Most VSDs are located dorsally in or around the membranous portion of the interventricular septum. The typical defect is “high” and perimembranous, but there certainly are variations in location, which are relevant to auscultation, echocardiographic diagnosis, cardiac catheterization, and either transcatheter closure or surgical repair. The typical VSD is located just below the base of the right or noncoronary cusp of the aortic valve when viewed from the left side of the ventricular septum. On the right side of the septum, the defect usually is adjacent to the cranial edge of the septal tricuspid valve leaflet and just caudoventral to the supraventricular crest that separates the inflow from the outflow tracts. These defects commonly are classified as membranous, perimembranous, or paramembranous and in older nomenclature were termed subcristal defects. A membranous defect can be very large, as in tetralogy of Fallot, encroaching on the supraventricular crest by extending toward the RV outflow tract. Other subaortic defects communicate with the supracristal, subpulmonic portion of the ventricular septum. These VSDs are termed subarterial or juxtaarterial, doubly-committed, supracristal, or outlet defects, depending on the reference cited. A defect located immediately ventral to the septal tricuspid leaflet is typical of a complete atrioventricular septal defect (endocardial cushion defect), a malformation seen most often in cats in conjunction with an ostium primum atrial septal defect and a common (straddling) atrioventricular valve leaflet. Defects of the muscular or trabecular ventricular septum are encountered infrequently in dogs and cats but can be single or multiple and apical or midventricular in location.
The clinical significance of the VSD depends on its size, location, and accompanying malformations. An uncomplicated, isolated VSD leads to left-to-right shunting. The smaller the VSD, the higher the velocity of the blood flowing across it from the higher-pressure LV into the lower-pressure RV. The velocity of the shunting typically exceeds 4.5 to 5 m/sec on continuous wave Doppler echocardiography, as predicted by the normal transventricular pressure difference of approximately 100 mm Hg. This produces a loud systolic murmur heard best over the right thorax as the turbulent shunt flow enters the RV adjacent to the tricuspid valve. The shunt volume enters the RV outflow tract into the pulmonary artery, using the RV largely as a conduit. From there the extra volume of blood flows into the lungs and returns to the left side of the heart, leading to a left-sided volume overload. Small to moderate VSDs may cause some cardiomegaly, but they cause clinically insignificant volume overloads of the left side of the heart. However, with larger defects the recirculated blood considerably increases left atrial (LA) and LV diastolic volumes and pressures, which leads to left-sided heart dilation and sometimes to LV failure. As a result of the increased diastolic volume and associated increase in wall tension, compensatory hypertrophy of the LV eventually occurs.
When left-to-right shunts are large or are complicated by other left-sided heart defects, left-sided or biventricular congestive heart failure (CHF) may develop. This is especially likely when pulmonary artery flow is 2.5 to 3 times greater than aortic flow or when substantial aortic or mitral regurgitation also is present. In general, an isolated VSD in a dog or cat does not commonly cause CHF unless the maximal VSD diameter is greater than 50% of the aortic diameter, and heart failure is diagnosed more often in cats than in dogs. Importantly, most puppies with an uncomplicated defect that survive to see a veterinarian for a first vaccination are likely to live a relatively normal life span. This probably relates to the fact that a large VSD should cause CHF very early in life, at the time pulmonary vascular resistance drops. When CHF develops before 6 weeks of age, the consequences are likely those of a “fading puppy” or one that appears to have died of pneumonia. However, CHF can occur in older puppies or even later in life. This is especially likely when a large membranous or a subarterial VSD allows the aortic root to prolapse. The subsequent loss of valvular support can allow progressively more severe aortic regurgitation. This additional volume overload to the LV predisposes to CHF. Also, small breeds of dogs are prone to developing chronic valvular heart disease of the mitral valve, which also contributes to the volume overload of the LV, and the combination of this acquired disease and a moderate to large VSD can lead to left-sided heart failure later in life.
The effects of a VSD on the right side of the heart and pulmonary circulation are variable and related to defect size and location, pulmonary valve function, pulmonary vascular resistance, and ventricular myocardial response. Usually, the larger and less restrictive a VSD, the more likely that the two ventricles will function as a common chamber with systolic and diastolic pressures becoming equal. This situation promotes significant RV dilation and hypertrophy. If the RV outflow is normal and pulmonary vascular resistance remains low, there is marked left-to-right shunting with pulmonary overcirculation. This increases the likelihood of left-sided or biventricular CHF. Pulmonary hypertension also can occur in this setting, but it is related mainly to increased pulmonary blood flow along with left-sided heart failure. Radiographs show a dilated main pulmonary artery with prominent pulmonary vascularity. In other animals with large shunts, the increased pulmonary blood flow damages the vasculature, which markedly increases pulmonary vascular resistance (Eisenmenger’s physiology). This leads to high-resistance pulmonary hypertension, which increases the pressure load on the RV; this, in turn, results in concentric hypertrophy. It also reduces left-to-right shunting and promotes bidirectional or predominantly right-to-left shunting. In these cases the proximal pulmonary arteries may be dilated, but the peripheral pulmonary vasculature appears undercirculated on radiographs.
Although this chapter focuses on the isolated VSD, other cardiac lesions may be evident that alter the pathophysiology, clinical findings, and management approaches. In addition to severe aortic regurgitation and Eisenmenger’s physiology described earlier, there are other obstructive conditions of the RV that can complicate a VSD. One of these is pulmonic (pulmonary) stenosis, which was reported to be the most common defect found in conjunction with a VSD (Oliveira et al, 2011). If the pulmonic stenosis is severe, then pressure on the right side of the heart equals or exceeds that on the left, which causes blood to shunt from right to left. This also occurs in tetralogy of Fallot in which there is an overriding aorta, pulmonic stenosis, severe RV hypertrophy, and right-to-left shunting of blood. In most cases, shunting is bidirectional, with shunt direction varying over the cardiac cycle. Patients with right-to-left shunting from the combination of VSD and pulmonic stenosis may develop clinical signs similar to those observed in tetralogy of Fallot. These include exertional weakness from hypoxemia, cyanosis, and secondary polycythemia.
A less common defect is the so-called double-chambered RV, which is characterized by fibromuscular reaction and proliferation within the RV cavity. This creates a midventricular obstruction just distal to the VSD in most cases, and the proliferation may close the VSD, leaving only a midventricular obstruction. Progressive tissue growth also can cause severe obstruction and the high pressures in the proximal RV lead to concentric hypertrophy of the proximal RV chamber. Affected dogs may develop signs of exercise intolerance, right-sided CHF, or right-to-left shunting across a residual VSD or a (patent) foramen ovale.
