Pulsatile Flow Leads to Intimal Flap Motion and Flow Reversal in an In Vitro Model of Type B Aortic Dissection

Abstract

Understanding of the hemodynamics of Type B aortic dissection may improve outcomes by informing upon patient selection, device design, and deployment strategies. This project characterized changes to aortic hemodynamics as the result of dissection. We hypothesized that dissection would lead to elevated flow reversal and disrupted pulsatile flow patterns in the aorta that can be detected and quantified by non-invasive magnetic resonance imaging. Flexible, anatomic models of both normal aorta and dissected aorta, with a mobile intimal flap containing entry and exit tears, were perfused with a physiologic pulsatile waveform. Four-dimensional phase contrast magnetic resonance (4D PCMR) imaging was used to measure the hemodynamics. These images were processed to quantify pulsatile fluid velocities, flow rate, and flow reversal. Four-dimensional flow imaging in the dissected aorta revealed pockets of reverse flow and vortices primarily in the false lumen. The dissected aorta exhibited significantly greater flow reversal in the proximal-to-mid dissection as compared to normal (21.1 ± 3.8 vs. 1.98 ± 0.4%, p < 0.001). Pulsatility induced unsteady vortices and a pumping motion of the distal intimal flap corresponding to flow reversal. Summed true and false lumen flow rates in dissected models (4.0 ± 2.0 L/min) equaled normal flow rates (3.8 ± 0.1 L/min, p > 0.05), validated against external flow measurement. Pulsatile aortic hemodynamics in the presence of an anatomic, elastic dissection differed significantly from those of both steady flow through a dissection and pulsatile flow through a normal aorta. New hemodynamic features including flow reversal, large exit tear vortices, and pumping action of the mobile intimal flap, were observed. False lumen flow reversal would possess a time-averaged velocity close to stagnation, which may induce future thrombosis. Focal vortices may identify the location of tears that could be covered with a stent-graft. Future correlation of hemodynamics with outcomes may indicate which patients require earlier intervention.

On the Impact of Vessel Wall Stiffness on Quantitative Flow Dynamics in a Synthetic Model of the Thoracic Aorta

“…-and under-damping in the system. Other studies with comparable flow circuit setups showed similar oscillating waveform shapes 21,30,31 . Additional engineering efforts to mitigate this phenomenon may benefit analyses of pressure and flow waveform shapes in multiple vessel geometries and/or under varying boundary conditions.…”

“…Previous work with advanced MRI-compatible flow circuit setups reported similar increased (i.e. >50 mmHg) pulse pressures 21,30,31 . We note that two main factors determine successful pressure tuning: (1) P sys or P MAP can easily be elevated by increasing flow resistance distal to the capacitors for which the pulse pressure remains constant; (2) pulse pressure is governed by the available capacitance, i.e.…”

Effect of intimal flap motion on flow in acute type B aortic dissection by using fluid‐structure interaction

“…Although flap dynamic behaviours were highly variable for each patient, the instantaneous flap movement, especially at peak systole, can reach up to 10.2 mm in the acute phase 13 . Drastic flap fluctuations under pulsatile flow were also demonstrated by previous experimental work on a compliant flow phantom of the aorta with a mobile intimal flap 19 and one‐way FSI simulation for acute and chronic TBAD 20 . These findings suggest that the assumption of constrained wall deformation of <1 mm in the existing FSI studies of aortic dissection needs to be relaxed for a more realistic and accurate prediction of hemodynamics.…”

“…Greater vortical volume occurs during diastole due to flow reversal in the FL (34.9 cm 3 FSI vs 25.0 cm 3 rigid) (13.5% FSI vs 9.8% rigid). Based on an in‐vitro model using a compliant flow phantom of TBAD, Birjiniuk et al 19 demonstrated that the action of a mobile intimal flap (maximum flap displacement = 14.3 ± 0.5 mm near the distal tear) led to significant flow reversal in the proximal FL, as quantified by reverse flow index (percentage of blood volume ejected retrograde at a given slice normal to the aortic wall). This further demonstrates that large flap movement cannot be neglected because it has a profound influence on the flow dynamics.…”

Wall Shear Stress Assessment of the False Lumen in Acute Type B Aortic Dissection Visualized by 4-Dimensional Flow Magnetic Resonance Imaging: An Ex-Vivo Study

“…3 Once complications occur the prognosis of TBAD dramatically declines to 30 days hospital mortality of over 10%. 1,2,5 Identification of clinical and imaging predictors of adverse outcomes in uTBAD seems mandatory in order to identify those patients who will benefit from early intervention by TEVAR. 6 The gold standard for imaging TBAD is Computed Tomography Angiography (CTA).…”