Alpha Adrenergic Antagonists (Alpha Blockers)
These drugs selectively block alpha1-adrenergic receptors without affecting alpha2-adrenergic receptors. Examples of agents used in the treatment of hypertension include prazosin, terazosin, indoramin, phenoxybenzamine, phentolamine, tamsulosin, tolazoline, urapidil and doxazosin. Terazosin is also used for treating urinary symptoms from benign prostatic hypertrophy.
Adverse effects include first-dose phenomenon,* dizzi-ness, tachycardia and palpitations. Peptic ulcer is frequently exacerbated by these agents. Headache and asthenia have been reported with therapeutic use of terazosin. Therapeutic use of prazosin has been associated with several cases of paranoia, hallucinations, and confusion. Based on observations from one study, it has been suggested that doxazosin should not be used as monotherapy for management of stage 1 or stage 2 hyper-tension, because of an increase in the incidence of congestive heart failure. Vomiting, abdominal pain, and diarrhoea may occur following large oral doses.
Overdose results in hypotension and CNS depression. Hypotension and reflex tachycardia are the most common manifestations. However, tachycardia may not always occur. Phentolamine may cause acute and prolonged hypotension. Death has occurred following IV administration of phen-tolamine when used in the diagnosis of pheochromocytoma. Seizures have been reported with indoramin overdose. Phenoxybenzamine produces miosis, while phentolamine and tolazoline produce mydriasis. Priapism may occur.
Treatment involves administration of IV fluid boluses, and vasopressors such as dopamine. However, dopamine being primarily a beta- adrenergic agent, has produced fatal cardiac arrest secondary to hypotension, when given with tolazoline. Adrenaline stimulates both alpha and beta receptors. In the presence of alpha blockade, the net effect may be vasodila-tion and worsening of hypotension. Noradrenaline primarily stimulates alpha receptors and is preferable. Priapism is a surgical emergency requiring immediate consultation with a urologist.
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