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Hypoplastic left heart
Historical Notes
In 1952, Lev called attention to congenital hypoplasia of major components of the left side of the heart. In 1958, Noonan and Nadas referred to these malformations as the hypoplastic left heart syndrome . At the severe end of the spectrum, the aortic and mitral valves are atretic, and the left ventricle is virtually nonexistent. , At the mild end of the spectrum, the aortic and mitral valves are patent, and there is a lesser degree of left ventricular hypoplasia. The hypoplastic left heart syndrome is a genetically heterogeneous disorder that affects 1 in 5000 live births and accounts for 7.5% of infants with congenital heart disease.
Anatomical considerations
Aortic atresia is accompanied by a hypoplastic ascending aorta that serves as a common coronary artery ( Figs. 28.1 and 28.2 ). A hypoplastic but patent mitral valve is accompanied by a hypoplastic but patent left ventricle (see Fig. 28.1 ). Mitral atresia is accompanied by a blind slit-like left ventricular cavity imbedded in the ventricular muscle ( Figs. 28.3 and 28.4 ; see Fig. 28.2 ). An experimental model in chick embryos is represented by atresia or hypoplasia of the mitral valve, and of the left ventricle, aortic valve, and thoracic aorta. Hypoplasia or atresia of the mitral valve leaves the left atrium without an exit except a restrictive patent foramen ovale (see Figs. 28.1 and 28.2 ), which is further compromised by hypoplasia of the limbus that is rotated and deviated close to the orifice of the superior vena cava. An intact atrial septum is accompanied by either a thick muscular septal wall and a small left atrium, or by a thick septum secundum, a thin septum primum, and an enlarged left atrium. Leftward displacement of the septum primum is common. Alternative decompression pathways for the obstructed left atrium consist of vascular channels from a levo-atrial cardinal vein to the innominate vein, an accessory vein from left atrium to the superior vena cava, a venous connection from left atrium to hepatic veins, a coronary venous connection from left atrium to coronary sinus, and a coronary sino-septal defect.
Vasoconstriction during early embryogenesis leads to decreased growth and development of pulmonary veins and to alveolar capillary dysplasia, forcing arterial blood to bypass the deficient capillary bed and drain through anomalous bronchial veins. Lymphatics are strikingly enlarged. Dilated pulmonary veins are thick and arterialized with multiple elastic laminae.
The subject of the first section of this chapter is aortic atresia with a hypoplastic but perforate mitral valve (see Fig. 28.1 ). The second section deals with aortic atresia and mitral atresia (see Fig. 28.2 ). As a physiologic group, this cluster of lesions is known as hypoplastic left heart syndrome, a designation that denotes the clinical presentation and surgical management.
Aortic atresia with hypoplastic but perforate mitral valve
The pathway to the systemic circulation is a single arterial trunk represented by pulmonary artery/ductus/descending aortic continuity (see Fig. 28.1 ). A hypoplastic ascending aorta serves as a common coronary artery. , Fifty percent to 75% of cases are accompanied by moderate coarctation of the aorta, located either proximal to the ductus (preductal) or distal to the junction of the ductus and aortic arch (paraductal) ( Fig. 28.5 ). The right ventricle is hypertrophied because it is the sole pumping chamber for the systemic and pulmonary circulations. It harbors histologic changes of ischemia and infarction. The blind hypoplastic left ventricle is thick-walled and lined with endocardial fibroelastosis. , Histological abnormalities of the left ventricular myocardium include myocardial fibrosis and aortic elastic fragmentation. Rarely, the left ventricle is characterized by isolated apical hypoplasia with fatty replacement. Isovolumetric contraction causes myofiber disarray, but does not cause direct ventriculocoronary artery communications. Pinpoint neonatal aortic stenosis with small left ventricular cavity (see Chapter 7 ) is associated with intramyocardial sinusoids, but not with direct ventriculocoronary arterial communications. The left ventricle is adequately formed in the presence of a ventricular septal defect and a patent aortic valve. ,
The tricuspid valve is abnormal in a distinct minority of patients. The leaflets may be dysplastic with nodular free edges, shortened chordae tendineae, obliterated interchordal spaces, and an accessory orifice, and only two leaflets may be identifiable. A major concern in hypoplastic left heart syndrome is that the brain is smaller and structurally less mature than normal, and abnormal cerebral structural and vascular development is detectable in utero. Scimitar syndrome has been reported in a child with a hypoplastic left heart.
The coronary circulation has been a matter of lively interest. , , , , , A hypoplastic ascending aorta functions as a common coronary artery that receives retrograde systolic and diastolic flow from the patent ductus (see Figs. 28.1 and 28.5 ). , , The tubular, hypoplastic ascending aorta is not an impediment to retrograde flow into the common coronary artery, but the preductal coarctation (see Fig. 28.5 ) is an impediment. Intra-myocardial coronary abnormalities analogous to those of pulmonary atresia with intact ventricular septum can be anticipated (see Chapter 21 ) because isovolumetric contraction generates excessive systolic pressure that acts as a driving force for direct ventriculocoronary arterial communications. However, the differences between pulmonary atresia with intact ventricular septum and aortic atresia with a hypoplastic but patent mitral valve are as great as the similarities. Myocardial sinusoids consist of restrictive vascular networks that spare the coronary arteries from the impact of high ventricular systolic pressure delivered through direct ventriculocoronary arterial communications. In aortic atresia with a hypoplastic but patent mitral valve (see Fig. 28.1 ), the intramyocardial communications are sinusoidal. Accordingly, epicardial and subepicardial coronary arteries do not receive the impact of high isovolumetric systolic pressure, and are spared the luminal obliterative features of pulmonary atresia with intact ventricular septum (see Chapter 21 ).
Physiologic consequences
The physiologic consequences of hypoplastic left heart with aortic atresia and a hypoplastic but perforate mitral valve are determined by the size of the ductus arteriosus, the pulmonary vascular resistance, and the condition of the atrial septum. Constriction of the ductus compromises flow into the systemic circulation and into the hypoplastic ascending aorta, which functions as a common coronary artery (see earlier). Right ventricular function suffers because of the ischemic effects of inadequate coronary blood flow , because of pulmonary hypertension caused by the non-restrictive ductus, because of high pressure in the obstructed left atrium, and because a large left ventricular mass has disadvantageous effects on right ventricular end-diastolic volume and right ventricular wall motion. Right ventricular mechanics are negatively influenced by left ventricular endocardial fibroelastosis. Competence of the tricuspid valve is important for survival, yet tricuspid dysplasia with multiple papillary muscles is common (see earlier). Low pulmonary vascular resistance permits increased pulmonary arterial blood flow that is received by the obstructed left atrium from which the only effective egress is a restrictive patent foramen ovale (see earlier) (see Fig. 28.1 ). In the presence of an adequate interatrial communication, increased pulmonary blood flow makes a large volume of oxygenated left atrial blood available for mixing in the right atrium, so systemic arterial oxygen saturation is relatively high. However, preferential blood flow into the lungs through the ductus is accompanied by a reciprocal fall in systemic blood flow and a shock-like state. When pulmonary vascular resistance is high, systemic blood flow is maintained at the price of increasing cyanosis.
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