In this ECG of a Boxer, the first 24 complexes are a fast ventricular tachycardia (320 bpm). When the ventricular tachycardia terminates, the dog goes into either a sinus tachycardia or a supraventricular tachycardia (7 complexes) and then a sinus rhythm with ventricular bigeminy (every other complex is a PVC).