Diagnosing Atrioventricular Canal Defects: A Comprehensive Guide for Automotive Experts and Beyond

Atrioventricular canal defects (AVCDs), also known as atrioventricular septal defects or endocardial cushion defects, represent a spectrum of congenital heart anomalies. These defects range from isolated atrial or ventricular septal defects to more complex conditions involving the atrioventricular valves and septa. Notably, atrioventricular canal defects also have a diagnosis of increased prevalence in individuals with chromosomal abnormalities, particularly trisomy 21 (Down syndrome), but they are also observed in non-syndromic patients. Congenital heart diseases, including AVCDs, remain a significant contributor to infant mortality, highlighting the critical need for early and accurate diagnosis and management. AV canal defects account for 3% to 5% of all congenital heart diseases (CHD) cases. This article delves into the critical aspects of atrioventricular canal defects, focusing on their diagnosis, underlying causes, and pathophysiology, to emphasize the importance of early detection and comprehensive care by an interprofessional team. This proactive approach aims to mitigate potential complications and improve outcomes for affected children.

Objectives

Upon completion of this article, readers will be able to:

• Identify conditions commonly associated with atrioventricular canal defects.
• Recognize the typical clinical presentation of patients diagnosed with atrioventricular canal defects.
• Articulate the potential complications that can arise in patients with uncorrected atrioventricular canal defects.
• Emphasize the crucial role of enhanced care coordination among interprofessional healthcare team members in facilitating early suspicion, accurate diagnosis, and effective management of atrioventricular canal defects, ultimately preventing cardiac failure and other significant morbidities in these patients.

Introduction to Atrioventricular Canal Defects

Atrioventricular canal defects (AVCDs) encompass a variety of heart malformations, including isolated ventricular septal defects (VSDs), atrial septal defects (ASDs), septal defects, atrioventricular valve anomalies, and endocardial cushion defects. A significant association exists between AVCDs and chromosomal abnormalities, most notably trisomy 21 (Down syndrome). However, it’s crucial to recognize that atrioventricular canal defects also have a diagnosis in individuals without identifiable syndromes.

Congenital heart diseases (CHD) are a leading cause of death in infants worldwide.[1] These conditions significantly impact morbidity, quality of life, and healthcare expenditure. Ventricular and atrial septal defects are considerably more prevalent than cyanotic heart diseases such as tetralogy of Fallot (TOF) and transposition of the great arteries (TGA). Advances in diagnostic echocardiography have empowered cardiologists to diagnose even subtle, asymptomatic atrioventricular (AV) canal defects at earlier stages. This improved diagnostic capability may contribute to the observed increase in reported CHD incidence in recent decades. While comprehensive epidemiological data from developing countries is still emerging, the incidence of CHDs appears to be comparable to that in developed nations.

Etiology of Atrioventricular Canal Defects

The majority of AV canal defects are linked to syndromic abnormalities. Several syndromes exhibit a known association with AV canal defects, including CHARGE syndrome, Down syndrome, Ellis-van-Creveld syndrome, Ivemark syndrome, Kaufman McKusick syndrome, Ritscher-Schinzel syndrome, Smith-Lemli-Opitz syndrome, and trisomy 3p. Among these, Down syndrome demonstrates a particularly strong correlation with AV canal defects.[2] Therefore, when considering atrioventricular canal defects also have a diagnosis, Down syndrome should be a primary consideration.

Epidemiology of Atrioventricular Canal Defects

Epidemiological studies estimate the incidence of AV canal defects to range from 0.24 to 0.31 per 1000 live births.[3] These defects constitute a notable proportion, approximately 3% to 5%, of all congenital heart diseases.[4, 5] While some research suggests a slight female predominance (female-to-male ratio of 1.3:1), particularly in cases associated with Down syndrome,[6] gender is not considered a primary risk factor. This potential gender difference might be an effect modifier related to the association between Down syndrome and CHD. Furthermore, maternal factors such as gestational diabetes mellitus (GDM), pregestational diabetes, and obesity have been identified as carrying a significant association with the development of non-syndromic AV canal defects.[7]

Pathophysiology of Atrioventricular Canal Defects

The formation of the four-chambered heart is a complex process requiring precise coordination and fusion of various mesenchymal tissues. Endocardial cushions, specialized mesenchymal tissues, play a critical role in this process, facilitating the separation of the common AV canal. The superior and inferior endocardial cushions within the AV canal normally fuse around 4 to 5 weeks of gestation. Disruptions or failures in the fusion of these endocardial cushions at different levels lead to a spectrum of septal defects, ranging from partial to complete AV canal defects.[8] Incomplete fusion of the interatrial septum results in communication in the lower portion of the septum, known as an ostium primum defect.

AV canal defects are broadly classified as either complete or partial, based on the specific anatomical abnormalities present.

Complete AV canal defects can be further categorized based on the morphology of the atrioventricular valve, utilizing the Rastelli classification. This classification system is predicated on the insertion of the chordae tendineae and the structural characteristics of the superior bridging leaflet.[11]

• Type A: The most prevalent type, especially in association with Down syndrome. The superior bridging leaflets are anchored to the left ventricle via chordal attachments.
• Type B: The least common type. The superior bridging leaflets extend to the right ventricle body through chordal insertions.
• Type C: Often associated with complex congenital heart conditions like tetralogy of Fallot (TOF) and transposition of the great arteries (TGA).[12] Characterized by free-floating superior bridging leaflets lacking chordal insertions.

History and Physical Examination in Atrioventricular Canal Defects

In patients with complete atrioventricular canal defects, symptoms of pulmonary overcirculation typically manifest before six months of age. The severity of these symptoms is directly related to the size and type of the defects.[4]

Early indicators of AV canal defects include tachypnea (rapid breathing) and failure to thrive (difficulty gaining weight). The presence and intensity of symptoms are influenced by the degree of atrioventricular valve regurgitation and any co-existing congenital heart diseases, which can expedite the onset of congestive heart failure (CHF).

Partial AV canal defects may present with subtle or delayed symptoms, potentially not becoming apparent until later in childhood. Notably, ostium primum atrial septal defects (ASDs) associated with AV canal defects tend to become symptomatic earlier than ostium secundum ASDs. A common initial presentation is the detection of a heart murmur during routine examination, resulting from increased blood flow across the pulmonary valve, best auscultated at the upper left sternal border.

Symptoms of Congestive Heart Failure: These include increased respiratory effort, diaphoresis (sweating) during feeding, poor feeding, lethargy, and excessive sleepiness.

Signs of Congestive Heart Failure: These include tachypnea, tachycardia (rapid heart rate), failure to thrive (growth deceleration across growth percentiles), wheezing or rales (abnormal lung sounds) on auscultation, S3 gallop rhythm (an extra heart sound), apical displacement of the apical impulse (heartbeat felt further out than normal), hepatomegaly (enlarged liver), or elevated jugular venous pressure (JVP).

A comprehensive cardiac examination is essential to assess for:

1. Wide and fixed split S2: This heart sound abnormality occurs due to left-to-right shunting across the ASD, leading to persistently increased blood flow to the right side of the heart, irrespective of the respiratory cycle.
2. S3: This extra heart sound arises from increased blood flow impacting the compliant left ventricular walls.
3. Additional Murmurs:
   • Holosystolic murmur: Caused by left atrioventricular valve regurgitation, best heard at the apex of the heart.
   • Mid-diastolic murmur: May be present if the shunt is substantial or if significant atrioventricular valve regurgitation exists.

General Physical Exam Signs:

Dysmorphic features suggestive of Down syndrome, such as a flat facial profile, upslanting palpebral fissures (eyes slanting upwards), a single palmar crease, and a sandal gap deformity (increased space between the first and second toes), should be carefully evaluated, particularly in patients whose karyotype (chromosomal analysis) is unknown. Approximately 40% to 45% of individuals with Down syndrome have AV canal defects.

A thorough assessment for other congenital anomalies, including cleft lip, cleft palate, and musculoskeletal abnormalities, should also be conducted.

Evaluation and Diagnosis of Atrioventricular Canal Defects

Echocardiogram:

Routine antenatal screening using fetal echocardiography has significantly improved the early detection of septal defects. However, cases diagnosed prenatally often carry a less favorable prognosis. When a defect is suspected during antenatal screening, a detailed fetal echocardiogram is typically performed. This advanced imaging technique can accurately define the type of defect, associated valve morphology, presence of shunts, and blood flow dynamics. Postnatal diagnosis is usually prompted by the presence of CHF signs and symptoms in a newborn or if a defect was identified or suspected during antenatal screening. Modern Doppler echocardiography has become the cornerstone of AV canal defect diagnosis, rendering cardiac catheterization largely unnecessary for diagnostic purposes.[13] However, cardiac catheterization may still be performed prior to surgical correction to precisely delineate the cardiac anatomy and hemodynamics in certain complex cases.

Chest X-ray:

In partial AV canal defects, chest X-ray findings typically reveal right heart enlargement and increased pulmonary vascularity. Intermediate and complete forms often demonstrate more generalized cardiomegaly (enlargement of the heart), affecting all chambers. However, left atrial enlargement may be less prominent, and left ventricular enlargement may be subtle due to masking by the more significantly enlarged right ventricle.[1]

Electrocardiogram (ECG):

A characteristic ECG finding in AV canal defects is the presence of a superior axis, directed more towards the left. The next most common ECG abnormality is the presence of rsR’ or rR’ patterns in the right precordial leads, indicative of volume or pressure overload on the right ventricle. Other ECG findings may include prolongation of the PR interval and evidence of right ventricular enlargement.[14, 15]

Treatment and Management of Atrioventricular Canal Defects

The definitive treatment for complete atrioventricular canal defect (CAVCD) is surgical correction, ideally performed as early as possible. Overall management strategies encompass three key phases: initial medical management, definitive surgical correction, and long-term follow-up care.[16]

Surgical correction is preferably undertaken before six months of age, as the risk of developing pulmonary vascular disease increases with prolonged duration of the defect.[17] Initial medical management focuses on optimizing myocardial function by reducing preload and afterload using diuretics and angiotensin-converting enzyme inhibitors (ACEIs) or angiotensin II receptor blockers (ARBs). Inotropic agents, such as digoxin, may be used to enhance cardiac contractility.[3] Surgical management of CAVCD is tailored to various factors, including the type of defect, valve morphology, associated valve and conduction abnormalities, presence of shunts, and other vascular anomalies.[18] Intraoperative mortality rates for surgical correction are reported to be around 3%, with a 10-year survival rate exceeding 90%.

Complete AVCD Surgical Strategies:

1. Balanced Lesion:

   • Primary complete repair is the preferred approach.
     • Techniques include single-patch, double-patch, and modified single-patch repair.

2. Unbalanced Lesion:

   • Palliative intervention: Single ventricle palliation may be necessary, similar to approaches used for hypoplastic left heart syndrome (HLHS) and tricuspid atresia.

Partial and Intermediate AVCD Surgical Strategies:

• Primary surgical repair is typically performed.
  • Techniques include patch closure of the septal defect and mitral valvuloplasty (valve repair) if needed.

Long-term Follow-up Care:

Annual evaluation by a cardiologist is recommended for individuals with AVCD, both those who have undergone surgical correction and those with uncorrected defects. This follow-up is crucial for monitoring for and preventing potential complications arising from the defect itself or as a consequence of surgery. Children with AVCD are at increased risk for neurological impairment; therefore, routine screening for neurological and developmental disorders is advised. Other long-term management considerations include infective endocarditis prophylaxis and risk assessment during pregnancy.

Differential Diagnosis of Atrioventricular Canal Defects

The differential diagnosis for AV canal defects primarily includes large ventricular septal defects (VSDs) and atrial septal defects (ASDs), particularly due to the shared symptom of congestive heart failure. While ECG and chest X-ray findings may exhibit some overlapping features, echocardiography is highly effective in differentiating AV canal defects from these other conditions and establishing an accurate diagnosis.

Prognosis of Atrioventricular Canal Defects

The mortality rate associated with treated atrioventricular canal defects is estimated to be approximately 3%.[16] The 10-year survival rate for treated patients is excellent, exceeding 90%. However, reoperation rates range from 10% to 20%.[19] The most common indication for reoperation, occurring in 5% to 10% of patients, is worsening mitral regurgitation.[20]

Complications of Untreated Atrioventricular Canal Defects

Regardless of the specific type of AV canal defect, patients will eventually develop a left-to-right shunt. The magnitude of shunting varies depending on the defect severity. In complete AV canal defects, blood shunts left-to-right at the atrial, ventricular, and atrioventricular valve levels. In partial defects, shunting occurs primarily through the ostium primum. If left uncorrected, these shunts lead to heart failure symptoms, often before the age of one year. Increased blood flow to the right heart circulation results in the development of pulmonary hypertension. A transient period of symptomatic improvement may occur during the early stages of pulmonary hypertension, which is a concerning prognostic indicator as it can progress to Eisenmenger syndrome, a severe and irreversible form of pulmonary hypertension. Incompetent atrioventricular valves contribute to significant valvular regurgitation within the heart chambers. Regurgitation predominantly occurs from ventricles to atria on the same side, although some left ventricle to right atrium regurgitation can occur through a cleft in the anterior mitral leaflet. These hemodynamic abnormalities can lead to atrial dilatation or ventricular hypertrophy, depending on the defect characteristics, accelerating the progression of congestive heart failure in affected children.

Deterrence and Patient Education Regarding Atrioventricular Canal Defects

Typically, an atrioventricular canal defect is suspected by a pediatrician or pediatric cardiologist based on symptoms of heart failure or the auscultation of a heart murmur during a physical examination. To confirm the suspicion of underlying heart disease, further diagnostic tests are typically ordered. These may include:

• Chest X-ray: Provides images of the heart and lungs to assess heart size and pulmonary blood flow.
• Electrocardiogram (ECG): Records the electrical activity of the heart to identify rhythm abnormalities and chamber enlargement.
• Echocardiogram (echo): Uses ultrasound waves to create detailed images of the heart’s structure and function, enabling visualization of septal defects and valve abnormalities.
• Pulse Oximetry: Measures the oxygen saturation level in the blood, helping to assess for cyanosis (low oxygen levels).
• Cardiac Catheterization: A more invasive procedure used to measure pressures and oxygen levels within the heart chambers and blood vessels. It can also visualize the heart’s anatomy. This is less frequently needed for diagnosis but may be used pre-operatively in complex cases.
• Cardiac MRI: Provides three-dimensional images of the heart, offering detailed anatomical information and helping to identify defects.

Treatment for AV canal defects typically involves early surgical intervention, ideally within the first six months of life. In some cases, medications may be used to manage symptoms and stabilize the patient until surgery can be performed. These medications may include:

• Diuretics (water pills): Help remove excess fluid from the body, reducing fluid overload in the lungs and improving breathing.
• Digoxin: A medication that strengthens heart muscle contractions, improving the heart’s pumping efficiency.
• ACE Inhibitors: Medications that relax blood vessels, reducing afterload and making it easier for the heart to pump blood.

Enhancing Healthcare Team Outcomes in Atrioventricular Canal Defect Management

The management of atrioventricular canal defects is intricate and demands a coordinated, multidisciplinary approach. Optimal outcomes rely on effective communication and collaboration between cardiac surgeons and the primary cardiology team involved in the patient’s care. A comprehensive understanding of the patient’s specific anatomy and a well-defined surgical strategy are essential prior to surgical intervention. In certain situations, factors such as a small ventricle or a straddling valve may preclude definitive biventricular repair, necessitating alternative palliative approaches. Therefore, multidisciplinary cardiac conferences involving cardiologists and cardiovascular surgeons are invaluable for collaborative decision-making regarding both preoperative and postoperative management strategies. As with any complex surgical procedure, thorough preoperative evaluation and optimization of the patient’s cardiac function with appropriate anti-failure medications are crucial. Given the potential risks associated with early-life surgery, nutritional support and monitoring by a nutritionist are essential to ensure adequate weight gain and nutritional status prior to surgery.

In the postoperative period, the roles of nurses and pharmacists are paramount. Nurses are critical in monitoring patients for pain, sternotomy site infections, chest tube drainage, and various common postoperative complications, including atelectasis, deep vein thrombosis, postpericardiotomy syndrome, and pain management. Pharmacists may be involved in parenteral nutrition if required until the patient can tolerate enteral feeding. Meticulous planning and open communication among all members of the healthcare team are strongly recommended to minimize morbidity and optimize outcomes for patients with atrioventricular canal defects.

Review Questions

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References

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