This page is for referring vets. If you are a pet owner please click here.

Case Report – Reverse shunting and polycythaemia in a dachshund with a congenital ventricular septal defect (VSD)

Signalment, history and clinical exam:

A two year old, male entire Dachshund presented with a chronic history of exercise intolerance that had been noted ever since he was a puppy. When exercising the dog would quickly become breathless, sit down and refuse to move. On examination, the dog was bright and alert and was in fair body condition. However, the mucous membranes were hyperaemic and the capillary refill time was prolonged. The resting heart rate was elevated and a few premature beats were noted. There was no heart murmur and the pulse quality was good. Auscultation of the lung fields was unremarkable but the dog became noticeably tachypnoeic following excitement. Ophthalmoscopy revealed wide and tortuous retinal blood vessels. Clinical examination was otherwise unremarkable and the systolic blood pressure was within normal limits.

Investigation:

fig.1 - ECG - shows occasional VPC's, tall P-waves, deep R-waves and mean electrical axis deviation consistent with right sided cardiac enlargement

Routine haematology revealed marked polycythaemia with a haematocrit of 81.4%. Serum biochemistry was unremarkable as was urinalysis. Relative polycythaemia was unlikely given the severity of the erythrocytosis so absolute erythrocytosis caused by cardiac, pulmonary, renal or neoplastic disease was likely. Given the premature cardiac beats an ECG trace was done (fig. 1).  This confirmed the presence of occasional ventricular premature complexes and also revealed tall p waves, deep R waves and mean electrical axis deviation consistent with right sided cardiac enlargement . Inflated thoracic radiographs confirmed right sided cardiac enlargement and revealed a mild bronchial pattern in the dorsal lung fields (figs. 2 & 3).  Abdominal ultrasonography was unremarkable. Echocardiography revealed marked thickening of the right ventricle which was similar in size to the left ventricle (figs. 4 & 5).  The inter-ventricular septum was flattened and the tricuspid valve appeared subjectively thickened with significant tricuspid regurgitation, findings which would be consistent with pulmonary hypertension. There was no evidence of any pulmonic stenosis. A bubble study performed during echocardiography was diagnostic for a ventricular septal defect (VSD) with right to left shunting, which explained the polycythaemia and associated clinical signs.

Management and treatment

fig.2 - lateral thoracic radiograph (inflated) reveals right-sided cardiac enlargement with a mild bronchial pattern in the dorsal lung fields

In order to reduce the detrimental effects of hyperviscosity syndrome phlebotomy was performed every 48 hours until the PCV had reduced to 68%. Phlebotomy was then continued as required to maintain the PCV at 65-70%. Given the associated thromboembolic risk, treatment with low dose aspirin was also started and sildenafil was used in an attempt to reduce the pulmonary hypertension. Short walks little and often were advised with avoidance of excessive exercise. The dog responded well to treatment but requires phlebotomy every 3 to 4 weeks in order to maintain the PCV within an optimal range. Exercise intolerance is still apparent but is significantly improved.

Discussion

fig.3 - ventrodorsal thoracic radiograph

Congenital VSDs with reverse shunting result in poor oxygenation and compensatory polycythaemia therefore ensues. Given the absence of significant pulmonic stenosis or pulmonary disease in this case, the reverse shunting could have occurred due to concurrent primary pulmonary hypertension or, more likely, due to Eisenmenger’s syndrome. This latter phenomenon occurs when the presence of a VSD with left to right shunting allows the normally high capacitance, low pressure pulmonary vasculature to be exposed to high pressures from birth. This prevents normal pulmonary vascular development, ultimately resulting in marked pulmonary hypertension and reverse shunting, usually at a very early age (<6 months).

 The main complications associated with this condition are the inherent risks associated with hyperviscosity syndrome; namely, local or systemic vascular compromise and reduction in microcirculation leading to hypoxia or thrombosis.  Neurological signs such as seizures, ataxia, blindness, tremor and behavioural changes are the most common sequelae. However, if the PCV can be effectively controlled and the detrimental effects of hyperviscosity syndrome minimised, the long term prognosis for secondary polycythaemia depends largely on the underlying cause. Since a VSD with reverse shunting is a rare condition in the dog there is little in the literature regarding specific survival times. However, in the closely related condition of reverse PDA, survival times of more than four years have been reported. In this dog an additional concern was the presence of VPCs which may well have been related to myocardial hypoxia. Myocardial ischaemia resulting from polycythaemia may worsen the prognosis. However, if further ischaemia can be controlled adequately through reduction of hyperviscosity then a fair survival time may still be achieved.

fig.5 - Echocardiography reveals marked thickening of the right ventricle; similar in size to the left.

fig.4 - Echocardiography revealed significant tricuspid regurgitation consistent with pulmonary hypertension