Anatomy

        Hypoplastic left heart syndrome (HLHS) can be subcharacterized into four anatomic subtypes based on the morphology of the left heart valves. The four types are:

• Aortic and mitral stenosis (narrowing of the aortic and mitral valves)

• Aortic and mitral atresia (absence of the aortic and mitral valves)

• Aortic atresia and mitral stenosis

• Aortic stenosis and mitral atresia

Differentiating between aortic stenosis where a biventricular repair is possible versus aortic stenosis that is incompatible with a biventricular circulation can be difficult.

Within children with the aortic atresia form of HLHS, the ascending aorta can be 2.5mm in diameter or even less. Within children with the aortic stenosis form of HLHS, the aorta is often 4-5mm in diameter. The ascending aorta is typically narrowest at its junction with the arch of the aorta and innominate artery (one of the large arteries that branches from the aortic arch). The diameter of the proximal arch is usually between 3 and 5mm. A coarctation shelf may even be formed. This shelf is formed by the abnormal extension of the ductal tissue into the aorta.

       In addition to the aorta, the pulmonary artery branches are also affected. The pulmonary artery branches are often smaller in HLHS, particulary in those with aortic atresia, than in other similar syndromes with a single ventricle and systemic outflow obstruction. This incomplete development (hypoplasia) of the pulmonary artery branches may be a consequence of decreased in utero pulmonary blood flow.

       HLHS also causes the formation of a smaller left atrium, a severly restrcitive foramen ovale, a thickend and fibrotic endocardium, stenotic pulmonary veins, and increased muscularization of the walls of the pulmonary veins. There are ofcourse other problems that come with HLHS, although these may not be present in every child with HLHS. For example, there could be structural abnormalities of the tricuspid and pulmonary vales, and total anomalous pulmonary venous connection ( the pulmonary veins that return oxygenated blood from the lungs connect to the right side of the heart, instead of the left atrium; this leads to the mixing of oxygenated and de-oxygenated blood).