Monday, September 13, 2010

Atrioventricular Septal Defects (AVSD)


Definition:


Atrioventricular septal defects (AVSD), also called: Common AV canal defects, Persistent common AV canal, or Endocardial cushion defects, are characterized by deficiency of the lower part of the atrial septum, deficiency of the upper part of the ventricular septum, and anomalies of the mitral & tricuspid valves in various combinations.


Embryology:


Septa of the heart:


There are four septa in the heart:


1. The Atrial septum:


The upper part of the atrial septum is formed by the septum secondum, while the lower part is formed by the septum primum. The embryology of the atrial septum is discussed in details in the section on “atrial septal defect”.


2. The Ventricular septum:


The ventricular septum is largely formed by the muscular septum, and only a small part is formed by the membranous septum, which lies immediately below the aortic valve, at the junction between its right-coronary & non-coronary cusps. The embryology of the ventricular septum is discussed in details in the section on “ventricular septal defect”.


3. The Atrioventricular canal septum (also called: the septum intermedium);


The Atrioventricular  canal (AVC) is a small tube connecting the primitive atrium to the primitive ventricle in the fetus.


The AVC connecting the the primitive atrium (not shown) and the primitive ventricle, which later on is divided into the left ventricle (LV) & the right ventricle (RV). The bulbus cordis (BC) is also shown.
http://www.lumen.luc.edu/lumen/meded/grossanatomy/thorax0/heartdev/av1.jpeg



Mesenchymal tissues called: the anterior (or superior) & posterior (or inferior) endocardial cushions, develop inside the AV canal, grow towards each other and unite to form the septum intermedium, which divides the common AV canal into: a right (tricuspid) AV canal, and a left (mitral) AV canal.


The endocardial cushions growing to divide the common AVC into the the right (tricuspid) & left (bicuspid: mitral) AVC. The sinus venosus (SV), primitive atrium (PA), primitive ventricle (PV), bulbus cordis (BC), and truncus arteriosus (TA) are also shown.
http://sprojects.mmi.mcgill.ca/embryology/cvs/graphics/ecc.gif



The right AV canal is then absorbed into the RA, and the left AV canal is absorbed into the LA.



Division of common AV canal into right & left AVC.
http://www.med.unc.edu/embryo_images/unit-cardev/cardev_gifs/cardev026.gif


Division of common AV canal into right & left AVC.
http://www.lumen.luc.edu/lumen/meded/grossanatomy/thorax0/heartdev/av2.jpeg



The anterior and posterior endocardial cushions also have other functions:
  • The endocardial cushions form the primitive mitral & tricuspid valves: the formation of the septum intermedium is the basis for the creation of separate mitral and tricuspid orifices. Definitive mitral & tricuspid valves are then derived from ventricular muscles. Later on they become thin & fibrous.
  • The endocardial cushions share in the formation of the membranous ventricular septum.
  • The endocardial cushions share in the closure of the ostium primum in the atrial septum.
  • Since AV canals are absorbed into the atria; it follows that the endocardial cushions also share in the formation of the atria.


4. The Aorticopulmonary septum:


The truncus arteriosus is the most distal part of the bulbus cordis. It is the common arterial trunk leaving the fetal heart, connected to both ventricles.


Mesenchymal tissues called: the major (right & left) and minor (anterior & posterior) bulbar cushions, develop inside the truncus arteriosus, dividing it into the ascending aorta connected to LV & the pulmonary trunk connected to RV.


The embryology of the bulbus cordis & Aorticopulmonary septum is discussed in details in the section on “Aorticopulmonary septal defect”.


The major & minor bulbar cushions also share with the endocardial cushions in the formation of the membranous ventricular septum; the membranous ventricular septum is contributed ventrally by the bulbar cushions, and dorsally by the endocardial cushions (ECC).

Thus; partition of the AVC will form the mitral (bicuspid) & the tricuspid valve, while partition of the truncus arteriosus (TA) will form the semilunar (aortic & pulmonary) valves.




http://sprojects.mmi.mcgill.ca/embryology/cvs/graphics/valve_formation.gif


As there is no septal tissue separating the AV ring from neither the primum ASD nor the AV canal-type membranous VSD; it follows that if both primum ASD & AV canal type VSD are present, they communicate together, creating a large communication between the four chambers of the heart, and resulting in a large left-to-right shunt at both atrial and ventricular levels.



http://www.donroy.com/downhome/images/avsdheart.gif



http://www.mountnittany.org/assets/images/krames/104530.jpg


Atrioventriuclar septal defects (AVSD) result from failure of fusion or incomplete development of the anterior & posterior endocardial cushions:
  • Deficiency of the endocardial cushion tissue sharing in closure of the ostium primum will result in a ostium primum ASD.
  • Deficiency of the endocardial cushion tissue sharing in the formation of the membranous ventricular septum will result in an AV canal-type membranous VSD.
  • Deficiency of the endocardial cushion tissue sharing in the formation of the mitral & tricuspid valves will result in mitral and/or tricuspid valve anomalies.


Pathological types of AVSD:


1. Partial AVSD, which consists of:
  • Ostium primum ASD.
  • Cleft of the anterior mitral leaflet (AML).
  • With or without cleft of the septal tricuspid leaflet (STL).
  • Separate mitral annulus and tricuspid annulus.


Normal cardiac valves and their relations to each other: the heart in systole.
http://www.prenhall.com/heartlungsounds/images/systole.jpg


Normal cardiac valves and their relations to each other: the heart in diastole.
http://www.prenhall.com/heartlungsounds/images/diastole.jpg




Ostium primum ASD: note that there is no septal tissue separating the defect from the AV ring.
http://heartkids.congenital.org/pics/endocardial3a.jpg


Partial atrioventricular septal defect (AVSD): The mitral and tricuspid annuli are separate. The cleft in the mitral leaflet is in the anterior position. This type of anatomy is usually associated with a primum atrial septal defect (ASD). Partial AVSD is more common than intermediate AVSD.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-894813-3129tn.jpg




Partial AV canal defect; note the cleft in the AML.
http://www.pted.org/pics/endocardial3c.jpg


2. Complete AVSD which consists of:
  • Ostium primum ASD.
  • Perimembranous AV canal-type VSD.
  • Common atrioventricular (AV) valve; and a single annulus.


The presence of a common AV valve in complete AVSD is due to failure of division of the common AV canal into two canals, thus only one orifice remains, connecting the two atria to the two ventricles.


The common AV valve consists of 5 leaflets:
  • Anterior or Antero-Superior Bridging Leaflet (ASBL): formed of the anterior half of the AML & the anterior half of the STL.
  • Posterior or Postero-Inferior Bridging Leaflet (PIBL): formed of: the posterior half of the AML & the posterior half of the STL.
  • Left mural (left lateral) leaflet.
  • Right superior leaflet.
  • Right mural (right inferior) leaflet.



The common AV valve: the mitral valve (red) is shown to the left hand side and the tricuspid valve (blue) to the right hand side. The 5 leaflets of the common valve are seen: The ASBL and the PIBL have components from both mitral and tricuspid valves. The left sided mitral leaflet is the left mural one. The right upper tricuspid leaflet is the right superior, while the right lower tricuspid leaflet is the right mural one. Note that the aortic valve (red) is anterior to the common valve while the pulmonary valve (blue) is the most anterior valve opf the heart.
http://www.pted.org/pics/endocardial3e.jpg


(a) An incomplete atrioventricular septal defect (AVSD) with right superior leaflet (RSL), right lateral leaflet (RLL), right inferior leaflet (RIL), left superior leaflet (LSL), left lateral leaflet (LLL), and left inferior leaflet (LIL). (b) A complete AVSD with superior bridging leaflet (SBL), inferior bridging leaflet (IBL), LLL, RSL, and RIL. The locations of the atrioventricular (AV) node and bundle of His are indicated. All images are surgeon's-eye views with cranial leftward, caudad rightward, superior upward, and posterior downward.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/902594-903127-1349tn.jpg


3. Intermediate AVSD which consists of:
  • Ostium primum ASD.
  • Perimembranous AV canal-type VSD.
  • Separate mitral and tricuspid orifices; due to fusion of the ASBL & the PIBL, inspite of the presence of a single annulus.

Intermediate AVSD: A single valve annulus is present. The anterior and posterior bridging leaflets are fused (whereas in complete AVSD the anterior and posterior bridging leaflets are not fused). Therefore, the atrioventricular valve has a tricuspid and a mitral component. Intermediate AVSD is the least common type of AVSD.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-894813-3130tn.jpg




Diagram representing the common atrioventricular junction in hearts with an ostium primum atrioventricular septal defect viewed from the base of the heart. The fusion of the superior and inferior leaflets across the ventricular septum creates separate valvar orifices into each ventricle.
http://www.sahha.gov.mt/showdoc.aspx?id=490&filesource=4&file=fig25.jpg


4. Besides the three main pathological types of AVSD mentioned before, other variations are possible:
  • Isolated ostium primum ASD.
  • Isolated perimembranous AV canal-type VSD.
  • Isolated cleft of anterior mitral leaflet.
  • Isolated cleft of septal tricuspid leaflet.
  • Any combination of the above.



Rastelli classification of complete AVSD:

According to the attachment of the ASBL, Rastelli classified complete AVSD into 3 subtypes:
  • In a Rastelli type A defect, the SBL is divided at the plane of the interventricular septum and attached to the crest of the VSD by numerous chordae. 
  • Type B complete atrioventricular septal defects, which are rare, are characterized by cordal attachments from the left AV valve to papillary muscles in the right ventricle (RV). 
  • In a Rastelli type C defect, the SBL is said to be "free floating" because it is undivided and unattached to the crest of the VSD.
Rastelli classification of AVSD.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/902594-903127-1350tn.jpg


In all types of Rastelli classification, the PIBL may have any or no attachment.


The definition of the Rastelli subtype is mainly of surgical importance. Type A is the most easily corrected surgically, while type B correction is very difficult.


The left ventricular outflow tract (LVOFT) in complete AVSD:


In patients with complete AVSD, failure of AV canal division by the septum intermedium, the presence of a common AV valve facing the four chambers of the heart, and the presence of a large VSD facing both ventricles, disturb the development of LVOFT which becomes elongated and takes the shape of a goose neck; what is called: goose-neck deformity.


Complications of AVSD:
  • Partial type: like ASD
  • Complete type: like big VSD
  • Infective endocarditis is rare.


Pathophysiology of AVSD:
  • Partial type: like ASD.
  • Complete type: like big VSD.
  • Presence of mitral regurgitation (MR) and/or tricuspid regurgitation (TR) adds to the pathophysiological changes, accordingly.


Clinical picture of AVSD:


Female/ Male ratio: 1.3/1

About two thirds of cases of AVSD are of the partial type, and present clinically similar to secondum ASD, while all other combinations constitute only one third of cases.



Symptoms of AVSD:

Partial type:
  • Usually presents similar to ASD.
  • In presence of MR: CHF manifestations may be present.

Complete type:
  • Usually presents similar to big VSD; with severe CHF.
  • In presence of pulmonary stenosis (PS) or elevated pulmonary vascular resistance (Rp): the pulmonary circulation is protected and patients may do better.


Signs of AVSD:

Partial type:
  • Usually presents similar to ASD.
  • signs of MR & CHF may be found.

Complete type:
  • Usually presents similar to big VSD, with CHF manifestations.
  • Plus wide fixed splitting of S2, indicating associated ASD.
  • Sings of MR & TR may be found.


Chest x-ray in patients with AVSD:

Partial type:
  • Usually presents similar to ASD.
  • In presence of MR: bi-ventricular enlargement may be present.

Complete type:
  • Usually presents similar to big VSD,
  • The absence of impressive left atrial enlargement (LAE) is the finding which suggests the presence of ASD.




PA radiograph demonstrates only eleven sets of ribs. There is marked cardiomegaly with the left apex extending to the lateral chest wall. The pulmonary vascular markings are bilaterally increased and again air trapping can be appreciated in the right middle and left lower lung segments.
http://www.bcm.edu/radiology/cases/pediatric/images/2c1a.jpg




Chest X-Ray in a 4 month old child with Down syndrome demonstrates cardiomegaly and pulmonary plethora. Echocardiography confirmed the presence of an AVSD.
http://radiopaedia.org/uploads/radio/0013/1267/af7a0a1afd24a36b31e4a24bb04b09_gallery.jpg?1257181714



Electrocardiography in patients with AVSD:

QRS axis:
  • Left-axis deviation is characteristic.
  • Right-axis deviation occurs only in 3 %.
PR interval: is prolonged in most cases.


Chamber enlargement:
Partial type:
  • Right ventricular hypertrophy (RVH) is usually present.
  • With MR: RVH + left ventricular hypertrophy (LVH) may occur.
Complete type:
  • Enlargement of the 4 cardiac chambers may occur.

ECG from a child with a primum atrial septal defect (ASD). Note the left-axis deviation with a counterclockwise vector of depolarization (small q waves in leads I and aVL) and right ventricular hypertrophy and/or volume overload (rSR' pattern and upright T wave in lead V1).
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-889394-3112tn.jpg





Left superior axis deviation in the frontal plane and rR' pattern in right precordial leads.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/1331339-1331342-894813-1371484tn.jpg




Partial AVSD: Note that AVF is negative and so the axis is superior. There is an RSR' pattern in V1 but the T wave inversion is appropriate for this age. There is no evidence of right ventricular hypertrophy which would be expected if this was a complete atrioventricular septal defect.
http://www.med.nus.edu.sg/paed/resources/cardiac_thumbnail/common/avsd/Partial%20AVSD_ecg.jpg




ECG of a 3-month-old female with a left ventricular (LV)–dominant atrioventricular (AV) canal. The ECG reveals left axis deviation with an initial counterclockwise frontal loop.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-901552-3095tn.jpg


Echocardiography in patients with AVSD:

Apical 4 chamber view usually shows the pathology: the atrial septum, ventricular septum, and the common or separate AV valves according to the type of AVSD.

The presence of mitral, tricuspid or common valve regurgitation is detected by Doppler & color Doppler echo.

TEE may be needed in some patients to demonstrate the pathology.




Apical echocardiographic view of a primum atrial septal defect (ASD). Note the position of the defect when compared with a secundum ASD. RA = Right atrium; LA = Left atrium; RV = Right ventricle; LV = Left ventricle.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-889394-3107tn.jpg




Apical echocardiographic view of a primum atrial septal defect (ASD). Note that the atrioventricular valves are at the same level (instead of mild apical displacement of the tricuspid valve), which is seen in the spectrum of atrioventricular canal defects. RA = Right atrium; LA = Left atrium; RV = Right ventricle; LV = Left ventricle.
http://img.medscape.com/pi/emed/ckb/pediatrics_cardiac/889392-889394-3108tn.jpg





Cardiac catheterization in patients with AVSD:

Cardiac catheterization is not needed in most cases, but may show:
  • O2 step-up: At RA level.
  • Pressures: mild increase in RV & PA pressures in partial type, but marked in complete type, and may reach systemic levels.
  • Angiography:
    • LV angiography:
      • Goose-neck deformity: in complete type.
      • MR may be detected.
      • Shunts are visualized.
    • Aortography: may be done to exclude an associated patent ductus arteriosus (PDA).
  • Catheter may pass from RA to LA, RA to LV, RV to LA, and from RV to LV


Natural history:

  • Partial type: natural history is similar to ASD but:
    • higher incidence of endocarditis due to mitral valve deformity.
    • in presence of MR: CHF may occur with its consequences. 
  • Complete type: natural history is similar to big VSD, but spontaneous narrowing or closure of the defects does not occur.


Medical management:
  • Partial: like ASD.
  • Complete: like big VSD, with:
    • Endocarditis prophylaxis indefinitely.
    • Pacing may be needed.

Surgical management: 

Partial AVSD: 

Management is similar to ASD:
  • With mitral & tricuspid repair as needed.
  • Hospital mortality is about 6 %, and increases in:
    • Infants < 1 year of age. 
    • Severe pre-operative disability. 
    • Cardiothoracic (CT) ratio in x-rays > 0.6
    • Severe MR.


Complete AVSD:
  • Primary correction: Most infants need primary correction at age 6-12 months, with:
    • Closure of the intracardiac communications with a single patch or separate atrial and ventricular patches.
    • Construction of two separate and competent AV valves using the available tissue from the common AV valve.
    • Repair of associated cardiac anomalies.
  • Palliative PA banding: followed by repair is needed in critically-ill infants with low birth weights.
  • With associated pulmonary stenosis (PS): systemic-to-pulmonary shunt may be needed.
  • Hospital mortality is about 20 %; and increases in:
    • Infants < 1 year of age.
    • Severe mitral regurgitation (MR).
    • Very high pulmonary vascular resistance (Rp).
Primary correction of a complete AVSD.
http://www.mountnittany.org/assets/images/krames/104531.jpg




Video: Echocardiography: Apical 4-chamber view: Complete AVSD.
http://www.youtube.com/watch?v=v_XvufukTZU


Video: Echocardiography: Apical 4-chamber view: Complete AVSD.
http://www.youtube.com/watch?v=0J48OaQAkzo&NR=1

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