Atrioventricular septal defects (AVSDs) are anatomic defects that arise from faulty development of the embryonic endocardial cushions. This spectrum ranges from a primum atrial septal defect and cleft mitral valve, known as a partial atrioventricular septal defect (partial AVSD), to defects of both the primum atrial septum and inlet ventricular septum and the presence of a common atrioventricular valve, referred to as complete atrioventricular septal defect (complete AVSD, CAVSD). The terms atrioventricular canal defect and endocardial cushion defect are used in reference to this group of defects; however, atrioventricular septal defect is now the preferred terminology. These defects, particularly the complete form, typically present in the fetal or neonatal period and are an important source of cardiac morbidity and mortality in this age group.
This article focuses on the complete form. Partial, intermediate, and unbalanced forms are reviewed in other chapters (see Atrioventricular Septal Defect, Partial and Intermediate and Atrioventricular Septal Defect, Unbalanced).
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Faulty development of the endocardial cushions, which represent the primordia of the atrioventricular septum and atrioventricular valves, plays a central role in the development of atrioventricular septal defects. [1, 2] The superior and inferior endocardial cushions appear at 4-5 weeks' gestation. During this time, the common atrioventricular canal is positioned over the primitive left ventricle.
Mesenchymal cells invade these masses of tissue, and, during the fifth week of gestation, the cushions approach each other and fuse. This divides the common atrioventricular canal into right and left canals.  The right and left lateral endocardial cushions develop shortly after the appearance of the superior and inferior cushions, followed by the dextrodorsal conus cushion. These structures are involved in the development of the mitral and tricuspid valves and their support apparatus (see the image below).
The endocardial cushions do not directly form the valve components but play an essential role in the process by which undermining and delamination of the myocardium forms the valve leaflets and chordal attachments.  Complete failure of fusion of the endocardial cushions results in deficiency of the inlet portion of the interventricular septum, a common atrioventricular valve annulus and common AV valve, as well as deficiency of the inferior (primum) portion of the atrial septum. This constellation of features results in a large defect in communication with all 4 chambers of the heart.
In complete atrioventricular septal defect, a single atrioventricular valve annulus, a common atrioventricular valve, and a defect of the inlet ventricular septum are observed. The deficiency of the atrioventricular septum also results in the presence of a large primum atrial septal defect. Details of the anatomy, particularly the morphology of the atrioventricular valve are crucial in planning surgical repair of this lesion. The common AV valve consists of at least 4 leaflets. These include the anterior and posterior bridging leaflets and 2 lateral leaflets. The anterior leaflet may be further subdivided to produce a total of 5 leaflets. The classification system initially described by Rastelli et al is used to describe the morphology of the atrioventricular valve. 
With the Rastelli type A valve, the anterior leaflet is divided into 2 portions of approximately equal size. The lateral portions of this leaflet attach to the anterior papillary muscles in each ventricle. Chordae tendineae attach the medial portion of this leaflet to the crest of the ventricular septum or slightly to the right ventricular side. Interventricular communication may occur between the anterior and posterior bridging leaflets and underneath the anterior leaflet in the interchordal spaces.
In type B valves, the rarest type, the anterior bridging leaflet is divided but overhangs the ventricular septum more so than in type A valves. The chordae from the medial portion of the divided anterior leaflet have no direct insertion to the ventricular septum but rather insert onto an anomalous papillary muscle positioned in the right ventricle near the ventricular septum. Because of the lack of chordal insertions to the septum, free interventricular communication occurs beneath the anterior leaflet.
In a Rastelli type C valve, the anterior bridging leaflet is larger and overhangs the septum more so than with a type A and type B valves. It is not attached in its mid portion to the ventricular septum or elsewhere and is referred to as being “free floating." Free interventricular communication also occurs underneath this valve leaflet.
Because of the deficient atrioventricular septum, the atrioventricular valves are displaced apically. As a result, the left ventricular inlet distance (distance from mitral valve annulus to apex) is shorter than the outlet distance (apex to aortic valve annulus). In the normal heart, these distances are nearly equal. In addition, the left ventricular outflow tract is displaced anteriorly, as opposed to wedged between the 2 atrioventricular valves. These features lead to the characteristic "gooseneck" deformity seen on anteroposterior angiography. Although this leads to a left ventricular outflow tract (LVOT) diameter that is smaller than normal, it usually does not cause clinically significant obstruction by itself. However, contribute to an LVOT obstruction when associated with a subaortic membrane or accessory atrioventricular valve tissue.
Defects commonly seen in association with complete atrioventricular septal defect include patent ductus arteriosus, coarctation of the aorta, atrial septal defects, absent atrial septum, and anomalous pulmonary venous return.  Abnormalities of the mitral valve also commonly occur, including single papillary muscle (“parachute mitral valve”) and double orifice mitral valve. Tetralogy of Fallot is also present in about 2.7-10% of cases. At least 75% of patients with tetralogy of Fallot and complete atrioventricular septal defect have Down syndrome. 
The pathophysiology of complete atrioventricular septal defect depends on the magnitude of blood flow through the ventricular septal defect (VSD) and the amount of atrioventricular valve regurgitation. Patients with little atrioventricular valve regurgitation and high pulmonary vascular resistance (PVR) are asymptomatic early in life, and their condition may be difficult to diagnose.
These patients occasionally remain relatively asymptomatic until their second or third decade, when they develop increasing cyanosis from advanced pulmonary vascular disease. In most cases, the PVR decreases normally over the first 6 weeks of life, and the patient develops a large left-to-right shunt through both the atrial and ventricular defects, resulting in congestive heart failure (CHF). Patients with clinically significant atrioventricular valve regurgitation may also have signs of CHF, such as tachypnea, excessive sweating, and failure to appropriately gain weight.
Atrioventricular septal defects account for 2-9% of congenital heart disease in various series. Most investigators report a prevalence rate in the range of 3-5%.  The male-to-female distribution of atrioventricular septal defect is approximately equal.  The incidence of atrioventricular septal defect is higher among stillborn infants, likely due to the higher number of chromosomal and other genetic anomalies in this group. The pooled frequency of atrioventricular septal defects from several series of congenital heart disease in stillborn infants was about 7%. 
Freeman et al reported a prevalence of 9.6 cases of Down syndrome per 10,000 live births.  Congenital heart disease is present in 44% of affected infants, and atrioventricular canal defects are present in 45% of infants with Down syndrome and congenital heart disease.
Familial clustering may occur with atrioventricular canal defects. About 14% of women with common atrioventricular canal pass on congenital heart disease to their children. In a pedigree analysis, 11.7% of probands had a family history of congenital heart disease. 
Race-, sex-, and age-related demographics
The occurrence does not appear to vary on the basis of race. Advanced maternal age is a risk factor for Down syndrome. Because at least two thirds of patients with uncomplicated complete atrioventricular septal defect have trisomy 21, ethnic groups in which advanced maternal age is common may have an increased incidence of complete atrioventricular septal defect.
The male-to-female ratio for complete atrioventricular septal defect is 1:1.
Patients with complete atrioventricular septal defect often present with symptoms early in life. CHF usually develops by 6 weeks as PVR decreases and pulmonary blood flow increases. A rare case of survival to the eighth decade with untreated complete atrioventricular septal defect was reported. In some patients, PVR never decreases, and symptoms of CHF do not develop. In these rare cases, patients may remain asymptomatic as their pulmonary vascular obstructive changes worsen until cyanosis develops because of a right-to-left shunt.
Without operation, the survival of patients with this lesion is poor. Death may occur during infancy secondary to heart failure or pneumonia. Death later in childhood results from progressive pulmonary vascular obstructive disease (PVOD). PVOD tends to develop more rapidly than in other congenital heart defects. Intimal fibrosis (Heath-Edwards grade 3 lesions) have been demonstrated to appear between age 6-12 months. Vascular dilation and plexiform lesions (Heath-Edward grade 4 lesions) may occur by age 1 year.  Evidence suggests that these changes develop earlier and progress more rapidly in infants with Down syndrome.
Risk factors for surgical and late mortality and morbidity are identified. Preoperative risk factors for mortality include small size, unbalanced ventricular size, New York Heart Association class IV, and severe atrioventricular valve insufficiency. The era of operation (before 1987), patient age at operation, presence of accessory atrioventricular valve orifices, and other congenital heart diseases also increase the surgical mortality risk. Down syndrome is surprisingly not an independent risk factor for morbidity and mortality and therefore should not limit intervention. In one retrospective study (2002-2010) that evaluated 53 consecutive patients aged 3 years and younger who presented with complete AVSD and underwent surgical repair, multivariate analysis revealed the only significant factor associated with moderate or severe left atrioventricular valve regurgitation was the absence of Down syndrome. 
Infants can successfully undergo surgery, with a published mortality rate of 3.6% and with minimal long-term morbidity. Late survival is approximately 96%, and the reoperation rate is approximately 11%. The need for reoperation affects long-term survival after congenital AVSD repair. 
Published 10-year survival rates are 81-91%. In one retrospective study (1974-2000) comprising 198 patients who underwent surgical repair, the overall estimated survival for the entire cohort was 85% at 10 years, 82% at 20 years, and 71% at 30 years after initial congenital AVSD repair.  The estimated freedom from reoperation was 88% at 10 years, 83% at 20 years, and 78% at 30 years after initial congenital AVSD repair. 
Risk factors strongly associated with early death or the need for repeat operation include operation before 1987, postoperative pulmonary hypertensive crisis, immediate postoperative severe left atrioventricular valve regurgitation, and double-orifice left atrioventricular valve. In the past, death was significantly most common when complete atrioventricular septal defect with RV outflow tract obstruction was corrected in children younger than 5 years or weighing less than 15 kg. In the current era, most centers operate by age 6 months. 
Patients with complete atrioventricular septal defect typically develop tachypnea and failure to thrive in the first few months of life. Tachypnea hampers normal feeding. In addition, respiratory tract infections, such as those due to respiratory syncytial virus (RSV), are poorly tolerated.
Patients may survive past the first few years of life without surgical intervention if the PVR remains elevated, although they may develop irreversible pulmonary vascular obstructive disease (PVOD) at a rapid rate. Surgical morbidity and mortality rates associated with this defect have dramatically improved over the years. A recent multicenter study demonstrated an in-hospital mortality rate of 2.5% and an overall 6-month mortality rate of 4%.  About 3% of patients with a surgical heart block require a pacemaker, and about 7% may require repeat operation for residual defects or surgically induced mitral insufficiency. Actuarial survival at 13 years is 81%.
In patients with a nonrestrictive VSD component, pulmonary vascular disease (Eisenmenger syndrome) eventually occurs unless the VSD component is surgically closed. Rare cases have occurred even when surgical repair is successfully accomplished in infants younger than 6 months. Cyanosis occurs when patients develop some degree of right-to-left shunt at either atrial or ventricular levels. Although patients' quality of life may be impaired at this point, their life expectancy may be 20-50 years.
Treatment for the complete atrioventricular septal defect is primarily surgical. Operative morbidity and mortality for this procedure has dramatically improved over the past 20 years. Tweddell et al identified risk factors for surgical and late mortality and morbidity; these are the era of operation, patient's age at operation, severity of left atrioventricular valve regurgitation, magnitude of preoperative heart failure, presence of accessory atrioventricular valve orifices, other congenital heart disease, and Down syndrome. 
Miller et al reviewed the long-term survival of infants with all types of atrioventricular septal defects with Down syndrome (n = 177) and without Down syndrome (n = 161).  In this cohort, born from 1979-2003, overall survival probability through 2004 was 70% in those with Down syndrome and 69% in those without. Mortality was higher in children with a complex atrioventricular septal defect and in those with 2 or more major noncardiac malformations, but was lower in children born in 1992-2003.
In infants, the published mortality rate for complete atrioventricular septal defect repair is 3.6% with minimal long-term morbidity; the 10-year survival rate is 81%. Bando et al found similar results while identifying risk factors for early death and the need for repeat operation.  Risk factors included postoperative pulmonary hypertensive crisis, immediate postoperative severe left atrioventricular valve regurgitation, and a double-orifice left atrioventricular valve. McElhinney et al described an occasional anomalous attachment or tissue of the atrioventricular valve, which may complicate operative repair. 
Postoperative complications include arrhythmias, low cardiac output, pulmonary hypertension, atrioventricular valve stenosis, and mitral insufficiency. Arrhythmias include heart block and junctional tachycardia; the latter usually subsides within 3-7 days after surgery. Postoperative left ventricular dysfunction may result in low cardiac output and even renal insufficiency. Inotropic drugs may be needed for several days after surgery.
In patients with pulmonary hypertension, sedation, paralysis, and mild hyperventilation with 100% oxygen may be required to prevent pulmonary hypertensive crisis and decreased right ventricular (RV) afterload. For pulmonary hypertension refractory to these measures, nitric oxide may be used to achieve pulmonary vasodilatation.
On occasion, patients may require long-term therapy, which might include phenoxybenzamine or calcium-channel–blocking drugs to manage an elevated pulmonary vascular resistance (PVR).
Severe mitral regurgitation may occur postoperatively and is ideally recognized on intraoperative transesophageal echocardiography (TEE). Residual mitral regurgitation is the most common indication for late reoperation after repair of complete atrioventricular septal defect. Approximately 5-10% of patients ultimately require mitral valve repair or replacment. [22, 23]
Anomalous attachment of atrioventricular valve tissue occasionally causes left ventricular (LV) outflow tract (LVOT) obstruction, in addition to the known tendency for patients with atrioventricular septal defect to have a small LVOT.  Such anomalous attachments may prevent complete relief of subaortic obstruction without mitral-valve replacement. Resection of some discrete obstructing tissue or, in some patients, a modified Konno procedure for tunnel-like LVOT obstruction or for obstruction caused by anomalous attachments of the mitral-valve apparatus may be performed. This procedure may complicate outflow-tract reconstruction and has had varied results.
A rare complication of the complete atrioventricular canal is subacute bacterial endocarditis. Successful repair during active endocarditis is reported.
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