A 15-year-old 15-kg male English Setter was evaluated because of polyuria, polydipsia, exercise intolerance, and intermittent panting for the preceding 3 weeks. The owners reported that episodes of panting were not related to either physical activity or emotional stress and that the dog had collapsed during a walk. The patient was an indoor dog that was up to date with vaccinations and parasite prevention and had no known exposure to toxic agents or human medications.
On initial examination, the dog was bright and alert. Cardiac auscultation revealed a grade 2/6 left apical systolic murmur; the heart rate was 140