Drug Information: Phentolamine
Detailed information regarding Phentolamine, a medication commonly used in clinical settings. Phentolamine is a non-selective alpha-adrenergic antagonist. It works by blocking alpha-1 and alpha-2 adrenergic receptors, resulting in vasodilation and a decrease in blood pressure. Its primary use is in the management of hypertensive emergencies, particularly those caused by pheochromocytoma. It is also used to prevent or treat dermal necrosis following extravasation of adrenergic drugs like norepinephrine or dopamine. Phentolamine’s ability to reverse vasoconstriction makes it a valuable tool in specific clinical scenarios.
Category
Antihypertensive
Mechanism of Action
Phentolamine functions as a non-selective alpha-adrenergic antagonist. This means it blocks both alpha-1 (α1) and alpha-2 (α2) adrenergic receptors. Alpha-adrenergic receptors are located throughout the body, primarily in blood vessels. When activated by catecholamines like norepinephrine and epinephrine, α1 receptors cause vasoconstriction (narrowing of blood vessels), while α2 receptors, primarily located presynaptically, inhibit further norepinephrine release.
By blocking α1 receptors, phentolamine prevents vasoconstriction, leading to vasodilation and a decrease in peripheral resistance, ultimately lowering blood pressure. The blockade of α2 receptors further contributes to vasodilation. Normally, α2 receptors act as a negative feedback mechanism, inhibiting norepinephrine release. By blocking these receptors, phentolamine allows for increased norepinephrine release, which paradoxically can enhance vasodilation in certain circumstances due to the increased stimulation of beta-adrenergic receptors (primarily β2) in blood vessels, which cause vasodilation. However, the overall effect is a reduction in blood pressure due to the dominant α1 blockade. The non-selective nature of phentolamine leads to a more pronounced effect compared to selective α1 blockers.
Clinical Uses
Phentolamine’s primary clinical use is in the management of hypertensive crises, particularly those associated with pheochromocytoma, a tumor of the adrenal gland that secretes excessive amounts of catecholamines (epinephrine and norepinephrine). It is also used to control hypertension caused by interactions between monoamine oxidase inhibitors (MAOIs) and tyramine-containing foods or medications.
Furthermore, phentolamine is employed to prevent or treat dermal necrosis and ischemia following the extravasation (leakage) of vasoconstricting agents like norepinephrine, epinephrine, or dopamine. In this setting, it is injected locally into the affected area to counteract the vasoconstriction and improve blood flow, preventing tissue damage. It has also been used to treat hypertensive emergencies related to clonidine withdrawal. Phentolamine can also be used to diagnose pheochromocytoma with the phentolamine test which has largely been replaced by biochemical testing.
Dosage
Adults:
- Pheochromocytoma:
- Preoperative: 5 mg IV or IM, 1-2 hours before surgery; repeat as needed.
- Intraoperative: 5 mg IV bolus as needed to control blood pressure elevations.
- Extravasation: Inject 0.5-1 mL of a 5-10 mg/mL solution subcutaneously into the area of extravasation within 12 hours.
Pediatrics:
- Pheochromocytoma:
- Preoperative: 0.05-0.1 mg/kg/dose IV or IM, 1-2 hours before surgery. Maximum dose 5mg.
- Intraoperative: 0.05-0.1 mg/kg/dose IV bolus as needed to control blood pressure elevations. Maximum dose 5mg.
- Extravasation: Inject 0.1-0.2 mL of a 5-10 mg/mL solution subcutaneously into the area of extravasation within 12 hours.
Indications
Common indications for phentolamine include:
- Hypertensive crisis secondary to pheochromocytoma.
- Prevention and treatment of dermal necrosis following extravasation of vasopressors.
- Management of hypertensive emergencies due to drug interactions (e.g., MAOI interactions).
- Hypertension associated with clonidine withdrawal.
Contraindications
Phentolamine is contraindicated in:
- Patients with a known hypersensitivity to phentolamine or any of its components.
- Patients with coronary artery disease, angina pectoris, or a history of myocardial infarction.
Caution should be exercised when administering phentolamine to patients with gastritis or peptic ulcer disease.
FAQ
- Q: What is phentolamine used for?
A: Phentolamine is primarily used to treat hypertensive crises, particularly those caused by pheochromocytoma. It’s also used to prevent tissue damage from vasopressor extravasation. - Q: How does phentolamine work?
A: Phentolamine blocks alpha-1 and alpha-2 adrenergic receptors, causing vasodilation and reducing blood pressure. - Q: What are the common side effects of phentolamine?
A: Common side effects include hypotension, tachycardia, and gastrointestinal disturbances like nausea and vomiting. - Q: Can phentolamine be used in children?
A: Yes, phentolamine can be used in children, but the dosage needs to be carefully calculated based on their weight and the specific indication. - Q: How is phentolamine administered for vasopressor extravasation?
A: It is injected locally into the area of extravasation to counteract the vasoconstriction. - Q: What should I do if I experience dizziness after receiving phentolamine?
A: Dizziness is a sign of low blood pressure. You should lie down and notify your healthcare provider immediately. - Q: Is phentolamine safe for patients with heart conditions?
A: Phentolamine is contraindicated in patients with coronary artery disease, angina, or a history of myocardial infarction. It should be used with caution in patients with other heart conditions. - Q: Can phentolamine be used during pregnancy?
A: The safety of phentolamine during pregnancy has not been fully established. It should only be used if the potential benefit justifies the potential risk to the fetus. Consult with a healthcare professional.
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