Phentolamine (Regitine) is a pure alpha blocker that may be difficult to find in an ED. Currently it is supplied in powdered form, with 5 mg of phentolamine that is reconstituted with 1 ml saline, with a final concentration of 5 mg/ml. Previous preparations had a different concentration (10 mg/ml), so check your stock. With the 5 mg vial, one is dealing with small volumes with this concentration, and 0.5 ml would contain 2.5 mg of phentolamine. Once reconstituted, phentolamine is then diluted with lidocaine. A 1% lidocaine solution is adequate. If one draws up 0.5 ml of phentolamine and mixes it with 0.5 ml of lidocaine, the resultant 1 ml syringe will contain 2.5 mg of phentolamine. This should be adequate for most cases. For local injection, the exact dose, volume, and dilution are variable, and there are no standard recommendations.
Success has been reported with the local injection of phentolamine in the 1.5- to 3-mg-dose range. No systemic effects should be encountered with these doses. Phentolamine local injection has been used extensively for reversing local vasoconstriction from the extravasations of dopamine and norepinephrine at a peripheral venous site. In these instances, it can avert tissue necrosis and the need for skin grafting.
For fingertip ischemia, it is best to draw up the phentolamine in a 1 ml syringe, then dilute it with 1% lidocaine. Use a 25- to 30-gauge needle. Insert the needle at the puncture site and slowly inject the solution. Multiple injections may be required if the area is large, but usually one works. Because the maximum volume of epinephrine that can be injected from the EpiPen is 0.3 ml, a 1 ml of volume of solution will usually be enough, but there is no maximum dose. Phentolamine itself has no local adverse effects. Theoretically, a compartment syndrome could develop if excessive phentolamine were used, but usually small amounts are rapidly effective so titration is key. Immediate hyperemia signals success, and repeat injections are not necessary.