Obstruction to pulmonary blood flow as a result of neoplasia in the right ventricular outflow tract is described in two dogs. Whereas one dog had exertional syncope and a systolic ejection murmur, the other had signs of congestive failure and hypoxia. In both animals the mass was detected in the right ventricle with two-dimensional echocardiography and confirmed angiographically. Although rare, primary right ventricular neoplasia represents a potentially treatable form of cardiac disease and should be considered as a cause of acquired outflow tract obstruction. (Journal of Veterinary Internal Medicine 1990; 4:12-16) PRIMARY tumors of the heart, although uncommon, occur with sufficient frequency in dogs that clinicians should be aware of the manifestations of cardiac neoplasia and should consider neoplasia in the differential diagnosis of heart disease.'.' However, the clinical manifestations of cardiac neoplasia are highly variable making antemortern diagnosis difficult. This study describes the clinical, echocardiographic, hemodynamic, and angiographic features of primary right ventricular neoplasia in two dogs. In both dogs the tumor produced obstruction of the right ventricular outflow tract. Case Reports Dog IAn eight-year-old, spayed female Bassett Hound cross weighing 30 kg was referred to the University of Tennessee Veterinary Teaching Hospital for evaluation and treatment of anasarca and dyspnea of two weeks duration. The dyspnea and edema had improved with furosemide therapy. Tests for circulating microfilaria and heartworm antigen were negative.During physical examination the heart rate was slightly elevated ( 180 beats per minute), but auscultation of the heart was otherwise normal. The jugular veins were distended. and the liver was palpably enlarged. The dog was tachypneic (60 breaths per minute).An electrocardiogram showed abnormally deep S waves in leads 11, 111, aVF, CV6LL, and CV6LU and a right axis deviation in the frontal plane (mean electrical axis, 260"). These ECG changes were suggestive of right ventricular enlargement.Survey thoracic radiographs revealed an enlarged right heart and caudal vena cava. The main pulmonary artery and peripheral pulmonary vasculature were unremarkable. The dog's packed cell volume and red blood cell count (RBC) were high normal (PCV = 5096, RBC = 8.66 X 106/pl), and there was a slight increase of the alkaline phosphatase (189 units/L). The central venous pressure was 20 cm HzO (normal < 7 cm HzO) indicating right heart failure.Two-dimensional and M-mode echocardiographic examination* demonstrated normal left heart structure and function. However, the right atrium and right ventricle were eccentrically hypertrophied, and there was an echodense mass located within the right ventricular outflow tract (Fig. I). The mass was best seen using left cranial and left caudal long-axis views.3Right heart catheterization revealed right ventricular systolic hypertension consistent with right ventricular outflow obstruction or pulmonary hypertension (peak systolic pressure = ...