Dopamine and Phenytoin: Interaction Details

a) Five patients whose blood pressure was stable while on DOPamine infusion developed hypotension and/or cardiac arrest after an intravenous infusion of phenytoin. All patients were critically ill, had multiple system failure and all required intravenous DOPamine therapy for blood pressure support at a time when volume restoration was clinically adequate. In each case, prior to the phenytoin infusion, there was documented seizure activity. The intravenous phenytoin loading dose was 11 mg/kg at a rate of 25 mg/min. Similar results were obtained in dog studies. A possible mechanism is that DOPamine is known to prevent uptake at sympathetic storage sites and increase the synthesis of norepinephrine, while phenytoin exerts a greater myocardial depressant effect in the face of catecholamine depletion ¹.

b) A patient receiving DOPamine did not experience changes in blood pressure when phenytoin was administered. A 45-year-old male being treated with DOPamine 7 mcg/kg/min for three days experienced a tonic-clonic seizure and a drop in his blood pressure. Dobutamine was instituted at 5 mcg/kg/min to increase his blood pressure, and a loading dose of phenytoin 250 mg was administered intravenously. Phenytoin was then maintained at 750 mg daily by continuous infusion. This therapy was continued for seven days without changes while the patient's blood pressure fluctuated between 110 and 140 mm Hg systolic and 60 and 80 mm Hg diastolic. One possible explanation for the lack of a hypotensive response to phenytoin in this patient may be the addition of the dobutamine infusion. Because dobutamine causes an increase in cardiac output, it may have counteracted the myocardial depressant effects of phenytoin ².