4
24. Which of the following anesthetic drugs produces powerful
stimulation of the cerebral cortex?
a. Cocaine
b. Procaine
c. Lidocaine
d. Tetracaine
e. Mepivacaine
(a) see explanation above
Questions regarding mechanism of action:
25. Local anesthetics block nerve conduction by
a. Depolarizing the nerve membrane to neutrality
b. Increasing membrane permeability to K+
c. Increasing membrane permeability to Na+
d. Preventing an increase in membrane permeability to K+
e. Preventing an increase in membrane permeability to Na+
(e) didn’t I make you memorize this? You should at keast
remember Na+ ions are involved, which limits your choices to
(c) and (e). (c) would increase or facilitate nervous impulse
conduction, which is the opposite of what you want the local
anesthetic to do, so pick (e).
26. Which of the following is true regarding the mechanism of action of
local anesthetics?
a. Usually maintain the nerve membrane in a state of
hyperpolarization
b. Prevent the generation of a nerve action potential
c. Maintain the nerve membrane in a state of depolarization
d. Prevent increased permeability of the nerve membrane to
potassium ions
e. Interfere with intracellular nerve metabolism
(b) this should be really obvious!
27. Local anesthetic agents prevent the generation of nerve impulses by
a. Decreasing threshold for stimulation
b. Decreasing resting membrane potential
c. Decreasing inward movement of sodium ion
d. Increasing inward movement of potassium ion
(c) Answer is (c)- straight memorization- nerve impulses are
generated by the influx of sodium resulting in depolarization.
repolarization and inactivity occurs when potassium moves out.
(sodium-potassium pump). LAs act by blocking Na+ movement.
28. Local anesthetics interfere with the transport of which of the following
ions during drug-receptor interaction
a. Sodium
b. Calcium
c. Chloride
d. Potassium
e. Magnesium
(a) see how many different ways they can ask the same
question?
Questions regarding pH effects on absorption of local anesthetics
30. If the pH of an area is lower than normal body pH, the membrane
theory of local anesthetic action predicts that the local anesthetic
activity would be
a. Greater, owing to an increase in the free-base form of the drug
b. Greater, owing to an increase in the cationic form of the drug
c. Less, owing to an increase in the free-base form of the drug
d. Less, owing to a decrease in the free-base form of the drug
e. None of the above
(d) the next three or four questions are all versions of the same
thing – see the explanation below
31. A local anesthetic injected into an inflamed area will NOT give
maximum effects because
a. The pH of inflamed tissue inhibits the release of the free base
b. The drug will not be absorbed as rapidly because of the
decreased blood supply
c. The chemical mediators of inflammation will present a chemical
antagonism to the anesthetic
d. Prostaglandins stabilize the nerve membrane and diminish the
effectiveness of the local anesthetic
(a) while some of the other alternatives sound plausible, think
about the factoids you were taught about local anesthetics and
variables that affect their action. An important one was the role
of pH and ionization factors. Remember, the free base or
nonionized form is the form that passes through membranes,
yet once inside the neuron only the ionized form is effective.
Inflamed tissue has a lower pH than normal tissue and will shift
the equilibrium of the LA solution such that most of it remains
ionized and thus unavailable to penetrate
32. The penetration of a local anesthetic into nervous tissue is a function
of the
a. Length of the central alkyl chain
b. Lipid solubility of the ionized form
c. Lipid solubility of the unionized form
d. Ester linkage between the aromatic nucleus and the alkyl chain
e. Amide linkage between the aromatic nucleus and the alkyl chain
(c) only options (b) and (c) are relevant here - the others have
nothing to do with LA penetration into membranes. Membrane
permeability is affected by whether or not the molecule is
“charged” or ionized or not (e.g., unionized). Only the latter form
passes readily through membranes. See, they’re asking the
same thing they asked in the previous question, just coming at it
from another angle. Remember the fact and you can cover the
angles.
35. At a pH of 7.8, lidocaine (pKa = 7.8) will exist in
a. the ionized form
b. the nonionized form
c. an equal mixture of ionized and nonionized forms
d. a mixture 10 times more ionized than nonionized forms
(c) the ratio of ionized to unionized forms is given by the
formula log A/AH= pH-pKa. In this instance the difference
between pH and pKa is 0. Thus lidocaine will exist as an equal
mixture ( so (c) is correct). Most local anesthetics are weak
bases with pKa ranging from 7.5 to 9.5. LA’s intended for
injection are usually prepared in salt form by addition of HCl.
They penetrate as the unionized form into the neuron where
they re-equilibrate to both charged and uncharged forms inside
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