The Ventricular Mass
The embryologic primitive ventricle is identified as a “mass”, since in its embryologic development, many things may happen to this structure. There may develop two discrete ventricles divided by and intact septum (normal), or there may be one dominant ventricle with one hypoplastic ventricle (typically a hypoplastic LV), there may be a single ventricle (rare), or there may be a “transposition” of the ventricles (right ventricle on the left and the left ventricle on the right such as L-transposition)
If you have read the previous blogs, you will see that the atria may develop separately from the ventricles and may be concordant, discordant or isomeric.
In other words, the left atrium may be attached to the left ventricle and the right atrium may be attached to the right ventricle (concordant),or the atria may be discordant in that the RA is attached to the LV and the LA is attached to the RV, or they may be isomeric in that there may be two morphologic right or left atria attached in any way.
Typically, each atrioventricular valve, mitral valve (MV), tricuspid valve (TV) will follow its respective ventricle MV to left ventricle (LV), TV to right ventricle (RV) , and each semilunar valve will follow its respective great artery connection, aortic valve to aorta, pulmonic valve to pulmonary artery.
Embryologically, the ventricular mass is known as the “spongy myocardium”. On the right side, the tricuspid valve, the papillary muscles and the chordae emerge from the spongy myocardium and grow superiorly until the three cusps of the tricuspid valve coapt into a fully functioning valve apparatus.
The spongy myocardium eventually develops into a trabeculated structure that allows the ventricle to become more and more elastic. In other words, it becomes a structure that is more compliant during diastole (the ability to relax in diastole).
If the tricuspid valve leaflet apparatus fails to separate from the “spongy myocardium”, this results in a defect known as “Ebstein’s Malformation.”
A true univentricular heart is rare. This means that there is one ventricle. Typically, there is one dominant ventricle and one hypoplastic ventricle.
If there is a hypoplastic ventricle, then there is often an underdeveloped ventricle, AV valve and outflow valve (very often left ventricle, aorta, and mitral valve), otherwise known as a hypoplastic left heart syndrome (HLHS).
There may be a dominant ventricle with two inlet AV valves that drain into one ventricle. This is known as a “Double Inlet” defect. If there are two outlet valves with one dominant ventricle, this is known as a “Double Outlet” defect.
The 50% rule applies to all valves. If the particular valve crosses over any septum greater than 50%, then it overrides.
For instance, If the interventricular septum is to the left of both AV valves, the MV and TV, then this valve may be a double inlet defect in that both atria drain into one ventricle via the AV valves.
As an echocardiographer, it is important to visualize each structure in its entirety. The atria are not that easy to differentiate, but pay particular attention to the ventricles. The right ventricle is more trabeculated.
If the great arteries are parallel, then this may be a transposition. If the great arteries are perpendicular to each other, then the ventricles and the great arteries are probably normally related.
Evaluate the size of the ventricles. Which artery is connected to which ventricle? Where are the vena cava and pulmonary veins connected?
Take all of these into consideration as you evaluate the pediatric heart.