Heath-Edwards classification		A histopathologic classification useful in assessing the potential for reversibility of pulmonary vascular disease. (Heath D, Edwards JE. The pathology of hypertensive pulmonary vascular disease: A description of six grades of structural changes in the pulmonary arteries with special reference to congenital cardiac septal defects. Circulation 1958;18:533-547.) Grade I - hypertrophy of the media of small muscular arteries and arterioles. Grade II - intimal cellular proliferation in addition to medial hypertrophy. Grade III - advanced medial thickening with hypertrophy and hyperplasia including progressive intimal proliferation and concentric fibrosis. This results in obliteration of arterioles and small arteries. Grade IV - 'plexiform lesions'’ of the muscular pulmonary arteries and arterioles with a plexiform network of capillary-like channels within a dilated segment. Grade V - complex plexiform, angiomatous and cavernous lesions and hyalinization of intimal fibrosis. Grade VI - necrotizing arteritis.
hemi-Fontan		The first part of a "staged Fontan", sometimes chosen to reduce the morbidity and mortality that might be associated with performing the complete Fontan at one operation. The hemi-Fontan includes a bi-directional cavo-pulmonary anastomosis and obliteration of central shunts. The second step to complete the Fontan procedure may be performed at a later time.
hemi-truncus		An anomalous pulmonary artery branch to one lung arising from the ascending aorta in the presence of a main pulmonary artery arising normally from the right ventricle and supplying the other lung.
heterograft		Transplanted tissue or organ from a different species.
heterotaxy		Abnormal arrangement (taxo in Greek) of viscera that differs from the arrangement seen in either situs solitus or situs inversus. Often described as "visceral heterotaxy".
heterotopic		Located in an anatomically abnormal site, often in reference to transplantation of an organ.
Holt-Oram syndrome		Autosomal dominant syndrome consisting of radial abnormalities of the forearm and hand associated with secundum ASD (most common), VSD, or, rarely, other cardiac malformations. (Holt M, Oram S. Familial heart disease with skeletal manifestations. Br Heart J 1960;22:236-242.) The gene for this syndrome is on 12q2. (Basson CT et al. The clinical and genetic spectrum of the Holt-Oram syndrome [heart-hand syndrome] N Eng J Med 1994;330:885-891.)
homograft		Transplanted tissue or organ from another individual of the same species.
Hunter syndrome		A genetic syndrome due to a deficiency of the enzyme iduronate sulphate (mucopolysaccharidase) with X-linked recessive inheritance. Clinical spectrum is wide. Patients present with skeletal changes, mental retardation, arterial hypertension and involvement of atrioventricular and semilunar valves resulting in valve regurgitation.
Hurler syndrome		A genetic syndrome due to a deficiency of the enzyme a-L-iduronidase (mucopolysaccharidase) with autosomal recessive inheritance. Phenotype presents with a wide spectrum including severe skeletal abnormalities, corneal clouding, hepatosplenomegaly, mental retardation and mitral valve stenosis.
hyperviscosity		An excessive increase in viscosity of blood, as may occur secondary to erythrocytosis in patients with cyanotic congenital heart disease. Hyperviscosity symptoms include: headache; impaired alertness, depressed mentation or a sense of distance; visual disturbances (blurred vision, double vision, amaurosis fugax); paraesthesiae of fingers, toes or lips; tinnitus; fatigue, lassitude; myalgias (including chest, abdominal muscles),and muscle weakness. (Perloff JK at al. Adults with cyanotic congenital heart disease: hematologic management. Ann Intern Med 1988;109:406-413.) Restless legs or a sensation of cold legs may reflect hyperviscosity (observation of Dr. E. Oechslin). As the symptoms are non-specific, their relation to hyperviscosity is supported if they are alleviated by phlebotomy. Iron deficiency and dehydration worsen hyperviscosity and must be avoided, or treated if present.
hypoplastic left heart syndrome		A heterogeneous syndrome with a wide variety and severity of manifestations involving hypoplasia, stenosis, or atresia at different levels of the left heart including the aorta, aortic valve, left ventricular outflow tract, left ventricular body, mitral valve and left atrium.
hypoplastic left heart syndrome		A heterogeneous syndrome with a wide variety and severity of manifestations involving hypoplasia, stenosis, or atresia at different levels of the left heart including the aorta, aortic valve, left ventricular outflow tract, left ventricular body, mitral valve and left atrium.