Re-evaluation of hypoplastic left heart syndrome from a developmental and morphological perspective.

BACKGROUND: Hypoplastic left heart syndrome (HLHS) covers a spectrum of rare congenital anomalies characterised by a non-apex forming left ventricle and stenosis/atresia of the mitral and aortic valves. Despite many studies, the causes of HLHS remain unclear and there are conflicting views regarding the role of flow, valvar or myocardial abnormalities in its pathogenesis, all of which were proposed prior to the description of the second heart field. Our aim was to re-evaluate the patterns of malformation in HLHS in relation to recognised cardiac progenitor populations, with a view to providing aetiologically useful sub-groupings for genomic studies. RESULTS: We examined 78 hearts previously classified as HLHS, with subtypes based on valve patency, and re-categorised them based on their objective ventricular phenotype. Three distinct subgroups could be identified: slit-like left ventricle (24%); miniaturised left ventricle (6%); and thickened left ventricle with endocardial fibroelastosis (EFE; 70%). Slit-like ventricles were always found in combination with aortic atresia and mitral atresia. Miniaturised left ventricles all had normally formed, though smaller aortic and mitral valves. The remaining group were found to have a range of aortic valve malformations associated with thickened left ventricular walls despite being described as either atresia or stenosis. The degree of myocardial thickening was not correlated to the degree of valvar stenosis. Lineage tracing in mice to investigate the progenitor populations that form the parts of the heart disrupted by HLHS showed that whereas Nkx2-5-Cre labelled myocardial and endothelial cells within the left and right ventricles, Mef2c-AHF-Cre, which labels second heart field-derived cells only, was largely restricted to the endocardium and myocardium of the right ventricle. However, like Nkx2-5-Cre, Mef2c-AHF-Cre lineage cells made a significant contribution to the aortic and mitral valves. In contrast, Wnt1-Cre made a major contribution only to the aortic valve. This suggests that discrete cardiac progenitors might be responsible for the patterns of defects observed in the distinct ventricular sub-groups. CONCLUSIONS: Only the slit-like ventricle grouping was found to map to the current nomenclature: the combination of mitral atresia with aortic atresia. It appears that slit-like and miniature ventricles also form discrete sub-groups. Thus, reclassification of HLHS into subgroups based on ventricular phenotype, might be useful in genetic and developmental studies in investigating the aetiology of this severe malformation syndrome.

Cardiopulmonary bypass in ewe's fetus: advances and setbacks in our learning curve.

Fetal cardiac surgery represents a surgical challenge and several centers are attempting to establish a suitable methodology in animals. We present our experience with extra-corporeal bypass procedures in preterm and term sheep fetuses. Twenty-two fetuses (103-139 days gestation, mean 115 days gestation) underwent a 1-hour period of right heart-to-pulmonary artery extracorporeal circulation followed by 1 hour of observation. Animals were divided into group 1 and group 2, according to gestational age (above and below 0.85). Three pumps were used: centrifugal without (group 1) reservoir, centrifugal with (group 2) reservoir, and roller with reservoir (group 2). Experiments were completed in 75% of fetuses in group 1 and in 37% of fetuses in group 2. Bleeding was the main cause of failure, especially for group 2. A slow deterioration of blood gas status was noted in group 1, while this trend could be partially reversed in group 2 with corrective measures. Complete heart bypass could not be achieved in either group, and residual fluctuations in arterial pressure were observed. During bypass, body temperature decreased more in group 2 than in group 1. We conclude that cardiac bypass is feasible over a short period in near-term fetuses. A successful outcome may also be obtained in younger fetuses, but better measures need to be implemented for the prevention of surgical bleeding.