Introduction

The largest group of congenital defects are related to the heart, accounting for over one-third of all cases of congenital abnormalities. Heart conditions in babies and children are also usually congenital – meaning that they are present from birth. Congenital heart defects describe a group of conditions related to structural abnormalities of the heart and its associated vasculature. Detecting such defects early is essential, due to being life-threatening in some cases or causing a failure to thrive in children. Some conditions occur within a few minutes of life, whilst others may take weeks, months or even years. It is possible that a few of these conditions will be so minor that they will never be diagnosed or cause complications to the affected person.¹

Basic anatomy and physiology of the heart

The heart is responsible for pumping blood around the body. It is divided into 4 anatomical areas, which are known as the chambers of the heart. Each chamber is separated from the others, except when a valve is present, which allows blood to flow between compartments. The heart pumps blood to two areas separately: the lungs and the rest of the body.²

The right side of the heart

On the right side of the heart, there is a right atrium and a larger right ventricle. An atrioventricular valve(AV) is present between the two. The right atrium receives returning blood that has travelled around the rest of the body, meaning that the oxygen in it has been used/taken up already. When the AV valve is opened, blood from the right atrium can go down to the right ventricle. Once the right ventricle contracts, it pumps blood up through the (open) pulmonary valve into the pulmonary artery. The pulmonary artery therefore carries blood from the right side of the heart to the lungs. This blood is deoxygenated indicating a low oxygen level and that it carries more carbon dioxide, returning to the lungs for gas exchange. The lungs are responsible for its oxygenation.³

The left side of the heart

The circulatory system is a closed loop. So, if deoxygenated blood has travelled from the right side of the heart to the lungs and has become oxygenated, where does it go next? It returns to the heart. This time it travels towards the left side of the heart, and it’s being carried by the pulmonary veins. Oxygenated blood enters the left atrium and is eventually facilitated down to the left ventricle. The left ventricle is known for having a thick muscular wall – more muscular than the right ventricle. The reason is that the left ventricle will now need to pump oxygenated blood to the whole body. Once the left ventricle contracts, blood rushes out through the aortic valve and into the aorta supplying throughout the body.⁴

Foetal circulation

Interestingly, foetal circulation doesn’t really work in the same way.

In a foetus, the lungs are not mature enough for gas exchange, which occurs at the placenta instead. The foetal circulation focuses on delivering oxygenated blood to the heart and the brain. As the vessels in the lungs are in a hypoxic(lack of oxygen) state, it results in pulmonary vasoconstriction, and this leads to increased pulmonary vascular resistance. 

In simpler terms, there is increased resistance in the lungs, so the right side of the heart has higher pressure. 

This contrasts with adult circulation, where the right side of the heart has lower pressure than the left side.⁵

The other key adaptations in foetuses are foramen ovale and ductus arteriosus. 

• Foramen ovale is an opening or ‘oval-shaped window’ in the wall separating the right and left atrium. This means that blood can directly flow from one side to the other
• The ductus arteriosus connects the aorta and pulmonary artery, allowing blood to mix. With high right heart pressures, the foramen ovale shunts blood from right to left. The ductus arteriosus redirects blood from the pulmonary artery back to the aorta. Together, these adaptations bypass the lungs and right ventricle, enabling blood to quickly enter left-sided circulation for systemic distribution⁶

Causes of congenital heart defects

Many congenital heart defects may not have a known cause. Conception and pregnancy can be complex and much still needs to be explored with regards to the development of congenital abnormalities. However, there are some known causes:

• Chromosomal abnormalities: E.g., Down’s syndrome, patau's syndrome, Turner’s syndrome
• Maternal illness/ infections (particularly during pregnancy): Rubella, Diabetes Mellitus
• Drugs/Medications: Teratogenesis (e.g., sodium valproate), warfarin, alcohol, smoking

There is also an increased risk by about 3-fold if the baby had a first-degree relative (parents or siblings) with a congenital heart defect.

The main types of congenital heart defects

The following conditions are divided according to whether they are cyanotic, cyanotic or outflow obstruction.

Acyanotic

Acyanotic heart defects occur when oxygen-rich blood flows back to the right side of the heart due to a left-to-right shunt. This prevents deoxygenated blood from entering the body’s circulation, so babies with these conditions usually don’t develop cyanosis (a bluish tint indicating low oxygen). However, they may experience symptoms such as difficulty breathing, rapid breathing, or increased effort while breathing.⁷

1. Ventricular septal defect (VSD)

Around 30% of heart defects at birth involve a hole between the two lower chambers of the heart (VSD). This often shows up after the first week of life. Blood flows from the stronger left side of the heart to the weaker right side. Small holes usually make a loud noise (murmur) but don’t cause big problems. Larger holes can lead to heart failure and make less noise. Treatment may include medicine, and in some cases, surgery is needed. The most common way to fix it is through a less invasive procedure using a catheter.

2. Atrial septal defect (ASD)

There are different types of atrial septal defects, but the most common (80%) is secundum ASD. This occurs when the foramen ovale doesn't close after birth, allowing blood to move from the left atrium to the right due to lower pressure on the right side. ASDs are typically asymptomatic but can cause breathlessness, wheezing, and recurrent chest infections. Secundum ASD is usually managed with cardiac catheterization.

3. Patent Ductus Arteriosus (PDA)

The ductus arteriosus is a small vessel that connects two major arteries in unborn babies, helping blood bypass the lungs before birth. It usually closes soon after birth, but if it stays open (called PDA), some blood flows back to the lungs even though it’s already carrying oxygen. This doesn’t usually cause a blue tint (cyanosis) but can be noticed during a baby’s check-up.

The ductus closes naturally as certain hormone levels drop after birth. Medicines like ibuprofen can help close it. If it stays open, doctors can use a small procedure with a thin tube to fix it around the first year of life. In rare cases, keeping it open can help babies with other heart problems.⁸

Outflow obstruction

Several conditions are associated with outflow obstruction: pulmonary stenosis, aortic stenosis, and coarctation of the aorta.

1. Pulmonary stenosis 

Pulmonary stenosis is a condition where the pathway from the right side of the heart to the lungs is narrowed. This makes the heart work harder, increasing pressure on the right side. If severe, it can cause blood to flow the wrong way (right-to-left shunting) and lead to the right side of the heart becoming enlarged or even failing.

2. Aortic stenosis 

Aortic stenosis is a condition where the opening of the aortic valve is narrowed, making it harder for blood to flow from the heart to the body. This can cause babies and children to feel very tired, unwell, or have weak pulses, especially when they are active or exerting themselves.

3. Coarctation of the aorta 

Coarctation of the aorta is a condition where part of the main artery (aorta) is narrowed, often near a small blood vessel called the ductus arteriosus. This narrowing is usually linked to genetic factors. It causes high blood pressure before the narrowed area and low blood pressure after it. This can lead to uneven pulse strength, such as weaker pulses in the legs and stronger pulses in the arms. Blood pressure readings will also show these differences. The severity of the narrowing can vary, from mild to serious.⁹

Cyanotic

Cyanotic conditions occur when blood flows directly from the right side of the heart to the left (right-to-left shunt) without being oxygenated in the lungs. This causes the heart to pump deoxygenated blood to the body, leading to cyanosis (a bluish tint to the skin). As the organs receive low-oxygen blood, affected children and babies often show symptoms earlier than those with cyanotic conditions.¹⁰

1. Tetralogy of Fallot (TOF)

Tetralogy of Fallot (TOF) is a heart condition that involves four specific abnormalities: a ventricular septal defect (a hole between the heart's lower chambers), an overriding aorta (misplacement of the aorta), pulmonary valve stenosis (narrowing of the pulmonary valve), and right ventricular hypertrophy (thickening of the right ventricle). These issues work together to create a right-to-left shunt, where deoxygenated blood bypasses the lungs and flows into the body, leading to cyanosis. The increased pressure in the right side of the heart, caused by the thickened ventricle and narrow outlet, contributes to this. Severe pulmonary stenosis can speed up the onset of heart failure in affected babies.¹¹ 

Tet Spells also called cyanotic spells, are episodes of worsened cyanosis in conditions like Tetralogy of Fallot, triggered by crying, waking, or exertion. Older children may squat to improve oxygen levels while bending a baby’s knees to the chest can help. Medical treatment is available and managed by pediatric specialists.

2. Transposition of the great arteries (TGA)

Transposition of the Great Arteries (TGA) occurs when the aorta and pulmonary trunk switch places, with the aorta leaving the right ventricle and carrying deoxygenated blood to the body. This causes severe cyanosis and requires surgical correction. Early detection is crucial to keep the ductus arteriosus open, allowing some mixing of oxygenated and deoxygenated blood. Prostaglandins are used to maintain this connection temporarily until surgery can be performed.¹²

3. Atrioventricular septal defect - complete (AVSD)

Atrioventricular Septal Defect (AVSD) occurs when there are holes between the heart’s upper and lower chambers and problems with the valves in between. It’s commonly seen in babies with Down Syndrome, making genetic factors a significant risk.

There are two types of AVSD:

• Complete AVSD: More likely to cause cyanosis and can put high pressure on the right side of the heart, potentially leading to heart failure if untreated
• Partial (Incomplete) AVSD: Less severe and less likely to cause cyanosis

AVSD can be diagnosed before or after birth through physical exams or screenings. Surgical repair is the only treatment, with timing based on the defect’s severity and the child’s health.¹³

Summary

Congenital heart defects, structural abnormalities present at birth, significantly impact a child’s health and development. Understanding heart anatomy and foetal circulation is essential to grasp their causes. These defects are categorized as cyanotic, acyanotic, or obstructive. Cyanotic defects, such as Tetralogy of Fallot, result in oxygen-poor blood bypassing the lungs, causing bluish skin (cyanosis). Acyanotic defects, like ventricular septal defects, involve abnormal blood flow without cyanosis, while obstructive defects, such as coarctation of the aorta, cause narrowed pathways that disrupt circulation and strain the heart. These conditions can range from mild to life-threatening, with early detection through prenatal screenings or newborn exams being crucial. Advances in treatment, such as minimally invasive surgeries and medications, have improved survival rates and outcomes. With timely diagnosis and management, many children with congenital heart defects can lead healthier, active lives, highlighting the importance of early intervention and ongoing care.