Atrioventricular (AV) septal defects comprise a broad spectrum of lesions, from partial or intermediate forms with no shunting at the ventricular level to complete AV septal defects with large atrial septal defects, large ventricular septal defects (VSDs), and a single common atrioventricular valve (AVV) orifice. Instead of separate mitral and tricuspid valve inlets, a common AVV has a single inlet (orifice) into the ventricular chambers. When this common AVV opens predominantly toward one ventricle or the other, an unbalanced AV canal (AVC) or AV septal defect forms, as shown below.
If the common AVV predominantly opens into the morphologic left ventricle, the defect is termed a left ventricular (LV)–type or LV-dominant AV septal defect (canal). If the common AVV opens predominantly into the morphologic right ventricle, the defect is termed a right ventricular (RV)–type or RV-dominant AV septal defect (canal). The degree of unbalance varies from mildly unbalanced with 2 nearly normal-sized ventricles to severely unbalanced with a single dominant ventricle and a second hypoplastic ventricle. This results in essentially single-ventricle physiology. Importantly, the ventricles, not the common AVV, are unbalanced. The development of the ventricles is unbalanced with hypoplasia of the inlet and outlet septum, resulting in hypoplasia of the chamber with malalignment of the ventricular septum.
AV septal defects occur at the embryonic age of 34-36 days when fusion of the endocardial cushions fails. This occurs when the endocardial cushion fibroblasts fail to migrate normally to form the septum of the AVC. As a result, a deficiency of the primum atrial septum, the ventricular septum, the septal leaflet of the tricuspid valve, and the anterior leaflet of the mitral valve occurs. The position of the AVVs becomes lower than normal. The anterior leaflet of the AVV extends across the ventricular septum and is shared between the left and right ventricles. If the leaflet opens preferentially toward either ventricle, blood flow is limited to the other ventricle, causing hypoplasia of that ventricle and creating unbalance between the 2 ventricles. [1, 2, 3]
Please see Atrioventricular Septal Defect, Complete and Atrioventricular Septal Defect: Surgical Perspective for general anatomic principles common to all patients with AV septal defects.
As noted above, 2 major types of unbalanced AV septal defects (canals) are recognized (ie, LV-dominant, RV-dominant). Generally, concomitant hypoplasia of the left-sided structures (LV, aortic) or the right-sided structures (RV, pulmonary artery [PA]) also occurs. Although a considerable spectrum of ventricular dominance occurs, the term unbalanced AV septal defect generally implies hypoplasia of one ventricle and its associated outflow tract with essentially single-ventricle physiology. RV-dominant AV septal defects occur more commonly than LV-dominant AV septal defects. The LV or RV is severely hypoplastic in approximately 7% of patients born with complete AVC defects.
The physiology of the lesion depends on the degree of ventricular unbalance, the size of AV septal defects, AVV competence, the degree of right-sided or left-sided outflow obstruction, and pulmonary vascular resistance. As with balanced AV septal defects, in the absence of significant left-sided or right-sided outflow obstruction, the physiology and clinical presentation of partially unbalanced AV septal defects are generally those of pulmonary overcirculation. Infants typically present with congestive heart failure (CHF) in the first month of life. Infants may present in extremis with acidosis if severe hypoplasia of left-sided structures with ductal-dependent systemic circulation is present, or they may present with severe cyanosis if severe hypoplasia of the right-sided structures with ductal-dependent pulmonary circulation is present.
When a VSD is present, the risk of pulmonary vascular disease is high. If the patient is deemed a poor candidate for 2-ventricle repair, effort should be made early to protect the pulmonary vascular bed to optimize a single-ventricle repair. PA banding in this situation allows additional time before a decision must be made about proceeding with either a univentricular or biventricular repair. If the VSD is small in the presence of LV hypoplasia, this may bode well for a possible biventricular repair because most cardiac output still is being carried by the small LV.
AV septal defects are relatively common forms of congenital heart disease, representing approximately 3% of all congenital heart disease; the estimated incidence is 0.19 per 1000 live births (one half of patients have Down syndrome). AV septal defects are present in 45-62% of infants with Down syndrome.
Unbalanced forms occur in approximately 7% of patients with AV septal defects. The vast majority of these do not occur in patients who have Down syndrome.
Unbalanced AV septal defects are frequently observed in patients with heterotaxy syndromes. They occur much more frequently in patients with asplenia than in those with polysplenia.
The prognosis following biventricular repair is generally good. The operative mortality rate is generally less than 3%. Most patients remain asymptomatic with a normal functional status. Less than 10-15% of patients require reoperation for residual AVV insufficiency or LV outflow tract obstruction. 
The prognosis following univentricular repair is reasonable and improving as surgical techniques and medical management improve. However, the true long-term function of a single ventricle, especially a single right ventricle, remains unknown.
In 1983, Emanuel et al reported that 14% of offspring of mothers with AV septal defects have congenital heart disease. 
Long-term morbidity and mortality rates are related to the development of pulmonary vascular obstructive disease. As many as 30% of patients with complete AV septal defects develop pulmonary vascular obstructive disease by age 7-12 months, and 90% develop it by age 3-5 years.
The true natural history is difficult to accurately determine because no group of infants born with this lesion has been monitored without surgical intervention.
Patients with unrepaired complete AV septal defects have a poor overall prognosis. Approximately 80% of patients with complete AV septal defects die by age 2 years. In 1979, a study of autopsied patients reported that only 54% of infants survived 6 months, 35% survived 1 year, and 4% survived 5 years.  In 1981, Somerville et al found that 55% of patients died or had significant medical problems in the first year of life.  In 1985, Bull et al found that this outlook was not as dismal for patients with Down syndrome, and that only 4 late deaths occurred over a 27-year period in patients aged 1 year with unoperated AV septal defects. 
Postoperative complications following biventricular repair include atrioventricular (AV) block, pulmonary hypertension, residual AVV regurgitation, AVV stenosis, and residual LV outflow tract obstruction.
Postoperative complications following univentricular repair include the following:
Atrial flutter and other atrial or, less commonly, ventricular arrhythmias
Pulmonary thromboembolism, stroke
Residual pulmonary branch stenosis
Formation of systemic venous collaterals resulting in a right-to-left shunt or the development of pulmonary arteriovenous fistulae
Low exercise capacity
Formation of systemic-to-pulmonary arterial collaterals that may result in a residual left-to-right shunt and excessive volume load on the systemic ventricle
No racial predilection is known.
No sex predilection is known.
AV septal defects are present at birth; most patients present within the first month of life.
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