Congenital Heart Defects

1. Acyanotic Heart Defects
   
   • Patent Ductus Arteriosus (PDA)
   • Atrial Septal Defect (ASD)
   • Ventricular Septal Defect (VSD)
   • Atrioventricular Septal Defect (AVSD)
   • Aortic/Pulmonary Stenosis
   • Coarctation of the Aorta
2. Cyanotic Heart Defects
   
   • Tricuspid Atresia
   • Pulmonary Atresia
   • Transposition of the Great Arteries
   • Tetralogy of Fallot
   • Hypoplastic Left Heart
   • Univentricular Heart
3. Quiz

Congenital heart defects are divided into two categories:

• Cyanotic defects - babies develop central cyanosis (a blue discolouration of the face, mouth and tongue), often in the first days or weeks of life. Oxygenated and deoxygenated blood mix in the heart, which lowers the partial pressure of oxygen (pO₂) of systemic blood. E.g. Tetralogy of Fallot and transposition of the great arteries.
• Acyanotic defects - the pO₂ of blood in the systemic circulation is maintained. E.g. atrial septal defects and coarctation of the aorta.

The development of the heart is complex. Unfortunately this can mean that there are opportunities for development to go wrong.

Congenital heart defects can be categorised into acyanotic or cyanotic defects. This describes whether the newborn will present with central cyanosis or not. Central cyanosis occurs when the partial pressure of oxygen (pO₂) in the systemic circulation is low. Babies have a blue discolouration to their face, mouth and tongue. On brown or black skin it may be easier to see on the palms, the soles of the feet, the gums and inside the eyelids. Peripheral cyanosis is different. It is a blue discolouration of the fingers and toes caused by poor blood flow, for example when the arteries there constrict in cold weather.

Acyanotic Congenital Heart Defects

This first section is about the category of acyanotic congenital heart defects. These are a group of heart defects that don't immediately present with central cyanosis. This is because the blood in the systemic circulation is fully saturated with oxygen so the pO₂ is maintained.

Patent Ductus Arteriosus (PDA)

• The ductus arteriosus connects the main pulmonary artery, close to the origin of its left branch, to the descending aorta (the part of the aorta that runs down the body after the aortic arch). It normally closes within the first few days of life. In PDA it stays patent (open).
• This patent connection allows blood under higher pressure in the aorta to flow into the pulmonary artery.
• This results in a higher volume of blood in the pulmonary circulation. This increases venous return to the left side of the heart (the flow of blood back to the heart along the pulmonary veins). The left side is given more blood than it can comfortably pump, which is called left-sided volume overload, and heart failure can follow if the PDA is untreated.
• Longstanding disease can also raise the pressure in the arteries of the lungs (pulmonary hypertension). This strains the right side of the heart and can lead to right-sided heart failure.
• This is an acyanotic heart defect because the blood reaching the systemic circulation is fully saturated with oxygen from the lungs.

Diagram - A heart with a patent ductus arteriosus heart defect

SimpleMed original by Dr. Bethany Turner

Atrial Septal Defect (ASD)

• Caused by underdevelopment of the septum primum or secundum (the two embryonic walls that together divide the atria), resulting in a hole in the atrial septum.
• This allows blood in the left atrium to flow into the right atrium.
• The increased volume in the right atrium leads to a higher volume of blood being pumped around the pulmonary circulation. If a large ASD is left untreated, this can cause damage to the blood vessels (vasculature) and fibrosis of the arteries in the lungs. This makes the arteries less distensible (less able to stretch), which increases resistance to blood flow and results in pulmonary hypertension (high blood pressure in the arteries of the lungs).
• Pulmonary hypertension means that the right side of the heart must work against an increased afterload (the resistance the ventricle has to pump against). This increased strain on the heart can lead to right-sided heart failure. 
• Pulmonary hypertension reflects increased resistance in the pulmonary vasculature. Pulmonary oedema is not a typical feature unless there is associated left-sided heart failure.
• If the pressure in the pulmonary circulation becomes high enough due to increased resistance, the pressure in the right atrium will exceed that in the left.
• This will cause the flow of blood to reverse, and deoxygenated blood will mix with oxygenated blood in the left atrium. The patient will become cyanotic due to the reduced pO2 in the systemic circulation. This whole sequence, from the raised pressure in the lungs to the reversed flow and cyanosis, is called Eisenmenger’s syndrome.
• A patent foramen ovale (PFO) is not classed as an ASD. PFO is caused by failure of the foramen ovale (the flap-like opening between the atria before birth) to close after birth. It is not an ASD because there is no true defect in the atrial septum. Most adults with a PFO have no symptoms. Left atrial pressure is usually higher than right atrial pressure, so there is no sustained right to left shunting (flow of blood from the right atrium to the left atrium). In some adults, though, blood does shunt from right to left through the PFO. A clot from a vein can then pass into the arterial circulation (a paradoxical embolism), which links PFO with cryptogenic stroke (stroke with no other identifiable cause).

Diagram - A heart with an atrial septal defect

SimpleMed original by Dr. Bethany Turner

Ventricular Septal Defect (VSD)

• During development of the interventricular septum, the membranous portion of the septum fails to develop properly. This means that the primary interventricular foramen (the opening between the developing ventricles) remains open.
• Blood in the left ventricle will flow into the right ventricle.
• The left ventricle is at a higher pressure than the right, which drives this flow. The size of the defect decides how much blood moves. A very small (restrictive) VSD lets little blood through and often causes no symptoms. A large VSD sends a lot of extra blood to the lungs and causes heart failure, usually from about 4 to 6 weeks of age, when the resistance in the lung vessels (pulmonary vascular resistance) has fallen.
• By the same mechanism as in ASDs, a moderate or large VSD left untreated can cause pulmonary hypertension and eventually right-sided heart failure.

Diagram - A heart with a ventricular septal defect

SimpleMed original by Dr. Bethany Turner

Atrioventricular Septal Defect (AVSD)

• Caused by failure of the endocardial cushions (masses of tissue in the developing heart that form the central part of the septa and the atrioventricular valves) to develop properly.
• Results in a hole in the middle of the heart. In a complete AVSD the hole joins the atria and the ventricles and there is one common atrioventricular valve, instead of separate mitral and tricuspid valves. In a partial AVSD the hole is only between the atria (an ostium primum ASD, low down in the atrial septum), and there are two separate valves, although the mitral valve has a cleft (a split). Blood flows from left to right through the hole, so the baby is not cyanosed at first, but the extra blood flow to the lungs leads to heart failure.
• AVSD is the heart defect most characteristic of Down’s syndrome. About half of babies with Down’s syndrome have a heart defect, and AVSD is the most common type in some studies, although ventricular septal defect is more common in others.

Diagram - A heart with an atrioventricular septal defect

SimpleMed original by Dr. Bethany Turner

Aortic/Pulmonary Stenosis

• One or both semilunar valves (the aortic and pulmonary valves) don’t develop properly and are narrow (this is called stenosis) when the baby is born.
• In the case of the aortic valve, a common defect is that the valve has only two leaflets (the flaps of the valve) instead of three. This is a bicuspid aortic valve.
• Aortic stenosis results in left ventricular hypertrophy and pulmonary stenosis results in right ventricular hypertrophy, as the ventricle has to generate more force to push blood through the narrowed valve. This hypertrophy can lead to heart failure.

Diagram - A heart with pulmonary stenosis. Notice the narrowed valve at the entrance to the pulmonary artery (circled)

Creative commons source by Mariana Ruiz LadyofHats; derivative work: MrArifnajafov (2010-11-30), translated and edited by Dr. Marcus Judge [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)] 

Coarctation of the Aorta

• This is a congenital narrowing of part of the aorta, commonly around the ductus arteriosus area.
• Upstream of the coarctation (towards the heart), blood pressure is high because blood struggles to flow past the coarctation. This increases the risk of aneurysms of the aortic arch. It also increases the risk of dilation of the aortic root (the first part of the aorta, next to the heart), which can lead to aortic valve regurgitation (blood leaking backwards through the valve). Coarctation is also linked to a bicuspid aortic valve and to aortic dissection (a tear in the wall of the aorta). People with coarctation are also more likely to have berry aneurysms (small bulges on the arteries of the brain), which can rupture.
• Downstream of the coarctation (towards the body), the femoral pulses are weak and delayed in older children and adults. Some older patients also have claudication (cramping pain in the legs on walking, in this case due to reduced perfusion).
• If the narrowing is after the three branches of the arch of the aorta, there will be a delay between feeling the radial and femoral pulses - radial-femoral delay.
• If the narrowing is between the brachiocephalic trunk (the first branch of the aortic arch) and the left subclavian artery, the left arm is supplied from beyond the narrowing. The left radial pulse is then weaker and later than the right, so there is a delay between feeling the radial pulses in the two wrists - radial-radial delay.
• Older children and adults with coarctation of the aorta typically present with radial-femoral delay (the femoral pulse is felt later than the radial pulse), strong radial pulses and weak or absent femoral pulses. This is the usual picture when the narrowing lies beyond the left subclavian artery, so that both arms are supplied by the aorta before the narrowing.
• A newborn with severe (critical) coarctation may look well at first. The obstruction becomes severe as the ductus arteriosus closes, usually in the first days to weeks of life. The baby can then develop heart failure and shock (not enough blood reaching the organs). This is an emergency. An infusion of a prostaglandin (which relaxes the duct) can reduce the obstruction until the baby is treated.

Diagram - A heart with coarctation of the aorta after the brachiocephalic artery

SimpleMed original by Dr. Bethany Turner

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