Congenital Heart Defects



        The fetal heart is much different from what is seen at the time of birth on.  The fetal circulation and heart is modified to bypass the non-functioning lungs and intestines.  All oxygen and nutrients are received from the mother via the placenta and these organs do not need to function until after birth. 

        The circulation to the gut is bypassed by the ductus venosus.  The left branch of the hepatic portal vein joins the umbilical vein and bypasses the liver by the ductus venosus.  The blood bypassing the liver is oxygenated and mixes with the deoxygenated blood of the inferior vena cava returning blood to the right atrium.  2/3 of this blood passes from the right atrium to the left atrium through the foramen ovale.

        In the beginning of embryological development the heart is not segregated into four distinct chambers, but rather a single chamber.  The atrioventricular chambers are formed by the fusion of the superior and inferior endocardial cushions that will separate the heart into right and left atrioventricular openings.  The atrial septa are created by the fusion of the septum primum and septum secundum.  The primary septum is the first to appear.  It grows downward from the posterosuperior portion of the developing chamber and does not reach the developing atrioventricular area.  The passage created is termed the ostium primum.  The ostium primum will be obliterated once it meets the endocardial cushions.  Before the ostium primum closes however, a second hole is created by apoptosis in a more superior part of the septum primum.  This hole is termed the ostium secundum.  The septum secundum develops from the roof of the primitive atrium.  It develops passed the ostium secundum, but will not reach the atrioventricular area.  The result is an oblique hole created by the incomplete septum secundum and ostium secundum (foramen ovale).  The formation of the foramen ovale is one of the ways fetal circulation is modified during fetal development.

        The blood that remains in the right atrium bypasses the lungs by the ductus arteriosus.  The ductus arteriosus shunts blood from the pulmonary trunk to the aorta and into the umbilical artery to receive O2 and nutrients from the placenta.  

        At the time of birth there is a decrease in resistance to the lungs due to air expansion and this increases blood to the lungs.  The increase in blood flow returning from the lungs increases the pressure in the left atrium and forces the septum primum against the septum secundum closing the oblique passage.

        The incidence of congenital heart disease in people with Down’s Syndrome is estimated to be between 40-50% (Van Dyke, D.C., Lang, D.J., Heide, F., van Duyne, S., and Soucek, M.J., 1990, and Pueschel, S.M., 2001) and 40-66% (Jackson, P.L., and Vessey, J.A., 2000).  The most common congenital heart anomalies in people with Down's Syndrome are patent ductus arteriosus, atrial septal defect, ventricular septal defect, atrioventricular canal defects, and Tetralogy of Fallot.

Patent Ductus Arteriosus (PDA)

        The physiological consequence of the ductus arteriosus failing to close at the time of birth is a left to right to shunt.  The systemic system is under more pressure than the pulmonary system and the left ventricle musculature becomes thicker, relative to the left ventricle, to compensate for its requirement to do more work.  The high pressures created by the left ventricle with a PDA causes blood to flow from the aorta into the pulmonary trunk.  The partial pressure of oxygen returning to the lungs is increased because the blood from the aorta is oxygenated and the blood in the pulmonary trunk is deoxygenated.  Poor oxygen saturation results in decreased perfusion at the lungs due to the reduced pressure gradient created for oxygen to travel across.

        Another physiological consequence of a PDA is the increase in the volume of blood that is in the pulmonary system and the decrease of blood in the systemic system.  The low volume of blood in the systemic system decreases the cardiac output by lowering the stroke volume (cardiac output = stroke volume x heart rate).  The volume overload in the pulmonary vasculature creates edema because the high pulmonary resistance forces plasma into the interstitial space.  The fluid can "wash" away surfactant.  Surfactant decreases the surface tension of alveoli by decreasing the adhesive forces between water molecules at the alveolar-capillary membrane.  It is able to do this because it has hydrophobic properties and when it gets in between the water molecules, the water molecules are farther apart and less likely to bond.  Surfactant also ensures that smaller alveoli do not empty into ones that are much larger.  By washing away the surfactant, there is more alveolar surface tension increasing the work of breathing.

Atrial Septal Defect

        The atrial septal defects that are most common to people with Down's syndrome are of three types, ostium secundum, ostium primum and sinus venosus.  An ostium secundum defect is an abnormally large hole in the foramen ovale, where the ostium secundum is located.  An ostium primum defect occurs lower in the atrial wall and is associated with problems in the atrioventricular valves.   A sinus venosus defect can occur anywhere between the superior and inferior vena cava and is associated with an anomalous connection of the venous system to the left atrium.  The physiological consequences of what happens in an atrial septal defect are similar to a PDA.  The pulmonary blood supply becomes overloaded resulting in pulmonary hypertension, decreased perfusion, and pulmonary edema.

Ventricular Septal Defect

        A ventricular septal defect is a hole in the ventricle that can range anywhere from the size of a pinhole to a single ventricle (Vaska, P.L., 1997).  Hypertrophy of the right and left atrium may occur.  Also, the aorta may develop an insufficiency due to lack of support from the defective ventricular wall.  A left to right shunt develops due to high pressures created in the right ventricle.

Atrioventricular Canal Defects (AVCD)

        There are two types of Atrioventricular defects common to Down’s Syndrome. In a partial atrioventricular canal defect the AV valve rings are normal with a cleft in one of the mitral valve leaflets (Vaska, P.L., 1997).  This results when the endocardial cushion fails to obliterate the ostium primum.  Normally the leaflets are free to move, but in a partial AVCD, two of the five leaflets (superior and inferior bridging leaflets) become tethered to the ventricle and only atrial communication may be left.  The left and right side of the heart are still separate (Seale, A., and Shinebourne, E.A., 2003).  A complete AVCD is when the AV valve is complete but the AV valve rings are incomplete resulting in a common AV orifice (Vaska, P. L., 1997).  The superior and inferior bridging leaflets of the mitral valve are free floating and incapable of functionally separating the left and right side of the heart.  There is subsequent interatrial and interventricular communication  (Seale, A., and Shinebourne, E.A., 2003).
   

Diagrams showing septal defects
(Picture from Seale, A., and Shinebourne, E.A., 2003)
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Tetralogy of Fallot

        Tetralogy of Fallot is associated with four anomalies seen together.  The four anomalies are ventricular septal defect, pulmonary stenosis, aortic valve positioned to override the right ventricle, and right ventricular hypertrophy secondary to the pulmonary outflow obstruction (Vaska, P.L., 1997).  The overall morphology of the heart is reminiscent of a boot and is sometimes referred to "coeur en sabot" to reflect the shape. (http://www.emedicine.com/emerg/topic575.htm#target11, retrieved April 2, 2004)  As a consequence to the four anomalies desaturated blood is ejected from the right ventricle through the ventricular septal defect and through the aorta.  A right to left shunt replaces the left to right shunt seen in many of the other congenital heart defects in individuals with Down’s syndrome.  Cyanosis ensues due to the deoxygenated blood being pushed through the systemic circulatory system.  The pulmonary artery muscles may suddenly constrict causing an acute increase in pulmonary vascular resistance.  A critical situation ensues that result in a hypoxic “tet” spell.  A hypoxic ‘tet” spell may be relieved by squatting to increase venous return (Van Dyke, et al., 1990).  This is seen in the exercising child.  These episodes are unpredictable and have the potentially to be lethal. (http://www.emedicine.com/emerg/topic575.htm, retrieved April 2, 2004)  If the individual is chronically hypoxic polycythemia may develop (Vaska, P.L., 1997).

Tetralogy of Fallot
Picture from http://www.mayoclinic.org/tetralogyfallot-rst/fourtypes.html

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Page created by Myles Dmyterko and Lorianne Earl
Last modified April 4, 2004