The most commonly used vasoconstrictors, the sympa-thomimetic drugs, are often added to local anesthetics to delay absorption of the anesthetic from its injection site. By slowing absorption, these drugs reduce the anesthetic's systemic toxicity and keep it in contact with nerve fibers longer, thereby increasing the drug's duration of action. Administration of lidocaine 1% with ep-inephrine results in the same degree of blockade as that produced by lidocaine 2% without the vasoconstrictor.
Many vasoconstrictors are available, but epineph-rine is by far the most commonly employed. Because epinephrine can have systemic a- and (3-adrenergic effects, precaution is needed when local anesthetics containing this amine are given to a patient with hypertension or an irritable myocardium. Sensitivity to epi-nephrine may be incorrectly diagnosed as an allergy to local anesthetics. Epinephrine-containing solutions should be used cautiously in persons taking tricyclic an-tidepressants or monoamine oxidase (MAO) inhibitors, since those drugs may enhance the systemic pressor effects of sympathomimetic amines.
Levonordefrin (Neo-Cobefrin) is an active optical isomer of nordefrin that has ^-adrenergic activity and possesses little or no (3-agonist properties. It is used exclusively in some dental anesthetic cartridges as a vasoconstrictor. Its theoretical advantage is that it causes less hypertension and tachycardia than does epinephrine.
Phenylephrine hydrochloride (Neo-Synephrine) is a pure a-agonist that is occasionally used for subarachnoid block and is marketed with procaine for use in dentistry. It has little direct cardiac effect.
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