Atrio-ventricular Canal Defects

Last updated 2015 - Written by Dr. Benjamin Marsh

Definition

During development two plump swellings arise from the ventrum and dorsum of theatrioventricular canal. Normally their fusion gives rise to a portion of the atrial septum, the tricuspid valves septal leaflet, the mitral valves anterior leaflet and the membraneous portion of the ventricular septum.

Failure of proper development of the endocardial cushion results in a ostium primum
ASD, an inlet VSD and common anterior and posterior cusps of the AV valve
extending across both ventricles resulting in a single valve orifice.

These defects culminate in AV regurgitation and LV->RA shunt often with pHTN.

Uncommonly the common valvular orifice favors one ventricle resulting in ventricle hypoplasia of the unfavored ventricle.

A partial AV canal does not include a VSD, and results in separate mitral and
tricuspid orifices.

AV canal comprises 2% of CHD, 47% have Downs syndrome.

Associated with Downs, Ellis Van Crevald and Ivemark syndromes.

Other defects associated with AV canal include:

• Tetrology of Fallot 6%,
• Double Outlet Right Ventricle 6%,
• Transposition of the great arteries 3%
• (these defects are more rare in Downs syndrome).

Clinical Manifestations

• Routine screening fetal echos can diagnose AV canal defects (17% of fetal CHD).
• Presentation: FTT, repeated respiratory infections, CHF in first 1-2 months. Downs children may not have an audible murmur 2/2 congenital pHTN but will rapidly progress to pulmonary vascular occlusive disease and will need early surgical intervention.
• PE: Hyperactive precodium, thrill at LLSB and loud P2. Murmur is 3-4/6 holosystolic at LLSB, mitral regurg (early SEM at apex with a click). A gallop or a mid-diastolic rumble or a gallop may result from relative mitral or tricuspid stenosis.
• EKG:  A superior QRS axis (-40 to -150) is pathoneumonic and RVH, incomplete or complete RBBB are usually present. Most have first degree AV block andLVH.
• CXR: cardiomegaly and increased pulmonary vascular markings.
• Echo is diagnostic.

Management

Treatment starts with medical management of CHF as surgical mortality is high in early infancy. In uncomplicated cases repair is indicated between 6-12wks of age. Surgical mortality is near 4%. 10 year survival is between 78-93%. Surgery is typically conducted through a right atrial approach and consists of ASD and VSD closure (using one or two patches), the AV valve tissue is fashioned into two separate functioning valves (competent mitral valve creation is prioritized) while on cardiopulmonary bypass and hypothermia.

Unbalanced AV canal may be treated by earlier PA banding and later modified Fontan operation.

Partial AV canals are operated on between 6-12mo, though concomitant AV regurge, coarctation, ect.,  may hasten surgery. Surgery has a 2% mortality rate and 10y survival is near 92%.

Post op

Complications include regurge, subaortic stenosis, SVT,  complete heart block and bradyarrhythmias (as a result of sinus node dysfunction).

Pt often will continue to need anticongestive medications (digoxin, diuretics, captopril) especially if AV regurge or ventricular shunts persist.

Endocarditis prophylaxis is mandatory.

Source:  Park Ch 3, Nadas’ Ch 38, Nelsons Ch 420