A seizure induced by concurrent lidocaine-tocainide therapy--is it just a case of additive toxicity?

A 79-year-old white male was admitted to the hospital for treatment of a right-lower-lobe pneumonia. His past medical history included: mild congestive heart failure, asymptomatic ventricular tachycardia, and ethanol abuse. He was initially treated with furosemide for his heart failure, lidocaine for his arrhythmias, and Bactrim for his pneumonia. On day 13 of hospitalization he experienced a tonic-clonic seizure during the time he was being converted from lidocaine to tocainide. At the time of the seizure both tocainide and lidocaine were well within their respective therapeutic ranges. Since the seizure, the patient has tolerated treatment with each drug separately, and at serum concentrations similar to those preceding the seizure, without neurological complications, indicating the possibility of a tocainide-lidocaine induced seizure.