ECG of the Month

Abstract

A 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 initial evaluation. Thoracic auscultation revealed an irregularly irregular tachyarrhythmia with associated femoral pulse deficits. No murmurs were detected, and bronchovesicular sounds were considered normal. A history of bird hunting, the last episode of which occurred > 1 year earlier, was reported. Vaccination status was current, and the dog was regularly receiving a heartworm preventative.Diagnostic procedures included thoracic radiography; echocardiography; ECG; systemic blood pressure evaluation; CBC; serum biochemical analyses; titer assessment of serum antibodies against Trypanosoma cruzi via an immunofluorescent antibody test; and measurements of serum concentrations of taurine, thyroid hormones (total thyroxine, free thyroxine [determined via equilibrium dialysis], and thyroid-stimulating hormone), and cardiac troponin I. Findings of thoracic radiography included mild generalized cardiomegaly with a mildly large left atrium, pulmonary venous congestion, and a mild unstructured interstitial pattern in the lungs. Echocardiographic findings included left ventricular internal dimensions indicative of dilatation, left ventricular systolic dysfunction, a mild centrally located jet of mitral valve regurgitation, a moderately large left atrium, and mild low-velocity tricuspid valve regurgitation. The mitral and tricuspid valves appeared normal. Indirect systemic blood pressure measured by use of an oscillometric device was within reference limits. Results of serum biochemical analyses and a CBC were unremarkable. The dog was seronegative for T cruzi; serum taurine and thyroid hormone concentrations were within reference limits. Serum cardiac troponin I concentration was increased at 3.86 ng/mL (reference range, < 0.2 ng/mL). ECG InterpretationElectrocardiographic findings during the initial examination included an irregularly irregular wide and bizarre QRS tachycardia, conducted with a right bundle branch block (RBBB) morphology, and all QRS complexes were conducted with an RS complex morphology (Figure 1). Instantaneous heart rates varied from 55 to 188 beats/min. The RS complex appearance varied depending on heart rate. With heart rates > 97 beats/min, the RS complexes were conducted with an RBBB (RS complex duration, 100 milliseconds). More-narrow (40 milliseconds in duration), upright RS complexes occurred at heart rates < 62 beats/min, suggesting more-n...