Phentolamine is a medication used in acute medical care as an antidote for extravasation, a serious complication. Extravasation occurs when a medication leaks out of a vein and into the surrounding soft tissue, causing potential damage. When the leaked medication is a vasopressor (a drug that constricts blood vessels), phentolamine is administered as an emergency treatment to prevent severe tissue injury.
Understanding Extravasation
Extravasation is the accidental infiltration of an intravenous drug into the subcutaneous space (tissue beneath the skin) outside the intended blood vessel. When the leaked substance is a vesicant, capable of causing blistering and severe tissue damage, the event is defined as a serious injury. Phentolamine is specifically used to treat extravasation caused by vasoconstrictive agents, which are drugs that tighten blood vessels.
The medications requiring phentolamine are primarily vasopressors, such as norepinephrine, dopamine, and epinephrine, often used to raise a patient’s blood pressure in critical conditions. These drugs cause tissue damage because they are potent vasoconstrictors, severely narrowing the blood vessels at the site of the leak. This extreme local constriction cuts off blood flow, leading to tissue blanching, coldness, and a lack of oxygen and nutrients, a condition called ischemia.
If ischemia is not rapidly reversed, the affected tissue will die, resulting in necrosis, which may require surgical intervention or amputation. Phentolamine is not a universal antidote and is ineffective for extravasation caused by irritant drugs that damage tissue through other mechanisms, such as high or low pH or direct chemical toxicity.
Phentolamine’s Mechanism of Action
Phentolamine functions as a non-selective alpha-adrenergic blocker, interfering with certain signals in the nervous system. Vasopressors cause their detrimental effect by binding to and activating alpha-adrenergic receptors located on the walls of blood vessels. When these receptors are stimulated, they trigger muscle cells to contract, resulting in intense vasoconstriction.
Phentolamine works by binding to these alpha-receptors, blocking the extravasated vasopressor drug from attaching. By occupying the receptor sites, phentolamine prevents the constricting signal from being transmitted. This blockade instantly reverses the localized spasm, allowing the blood vessels to relax and widen (vasodilation).
Vasodilation restores blood flow to the ischemic tissue, flushing the trapped vasopressor out and delivering oxygen and nutrients. This reversal is visible as the blanched tissue quickly “pinks up” and warms, demonstrating the restoration of perfusion. The swift return of circulation prevents the progression from ischemia to irreversible tissue necrosis.
Clinical Protocol for Phentolamine Administration
The treatment protocol for vasopressor extravasation begins with immediate recognition and action. The first step is to stop the infusing medication immediately, but the catheter or needle should not be removed right away. A syringe is then attached to the line to gently aspirate as much of the leaked drug from the subcutaneous tissue as possible.
Following aspiration, the injury site should be clearly marked with a pen to track the extent of the damage and monitor treatment success. Phentolamine must be prepared for subcutaneous injection by diluting the available medication (typically 5 to 10 milligrams) in 10 milliliters of 0.9% sodium chloride (normal saline). The goal is a final concentration of approximately 0.5 to 1 milligram per milliliter.
The diluted solution is then injected subcutaneously using a fine-gauge needle (e.g., 25- or 27-gauge). The dose is divided into multiple small aliquots and injected circumferentially around the entire blanched and hardened area of extravasation. This technique ensures the antidote is spread throughout the affected area to maximize alpha-blockade and reverse vasoconstriction in all compromised tissue.
Phentolamine is most effective when administered as soon as extravasation is noted, ideally within the first hour, though benefit may be seen up to 12 hours after the event. After the injection, the affected limb should be elevated to encourage venous return. A dry, warm compress should be applied to the site, which assists the drug’s action by promoting further vasodilation. The site must be continuously monitored, and if blanching or signs of ischemia return, a second dose of phentolamine may be necessary.