10-year-old 20-kg (44-lb) spayed female Brittany was examined by a veterinarian because of acute onset of lethargy and inappetence. Tachycardia was detected on physical examination and confirmed via ECG; in all leads, the QRS complexes were wide. Lidocaine hydrochloride (2 mg/kg [0.9 mg/lb]) was administered IV, which slowed the heart rate slightly. The dog was subsequently transferred to a local emergency clinic for supervision overnight. Because of the incomplete response to lidocaine, the dog was referred to the Veterinary Medical Teaching Hospital at Texas A&M University for further evaluation the following morning.
The dog was bright and alert at the