terminal groups on the tail and aromatic ring
What is exparel?
Bupivacaine extended release liposome injection, only approved for bunionectomy and hemorrhoidectomy
When mixed with another local, it will change the effects and increase chance of toxicity
How many nodes of ranvier must be blocked by LA to stop the propagation of action potentials?
THREE
What characteristics govern LA systemic absorption?
Physiochemical factors: pKa, pH, and lipid solubility, blood flow
Physiologic conditions: tissue pH, pCO2, temp, characteristics (age, pregnancy)
Note: elderly and pregnant are more at risk for toxicity
Absorption by type of block from high to low?
I Think I Can Place Epidural Blocks So Smooth
Intravenous, Tracheal, Intercostal, Caudal, Paracervical, Epidural, Brachial plexus, Subarachnoid, Subcutaneous
Do ionized or non-ionized drugs cross a lipid membrane?
NONionized
What are the pros/cons of ionized LAs vs. non-ionized?
Ionized binds to the receptor easier
Nonionized gets through the barrier easier
The ideal LA drug would be 50/50 (half ionized, half nonionized)
The ionized form is favored when?
The non-ionized form is favored when?
(acid/base drug in acid/case environment)
Ionized favored when acidic drug in basic environment, basic drug in acidic environment
Nonionized favored when acidic drug in acidic environment, basic drug in basic environment
Local anesthetics are strong/weak acids/bases
Weak bases
Packaged in acidic formulations but basic upon injection
When looking at %non-ionized and onset, which LA doesn’t follow the rule?
Rule: higher % non-ionized = faster onset
Chloroprocaine is the exception to the rule because it is only 2 % non-ionized and has a fast onset (we give it in higher concentrations to have this effect)
Which LAs have slow, moderate, and fast onsets?
Slow: procaine, tetracaine (esters)
Intermediate: bupivacaine
Fast: chloroprocaine, lidocaine, etidocaine, mepivacaine
