Phenytoin sodium as a treatment for ventricular dysrhythmia in horses

Five adult horses with ventricular extra systoles (VES) and 2 with ventricular tachycardia (VT) refractory to treatment with rest, anti-inflammatory drugs, lidocaine, or procainamide were treated with phenytoin sodium p.o. q12h. The starting dosage of phenytoin was 20 or 22 mg/kg body weight (BW) q12h, and the maintenance dosage varied from 8 to 17 mg/kg BW q12h. The mean +/- standard deviation therapeutic blood concentration of total phenytoin was 8.8 +/- 2.1 mg/L, and the mean concentration of free phenytoin of 2.5 +/- 0.5 mg/L was relatively constant at a range of 24 to 29% of the total phenytoin concentration. In all horses, both VES and VT were abolished after treatment with phenytoin. On the basis of the results of this clinical study, the authors propose an initial dose of 20 mg/kg BW q12h for the first 3 or 4 dosages, followed by a maintenance dose of 10 to 15 mg/kg BW q12h. Phenytoin plasma concentrations should be monitored during therapy. High plasma concentrations were associated with adverse effects such as recumbency and excitement. In this study, which included a limited number of diverse patients, phenytoin sodium appeared to be an inexpensive and effective treatment for persistent VES or VT in cases where conventional treatment had failed.