Modeling single ventricle physiology: review of engineering tools to study first stage palliation of hypoplastic left heart syndrome

Abstract

First stage palliation of hypoplastic left heart syndrome, i.e., the Norwood operation, results in a complex physiological arrangement, involving different shunting options (modified Blalock-Taussig, RV-PA conduit, central shunt from the ascending aorta) and enlargement of the hypoplastic ascending aorta. Engineering techniques, both computational and experimental, can aid in the understanding of the Norwood physiology and their correct implementation can potentially lead to refinement of the decision-making process, by means of patient-specific simulations. This paper presents some of the available tools that can corroborate clinical evidence by providing detailed insight into the fluid dynamics of the Norwood circulation as well as alternative surgical scenarios (i.e., virtual surgery). Patient-specific anatomies can be manufactured by means of rapid prototyping and such models can be inserted in experimental set-ups (mock circulatory loops) that can provide a valuable source of validation data as well as hydrodynamic information. Such models can be tuned to respond to differing the patient physiologies. Experimental set-ups can also be compatible with visualization techniques, like particle image velocimetry and cardiovascular magnetic resonance, further adding to the knowledge of the local fluid dynamics. Multi-scale computational models include detailed three-dimensional (3D) anatomical information coupled to a lumped parameter network representing the remainder of the circulation. These models output both overall hemodynamic parameters while also enabling to investigate the local fluid dynamics of the aortic arch or the shunt. As an alternative, pure lumped parameter models can also be employed to model Stage 1 palliation, taking advantage of a much lower computational cost, albeit missing the 3D anatomical component. Finally, analytical techniques, such as wave intensity analysis, can be employed to study the Norwood physiology, providing a mechanistic perspective on the ventriculo-arterial coupling for this specific surgical scenario.

Keywords: Norwood procedure; computational modeling; experimental modeling; shunting; single ventricle.

Figure 1

Different shunting options for first stage palliation of HLHS, shown from idealized drawings: (A) modified Blalock-Taussig shunt from the innominate artery to the right pulmonary artery; (B) Sano shunt from the right ventricle (Rv) to the pulmonary artery (Pa); (C) central shunt from the ascending aorta (Ao) to the pulmonary artery.

Figure 2

Experimental set-ups for simulating the circulation following the Norwood operation, with modified Blalock-Taussig shunt (A) and Sano shunt (B). The mock loops include a 3D patient-specific anatomical model. The Berlin Heart EXCOR simulates the single ventricle. C_UB = lumped compliance for upper body district, R_UB = lumped resistance for upper body district, C_LB = lumped compliance for lower body district, R_LB = lumped resistance for lower body district, C_S = lumped compliance for pulmonary district, R_S = lumped resistance for pulmonary district, C_prox = proximal compliance compensating for rigid 3D model.

Figure 3

Example of particle image velocimetry (PIV) data, obtained in a patient-specific anatomical model of Stage 1 physiology, showing velocity vectors at early (A), peak (B) and end (C) systole.

Figure 4

Example of pressure and velocity maps in the 3D domain of a multi-scale simulation of HLHS post Stage 1 including a significant aortic coarctation, highlighting features such as pressure drop across the aortic narrowing as well as the velocity jet across the coarctation itself.

Figure 5

Example of results from fluid structure interaction (FSI) simulations, providing information on displacement of the myocardium, as well as showing vortex formation and blood streamlines in a patient after Stage 1 palliation.