A 1-year-old 20.2-kg neutered male mixed-breed dog was presented to the emergency service at the BluePearl Pet Hospital in New Braunfels, Texas, because of acute collapse and lethargy. On presentation, the dog was quiet, dull, and responsive. Cardiac auscultation revealed severe tachycardia (350 beats/min) with a regular rhythm and no murmur. Bronchovesicular sounds were considered normal. Femoral pulses were weak, and frequent pulse deficits were detected. A lead II ECG tracing was obtained (Figure 1).
Initial lead II ECG tracing obtained from a 1-year-old mixed-breed dog following an episode of acute collapse and