Drug Interactions between Lopressor and Phenergan VC

MONITOR: A case report suggests that beta-blockers may enhance the pressor response to phenylephrine. The proposed mechanism involves blockade of beta-2 adrenergic receptors in the peripheral vasculature, resulting in unopposed alpha-adrenergic effect of phenylephrine that is responsible for vasoconstriction. Additionally, beta-blockers may desensitize baroreceptors that normally modulate heart rate in response to blood pressure elevations by increasing vagal activity on the sinoauricular node. In the case report, a woman with a history of hypertension treated with hydrochlorothiazide (50 mg twice a day) and propranolol (40 mg four times a day) developed sudden bitemporal pain and became unconscious shortly after she was given one drop of a 10% phenylephrine solution in each eye during an ophthalmic examination. She subsequently died of intracerebral hemorrhage due to rupture of a berry aneurysm. The authors noted that the patient had received the same eye drop without incident on two previous occasions when she was not receiving blood pressure or other medications. Nevertheless, an interaction between phenylephrine and beta-blockers is not well established. Phenylephrine acts predominantly on alpha-adrenergic receptors and has little or no direct effect on beta-2 adrenergic receptors, although it may affect them indirectly by enhancing release of norepinephrine from adrenergic nerve terminals. In a study of 12 patients with hypertension, mean phenylephrine doses required to increase systolic blood pressure by 25 mmHg were not significantly different following 2 weeks on propranolol, metoprolol, and placebo (4.8 mcg/kg, 4.7 mcg/kg, and 5.3 mcg/kg, respectively). Baroreceptor-mediated decreases in heart rate during phenylephrine infusion were also in the same range on propranolol, metoprolol, and placebo over baseline heart rate values. In another study, no changes in blood pressure or heart rate were observed in hypertensive patients treated with metoprolol who were given 0.5 to 4 mg doses of phenylephrine intranasally every hour up to a total of 7.5 to 15 mg, or 4 to 30 times the usual recommended dose, compared to placebo or baseline values. These results support the lack of a significant interaction between beta-blockers and phenylephrine.

MANAGEMENT: Until more information is available, caution should be exercised when phenylephrine is used in combination with beta-blockers including ophthalmic formulations, which may be systemically absorbed and can produce clinically significant systemic effects even at low or undetectable plasma levels. Monitoring of blood pressure should be considered, particularly when phenylephrine is administered intravenously or intraocularly. Although an interaction is not likely to occur with cardioselective beta-blockers, caution may be advisable when high dosages are used, since cardioselectivity is not absolute and may be lost with larger doses. A beta-blocker such as propranolol may be used to treat cardiac arrhythmias that occur during administration of phenylephrine.

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MONITOR: Phenothiazines, tricyclic antidepressants (TCAs), and some antipsychotic (neuroleptic) agents may potentiate the blood pressure lowering capabilities of other drugs with hypotensive effects due to their peripheral alpha-1 adrenergic blocking activity. Orthostatic hypotension and syncope associated with vasodilation may occur, particularly during initial dosing and/or parenteral administration of the phenothiazine, TCA, or neuroleptic. The severity of this interaction may be affected by the agent's affinity for the alpha-1 adrenoceptor. One in vitro study demonstrated an affinity for the alpha-1 adrenoceptor for some of these medications that was similar to, or greater than, those of alpha blocker medications used to treat hypertension. Examples of drugs evaluated in this study with a high affinity included amitriptyline, clomipramine, chlorpromazine, clozapine, doxepin, flupenthixol, lurasidone, nortriptyline, perphenazine, paliperidone, quetiapine, risperidone, sertindole, and ziprasidone. On the other hand, examples of those with lower affinities included aripiprazole, lofepramine, protriptyline, sulpiride, and amisulpride.

MANAGEMENT: Close clinical monitoring for development of hypotension is recommended if phenothiazines, tricyclic antidepressants (TCAs), or certain antipsychotic (neuroleptic) agents are used in patients receiving antihypertensive medications or vasodilators. A lower starting dosage and slower titration of the phenothiazine, TCA, or neuroleptic may be appropriate, especially in the elderly. It may also be advisable to consider using a phenothiazine, TCA, or neuroleptic medication with a lower affinity for the alpha-1 adrenoceptor when possible. Patients should be counseled to avoid rising abruptly from a sitting or recumbent position and to notify their healthcare provider if they experience dizziness, lightheadedness, syncope, orthostasis, or tachycardia. Patients should also avoid driving or operating hazardous machinery until they know how the medications affect them.

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