| disease | Atrioventricular Canal Defect |
| alias | Endocardial Cushion Defect |
Atrioventricular canal malformation, previously known as endocardial cushion defect, ostium primum defect, common atrioventricular canal, or primary atrial septal defect, is a series of congenital heart malformations caused by the hypoplasia of the septum primum and endocardial cushion. The primary pathological feature is the partial absence of atrial and ventricular septal tissues above and below the atrioventricular ring, which may be accompanied by varying degrees of atrioventricular valve malformation. Atrioventricular canal malformation is relatively rare, accounting for only 5% of all atrial septal defect cases.
bubble_chart Pathological Changes
The pathophysiological changes caused by atrioventricular canal malformations primarily involve varying degrees of left-to-right shunting. The magnitude of the shunt depends on the severity of the lesion. In cases with an intact ventricular septum and no atrioventricular valve insufficiency, left-to-right shunting occurs only at the atrial level, leading to circulatory effects mainly characterized by right ventricular volume overload, increased right ventricular stroke volume, and elevated pulmonary blood flow, similar to secundum atrial septal defects. In cases with concomitant mitral valve insufficiency, blood from the left ventricle regurgitates into the left atrium during systole, significantly increasing the left-to-right shunt volume and enlarging the stroke volumes of both ventricles. These patients may exhibit marked cardiac enlargement and heart failure early in life. In complete atrioventricular canal cases, due to the presence of a large ventricular septal defect, left atrial blood can flow into the right ventricle during diastole, and left ventricular blood can flow into the right atrium during systole, resulting in an even greater left-to-right shunt. Approximately two-thirds of these cases have right ventricular and pulmonary artery pressures approaching systemic levels, and pulmonary vascular resistance can rise rapidly within the first year of life, leading to earlier onset of right heart failure. In advanced-stage cases, as pulmonary vascular resistance increases, the left-to-right shunt gradually diminishes, eventually resulting in right-to-left reverse shunting.
bubble_chart Clinical Manifestations
Atrioventricular canal malformations have various forms, which can be classified into the following types from simple to complex (Figure 3).
Figure 3 Classification of atrioventricular canal malformations
(1) Simple primum atrial septal defect (2) Partial atrioventricular canal malformation (3) Complete atrioventricular canal malformation
1. Primum atrial septal defect During embryonic development, the endocardial cushions are underdeveloped and fail to fully fuse with the septum primum, resulting in the persistent patency of the foramen primum after birth. The defect is crescent-shaped, with the lower border being the annulus of the atrioventricular valves and the upper border being the lower edge of the septum primum. The coronary sinus opening is located posteriorly and superiorly to the defect. The mitral and tricuspid valve leaflets are normal, and the ventricular septum is intact. This type is very rare.
2. Partial atrioventricular canal In addition to the primum atrial septal defect, there is a cleft in the central portion of the mitral valve's six leaflets. The length of the cleft varies, ranging from a small notch at the free edge of the leaflet to a complete split along the entire length of the leaflet. The edges of the cleft are rolled and have chordae tendineae attached. In most patients, the tricuspid valve leaflets are not cleft, and the leaflets are attached to the ventricular septum at their base, with no communication between the two ventricular cavities. The cleft in the mitral valve's large leaflet causes regurgitation, allowing blood to flow back from the left ventricle into the left atrium during systole, and from the primum atrial septal defect into the right atrium. This type of defect is the most common among atrioventricular canal malformations.
Figure 4 Complete atrioventricular canal, viewed from the right atrium
In cases of complete atrioventricular canal, because the chordae tendineae of the posterior leaflet are generally shorter than those of the anterior leaflet, the leaflets do not coapt well during ventricular systole, leading to more severe regurgitation and a larger volume of backflow. The left ventricular cavity is usually smaller than the right ventricular cavity.
Atrioventricular canal malformations may also be associated with other congenital cardiovascular anomalies, such as patent ductus arteriosus, tetralogy of Fallot, and pulmonary stenosis.
The clinical manifestations of atrioventricular canal malformations vary depending on the type of lesion, the magnitude of left-to-right shunting, the severity of atrioventricular valve regurgitation, and the degree of pulmonary vascular resistance elevation. The clinical presentation of a simple primum atrial septal defect is similar to that of a general atrial septal defect. Most patients may not exhibit clinical symptoms in early childhood, but as they grow older, pulmonary hypertension leading to pulmonary vascular obstructive disease can cause symptoms such as palpitations after exertion, shortness of breath, reduced exercise tolerance, respiratory infections, and right heart failure.
In some cases of partial atrioventricular septal defect, physical growth is poor, and pulmonary hypertension occurs earlier. The aforementioned clinical symptoms and heart failure can manifest early in life and progressively worsen.
Complete atrioventricular canal cases may present with pulmonary hypertension, heart failure, and progressive worsening within 1 year or even 1 month after birth. Rapid breathing, poor peripheral circulation perfusion, enlarged heart, and possible cyanosis are observed, often leading to early death. About half of the patients are accompanied by congenital dementia.
Physical examination: Most affected children exhibit delayed growth and development. The anterior chest is full and bulging, with strong apical impulses and an enlarged cardiac dullness area. The chest signs of a simple ostium primum atrial septal defect are similar to those of an ostium secundum atrial septal defect. In cases with cleft atrioventricular valves and complete atrioventricular canal, a rough pansystolic murmur can be heard over the precordium, loudest at the apex, and a tremor may be palpable. In pulmonary hypertension cases, the second sound at the pulmonary valve area is intensified with fixed splitting. A diastolic rumbling murmur may also be heard at the lower left parasternal and apical areas. In heart failure cases, the liver is enlarged, and cyanosis may sometimes appear.
bubble_chart Auxiliary Examination
Chest X-ray examination: The chest X-ray findings of simple ostium primum atrial septal defect are similar to those of larger ostium secundum atrial septal defects. In cases of partial or complete atrioventricular canal with atrioventricular valve regurgitation, the heart is significantly enlarged, with enlargement of the left ventricle, right ventricle, and right atrium, marked pulmonary vascular congestion, and increased pulsations. In cases with significantly increased pulmonary vascular resistance, the pulmonary vascular markings in the hilar region are thickened, while the peripheral pulmonary vascular markings are sparse.
Electrocardiogram examination: Prolonged P-R interval. Significant right ventricular hypertrophy or incomplete right bundle branch block may be present, with left ventricular hypertrophy and left axis deviation. The frontal plane vectorcardiogram shows a counterclockwise QRS loop, with the main body of the QRS loop shifted upward and to the left or even to the right, and the mean electrical axis between -30° and -140°. Some authors have noted that in partial atrioventricular canal cases, the frontal plane QRS loop runs counterclockwise to the left and upward, while the horizontal plane QRS loop runs counterclockwise to the left and backward. In complete atrioventricular canal, the frontal plane QRS loop runs counterclockwise to the right and upward, and the horizontal plane QRS loop runs counterclockwise forward and to the right.
Echocardiographic examination: The right atrium, right ventricle, and pulmonary artery are dilated. The motion of the ventricular septum is dependent on right ventricular contraction, showing paradoxical anterior movement during ventricular ejection. Cross-sectional echocardiography reveals the disappearance of the normal crux formed by the lower atrial septum, membranous ventricular septum, anterior mitral leaflet, and septal tricuspid leaflet. There is an interruption in the echo reflection of the lower atrial septum and upper ventricular septum, with communication between all four cardiac chambers. The mitral valve is displaced anteriorly, with the anterior leaflet approaching the septal tricuspid leaflet during systole and the ventricular septum during diastole. The mitral orifice appears "hammock-like" during diastole, and the left ventricular outflow tract is narrowed. After intravenous injection of contrast medium, due to higher left ventricular systolic pressure and slightly higher right ventricular diastolic pressure, contrast can be seen shunting back and forth at the upper ventricular septal defect.
Right heart catheterization: The catheter can pass from the right atrium into the left atrium or directly into the left ventricle. Left-to-right shunting is present at the atrial and ventricular levels, with higher oxygen saturation in the right atrium than in the vena cava and further elevation in the right ventricle. Pulmonary hypertension may show right-to-left shunting and increased pulmonary vascular resistance. In cases with mitral cleft, the right atrial pressure waveform may show elevated V waves due to atrioventricular valve regurgitation.
Selective left ventriculography demonstrates contrast flow through the atrial and ventricular septal defects into the right ventricle and/or right atrium, as well as regurgitation through the atrioventricular valve clefts into the left and right atria. It also reveals the size of the septal defects and the morphology of the atrioventricular valve anomalies. In partial atrioventricular canal, contrast injected into the left ventricle regurgitates into the left atrium before sequentially entering the right atrium, right ventricle, and pulmonary artery. In complete atrioventricular canal, contrast regurgitates into both atria while also directly entering the right ventricle through the ventricular septal defect, resulting in simultaneous opacification of both atria, the right ventricle, and the pulmonary artery. Due to the displacement of the atrioventricular valve orifice toward the apex and the upper ventricular septal defect, the left ventricular outflow tract becomes narrow and elongated, producing a "gooseneck" sign on anteroposterior left ventriculography. Thus, selective left ventriculography can differentiate between ostium primum and secundum atrial septal defects and is also valuable in distinguishing partial from complete atrioventricular canal.
In cases of atrioventricular canal defect with an intact ventricular septum and no pulmonary hypertension, if arterial oxygen saturation is reduced, the possibility of a common atrium malformation should be considered.
bubble_chart Treatment Measures
In 1954, Kirklin successfully corrected partial atrioventricular canal defects using Gross's "atrial well" method; in 1955, he further performed open-heart surgery under extracorporeal circulation. Lillihei successfully corrected a case of complete atrioventricular canal defect in 1954 using the cross-circulation method. In the early stages of surgical treatment, severe complications such as complete heart block, residual atrioventricular valve insufficiency, and subaortic stenosis were relatively common, and the surgical mortality rate was high. In 1958, Lev conducted a detailed study of the anatomical positions of the atrioventricular node and bundle in cases of atrioventricular canal defects and improved surgical techniques accordingly. Subsequently, the incidence of postoperative conduction tissue injuries significantly decreased.
The goals of surgical treatment for atrioventricular canal defects should include: closing the atrial septal defect, closing the ventricular septal defect, restoring atrioventricular valve function to eliminate insufficiency without causing stenosis, and avoiding injury to the atrioventricular node and conduction bundle.
There is no consensus on the surgical techniques for correcting atrioventricular canal defects. Due to the complexity of the lesions, the procedure is time-consuming and requires the use of extracorporeal circulation combined with hypothermia and cold cardioplegia. For infant cases, deep hypothermia with circulatory arrest may also be employed. Autologous pericardial patches or Dacron patches can be used to repair the septal defects. The advantage of autologous pericardial patches is that they reduce postoperative residual atrioventricular valve insufficiency or hemolysis, while the drawback is that they may lead to aneurysmal dilation when used for ventricular septal defect repair. In recent years, some have advocated valve replacement for cases where severe atrioventricular valve deformities are difficult to repair.
Surgical technique: A median sternotomy is performed. The pericardial patch for repairing the septal defects is prepared during the pericardiotomy. The presence of concurrent anomalies such as a left superior vena cava is examined. A finger is inserted through the right atrial appendage to assess the lesions, noting the location and size of the atrial and ventricular septal defects, as well as the presence and severity of mitral and tricuspid regurgitation. After the intracardiac finger exploration, the superior and inferior vena cavae and the ascending aorta are dissected and encircled with tapes for occlusion. Cannulas are inserted into the vena cavae near their orifices and the ascending aorta for venous drainage and arterial return, respectively, initiating extracorporeal circulation and lowering body temperature. A left ventricular vent is placed through a small apical incision. The ascending aorta is clamped, and cold cardioplegia is injected at its root while cold saline is poured into the pericardial cavity to further lower myocardial temperature. The vena cavae tapes are tightened, and a long right atriotomy is made for intracardiac repair.
Repair of Primum Atrial Septal Defect. The right atrium is incised to expose the atrial septal defect, with the inferior edge of the defect being the atrioventricular ring. Primum atrial septal defects require patching with pericardium or a Dacron graft, as direct suturing of the defect creates excessive tension, making the tissue prone to tearing and leading to recurrence of the defect. Patching the defect also reduces the incidence of injury to the conduction tissue. The pericardial patch or graft is trimmed according to the shape and size of the defect. The patch is first sutured and fixed to the mitral annulus at the central part of the base of the anterior mitral leaflet, followed by continuous suturing of the edges of the defect and the patch. Care should be taken to avoid injury to the conduction tissue when suturing the anterior-inferior edge of the defect. In cases of primum defects, the atrioventricular node is displaced downward into the posterior wall of the right atrium between the coronary sinus orifice and the ventricular crest. The atrioventricular bundle runs forward and upward along the edge of the defect to the junction of the ventricular crest and the atrioventricular ring, where it then gives off the left bundle branch at the top of the ventricular septum. In the area of the atrioventricular nodal triangle, sutures should be placed at the annular attachment of the anterior mitral leaflet. When the entire circumference of the defect is nearly sutured to the patch, residual air in the right atrium is first expelled, followed by releasing the caval tapes to remove air from the right atrium. A catheter inserted through the apex is used to expel any remaining air in the left ventricle. A venting needle is inserted into the root of the ascending aorta, and the aortic cross-clamp is released. Once the heartbeat resumes with strong contractions and the body temperature rises above 35°C, extracorporeal circulation can be discontinued. All intracardiac and aortic cannulas are removed, and the atrial, ventricular, and aortic incisions are sutured. Drainage tubes are placed in the pericardial cavity and anterior mediastinum, followed by closure of the sternum and chest wall incisions.
Partial Atrioventricular Canal Defect Repair After establishing extracorporeal circulation, the right atrium is incised to expose the lesion. Pressurized injection of saline via a left ventricular catheter aids in observing the coaptation of the atrioventricular valves. The cleft in the anterior mitral leaflet is intermittently sutured with 4-0 sutures, typically requiring only 3–4 stitches. Care is taken to avoid excessive tissue tension at the suture site, which could deform the leaflet and worsen regurgitation. If the edges of the cleft are curled, they can be flattened and sutured to reduce tension. In cases where short chordae or abnormal papillary muscles cause poor coaptation of the leaflet cleft, partial incision of the papillary muscle apex may improve alignment. The atrial septal defect can be repaired using the previously described method or by first suturing a larger pericardial patch to the base of the tricuspid septal leaflet at the apex of the anterior mitral leaflet cleft. The patch is then placed over the atrial septal defect and the atrioventricular node triangle, followed by suturing along the base of the tricuspid septal leaflet, the superficial tissue of the superior wall of the coronary sinus opening, or the inferior edge of the coronary sinus opening and the upper edge of the atrial septal defect (Figure 1).
(1) Right atrial incision (2) Suturing the mitral cleft
(3) Patching the atrial defect with a patch (4) Completion of repair
Figure 1 Partial Atrioventricular Canal Defect Repair
Complete Atrial Septal Defect Repair The morphology of complete atrioventricular canal defects varies. After incising the right atrium, the lesions of the atrioventricular valves and ventricular septum must be carefully identified. In cases with a cleft in the common anterior leaflet, the coaptation of the mitral leaflets and the chordal attachments at the edges of the anterior mitral leaflet cleft should be noted. A suture is then passed through the apex of the cleft at the base of the anterior mitral leaflet. For cases where the anterior mitral leaflet and tricuspid septal leaflet are fused, the connection between the mitral and tricuspid leaflets of the common anterior leaflet is incised above the ventricular septal crest to expose the ventricular septal defect. A Dacron patch is prepared according to the size of the ventricular septal defect and continuously or intermittently sutured with 5-0 sutures to the right side of the ventricular septal crest, leaving the tricuspid chordae on the right ventricular side of the patch and the mitral chordae on the left ventricular side. If a few chordae interfere with patch suturing, they may be excised. The anterior mitral leaflet on both sides of the cleft is then lifted to tighten the chordae, and the leaflet base is sutured to the patch edge, ensuring proper coaptation without deformation. The base of the tricuspid septal leaflet is sutured to the patch, ensuring the atrioventricular annuli are at an appropriate height. The cleft in the anterior mitral leaflet is sutured, while any cleft in the tricuspid valve usually does not require suturing. Saline is injected under pressure via a left ventricular catheter to observe atrioventricular valve function. Post-repair, about half of the cases may still exhibit mitral regurgitation, though less severe than preoperatively. If grade III mitral regurgitation persists, mitral valve replacement may be considered. Grade I tricuspid regurgitation has minimal impact on cardiac function, but if the tricuspid annulus is overly dilated, causing grade III regurgitation, annuloplasty can be performed at the junction of the septal and posterior leaflets. The pericardial or synthetic patch is trimmed to fit the shape and size of the atrial septal defect, and a continuous suture is passed through the base of the anterior mitral leaflet, Dacron patch, tricuspid septal leaflet base, and pericardial or synthetic patch. The primum atrial septal defect is then repaired as described earlier. Care must be taken to avoid injury to the atrioventricular node and conduction bundle (Figure 2).
(1) Right atrial incision (2) Incising the common leaflet
(3) Suture the mitral valve cleft (4) Patch the ventricular septal defect
(5) Patch the atrial septal defect (6) Suture the mitral valve cleft (7) Suture the tricuspid valve leaflet to the patch
Figure 2 Repair of complete atrioventricular canal defect
Treatment outcomes: The surgical mortality rate for partial atrioventricular canal cases is approximately 6%. Higher surgical mortality rates are associated with patients under 2 years of age, severe preoperative symptoms, cardiothoracic ratio above 0.6, cardiac function grade III or higher, and the presence of other cardiovascular anomalies.
Cases of complete atrioventricular canal defect are far more complex than partial atrioventricular canal defects and often involve other congenital cardiovascular anomalies, making the surgical procedure more challenging and resulting in higher mortality rates. In the early stages of surgical treatment, the mortality rate was as high as 60%, but it has now been reduced to below 30%. Factors such as infant cases, grade III atrioventricular valve regurgitation, cardiac function grade III or higher, cardiomegaly, smaller ventricular cavities, and the presence of other congenital cardiovascular anomalies all increase the risk of surgical treatment.
The natural course of unoperated atrioventricular canal defects varies depending on the type of lesion and the severity of functional impairment. In cases of ostium primum defects and partial atrioventricular canal with only grade I mitral regurgitation, if not accompanied by other congenital cardiovascular anomalies, the natural progression resembles that of large secundum atrial septal defects. Approximately 10-15% of cases develop pulmonary vascular obstructive disease due to pulmonary hypertension between the ages of 20 and 40.
In partial atrioventricular canal defects with moderate or greater mitral insufficiency, the left-to-right shunt is large, and pulmonary arterial pressure rises. At least 20% of cases show significant symptoms during infancy and die before the age of 10.
Complete atrioventricular canal defects are relatively rare, and there is insufficient data on their natural course. It is estimated that about 80% of unoperated cases die by age 2 due to pulmonary vascular obstructive disease and congestive heart failure. Therefore, surgical treatment should be pursued early for all types of atrioventricular canal defects. However, surgery is contraindicated if pulmonary vascular resistance exceeds 70% of systemic vascular resistance.
