Atrioventricular septal defects (or AV Canal) could be described as a failure of the fibrous skeleton of the heart or the apparatus of the heart that contains the atrioventricular valves (mitral and tricuspid valves) and the truncal valves (aortic and pulmonic valves), to fuse appropriately to the muscular portions of the heart which include the atria, ventricles and the atrial and ventricular septums, which in turn form the “crux” or the center of the heart. This malformation essentially creates a “hole” in the center of the heart.
The crux of the heart  could also be described as the “atrioventricular junction”.
The so called “fibrous skeleton” is really not a contiguous fibrous structure. Parts of it are entirely fibrous, other parts are fibro-muscular, and another part is fibro-fatty.
The valves of the heart attach to the muscular portions of the heart in varying ways.
For instance, the aortic valve attachment is almost completely fibrous, whereas the infundibulum (RVOT of right ventricular outflow tract) is a conically shaped fibro-muscular segment that connects the right ventricle to the pulmonary artery.
The AV valves (mitral and tricuspid valves) connect to the ventricles and the atria via both fibrous and fibro-muscular attachments.
The fibro-fatty layer follows the atrioventricular groove and serves to insulate  the electrical conduction systems of the atria from the ventricles.
The segment that houses the mitral and tricuspid valves are also known as the “endocardial cushions”, while the segment that houses the aortic and pulmonic valves are also known as the “conus cushions”.
The hallmark of an atrioventricular septal defect is a primum ASD, a membranous VSD, and a single overriding (or merged) AV valve that has (typically) five leaflets. The right side of this structure has two of the tricuspid valve leaflets while the left side has one of the mitral valve leaflets, with two leaflets at the center called “bridging leaflets” that override the ventricular septum.
Shunting can occur across this defect in varying ways and may be right-to left, left-to-right, or most commonly bi-directional since there tends to be equalization of pressures throughout the heart.
Shunting is also dependent on how the bridging leaflets attach. If they are situated superiorly towards the atrial septum, then most of the shunting occurs at the ventricular level. If the bridging leaflets are situated inferiorly towards the ventricular septum, then most of the shunting will occur at the atrial level. If they “float” then shunting occurs at both the atrial and ventricular levels.
A final variant occurs when the bridging leaflets close the septal defect so that there is minimal or no shunting across the common atrioventricular junction.
There are numerous types of variants of the common AV valve associated with this defect and are really not that relevant as it concerns the sonographer. This is the job of the surgeon. Suffice it to say that the five leaflets may attach themselves to each other in several different ways, along with their attachments to the atria and ventricles.
The papillary muscles are often abnormal. For instance, a “parachute mitral valve” could be described as hypoplasia of one or more of the papillary muscles and their attachments that creates a funnel type of inlet across the already abnormal mitral valve.
To further complicate this situation, the bridging leaflets may attach themselves in such a way that one or the other ventricles become dominant. For instance, if the common valve is situated in such a way that most of the atrial inflow enters one of the ventricles more than the other ventricle, then this is known as a “double inlet syndrome”, most commonly a “double inlet right ventricle”.
A “partial AV canal” might be described as a defect that involves a common AV junction in which there is a trileaflet tricuspid valve and a trileaflet mitral valve ( a bileaflet mitral valve with a cleft that essentially makes this a trileaflet valve).
The atrial shunt tends to be left-to-right, and the ventricular shunt tends to be bi-directional. This causes volume overload in both sides of the heart and pressure overload of the right side of the heart (pulmonary hypertension or PHTN).
Echocardiography
Evaluate the heart in all of the standard views and determine that there is a common AV junction. Determine the direction and size of both the atrial and ventricular shunts.
Look for outflow obstructions across the RVOT or LVOT. These obstructions may occur as a result of the way the bridging leaflets are attached to their atrioventricular and ventriculo-arterial components. This may best be determined by looking for “ventricular imbalance”. If one ventricle tends to be hypoplastic, then it is likely due to an outflow obstruction.
If there is bi-directional shunting at the ventricular level, this indicates severe pulmonary hypertension since pressures have equalized across the chambers of the heart.
The four chamber and subcostal views are best views used to determine whether there is a common AV junction, and which way and how large the shunts are.
Look for any valvular regurgitation. Valvular stenosis is difficult to determine in this lesion since there is so much mixing of right and left flow, and true gradients are hard to determine.
As outlined previously, there are numerous variations to this defect, but it is most commonly presented as a common, five leaflet, overriding AV valve with a primum ASD, and a large membranous VSD with bi-directional shunting and pulmonary hypertension.
Be sure to evaluate the pressure gradient across the VSD. This pressure gradient tends to be small, since the VSD is large and pressures tend to equalize throughout the heart. Subtract this pressure gradient from the systolic blood pressure, and this will give you the pulmonary artery systolic pressure. (see my post on “evaluating right ventricular systolic pressure”).
This defect is highly associated with Down Syndrome. For the sonographer, remember the axiom that “where there is one left-sided defect, always look for others”. Always check for ASD’s, VSD’s, coarctation of the aorta, right sided aorta, interruptions of the aorta and any outflow obstructions.
Surgical Repair
Surgery is always required to correct an AV canal defect, and is usually done within the first year of life.
The goal of this repair is to close the septal defects so that pulmonary congestion is minimized, and to reconstruct the common AV valve. Since there are so many permutations in which the common AV valve may present, there are numerous ways in which the common valve may be repaired and is highly individualistic.
The ASD and VSD are repaired with synthetic patches. The common AV valve is sutured together in such a way as to create a separate inflows for the right and left sides, and the septums are attached appropriately.
Any outflow obstructions are repaired, as well as other extraneous defects such as a coarctation of the aorta or a PDA (patent ductus arteriosis). The object of valvular repair is to do it in such a way as to limit post-surgical regurgitation or stenosis of the newly created AV valves
Outcomes
Surgical outcomes are highly dependent upon the severity of the defect, the level of shunting, and the severity of pulmonary hypertension. Surgical techniques have improved over the last 40 years or so and long term survival is very good, about 80-90%.