Complete Atrioventricular Canal Defects

Introduction

• AVC defects account for approximately ~4% of congenital heart defects, occurring in 0.19 in 1,000 live births.¹
• AVC defects exist on a spectrum, with CAVC being the most severe.
• CAVC consists of three defects (Figure 1):
  • Primum ASD
  • Common AVV orifice
  • Nonrestrictive, inlet VSD

• CAVC be classified as: (1) balanced if both ventricles are similar in size and the AV valve distributes blood flow equally to both ventricles, or (2) unbalanced if one of the ventricles is larger than the other and the AV valve preferentially opens to one of the ventricles, which significantly affects surgical options and long-term outcomes.
• The Rastelli classification is used to categorize CAVC based on the anatomy of the superior bridging leaflet and the attachment of its leaflets to the interventricular septum (IVS). There are 3 types (Figure 2):
  • Type A (most common, ~75%): The superior bridging leaflet is completely divided, and there are chordal attachments between the valve and the crest of the IVS.
  • Type B (rare, 1-2%): The superior bridging leaflet is partially divided to the right of the IVS and has anomalous chordal attachments that straddle the IVS and attach to a papillary muscle. This type is associated with unbalanced defects.
  • Type C (~25%): The superior bridging leaflet is undivided, and there are no chordal attachments from the superior bridging to any portion of the IVS (free-floating leaflet).

Etiology⁴

Embryology⁴

• CAVC results from abnormal development of the endocardial cushions between the fifth and eighth weeks of gestation. The endocardial cushions contribute to the lower part of the atrial septum, the upper part of the ventricular septum, and the septal leaflets of the AV valves. Incomplete fusion leads to the septal defects and valve malformations seen with AVC defects.

Genetic Factors²

• There is a strong association between CAVC and trisomy 21.
• Other genetic syndromes associated with AVC defects include:²
  • CHARGE syndrome (coloboma, heart defects, choanal atresia, growth/developmental delay, genital hypoplasia, ear anomalies/hearing loss)
  • Noonan syndrome
  • Ellis-van Creveld syndrome
  • Smith-Lemli-Opitz syndrome
  • 3p deletion syndrome
• Nonsyndromic cases: AVC defects may also result from autosomal dominant mutations independent of syndromic conditions.

Epigenetic Factors

• Although less common, environmental factors such as gestational diabetes and maternal obesity are associated with AVC defects.

Associated Anomalies^5,6

CAVC has associated with almost all forms of both minor and major CHD, including:

• Transposition of great arteries (D-transposition Link) (congenitally corrected Link)
• Truncus arteriosus
• Tetralogy of Fallot Link
• Double outlet right ventricle
• Atrial isomerism
• Subaortic stenosis
• Ventricular hypoplasia
• Coarctation of aorta Link
• Right aortic arch
• Persistent left superior vena cava (LSVC)

Pathophysiology^2,3

• Pathophysiology depends on the sizes of the ASD and VSD, degree of AVV regurgitation, and whether the defect is balanced or unbalanced.
• There is almost always shunting at the atrial level. If the ASD is large, there will be significant mixing at the atrial level, and the patients may present with some degree of cyanosis.
• At the level of the VSD, the degree of shunting is determined by the balance between pulmonary vascular resistance (PVR) and systemic vascular resistance (SVR). Large, nonrestrictive VSDs may have left-to-right, bidirectional, or right-to-left shunting.
• As PVR decreases over the first weeks to months of life, left-to-right shunting increases, leading to volume overload of both atria and ventricles and eventually signs and symptoms of heart failure due to pulmonary overcirculation.
• Over time, increased pulmonary blood flow (Qp) leads to increased PVR, and pulmonary hypertension can develop (See OA summaries “Pediatric Pulmonary Hypertension: Classification, Pathophysiology, and Diagnosis” Link and “Pediatric Pulmonary Hypertension: Anesthetic Considerations” Link) As right-sided pressures increase, shunt reversal occurs, causing cyanosis (Eisenmenger syndrome).
• Patients with trisomy 21 are at an elevated risk of developing early pulmonary hypertension.

Diagnosis³

Clinical Presentation

• CAVC is often diagnosed in utero or after an evaluation for murmur.
• Patients with CAVC may present during the first year of life with frequent upper respiratory tract infections or signs and symptoms of heart failure including tachypnea, tachycardia, subcostal retraction, hepatomegaly, and feeding difficulty leading to failure to thrive.
• Physical examination shows a narrowly split S₂ with accentuation of P2 from increased pulmonary pressure and a holosystolic murmur.

Diagnostic Studies

• Electrocardiography (ECG): left-axis deviation with right ventricular hypertrophy
• Chest radiograph (CXR): typically shows cardiomegaly with increased pulmonary vascular markings
• Echocardiogram: Mainstay of diagnosis. Attention should be paid to the size of ASD, VSD, the morphology and degree of AV valve regurgitation, the presence of LVOT obstruction, and associated lesions such as the presence of a persistent LSVC or CoA.
• Cardiac catheterization: Usually not necessary for diagnosis. It may be useful in late- presenting patients with concerns for pulmonary hypertension.

Treatment Strategies⁴

Medical Management

• Medical management aims to optimize the patient for surgery and includes addressing failure to thrive through supplemental caloric intake, often via tube feeding, and pharmacologic treatment of CHF, including:
  • Diuretics to reduce preload
  • Angiotensin-converting enzyme inhibitors to reduce afterload
  • Digoxin to improve contractility

Surgical Management

• Surgical correction is preferably performed before 6 months of age, as the risk of developing pulmonary hypertension is related to the duration of exposure of the pulmonary arteries to increased pulmonary blood flow. Patients with trisomy 21 are often repaired around 3 months of age, given their increased risk.
• The surgical management of CAVC is determined by multiple factors, including the type of defect, valve morphology, associated valvular and conduction abnormalities, the presence of intracardiac shunts, and concomitant vascular anomalies.
• Balanced lesions undergo primary repair:
  • Single-patch repair: This approach involves using a single patch to close the ASD and VSD simultaneously, using autologous pericardium. Following the division of the common AVV, the right and left valve components are reattached to the newly created septal structure. Closure of the left AVV cleft is routinely performed.
  • Modified single-patch: The VSD is closed by directly suturing the common AVV leaflets to the crest of the IVS. A separate pericardial patch is then used to close the ASD. Left AVV cleft is then closed.
  • Two-patch repair: This approach uses separate patches to close ASD and VSD, respectively. Typically, a Dacron patch is used to close the VSD, while the ASD is closed with an autologous pericardium patch. As with the single-patch technique, the cleft in the left AVV is closed primarily.
• Unbalanced lesions are usually palliated along the single ventricle pathway if there is severe hypoplasia of one ventricle.

Anesthetic Consideration for Surgical Management^1-4

Preoperative

Labs

• Complete blood cell count
• Type and crossmatch
• Basic metabolic profile to assess renal function and electrolyte disturbances due to diuretic therapy
• Thyroid function tests in patients with trisomy 21

Imaging/Diagnostics

• ECG
• Echocardiogram
• CXR
• Cardiac catheterization to determine the PVR in selected patients who present for late repair, or those with cyanosis and a concern for pulmonary hypertension.

Intraoperative

Monitoring

• Standard American Society of Anesthesiologists monitors
• Core temperature monitoring
• Near-infrared spectroscopy
• Invasive pressure monitoring: arterial pressure and central venous pressure
• Transesophageal echocardiography to assess for residual intracardiac shunts, and AV valve function, as well as ventricular function and volume status following repair

Access

• Two large-bore peripheral IV catheters
• Arterial line
• Central line

Anesthesia Considerations

Induction

• General endotracheal anesthesia is used.
• Air bubbles must be avoided in the IV lines.
• Patients with CAVC with significant pulmonary overcirculation are often on multiple diuretics, leaving them intravascularly volume-depleted. Additional fluids may be needed.
• They also may have poor lung compliance and a highly reactive pulmonary vasculature that makes mask ventilation without gastric insufflation challenging and predisposes the patient to rapid desaturation once apneic.
• Patients with trisomy 21 pose additional concerns: 1) potential for upper airway obstruction; 2) bradycardia with anesthesia, particularly with volatile agents; 3) difficulty with vascular access; 4) cervical spine injury from atlanto-occipital instability; and 5) hypothyroidism.
• An inhalation induction is usually well-tolerated in clinically stable patients without overt heart failure, pulmonary hypertension, or severe AV valve regurgitation.
• An intravenous induction and maintenance with high-dose fentanyl may provide improved hemodynamic stability, especially for patients with a reactive pulmonary vasculature. High-dose fentanyl blunts the increases in PVR and SVR associated with surgical stimulation.

Anesthetic Goals

• The hemodynamic goals of induction and the pre-CPB period include minimizing left-to-right shunting and left AVV regurgitation in the pre-CPB period. FiO₂ should be minimized once the airway is secured. Hyperventilation should be avoided.
• Inotropic support may be required for low cardiac output syndrome post-CPB in patients who presented with severe heart failure. Hypertension should be avoided; however, in order to reduce stress on the newly repaired AVV.
• PVR may remain elevated after CPB. Management strategies for pulmonary hypertension include: supplemental oxygen, hyperventilation, inhaled nitric oxide, and the correction of acidosis, as well as the avoidance of hypothermia and ensuring an adequate depth of anesthesia. (See OA summary “Pediatric Pulmonary Hypertension: Anesthetic Considerations” Link)
• The timing of extubation depends on the severity of preoperative symptoms but may be performed in the operating room in select patients with smaller, less hemodynamically significant lesions. Extubation directly to noninvasive ventilatory support in the ICU may be beneficial to reduce complications and support recovery in patients with trisomy 21 and obstructive sleep apnea.

Postoperative

• CAVC are pressure- and volume-loading lesions. Immediate postoperative care should focus on managing PVR, SVR, and intravascular volume.
• Potential postoperative complications include:
  • Low cardiac output syndrome: This is generally managed with inotropic drugs rather than volume resuscitation. Milrinone’s dual actions as an inotrope and vasodilator make it particularly attractive for improving contractility, reducing stress on the repaired AVV valve, and modulating PVR.
  • AV block. The conduction system can be injured during repair. Temporary pacing may be required. If complete heart block persists more than 7-10 days after surgery, permanent pacemaker placement should be considered.
  • AV valve regurgitation or stenosis. Patients may develop progressive AVV stenosis or regurgitation. The AVV in small infants is thin and friable, putting them at risk for dehiscence of the left zone of apposition. A transthoracic echocardiogram should be performed in patients who are not progressing appropriately after surgery to rule out AV valve complications.
• Subacute bacterial endocarditis prophylaxis is recommended for six months after surgical or closure. This is continued indefinitely for patients with residual lesions adjacent to the sire of prosthetic materials.