(See part one of this series for an appropriate introduction.)
As explained in part one, it is best to divide the heart into four sections- the atrial mass, ventricular mass, the outflow arteries and valves (Truncus) and the inflow portion (the endocardial cushions. For the echocardiographer, it will alleviate much anxiety to evaluate the heart in this way, identifying each structure of the heart in a systematic and organized way.
As far as the atria and the ventricles are concerned, there may be concordance (the atria attach to their respective ventricles), there may be discordance (the atria do not attach to their respective ventricles) which means that either the ventricles or atria are transposed, or both are transposed.
There are also single atrium and single ventricle syndromes in which the morphology of these structures are either right or left oriented.
The morphology of the AV valves (mitral and tricuspid) are independent of the atrioventricular attachments and there are a variety of ways that the AV valves may present.
In a normal heart where there is a biventricular and biatrial arrangement or where there is discordance and transposition with biventricular arrangement, the AV valves will always follow their respective ventricles. In other words, anytime there are two ventricles and two AV valves, the tricuspid valve (TV)will always be attached to the right ventricle (RV) and the mitral valve will always be attached to the left ventricle (LV).
There are four typical configurations for the AV valves. All of these arrangements can be found with concordant, discordant, biventricular, mixed and double inlet types of connections of the atria and ventricles.
The first type is the concordant, or a normal configuration (described above). The second type is a two valve structure where one valve is imperforate or atretic. The third type is a common valve that overrides or straddles the ventricular septum and the fourth type is a single valve of right or left sided morphology that overrides or straddles the venticular septum.
A double inlet defect would be a two valve structure in which one of the valves straddle the ventricular septum and therefore both atria tend to drain into the dominant ventricle. Most often the dominant ventricle is the left ventricle.
A single valve may override the ventricular septum. This valve may be of right or left morphology. There is typically a biventricular arrangement; papillary muscles and the cords tend to attach to both ventricles rather than on just one side of the heart and there is a “bridging” leaflet that connects the valvular arrangement.
If there is a common valve, then it is not possible to determine if there is a right or left sided morphology to the valve structure. It is incorrect to consider the common valve as having mitral or tricuspid components. Even in the case of double inlet defects, it is best to describe the valves as just right sided and left sided as opposed to identifying them as a tricuspid or mitral valve.
A common valve configuration is really an endocardial cushion defect, absent the septal connections that seperate the right and left hearts. A true AVSD or atrioventricular septal defect with an overriding common valve is the hallmark of an endocardial cushion defect.