Phenylephrine dosage

A 22-year-old African-American with chronic undifferentiated schizophrenia developed priapism after taking risperidone 4 mg bd, clonazepam 0.5 mg bd, vitamin E 400 IU bd, and a multivitamin for over 6 months. He did not respond to subcutaneous terbuta-line 0.25 mg. Irrigation of the corpora with phenylephrine 200 p resulted in detumescence risperidone was withdrawn. A few months later he took ziprasidone 20 mg bd for 1 week, clonazepam 1 mg bd, and vitamin E 400 IU bd. The ziprasidone dosage was increased to 40 mg bd, but early the next morning he developed a firm erection with some discomfort that lasted about 2 hour and resolved when he urinated the next morning he had a similar erection that also lasted 2 hour and resolved. 

The threshold dosage of phenylephrine in the average adult has been estimated to be 0.4 mg intravenously, 2 mg subcutaneously, and 50 mg orally. The upper limit for safe dosage in normal adults is approximately 1.5 mg intravenously and 300 mg subcutaneously. Because a 50-ml drop of 10% phenylephrine contains 5 mg of drug, multiple applications can result in overdosage, especially if absorption from the site of administration is enhanced or if the patient is compromised by age, body size, use of concomitant medications, or trauma. Furthermore, the extent of the absorption into the systemic circulation of topically applied phenylephrine is unknown because absorption has been shown to be possibly diminished due to local vasoconstriction. 

The authors commented that changes in arterial blood pressure are well described with phenylephrine eye-drops, especially in infants. Qearly precise dosage is difficult in these very young patients and they suggested that microdrops might be a safer mode of administration. 

Despite its purported use in refractory septic shock, very little information is published regarding the clinical efficacy of phenylephrine. Nevertheless, it is an attractive agent for use in sepsis owing to its selective a-agonism and primarily vascular effects and its rapid onset < and short duration of action. It is generally initiated at dosages of 0.5 mcg/kg per minute and may be titrated quickly to desired effect. 

M. Knochen, J. Giglio, Flow-injection determination of phenylephrine hydrochloride in pharmaceutical dosage forms with on-line solid-phase extraction and spectrophotometric detection, Talanta 64 (2004) 1226. 

Schieffer, G.W. Smith, W.O. Lubey, G.S. Newby, D.G. Determination of the structure of a synthetic impurity in guaifenesin modification of a high-performance liquid chromatographic method for phenylephrine hydrochloride, phenylpropanolamine hydrochloride, guaifenesin, and sodium benzoate in dosage forms. J.Pharm.Sci., 1984, 73, 1856-1858... 

Schieffer, G.W. Hughes, D.E. Simultaneous stability-indicating determination of phenylephrine hydrochloride, phenylpropanolamine hydrochloride, and guaifenesin in dosage forms by reversed-phase paired-ion high-performance liquid chromatography. J.Pharm.Sci., 1983, 72, 55-59... 

The shrinking of mucous membranes decreases operative bleeding while improving surgical visualization. Comparable vasoconstriction can be achieved with other local anesthetics by the addition of a low concentration of a vasoconstrictor such as phenylephrine (0.005%). Epinephrine, topically applied, does not prolong the duration of action of local anesthetics applied to mucous membranes because of poor penetration. Maximal safe total dosages for topical anesthesia in a healthy 70-kg adult are 300 mg for lidocaine, 150 mg for cocaine, and 50 mg for tetracaine. 

These are established interactions, but the paucity of clinical information suggests that in practice they do not present many problems, perhaps because the effects of these vasopressors are so closely monitored, and titrated to effect. If a pressor drug is required, a directly-acting drug such as noradrenaline (norepinephrine) or phenylephrine may be expected to be effective. The receptors may show some supersensitivity so that a dosage... 

The interaction between the MAOIs and oral phenylephrine is established, serious and potentially life-threatening. Phenylephrine commonly occurs in oral non-prescription cough, cold and influenza preparations, so patients should be strongly warned about them. Whether the effects of nasal drops and sprays and eye drops are also enhanced is uncertain, but it would be prudent to avoid them until they have been shown to be safe. The response to parenteral administration is also approximately doubled, so that a dosage reduction is necessary. 

Dose responsiveness In a study of the equivalently effective doses of ephedrine and phenylephrine in preventing hypotension after spinal anesthesia for cesarean section, the effective dosage ratio between ephedrine and phenylephrine was 80 1 (95% Cl =73, 90) the mean effective total doses were phenylephrine 500 micrograms and ephedrine 40 mg. 

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