Abstract
Atrioventricular septal defects (AVSDs), also referred to as atrioventricular canal defects, endocardial cushion defects, and atrioventricular communis, represent a spectrum of congenital heart disease. They are characterized by a variable deficiency of the atrioventricular septum and abnormal atrioventricular valves (AVVs).
Classification of AVSDs includes incomplete (partial AVSDs or ostium primum atrial septal defects), transitional (intermediate), or complete. The latter are further subcategorized by the superior bridging leaflet using the Rastelli classification and also by the degree of balance.
There is a strong association of AVSDs with Down syndrome and other cardiac anomalies. The pathophysiology is caused by left-to-right shunting across the septal defects and the degree of AVV regurgitation. The natural history leads to progressive congestive heart failure, late irreversible pulmonary hypertension, and, if left untreated, ultimately death.
The goal of surgical repair of AVSDs is to create normal anatomy and physiology by septation of the heart with no residual intracardiac shunts and normal function of the AVV. Complete AVSDs can be repaired using either a single, modified single, or two-patch technique. Special considerations are made in the presence of a left superior vena cava, left AVV abnormalities, unbalanced AVSDs, or complete AVSD with tetralogy of Fallot.
Overall survival is excellent following repair. However, significant morbidity may exist due to left AVV regurgitation and left ventricular outflow tract obstruction. Despite advances in care, these continue to be problematic and are the most common indications for reoperation.
Key Words
atrioventricular septal defect, atrioventricular canal defect, endocardial cushion defect, partial or incomplete AVSD, transitional or intermediate AVSD, complete AVSD, left atrioventricular valve regurgitation
Definition
Atrioventricular septal defects (AVSDs), also referred to as atrioventricular (AV) canal defects, endocardial cushion defects, and AV communis, represent a spectrum of congenital heart disease. They account for approximately 4% of congenital heart defects and are strongly associated with Down syndrome. They are characterized by a variable deficiency of the AV septum and abnormal AV valves (AVVs). The specific type is determined by the septal defects, AVVs, and the relationship to the ventricles.
Classification and Anatomy
The embryologic cause of AVSDs is the failure of endocardial cushions to properly develop. They derive from mesenchymal origin and migrate to septate the heart, ultimately contributing to the lower portion of the atrial septum, division of the AVV, and the inlet portion of the ventricular septum. When this does not completely occur, the result is an AVSD.
The pertinent anatomy of AVSDs includes the size and type of the atrial (ASD) and ventricular septal defects (VSD), the characteristics of the AVV(s), the left ventricular outflow tract (LVOT), and the location of the conduction system. The ASD is an ostium primum defect located in the lower portion of the atrial septum directly superior to the AVVs. The VSD is an inlet-type defect located directly inferior to the AVVs. The AVV(s) in AVSDs can have either a single or two separate orifices. In either case they are in the same plane, in contrast to a normal heart, where the mitral valve attaches to the septum more superiorly than the tricuspid valve ( Fig. 50.1 ). The leaflets constituting these valves are termed the anterior or superior bridging leaflets (SBLs), posterior or inferior bridging leaflets (IBLs), and mural or lateral leaflets (LLs) ( Fig. 50.2 ). The SBL is toward the surgeon’s left hand in a typical surgeon’s view from the right atrium. There are typically five to six total leaflets, consisting of the SBL, which may be divided into left and right components, the left and right IBL, and the left and right LL. The left (LAVV) and right AV valve (RAVV) components are often incorrectly described as the mitral and tricuspid valve, respectively. The LVOT is elongated with AVSDs. The aortic valve becomes displaced anterior and superiorly. This increases the outlet portion of the LVOT in comparison to the inlet portion, producing the classically described “gooseneck” deformity on angiography. It is postulated that this contributes to the predisposition of AVSD patients to develop LVOT obstruction (LVOTO) in the future ( Fig. 50.3 ). Last, because of the absence of the AV septum, the conduction system is not at the usual apex of the triangle of Koch. It is displaced posterior and inferior toward the coronary sinus (CS) in the nodal triangle defined by the CS, rim of the ASD, and posterior attachment of the IBL. The bundle of His then courses under the IBL to travel anterior and superior on the leftward aspect of the crest of the ventricular septum.
Classification of AVSDs includes incomplete, transitional, and complete. They should be distinguished on the basis of the AVV. If there is a common AVV orifice, the defect is complete, and if there are two separate AVV orifices, then the defect is incomplete. Incomplete AVSDs are also known as partial AVSDs or ostium primum ASDs. They consist of an ostium primum ASD, no VSD, and two separate AVV orifices with typically a cleft in the LAVV between the SBL and IBL (see Fig. 50.1 ). Of note, some prefer zone of apposition to cleft because it is thought to instead be a commissure and is not considered a true mitral valve. Throughout the chapter the term cleft will be used preferentially. A complete AVSD consists of an ostium primum ASD, a typically large and nonrestrictive VSD, and a common AVV orifice (see Fig. 50.1 ). Transitional or intermediate AVSDs have been described as an additional subtype, which is not used primarily by our center. They consist of an ostium primum ASD, a common AVV (single orifice), and no ventricular level shunting (either due to the absence of a VSD or closure of the VSD by aneurysmal tissue). Others have defined transitional AVSD (also known as intermediate AVSD) as an AVSD with two distinct left AVV and right AVV orifices and also both an ASD just above and a VSD just below the AV valves. Although these AV valves in the intermediate form do form two separate orifices, they remain abnormal valves. The VSD in this lesion is often restrictive.
Complete AVSDs are also subcategorized based on the SBL using the Rastelli classification. Type A has a SBL that is divided and attached to the crest of the ventricular septum. Type B has straddling chordae from the left SBL to right ventricular (RV) papillary muscles. Type C is a free-floating SBL that is not divided or attached to the crest ( Fig. 50.4 ). Type A is the most common, type B is rare and more likely associated with single-ventricle anatomy, and type C is more commonly associated with conotruncal anomalies.
Complete AVSDs are then further distinguished as balanced or unbalanced. This is determined not by the size of each ventricle but the proportion of the common AVV orifice distributed over each ventricle. This becomes an important determinant of surgical repair. Balanced and some unbalanced AVSDs (uAVSDs) are amenable to biventricular repair, whereas severely uAVSDs are treated with single-ventricle palliation. This is particularly important in patients with Down syndrome, who are not considered for single-ventricle palliation by many institutions given their poor outcomes with a Fontan procedure.
Associations
There is a strong association with Down syndrome. Approximately 30% to 40% of cardiac abnormalities in patients with Down syndrome are AVSDs. Other cardiac defects are also associated with AVSDs. A left superior vena cava (LSVC) can be present. Additional ASDs can be seen. Abnormalities of the AVV, such as a single papillary muscle with parachute left AV valve (PLAVV) (2% to 6%) or double-orifice left AV valve (DOLAVV) (8% to 14%) can be seen. There is an association with conotruncal anomalies, either tetralogy of Fallot (TOF) (10%) or double-outlet right ventricle (DORV) (2%). A patent ductus arteriosus (PDA) is commonly present but not always recognized on preoperative studies in the setting of a nonrestrictive VSD. Other defects can also be seen rarely.
Pathophysiology and Natural History
The pathophysiology of AVSDs is similar regardless of the type. The mechanism is left-to-right shunting across the septal defects, which can be exacerbated by AVV regurgitation (AVVR). The degree of left-to-right shunting at the ASD and VSD levels is dependent on the ventricular compliance and the pulmonary (PVR) and systemic vascular resistance (SVR), respectively. Initially after birth the PVR is high, and the degree of shunting (relative pulmonary and systemic blood flow; Q p :Q s ) is less. As the PVR falls, there is increased pulmonary blood flow, leading to a higher Q p :Q s . If left unrepaired, this increased pulmonary blood flow will induce vascular changes that lead to pulmonary vascular occlusive disease. The result is an increase in the PVR, leading to pulmonary hypertension. The Q p :Q s will decrease or can even reverse, leading to cyanosis, as in Eisenmenger syndrome.
The presence of AVVR further worsens the pathophysiology. It creates ventricular volume overload and can lead to congestive heart failure (CHF). The regurgitant jet can also create an obligate left ventricle (LV)-to-right atrial (RA) shunt via the left atrium (LA).
The presentation therefore varies depending on these factors. Incomplete AVSDs can present similarly to other ASDs and may be well tolerated for several decades. Complete AVSDs present earlier during infancy. Those with Down syndrome may have a delayed presentation due to higher PVR and decreased Q p :Q s . In contrast, those with a large VSD, AVVR, or left-sided obstruction such as subaortic stenosis (sub-AS) or coarctation of the aorta (CoA), have an accelerated presentation. However, the natural history in all cases leads to progressive CHF and associated sequelae. Advanced stages can also lead to irreversible pulmonary vascular obstructive disease. In this setting, CHF symptoms will improve, which is an ominous sign. It has been reported that up to 90% of patients with complete AVSDs will have pulmonary vascular disease at 1 year of age. If left untreated, AVSDs are ultimately fatal.
Diagnostic Assessment
Patients with AVSDs warrant a thorough diagnostic evaluation, including a birth history and physical examination, laboratory studies, electrocardiogram (ECG), chest radiograph (CXR), and transthoracic echocardiogram (TTE). In select cases, additional evaluation such as transesophageal (TEE) or three-dimensional echocardiography, cardiac catheterization, and cardiac magnetic resonance imaging may be indicated to answer specific clinical questions.
Physical examination may reveal stigmata of Down syndrome. Patients may have overt CHF signs. There is typically an active precordium and possibly a prominent thrill. There can be a pulmonary outflow murmur and a fixed, widely split second heart sound. A regurgitant systolic murmur may be heard in cases of AVVR. An ECG will have left axis deviation and may also demonstrate atrial enlargement and a prolonged PR interval. A CXR may demonstrate cardiomegaly and increased pulmonary vascular markings.
A TTE is the mainstay of diagnosis (see Figs. 50.1 and 50.2 ). Pertinent details to guide preoperative planning include the type of systemic venous connections, the size and location of ASD(s) and VSD, AVV anatomy and function, the degree of balance, size and function of the ventricles, the presence of systemic outflow tract obstruction, presence of a PDA, and any other associated cardiac defects.
The degree of unbalance in complete AVSDs can be challenging to evaluate. The anatomy of an AVSD makes standard measurements difficult and often unreliable. For example, z scores are not applicable, and ventricular size is not a consistent marker for unbalance because often the RV is larger than usual given the presence of a large atrial level shunt. Therefore a left-dominant uAVSD may have a nondominant RV that appears normal in size.
In cases of right-dominant uAVSDs with LV hypoplasia, attempts have been made to apply similar diagnostic algorithms used in other cardiac anomalies with hypoplastic left-sided structures to guide management. For example, the Rhodes score and the Congenital Heart Surgeons Society (CHSS) score used to assess LV adequacy in critical aortic stenosis have been used without success. Likewise, with left-dominant uAVSDs with RV hypoplasia, measures used for lesions such as pulmonary atresia with intact ventricular septum have not correlated well either.
It is therefore important to look at additional variables. In an effort to better guide these decisions specifically in patients with uAVSDs, extensive work has been done at Children’s Hospital of Philadephia. They have studied morphometric analysis of AVSDs in a retrospective fashion in an effort to use echocardiographic variables to estimate the degree of unbalance, guide surgical management, and predict outcomes. They first described the atrioventricular valve index (AVVI). Using a subxiphoid or subcostal left anterior oblique view, they bisect the common AVV by a line connecting the conal septum to the crest of the muscular septum. The smaller orifice area is then divided by the larger orifice area to give a ratio. A value equal to 1 is a balanced defect. An AVVI value of 0.67 was found retrospectively to distinguish those defects determined clinically to be balanced or unbalanced. Furthermore, in their series all patients with an AVVI less than 0.27 were found to undergo single-ventricle palliation. More recently, to eliminate ambiguity associated with this index, the modified AVVI was introduced. This is calculated as the LAVV area divided by the total AVV area. A value equal to 0.5 is a balanced defect. A modified AVVI less than or equal to 0.4 is right dominant and greater than or equal to 0.6 is left dominant. In retrospective analysis it was found that all patients with modified AVVI less than 0.19 underwent single-ventricle palliation, and all those with values between 0.4 and 0.6 underwent biventricular repair.
Other echocardiographic variables studied have included RV/LV inflow angle, LV inflow index (LVII), the amount of LA overriding the RAVV, VSD size, and the presence of retrograde flow in the transverse aortic arch. The RV/LV inflow angle is described as the angle between the base of the RV and LV free walls using the crest of the ventricular septum as the apex of the angle. The LVII is the assessment of the LV color Doppler inflow jet. Used together, these variables help stratify patients into appropriate surgical management.
In patients who present late or with cyanosis a diagnostic cardiac catheterization is indicated to determine the PVR. If the PVR is elevated, pulmonary vasoreactivity testing should be performed. Surgical closure of septal defects is contraindicated with a fixed PVR of greater than 10 U/m 2 .
Preoperative Critical Care Management
The management of all AVSDs follows the same principles. Treatment is directed at management of the underlying pathophysiology of left-to-right shunting and AVVR. Digoxin will increase cardiac inotropy. Afterload-reducing agents will decrease the SVR to decrease left-to-right shunting and improve AVVR. Diuretics will optimize the patient’s fluid status and preload. The goal is to control associated symptoms, avoid the complications of disease, and optimize the patient for anticipated surgery.
Down syndrome poses multiple management challenges with important implications. These patients are more difficult to sedate, they may have cervical spine instability that requires special precautions, and they are at increased risk of advanced pulmonary vascular disease. This is due in part to a combination of small airways, macroglossia with obstructive symptoms, chronic hypoventilation, and hypercarbia. This predisposition therefore affects clinical presentation, timing of surgical repair, and their candidacy for single-ventricle palliation in the setting of uAVSDs.
For patients who can be medically optimized, timing of surgical management of incomplete AVSDs is similar to that for secundum ASDs at approximately 2 to 4 years of age. Recommended timing for complete AVSDs is 4 months of age, earlier than for isolated VSD closure due to increased risk of advanced pulmonary vascular disease due to combined atrial and ventricular level shunting. However, in cases in which patients are unable to be managed medically and present symptomatic growth failure, earlier surgical management is recommended. The decision of complete repair or palliation is dependent on each patient. In general, complete repair is favored except in cases when contraindicated by comorbidities impacting surgical risk. Examples include patients not suitable for cardiopulmonary bypass due to poor clinical state or comorbidities such as prematurity, end-organ dysfunction, extracardiac anomalies, or neurologic factors. Additional reasons are those patients with complex congenital heart disease requiring more complex operations and uAVSDs that require single-ventricle palliation.
Surgical Correction
The first successful repair of an AVSD was by C. Walton Lillehei using cross-circulation in 1955. This was followed with repair using cardiopulmonary bypass by Kirklin, McGoon, and Cooley. The single-patch technique was then reported by Maloney et al. in 1962, the two-patch technique by Trusler et al. in 1976, and the modified single-patch technique by Wilcox et al. in 1997, followed by Nicholson et al. in 1999.
The goal of surgical repair of AVSDs is to create normal anatomy and physiology by septation of the heart with no residual intracardiac shunts and normal function of the AVVs. Depending on the type of AVSD, various techniques are employed. In addition, some types of anatomy and associated defects warrant special consideration and will be discussed separately.
Incomplete Atrioventricular Septal Defects
A standard median sternotomy is performed. We use autologous pericardium, which is harvested and placed in dilute glutaraldehyde solution, although other patch materials can be used. Polyethylene terephthalate (Dacron) is generally avoided because the roughness of the material can lead to hemolysis in the setting of an AVVR jet impacting the surface of the patch. Bicaval cannulation with aortic inflow is established with mild hypothermia and an LV vent. Antegrade cardioplegia is administered. A right atriotomy is performed close to and parallel to the AV groove to provide maximal exposure. The anatomy is inspected. The ostium primum ASD is closed by placing a series of horizontal mattress sutures on the rightward aspect of the crest of the ventricular septum, through the midpoint of the two AVV orifices, and then through the autologous pericardial patch. The sutures are then tied down. Before the patch closure is completed, attention is directed to the LAVV. The cleft between the SBL and IBL of the LAVV is tested by injecting saline into the LV to assess for coaptation and the degree of regurgitation. Identifying the last chords on the SLB and the IBL also assists with alignment of the cleft. The cleft is then closed with interrupted polypropylene sutures. The valve is again tested. The RAVV is then tested for competence. The pericardial patch is trimmed to appropriate size and sutured to the rim of the ASD using a running polypropylene suture. Care is taken to place superficial sutures in the area of the conduction system. It is our practice to leave the CS on the RA side, although others choose to place it on the left. The left heart is deaired before completion. The aortic cross-clamp is removed, the right atriotomy is closed, and the patient weaned from cardiopulmonary bypass.
Complete Atrioventricular Septal Defects
The exposure, harvesting of autologous pericardium, and cannulation strategy are the same as discussed earlier. The anatomy is inspected. The size and type of ASD and VSD are confirmed. The Rastelli type is determined. The LAVV papillary muscle configuration and adequacy of the left lateral leaflet are noted ( Fig. 50.5A ). The common AVV is then tested with cold saline to identify the central point of apposition of the SBL and IBL. This is the critical step of the operation. This location is marked with fine polypropylene suture to define the plane for septation of the common AVV into the left and right components.
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