A 5.0-kg (11.0-lb) 3-year-old sexually intact male Shih Tzu was referred for investigation of an enlarged heart that had been observed on routine thoracic radiographs by the referring veterinarian. When the patient was a puppy, medical records from the referring veterinarian indicated that a loud left-sided heart murmur was auscultated (murmur timing was not documented). This heart murmur decreased in intensity and was no longer present by 6 months of age. The patient had a recent history of gastrointestinal upset, but had been otherwise apparently healthy.
On physical examination, the patient was bright, alert, and responsive, with a heart