Case Studies In Small Animal

Cardiovascular Medicine

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Case 8

Right-To-Left Shunting VSD Section from "Small Animal Cardiovascular Medicine" On-Line

Text from "Small Animal Cardiovascular Medicine"

Right-to-Left Shunting Ventricular Septal Defect (Eisenmenger’s Complex)


Eisenmenger’s complex is the combination of a ventricular septal defect with pulmonary vascular disease leading to right-to-left shunting and cyanosis. Eisenmenger’s syndrome is the combination of other systemic-pulmonary communications, pulmonary vascular disease and cyanosis. Eisenmenger’s complex is rare in dogs and cats. Most ventricular septal defects are small and do not result in this type of significant hemodynamic abnormality.


Eisenmenger’s complex occurs most readily in a patient with a large ventricular septal defect. When a VSD is large (same cross-sectional area of the aorta or more) and so produces no resistance to blood flow, blood flow distribution is based purely on the relative impedances (primarily resistances) of the systemic and pulmonary vascular beds. Systolic pressures are identical in both ventricles and the great arteries no matter what the pulmonary vascular resistance is. Because pulmonary vascular resistance is normally much lower than systemic vascular resistance, massive left-to-right shunting occurs if pulmonary vascular resistance decreases into the normal range in a neonatal person or animal. In infants, pulmonary vascular resistance decreases into the normal range in the third to fourth months of life. At this time, they present in severe left heart failure (due to the massive left-to-right shunting and volume overload and despite the fact that they have severe pulmonary hypertension). It is most likely that most puppies and kittens with large VSDs die of heart failure within the first several weeks of life. Consequently, they are never examined by a veterinarian. Rarely, the pulmonary vascular resistance does not decrease to the point of producing severe heart failure and the dog or cat lives long enough to develop other problems. One potential problem is severe pulmonary vascular disease with shunt reversal and cyanosis.

In the situation of a large VSD, it is most likely that pulmonary vascular disease occurs primarily in response to the obligatory flow-related pulmonary hypertension. The pulmonary vasculature must thicken in response to the markedly increased pressure in an attempt to bring wall stress back toward normal and to prevent rupture. The damage to the pulmonary vasculature is progressive over time. When pulmonary vascular resistance increases to a value greater than systemic vascular resistance, right-to-left shunting occurs. In humans, pulmonary vascular disease can also occur and progress in children with moderate sized VSDs. The reason for this is less clear. The only time the authors have observed this situation in dogs is when the dogs were born and raised at high-altitude or moved to high-altitude. Hypoxia is a known stimulant of pulmonary vasoconstriction and appears to be a modifying factor in dogs.

Clinical Findings

Eisenmenger’s complex has been described in 3 dogs in one series of case reports. Unfortunately, the size of the VSDs in these dogs could not be well documented other than to note that the VSD was large in one dog. This dog also had a patent ductus arteriosus. All dogs presented with cyanosis, either at rest or with exercise, and polycythemia (PCV = 55-65%). Interestingly, each of these dogs still had a heart murmur, two in locations characteristic of a VSD. The primary owner complaints consisted of exercise intolerance, exercise induced dyspnea, cyanosis, and seizure activity. Radiographs revealed a main pulmonary artery bulge in 2 dogs, enlarged proximal pulmonary arteries with normal distal arteries in one dog, decreased pulmonary artery size in one, and markedly enlarged caudal lobar pulmonary arteries in one. The cardiac silhouette was enlarged in all dogs. The ECG suggested right ventricular enlargement in all 3 dogs. The arterial oxygen tension was 46 mmHg in one dog that was not cyanotic at rest. In 2 dogs, cardiac catheterization confirmed the presence of a VSD and right-to-left or bidirectional shunting of blood. At necropsy, the VSDs were identified in each case along with thickened right ventricular walls and dilated right ventricular chambers. The proximal main pulmonary artery was dilated in all of the dogs. The small pulmonary vasculature was abnormal but similar in each case. Dilation of the large elastic pulmonary arteries was present. The smaller elastic arteries had hypertrophied walls and large lumina. The muscular arteries had a marked increase in connective tissue and fibroblasts, focal subintimal proliferations which often obliterated the vessel lumen, medial muscular hypertrophy with severe disruption of the internal and external elastic laminae, and plexiform lesions. Based on Heath-Edwards classification the lesions in these dogs were grade 3 to grade 4.

A similar case has been reported of a two-year-old, 25-kg Labrador retriever that presented with dyspnea at rest and exercise intolerance. This dog had no cardiac murmur, a PCV of 55%, and an arterial oxygen tension of 39 mmHg. The right ventricular systolic pressure was 60 mmHg at cardiac catheterization and a ventricular septal defect was identified using contrast radiography. At the postmortem examination, this dog had a 1.2 cm ventricular septal defect and concentric right ventricular hypertrophy. The diameter of this VSD was smaller than the normal aortic diameter of a 25-kg dog. The dog had medial hypertrophy and intimal fibrosis in the small muscular pulmonary arteries along with some plexiform lesions.


Surgical closure of a right-to-left shunting VSD is generally contraindicated. Phlebotomy to relieve severe polycythemia or hydroxyurea administration may be tried to improve clinical signs. Long-term prognosis is poor.



Mark D. Kittleson, D.V.M., Ph.D. All rights reserved.