Hypoplastic Left Heart Syndrome Rather than being a single anatomic entity, the hypoplastic left heart syndrome groups together entities with characteristic physiologic and clinical properties The earliest grouping was made by Lev, but he described the syndrome as “hypoplasia of the aortic tract.”8 Hypoplasia of the left heart is not always the same as aortic atresia or hypoplasia On rare occasions, the aortic lesions can be found in the setting of a left ventricle of normal size, particularly when the aorta takes its origin from the morphologically right ventricle It was Noonan and Nadas, as far as we can establish, who first used the term “hypoplastic left heart syndrome.”9 Although the combinations of anatomic lesions that constitute the syndrome are variable, the features when seen together are unmistakable Thus the syndrome includes the majority of cases of aortic atresia and many of those with critical aortic stenosis The mitral valve can similarly be stenotic or atretic In all instances, however, a key feature is the integrity of the ventricular septum In most of the cases described in the initial investigation of Noonan and Nadas,9 aortic coarctation was the major anatomic feature, although the left ventricle was deemed to be hypoplastic It is questionable whether patients with such findings would today be included as having hypoplastic left heart syndrome Rare examples are found in which the mitral and aortic valves are of normal dimensions relative to the coexisting hypoplasia of the left ventricle Biventricular repair is possible in such cases, following simple relief of any obstruction within the aortic arch and descending aorta.10 This shows that major difficulties remain in providing an all-encompassing anatomic definition for hypoplasia of the left heart.11 It is more realistic, therefore, to analyze the unifying hemodynamic features In physiologic and clinical terms, hypoplasia of the left heart can be defined as the situation where the systemic circulation is dependent on the morphologically right ventricle in the setting of atresia or severe hypoplasia of the aortic valve.12 Such hypoplasia of the left heart is usually found in the setting of usual atrial arrangement with concordant ventriculoarterial connections There can be concordant atrioventricular connections (Fig 69.7, left) or mitral atresia due to absence of the left atrioventricular connection (Fig 69.7, right) In those with mitral atresia, the left ventricular cavity is often no more than a slit in the posteroinferior wall of the ventricular mass It is almost impossible to determine on morphologic grounds when the left ventricle is to be considered hypoplastic in patients with aortic valvar stenosis rather than atresia When there is obvious fibroelastosis in the small ventricle and the right ventricle is apex-forming, patients with such hearts certainly fulfill the anatomic criterions for the syndrome (Fig 69.8) The left ventricular walls are hypertrophied when there is mitral stenosis rather than atresia (see Fig 69.7, left) Indeed, the mass of the left ventricle may rarely be increased rather than decreased despite the cavitary hypoplasia The left atrium is usually small, although there can be hypertrophy of the left atrial walls Some patients can have an enlarged atrium, particularly the appendage The oval foramen is usually patent, but in up to one-tenth of patients, the atrial septum will be intact It can be aneurysmal, with protrusion into the cavity of the right atrium (Fig 69.9) FIG 69.7 Variations found at the atrioventricular junction in the setting of hypoplastic left heart syndrome Left, Mitral stenosis Right, Mitral atresia due to absence of the left atrioventricular connection Fibroelastosis of the left ventricle is found only in the setting of mitral stenosis FIG 69.8 Critical aortic stenosis that fulfills the criteria for inclusion as an example of hypoplastic left heart syndrome The right ventricle is apex forming, and the hypoplastic left ventricle is thick-walled, with a fibroelastotic lining