Hemodynamics in a bidirectional Glenn Shunt supplemented with a modified Blalock-Taussig shunt: Computational fluid dynamics assessment

Abstract

The bidirectional Glenn Shunt (BGS), superior vena cava (SVC) to right pulmonary artery (RPA) is occasionally supplemented with a left modified Blalock-Taussig shunt (mBTS) when PA growth is inadequate. Little is known about hemodynamics when BGS's low-velocity steady flow interacts with mBTS's high-velocity pulsatile flow. We employed simulations of flow in cavopulmonary pathways consisting of internal jugular, subclavian and innominate veins, SVC, RPA, LPA and mBTS, using computational fluid dynamics (CFD) to numerically solve (finite volume method) the Navier-Stokes equations subject to in vivo flow rates at the inlets and measured or calculated pressures at the outlets of the cavopulmonary system. For normal size (9 mm) PAs and a BGS alone, SVC pressure was 12.3 mmHg, RPA and LPA 12.2 mmHg; PA outlets 12 mmHg. We then compared results for the BGS with a 4 mm mBTS. I) With normal-size PAs, SVC pressure was 14.3 mmHg, RPA 13.3 mmHg and LPA 12.9 mmHg; PA outlets 12 mmHg. Wall shear stress (WSS), RPA 6.2 Pa and LPA 53.7 Pa. II) With normal size (9 mm) RPA but small (5.1 mm) LPA, SVC pressure was 15.1 mmHg, RPA 13.8 mmHg and LPA 17.4 mmHg; RPA outlet 12 mmHg and LPA 14 mmHg. WSS, RPA 13.2 Pa and LPA 71 Pa. Normal PA WSS is ~ 2 Pa. *All parameters were averaged over the cardiac cycle. The mBTS created vortices which markedly elevated WSS. High WSS can induce PA luminal cell expression of coagulatory molecules and initiate platelet aggregation. However, WSS lessened as flow velocity decreased from LPA to RPA. This is important since in vitro studies demonstrate WSS's harmful effect on PA endothelium is ameliorated under decelerating flow. In conclusion, for a BGS with a mBTS, SVC pressure became cyclic and significantly increased and thrombogenic vortices were established in the PAs.