This heart sound demonstrates a regular and rapid heart rate (approximately 210-220 bpm) in a 9-year-old Labrador with a supraventricular tachycardia. An S3 gallop sound, as well as a murmur associated with mitral insufficiency, is also audible. In addition to development of the SVT, likely due to atrial enlargement, this patient also had degenerative mitral valve disease and volume overload. It would be impossible to distinguish a supraventricular versus ventricular tachycardia based on auscultation alone; thus an ECG would give a definitive diagnosis.