Phenytoin administration by constant intravenous infusion: selective rates of administration. STUDY OBJECTIVES: To determine the adequacy of seizure control and the adverse effects of administering an IV loading dose of phenytoin by constant infusion pump. DESIGN: A prospective study of patients presenting with acute onset of seizures. Patients were divided into two groups. Group 1 comprised all patients 50 years of age or younger without a history of atherosclerotic cardiovascular disease (ASCVD). Group 2 comprised all patients older than 50 years or with a history of ASCVD. SETTING: A rural community hospital emergency department. TYPE OF PARTICIPANTS: Forty-two adult patients. INTERVENTIONS: Both groups received an IV loading dose of phenytoin at 15 mg/kg. Infusion rates were 50 mg/min and 25 mg/min for groups 1 and 2, respectively. Cardiac rhythm and vital signs were monitored throughout and after infusion. MEASURES AND MAIN RESULTS: Group 2 demonstrated significantly more cardiovascular side effects (hypotension and bradycardia) than did group 1 (Fisher's exact test, P less than .05). CONCLUSION: Phenytoin provided adequate seizure control in both groups. For individuals with ASCVD, IV phenytoin administration rates should not exceed 25 mg/min. For individuals without ASCVD, phenytoin administration at 50 mg/min appears safe and without significant cardiovascular side effects. 