Local Anesthetics Flashcards
Define dysesthesia.
abnormal or unpleasant sensation to normal stimuli
Define hyperesthesia.
excessive pain response to non-noxious stimuli
Define hypoesthesia.
decreased sensitivity to stimuli (aka numbness)
Define anesthesia
the loss of sensation caused by depression of excitation or inhibition of conduction, without inducing loss of consciousness
Define paresthesia.
altered sensation (pins and needles)
What are the desirable qualities we want in a local anesthetic?
adequate duration, no permanent nerve damage, must remove sensation, low systemic toxicity, non-irritating to tissue, reversible, has short onset, stable in solution, non-allergenic, can be sterilized w/o deteriorating
How many millivolts does a resting membrane potential possess? What amount is the threshold for signal conduction to occur?
resting membrane = -70mV
threshold = -55mV
What are the types of nerve fibers?
Types A (α, β, γ, δ), B and C. Type A are myelinated, α and β can be afferent or efferent, γ is only efferent, δ only afferent. Type B are myelinated sympathetic nerves. Type C are not myelinated
What are the two periods of the repolarization process of the nerve membrane?
Absolute refractory: Na channels are shut and conduction cannot occur.
Relative refractory: Na channels can function, but large stimulus is needed to induce conduction
What is impulse propagation?
When a stimulus causes the reversal of the membrane potential at a localized site on the unmyelinated nerve, that then spreads to adjacent areas
What is salutatory conduction?
How myelinated nerves send signals: depolarization jumps from node to node by raising the membrane potential closer to -55mV in adjacent nodes, making them easier to excite.
Nerve membranes are freely permeable to what two ions?
K+ and Cl-
What is the sequence of events for local anesthetics?
LA molecule diffuses across nerve membrane, LA displaced Ca ion from Na channel, LA binds intracellularly to Na channel, Na conduction inhibited, depolarization depressed
How is the effect of LA different in healthy vs. unhealthy tissue?
In healthy tissue the pH is ~7.4 so the LA can penetrate the nerves; In unhealthy tissue the pH is acidic, so the LA cannot penetrate the nerves and remains toxic in tissue
What are the two general types compounds used as local anesthetics?
Amino esters, and amino amides
How does LA distribute in the tissue?
It accumulates in highly perfused tissue (especially muscle), can cross the blood-brain barrier and placenta
What metabolizes the LA compounds? (esters and amides)
Esters are metabolized in blood plasma by pseudocholinesterases; Amides are metabolized in liver
How does methemoglobinemia occur?
It happens if the amino amide compound cannot be properly metabolized, and accumulates in the tissue.
How are local anesthetics excreted from the body?
They’re excreted in the urine via the kidneys
*renal problems raise the risk for toxicity
Which molecular form of the LA compound can diffuse through the nerve membrane?
The basic form (RN) as compared to the acid/ionized form (RNH+)
What are the five amino ester local anesthetics?
Benzocaine, Procain, Chloroprocaine, Cocaine, and Tetracaine
What are the five amino amide local anesthetics?
Bupivacaine, Lidocaine, Articaine, Mepivocaine, and Prilocaine
*acronym B-LAMP
Why are vasoconstrictors important?
They’re crucial because they cause the vessel to constrict, allowing the LA to have longer and better action on the nerve
What are the three common vasoconstrictors added to LAs?
Epinephrine, Norepinephrine, and Levonordefrin
**All catecholamines, and all act on adrenergic receptors of vascular smooth muscle
