Hypoplastic left heart syndrome: Year of the Zebra 2024

Hypoplastic left heart syndrome, or HLHS for short, is a congenital heart defect that affects the left side of the heart, most notably the left ventricle and the aorta.
The exact mechanism that causes HLHS is unknown, but one theory is that another primary heart defect reduces blood flow through or out of the left ventricle during fetal development, so that part of the heart does not grow and develop as expected.
HLHS is often associated with other heart defects, particularly an atrial septal defect, which is an opening between the left and right atria.
Normally, the left ventricle receives oxygenated blood and pumps that blood out to the body.

In babies with HLHS, oxygenated blood, has a difficult time getting into the underdeveloped left ventricle, so instead, most of the blood flows from the left atrium to the right atrium through the atrial septal defect, where it mixes with deoxygenated blood.
This mixture of blood then goes to the right ventricle, where most of it is sent to recirculate through the lungs.
However, some of this blood can exit the right ventricle and be sent to the body, through the ductus arteriosus, a duct connecting the pulmonary artery and aorta.
This duct is present in all babies at birth, fully closes within several days following birth, and, in babies with HLHS, provides an essential pathway for some oxygenated blood to get to the body.

Signs and symptoms of HLHS begin in the first couple days after birth, when the ductus arteriosus begins closing. As it closes, blood flow becomes restricted, limiting the amount of oxygenated blood that goes out to the body.
As a result, babies with HLHS develop signs of circulatory insufficiency, including low blood oxygen levels; cyanosis, a bluish skin discoloration; fast breathing; poor feeding; and cold or pale extremities.