In some defects, oxygen-rich blood circulates back through the lungs instead of circulating to the rest of the body. This can cause increased pressure and stress on the lungs, which can lead to difficulty breathing and feeding, and can impact growth.

Defects in which too much blood flows to the lungs include:

• Atrial septal defect (ASD): An opening between the two upper chambers of the heart (the right and left atria) allows blood to flow abnormally through the heart. Patients may not show symptoms or have minimal symptoms, but for patients with a larger opening, later in life it can lead to problems like pulmonary hypertension, heart rhythm abnormalities, or heart failure.
• Patent ductus arteriosus (PDA): The ductus arteriosus is an opening that connects the aorta (which carries blood from the heart to the rest of the body) and the pulmonary artery (which carries blood from the right side of the heart to the lungs), allowing a fetus to receive oxygenated blood from the placenta before birth. It typically closes after birth. When it does not, this causes extra blood to flow into the lungs, which can lead to trouble breathing and poor weight gain.
• Ventricular septal defect (VSD): A hole in the ventricular septal (which separates the two lower chambers of the heart) allows blood from the left ventricle to flow back into the right ventricle. This causes the right ventricle to pump too much blood into the lungs, which can cause trouble breathing and poor weight gain, as well as pulmonary hypertension in larger holes that go uncorrected.
• Atrioventricular canal (AVC or AV canal) or atrioventricular septal defect (AVSD): In this condition, the heart has several structural abnormalities, including atrial septal defect, ventricular septal defect, and improperly formed mitral and/or tricuspid valves. Blood mixes freely throughout the four chambers of the heart, causing issues in both the heart and lungs, causing poor weight gain, breathing problems and cyanosis or turning blue.

In these types of defects, there is not enough blood flow to the lungs to pick up oxygen, so the blood that flows to the body has less oxygen in it. Children with these types of defects may have a blueish tint to their skin, lips or nails due to the lack of oxygen throughout the body.

Defects in which too little blood flows to the lungs include:

• Double outlet right ventricle (DORV): The aorta and the pulmonary artery are both connected to the right ventricle, and no arteries are attached to the left ventricle, affecting blood flow.
• Pulmonary atresia: The pulmonary valve (which is found between the right ventricle and the pulmonary artery) does not develop correctly, which inhibits blood flow from the right side of the heart into the lungs.
• Pulmonary stenosis: The area where blood exits the right ventricle is too narrow, inhibiting blood flow into the lungs.
• Tetralogy of Fallot: This condition mainly affects the right side of the heart, causing inadequate blood flow into the lungs. Tetralogy of Fallot involves four defects: a ventricular septal defect, which allows blood to flow between the right and left ventricle; a narrowing of the pulmonary valve, which blocks blood flow from the right side of the heart to the lungs; thickening of the right ventricle; and the aorta lying directly over the ventricular septal defect.
• Transposition of the great arteries: The positions of the aorta and pulmonary artery are switched, meaning the right ventricle pumps blood returning from the rest of the body back into the body, while the left ventricle pumps blood returning from the lungs back into the lungs.
• Tricuspid atresia: The tricuspid valve (which allows blood flow from the right atrium to the right ventricle) does not form, meaning blood does not flow from the right atrium to the right ventricle, causing poor blood flow into the lungs.
• Truncus arteriosus: The aorta and the pulmonary artery start as one blood vessel and eventually divide into two separate arteries in normal fetal development. Truncus arteriosus results when this divide does not occur, and the aorta and pulmonary artery remain connected.

In some defects, the heart is not able to pump enough blood to the body due to small or blocked valves or vessels in the heart.

These defects include:

• Hypoplastic left heart syndrome: This is a combination of several defects, in which most of the structures on the left side of the heart – including the left ventricle, mitral valve, aorta, and aortic valve – are underdeveloped and cannot pump enough blood through the heart to the body. Interrupted aortic arch: The aorta does not develop normally before birth, inhibiting blood flow. This usually occurs with other heart defects such as truncus arteriosus, ventricular septal defect, transposition of the great arteries, or aortic stenosis.
• Coarctation of the aorta: The aorta is narrowed, which obstructs blood flow to the lower part of the body and increases blood pressure above the point where the narrowing occurs.
• Complex single ventricle: The heart develops abnormally so that it has one large ventricle instead of two, meaning the patient only has one working ventricle instead of two, which may cause either too much or too little blood flow to the lungs.
• Aortic stenosis: The aortic valve between the left ventricle and the aorta is narrowed, making it difficult for the heart to pump blood out to the body.

Some defects affect the valves or vessels of the heart, which impacts the way blood flows through the heart. These include:

• Anomalous left coronary artery: This defect occurs when the left coronary artery (which supplies blood to the left side of the heart) is connected to the pulmonary artery instead of the aorta, which inhibits the flow of oxygen-rich blood into the heart.
• Anomalous pulmonary venous return: The heart typically has four pulmonary veins that bring oxygen-rich blood back from the lungs to the left atrium (upper left chamber of the heart). In this defect, one or more of the pulmonary veins returns back to the right side of the heart instead of the left.
• Bicuspid aortic valve: This occurs when the aortic valve does not develop normally. The aortic valve is a thin flap of tissue between the left ventricle and the aorta that has three flaps to help blood flow into the aorta. In bicuspid aortic valve, the valve has two just two thickened flaps, which limits blood flow or leaks.
• Corrected transposition of the great arteries: Both the ventricles and the arteries are reversed. Typically, the left ventricle pumps blood to the body, and the right ventricle pumps blood to the lungs. Because of this, the left ventricle is stronger than the right. In corrected transposition of the great arteries, because the ventricles are reversed, the weaker ventricle is tasked with pumping blood to the entire body. Some patients with this defect will have no or few symptoms as children and may not require immediate treatment. Some may need surgery; the type of surgery depends on the individual patient’s condition and any other associated heart defects they may have.
• Ebstein anomaly: The tricuspid valve separates the upper right chamber (the right atrium) and the lower right chamber (the right ventricle) of the heart. It has flaps that enable blood to flow forward into the heart. Ebstein anomaly occurs when the flaps of the tricuspid valve do not form normally and allow blood to leak backwards into the right atrium, causing the right atrium to become enlarged. Patients with a milder defect may need little to no treatment or medication. Patients with a more severe case may need surgery to repair the valve.
• Mitral valve prolapse: The mitral valve separates the upper left chamber (the left atrium) and the lower left chamber (the left ventricle) of the heart. It has flaps that enable blood to flow from the left atrium into the left ventricle. Mitral valve prolapse occurs when the flaps of the mitral valve do not form normally and allow blood to leak backwards into the left atrium. If the defect is mild, patients may require periodic monitoring and medication to treat symptoms. In rare severe cases, a patient may require surgery to repair the valve.