Pulmonary hypertension
Pulmonary hypertension is high pressure in the pulmonary artery. In dogs it can be caused by heartworm disease or pulmonary thromboembolism. It can result in right-sided heart disease (cor pulmonale). Signs include difficulty breathing, cyanosis, and exercise intolerance.

Pulmonic stenosis
Pulmonic stenosis is a congenital disease in dogs characterized by right ventricular outflow tract obstruction. Most commonly the narrowing occurs at the pulmonary valve. The most commonly affected breeds include terriers, Bulldogs, Miniature Schnauzers, Chihuahuas, Samoyeds, Beagles, Keeshonds, Mastiffs, and Bullmastiffs. Signs include exercise intolerance, but often there is only a heart murmur.

Sick sinus syndrome
Sick sinus syndrome is most commonly seen in female Miniature Schnauzers. It is characterized by sinoatrial node dysfunction and often includes atrioventricular node disease and bundle branch block. Electrocardiogram findings include sinus bradycardia, sinus arrest, sinoatrial heart block, and asystole. The major sign is fainting.

Tetralogy of Fallot
Tetralogy of Fallot is a congenital heart defect in dogs that includes four separate defects: pulmonic stenosis, a ventricular septal defect, right ventricular hypertrophy, and an overriding aorta. Keeshonds and Bulldogs are predisposed. Signs include cyanosis and exercise intolerance.

Thrombocytopenia
Thrombocytopenia is a common condition in dogs characterized by low platelet counts. Platelets are used in clotting the blood, so dogs with this condition may have spontaneous bleeding or prolonged bleeding following surgery, injury, or during an estrous cycle. Causes include some rickettsial infections such as ehrlichiosis, cancers such as hemangiosarcoma, or immune-mediated disease.

Thrombocytosis
Thrombocytosis is a condition characterized by an excess of platelets. Most cases are physiologic (caused by exercise) or reactive (secondary to some cancers, blood loss, or certain drugs). Rarely the condition is caused by a primary bone marrow disorder. In this last case, the platelets may not function normally, causing the blood to not clot properly.

Ventricular septal defect
Ventricular septal defect is a hole in the division between the heart ventricles. It is a congenital disease in dogs. There usually are no signs in dogs except for a heart murmur.

von Willebrand disease
von Willebrand disease is an inherited, common disease found in both dogs and humans. It is characterized by a deficiency of a protein called von Willebrand factor, which is involved in blood clotting. The disease varies from mild to severe, depending on the amount of von Willebrand factor present in the dog. Signs include spontaneous bleeding and excessive bleeding following surgery, injury, or during an estrous cycle.