These drugs are directly acting adrenomimetic amines that exert their effects primarily through an action on a-adrenoceptors. Consequently, these agents have little or no direct action on the heart. All three drugs increase both systolic and diastolic blood pressures through their vasoconstrictor action. The pressor response is accompanied by reflex bradycardia, no change in the contractile force of the heart, and little change in cardiac output. They do not precipitate cardiac arrhythmias and do not stimulate the CNS.
Phenylephrine is not a substrate for COMT, while metaraminol and methoxamine are not metabolized by either COMT or MAO. Consequently, their duration of action is considerably longer than that of norepineph-rine. Following intravenous injection, pressor responses to phenylephrine may persist for 20 minutes, while pres-sor responses to metaraminol and methoxamine may last for more than 60 minutes.
The clinical uses of these drugs are associated with their potent vasoconstrictor action. They are used to restore or maintain blood pressure during spinal anesthesia and certain other hypotensive states. The reflex bradycardia induced by their rapid intravenous injection has been used to terminate attacks of paroxysmal atrial tachycardia. Phenylephrine is commonly used as a nasal decongestant, although occasional nasal mucosal damage has occurred from injudicious use of the nasal spray. It is also employed in ophthalmology as a mydri-atic agent. Phenylephrine, however, should not be given to patients with closed-angle glaucoma before iridec-tomy, since further increases in intraocular pressure may result. In dentistry, phenylephrine is used to prolong the effectiveness of a local anesthetic.
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