Patient-specific Fluid Dynamical Evaluation of Hypoplastic Left Heart Syndrome Surgical Treatment

Abstract

Background: Hypoplastic Left Heart Syndrome (HLHS) is a congenital condition which can severely impair the subject’s health. This syndrome, if left untreated, is charactereized by a high mortality. In subjects with HLHS, the left ventricle is underdeveloped or, in the worst cases, not functional at all: this condition evidently hampers the circulatory support usually exerted by the ventricle. Starting from the 1980s, the clinical option for a treatment (or, more precisely, a palliation) of the HLHS subject has been given by exploiting the right ventricle in order to support systemic circulation (usually supported by the left ventricle). This has been realized with a multi-step approach, with different surgical operations, the final step being the realization of the Total Cavopulmonary Connection (TCPC), with the venae cavae directly connected to the pulmonary arteries. Since the complexity of the condition, it is not surprising that a multidisciplinary approach is required; in particular, the necessity of minimizing the power lost in the connection is essential. The objective of this study is to characterize hemodynamically a HLHS subject, previously operated on at the Bambino Gesù Children's Hospital (Rome).Methods: The diagnostic MRI sets of the subject, obtained at follow-up, were analyzed using image processing software. The binarization of the images, required to label the blood domain, was performed via two-level thresholding, thus labeling the blood domain of the compartments of interest. A manual processing of the resulting volume was needed to isolate as much as possible the blood compartment from the rest of the anatomy, though, since two constant thresholds are generally insufficient to perfectly segment vessels throughout the image. The blood compartment volume was then discretized, using an unstructured mesh (more than 1 million tetrahedral cells). Suitable boundary conditions (BC) were set, using clinical data obtained in the follow-up. A computational fluid dynamics (CFD) study was executed, using the mathematical model of the anatomy and the boundary conditions, in stationary conditions.Results: The morphology of the calculated flow field was highly dependent on the closeness of the IVC and SVC anastomoses to the right pulmonary bifurcation: this entailed a complex flow field, with the pathlines’ direction showing large deviations, in particular along the path from the inferior vena cava to the inferior branch of the right pulmonary artery. The analysis of the pathlines demonstrated that the IVC flow (carrying hepatic factors from the liver) contributes to either RPA branch, as well as to the LPA. This has clinical relevance, considering that the hepatic factors carried by the IVC flow are essential for the physiological growth of the vessels. The good hemodynamical performance of the connection is also confirmed by its high hydraulic efficiency (94%).Conclusion: Patient-specific studies of HLHS or other relevant congenital pathologies are useful for evaluating the hemodynamical outcome of the affected subject at follow-up, verifying the fluid dynamical features at very fine spatiotemporal scales, otherwise unattainable by the current diagnostic techniques.