ESTERS
Cocaine hydrochloride remains
useful primarily be-cause of the vasoconstriction it provides with topical use.
Toxicity prohibits its use for other than topical anesthesia.
Cocaine has a rapid onset of
action (1 minute) and a duration of up to 2 hours, depending on the dose or
con-centration. Lower concentrations are used for the eye, while the higher
ones are used on the nasal and pha- ryngeal mucosa. Epinephrine plus cocaine,
although still used occasionally, is hazardous because the cate-cholamine
potentiates the cardiovascular toxicity (e.g., arrhythmia, ventricular
fibrillation) of cocaine.
Cardiovascular effects are
related to both central and peripheral sympathetic stimulation. Initial
brady-cardia appears to be related to vagal stimulation; this is followed by
tachycardia and hypertension. Larger doses are directly depressant to the
myocardium, and death results from cardiac failure.
Cocaine is readily absorbed
from mucous mem-branes, so the potential for systemic toxicity is great. The CNS
is stimulated, and euphoria and cortical stim-ulation (e.g., restlessness,
excitement) frequently result. Overdosage leads to convulsions followed by CNS
de-pression. The cortical stimulation it produces is respon-sible for the
drug’s abuse .
Benzocaine is a PABA
derivative used primarily for topical application to skin and mucous membranes.
Its low aqueous solubility allows it to stay at the site of ap-plication for
long periods. Its minimal rate of absorption after topical administration is associated
with a low in-cidence of systemic toxicity. Benzocaine is contraindi-cated in
patients with known sensitivity to ester-linked anesthetics or PABA-containing
compounds.
Chloroprocaine hydrochloride
(Nesacaine) is obtained from addition
of a chlorine atom to procaine, which re-sults in a compound of greater potency
and less toxicity than procaine itself. This local anesthetic is hydrolyzed
very rapidly by cholinesterase and therefore has a short plasma half-life.
Because it is broken down rapidly, chloroprocaine is commonly used in
obstetrics. It is be-lieved that the small amount that might get to the fetus
continues to be rapidly hydrolyzed, so there may be no residual effects on the
neonate.
Procaine hydrochloride (Novocain) is readily hy-drolyzed by
plasma cholinesterase, although hepatic metabolism also occurs. It is not
effective topically but is employed for infiltration, nerve block, and spinal
anesthesia. It has a relatively slow onset and short (1 hour) duration of action.
All concentrations can be combined with epinephrine. It is available in dental
car-tridges with phenylephrine as the vasoconstrictor.
Tetracaine hydrochloride (Pontocaine) is an ester of PABA that is
an effective topical local anesthetic agent and also is quite commonly used for
spinal (subarach-noid) anesthesia. Epinephrine is frequently added to prolong
the anesthesia. Tetracaine is considerably more potent and more toxic than
procaine and cocaine. It has approximately a 5-minute onset and 2 to 3 hours of
action.
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