Chapter: Modern Pharmacology with Clinical Applications: Local Anesthetics

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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.

ESTERS

Cocaine

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

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

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

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

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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