PHENOXYBENZAMINE HYDROCHLORIDE

PHENOXYBENZAMINE HYDROCHLORIDE
(fen-ox-ee-ben'za-meen)
Dibenzyline
Classifications: alpha-adrenergic antagonist; antihypertensive agent;
Therapeutic: antihypertensive

Prototype: Prazosin
Pregnancy Category: C

Availability

10 mg capsules

Action

Long-acting alpha-adrenergic antagonist. Apparently produces noncompetitive blockade of alpha-adrenergic receptor sites at postganglionic synapse. Alpha-receptor sites are thus unable to react to the endogenous or exogenous sympathomimetic agents epinephrine and norepinephrine.

Therapeutic Effect

Blocks excitatory effects of epinephrine, including vasoconstriction, but does not affect adrenergic cardiac inhibitory actions. It produces a "chemical sympathectomy" and it can maintain it.

Uses

Management of pheochromocytoma.

Unlabeled Uses

To improve circulation in peripheral vasospastic conditions such as Raynaud's acrocyanosis and frostbite sequelae, for adjunctive treatment of shock, hypertensive crisis.

Contraindications

Instances when fall in BP would be dangerous; pregnancy (category C), lactation.

Cautious Use

Marked cerebral or coronary arteriosclerosis, compensated congestive heart failure, coronary artery disease; older adults; renal insufficiency; respiratory infections.

Route & Dosage

Management of Pheochromocytoma
Adult: PO 10 mg b.i.d., may increase by 10 mg/d at 4-d intervals to desired response (usual range 20–40 mg/d in 2–3 divided doses)
Child: PO 0.2 mg/kg/d, may increase by 0.2 mg/kg/d to desired response (usual range 0.4–1.2 mg/kg/q6–8h)

Administration

Oral
  • Give with milk or in divided doses to reduce gastric irritation.
  • Preserve in airtight containers protected from light.

Adverse Effects (≥1%)

Body as a Whole: Dizziness, fainting, drowsiness, sedation, tiredness, weakness, lethargy, confusion, headache, shock. CNS: CNS stimulation (large doses). CV: Postural hypotension, tachycardia, palpitation. GI: Dry mouth. Urogenital: Inhibition of ejaculation. Respiratory: Nasal congestion. Skin: Allergic contact dermatitis. Special Senses: Miosis, drooping of eyelids.

Interactions

Drug: Inhibits effects of methoxamine, norepinephrine, phenylephrine; additive hypotensive effects with antihypertensives.

Pharmacokinetics

Absorption: Variably (approximately 30%) from GI tract. Onset: 2 h. Peak: 4–6 h. Duration: 3–4 d. Distribution: Accumulates in adipose tissue. Elimination: 80% in urine and bile within 24 h. Half-Life: 24 h.

Nursing Implications

Assessment & Drug Effects

  • Monitor BP and note pulse quality, rate, and rhythm in recumbent and standing positions during period of dosage adjustment. Observe patient closely for at least 4 d from one dosage increment to the next; hypotension and tachycardia are most likely to occur in standing position.
  • Drug has cumulative action, thus onset of therapeutic effects may not occur until after 2 wk of therapy, and full therapeutic effects may not be apparent for several more weeks.

Patient & Family Education

  • Make position changes slowly, particularly from reclining to upright posture, and dangle legs and exercise ankles and feet for a few minutes before standing.
  • Be aware that light headedness, dizziness, and palpitations usually disappear with continued therapy but may reappear under conditions that promote vasodilation, such as strenuous exercise or ingestion of a large meal or alcohol.
  • Pupil constriction, nasal stuffiness, and inhibition of ejaculation generally decrease with continued therapy.
  • Do not take OTC medications for coughs, colds, or allergy without approval of physician. Many contain agents that cause BP elevation.

Common adverse effects in italic, life-threatening effects underlined; generic names in bold; classifications in SMALL CAPS; Canadian drug name; Prototype drug

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