The advent of new non-invasive imaging modalities (i.e. 4D MRI, 3D Echocardiography) in recent years have facilitated the study and growing recognition that some of the blood flow in the cardiovascular system is naturally spiral and three-dimensional. The helical organization of the myocardial fibers, the heart’s torsional contraction dynamics, aortic valve structure, the outof-plane geometry of the aorta and tortuosity of vessels all contribute to the generation of spiral patterns of blood flow. In nature, many forms of fluid transport (e.g. whirlpool, cyclones) demonstrate high efficiency, flow entrainment, and stability due to their spirality. Flow in the cardiovascular system may also benefit from similar self-stabilizing impulsion dynamics.

Although spiral blood flow structures have been observed in the aorta and other large arteries, many questions remain unanswered regarding its influence on normative cardiovascular physiology and pathophysiology. The research work herein aims to study spiral flow dynamics and to understand its specific characteristics, especially those in athero-susceptible regions. Computational fluid dynamics (CFD) was used to study the modulation of spiral flow and its impact in idealized vascular phantoms (Aim 1) and realistic vascular geometries, namely the aortic arch with an anastomosed cannula, representative of the outflow graft of a mechanical circulatory support device (Aim 2). Aim 1 served as a test platform for studying spiral flow characteristics. Aim 2 provided an example of the translational applicability of spiral flow. Benchtop flow circuits were used to validate key aspects of the in-silico simulations. This research work brought together computational fluid dynamics with 3D vascular printing and benchtop mock circulatory flow loop visualization and analysis methodologies.

The ability of spiral flow to clear and reduce the size of recirculation zones in a set of idealized vascular phantoms was demonstrated in Aim 1. The phantoms tested were angled conduits with 45°, 90°, and 135° turns and idealized asymmetric and axisymmetric stenosis models. A spiral flow inducer was utilized to enable in-silico to in-vitro comparisons, while standalone phantoms were used to test the impact of spiral flow modulation. In the vascular phantoms coupled to spiral flow inducer models, the recirculation zones at the corners of the angled conduits and the flow separation post-coarctation in stenosis models demonstrated a marked decrease in size of regions of low velocities (< 5 cm/s) and low wall shear stress (WSS < 3 dyn/cm²). In these idealized stenosis models, spiral flow diminished the post-coarctation reattachment length by up to 45%. These results were validated using a benchtop experimental setup incorporating ultrasound velocity visualization and 3D printed parts. Simulations with standalone vascular phantoms demonstrated that the recirculation zones decreased as the helical content increased. In the 45° angled conduit, there was a 6.3-fold decrease in volume of thresholded low velocities and 1.8-fold decrease in low WSS areas when comparing straight flow to spiral flow with λ=L* (wavelength, λ, equal to the characteristic length, L*, defined for each phantom). For the 90° case there was a 2.6-fold decrease in low velocities and a 5.4-fold reduction in low WSS between straight and λ=L*. In the 135° case, there was a 4.2-fold decrease in low velocity volumes, and a 3.2-fold decrease in low WSS areas between straight and λ=L*. Compared to straight flow, spiral flow exhibited a swirling, non-colliding, and enhanced washout dynamics in the vascular phantoms tested.