Phentolamine Mesylate for Injection USP, 5mg

Tachycardia and cardiac arrhythmias may occur with the use of Phentolamine Mesylate for Injection, USP or other alpha-adrenergic blocking agents. When possible, administration of cardiac glycosides should be deferred until cardiac rhythm returns to normal.

See DOSAGE AND ADMINISTRATION, Diagnosis of pheochromocytoma, Preparation.

Long-term carcinogenicity studies, mutagenicity studies, and fertility studies have not been conducted with Phentolamine Mesylate for Injection, USP .

Administration of Phentolamine Mesylate for Injection, USP to pregnant rats and mice at oral doses 24-30 times the usual daily human dose (based on a 60-kg human) resulted in slightly decreased growth and slight skeletal immaturity of the fetuses. Immaturity was manifested by increased incidence of incomplete or unossified calcanei and phalangeal nuclei of the hind limb and of incompletely ossified sternebrae. At oral doses 60 times the usual daily human dose (based on a 60-kg human), a slightly lower rate of implantation was found in the rat. Phentolamine Mesylate for Injection, USP did not affect embryonic or fetal development in the rabbit at oral doses 20 times the usual daily human dose (based on a 60-kg human). No teratogenic or embryo toxic effects were observed in the rat, mouse, or rabbit studies.

There are no adequate and well-controlled studies in pregnant women. Phentolamine Mesylate for Injection, USP should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus.

It is not known whether this drug is excreted in human milk. Because many drugs are excreted in human milk and because of the potential for serious adverse reactions in nursing infants from Phentolamine Mesylate for Injection, USP, a decision should be made whether to discontinue nursing or to discontinue the drug, taking into account the importance of the drug to the mother.

See DOSAGE AND ADMINISTRATION

No deaths due to acute poisoning with Phentolamine Mesylate for Injection, USP have been reported.
Oral LD₅₀'s (mg/kg): mice, 1000; rats, 1250.

Overdosage with Phentolamine Mesylate for Injection, USP is characterized chiefly by cardiovascular disturbances, such as arrhythmias, tachycardia, hypotension, and possibly shock. In addition, the following might occur: excitation, headache, sweating, pupillary contraction, visual disturbances; nausea, vomiting, diarrhea; hypoglycemia.

There is no specific antidote.

A decrease in blood pressure to dangerous levels or other evidence of shock like conditions should be treated vigorously and promptly. The patient's legs should be kept raised and a plasma expander should be administered. If necessary, intravenous infusion of norepinephrine, titrated to maintain blood pressure at the normotensive level, and all available supportive measures should be included. Epinephrine should not be used, since it may cause a paradoxical reduction in blood pressure.

For preoperative reduction of elevated blood pressure, 5 mg of Phentolamine Mesylate for Injection, USP (1mg for children) is injected intravenously or intramuscularly 1 or 2 hours before surgery, and repeated if necessary.

During surgery, Phentolamine Mesylate for Injection, USP (5 mg for adults, 1 mg for children) is administered intravenously as indicated, to help prevent or control paroxysms of hypertension, tachycardia, respiratory depression, convulsions, or other effects of epinephrine intoxication. (Postoperatively, norepinephrine may be given to control the hypotension that commonly follows complete removal of a pheochromocytoma.)

For Prevention: 10 mg of Phentolamine Mesylate for Injection, USP is added to each liter of solution containing norepinephrine. The pressor effect of norepinephrine is not affected.

For Treatment: 5-10 mg of Phentolamine Mesylate for Injection, USP in 10 mL of saline is injected into the area of extravasation within 12 hours.

The test is most reliable in detecting pheochromocytoma in patients with sustained hypertension and least reliable in those with paroxysmal hypertension. False-positive tests may occur in patients with hypertension without pheochromocytoma.

If the intramuscular test for pheochromocytoma is preferred, preparation is the same as for the intravenous test. Five milligrams of Phentolamine Mesylate for Injection, USP is then dissolved in 1mL of Sterile Water for Injection. The dose for adults is 5 mg intramuscularly; for children, 3 mg. Blood pressure is recorded every 5 minutes for 30-45 minutes following injection. A positive response is indicated when the blood pressure is reduced 35 mmHg systolic and 25 mmHg diastolic, or more, within 20 minutes following injection.