Computational study of aortic hemodynamics for patients with an abnormal aortic valve: The importance of secondary flow at the ascending aorta inlet

Blood flow in the aorta is helical, but most computational studies ignore the presence of secondary flow components at the ascending aorta (AAo) inlet. The aim of this study is to ascertain the importance of inlet boundary conditions (BCs) in computational analysis of flow patterns in the thoracic aorta based on patient-specific images, with a particular focus on patients with an abnormal aortic valve. Two cases were studied: one presenting a severe aortic valve stenosis and the other with a mechanical valve. For both aorta models, three inlet BCs were compared; these included the flat profile and 1D through-plane velocity and 3D phase-contrast magnetic resonance imaging derived velocity profiles, with the latter being used for benchmarking. Our results showed that peak and mean velocities at the proximal end of the ascending aorta were underestimated by up to 41% when the secondary flow components were neglected. The results for helical flow descriptors highlighted the strong influence of secondary velocities on the helical flow structure in the AAo. Differences in all wall shear stress (WSS)-derived indices were much more pronounced in the AAo and aortic arch (AA) than in the descending aorta (DAo). Overall, this study demonstrates that using 3D velocity profiles as inlet BC is essential for patient-specific analysis of hemodynamics and WSS in the AAo and AA in the presence of an abnormal aortic valve. However, predicted flow in the DAo is less sensitive to the secondary velocities imposed at the inlet; hence, the 1D through-plane profile could be a sufficient inlet BC for studies focusing on distal regions of the thoracic aorta.

On the choice of outlet boundary conditions for patient-specific analysis of aortic flow using computational fluid dynamics

Boundary conditions (BCs) are an essential part in computational fluid dynamics (CFD) simulations of blood flow in large arteries. Although several studies have investigated the influence of BCs on predicted flow patterns and hemodynamic wall parameters in various arterial models, there is a lack of comprehensive assessment of outlet BCs for patient-specific analysis of aortic flow. In this study, five different sets of outlet BCs were tested and compared using a subject-specific model of a normal aorta. Phase-contrast magnetic resonance imaging (PC-MRI) was performed on the same subject and velocity profiles extracted from the in vivo measurements were used as the inlet boundary condition. Computational results obtained with different outlet BCs were assessed in terms of their agreement with the PC-MRI velocity data and key hemodynamic parameters, such as pressure and flow waveforms and wall shear stress related indices. Our results showed that the best overall performance was achieved by using a well-tuned three-element Windkessel model at all model outlets, which not only gave a good agreement with in vivo flow data, but also produced physiological pressure waveforms and values. On the other hand, opening outlet BCs with zero pressure at multiple outlets failed to reproduce any physiologically relevant flow and pressure features.

How should perineal wounds be closed following abdominoperineal resection in patients post radiotherapy--primary closure or flap repair? Best evidence topic (BET)

This best evidence topic was investigated according to a described protocol. The question posed was: should the irradiated perineal wound following abdominoperineal resection (APR) be closed with primary repair or a myocutaneous flap. Using the reported search 364 papers were found of which eight represented the best evidence to answer the clinical question. The conclusion drawn is that there is some limited evidence for recommending flap closure in abdominoperineal resection post radiotherapy. The best evidence available was from a systematic review of cohort studies and case series. Although no meta-analysis was performed, overall wound healing was improved using flap closure with a low frequency of flap necrosis. Other studies providing evidence were case-control series or cohort studies. Three papers prospectively compared vertical rectus abdominus muscle (VRAM) flap with primary closure; two of which demonstrated statistically significant improvement in complication rates with flap closure. Two retrospective case control series showed significant improvement in major wound complication rates in the flap group. Two studies retrospectively compared gracilis flap repair with primary closure and showed significantly lower incidence of major perineal complications. Most studies suffered from significant limitations, small sample sizes and no direct comparisons between matched groups with respect to type of anatomic flap, wound size, tumour recurrence or radiation dose. Whilst there is evidence that myocutaneous flap closure following APR in radiotherapy patients can reduce wound related complications, prospective randomized controlled trials are warranted.