Session E18: Biological fluid dynamics: Blood Flow in Organs

Impact of cervix geometry and stitch material on success of cerclage procedure

Cervical insufficiency is a medical condition during pregnancy in which the uterine cervix softens and begins to dilate before reaching full term. It is the most common cause of second trimester pregnancy loss. One clinical technique used to treat cervical insufficiency is the cervical cerclage. There are conflicting findings on the efficacy of a cerclage. The purpose of this investigation is to examine the mechanical limitations of a cervical cerclage in different presentations of cervical insufficiency. Generalized synthetic models of the cervix are created using ultrasound images collected by clinical collaborators and fabricated with silicon to imitate physiological properties of the cervix.  Because precise data do not exist on the material properties of the softened cervix, we quantified the qualitative assessment of trained obstetricians. Physicians stitch the synthetic cervixes and pressure transducers record the maximum force on the stitch before rupture. The cervical geometry (e.g. how wide the cervix has dilated when the stitch is performed) and stitch material are varied to investigate their impact on the success of the cerclage stitch. The results of this study will provide insight into the most effective clinical interventions and the mechanism of their success.

Optic nerve sheath bleeding driven by rapid cerebrospinal fluid pressure amplification

The optic nerve is a collection of dendrite fibres connecting the eye to the brain, surrounded by a sheath which contains a thin layer of cerebrospinal fluid (CSF). This CSF pressure is directly influenced by pressure changes in the brain. Bleeding of the optic nerve sheath is often observed in conjunction with traumatic brain injury.  We propose a theoretical model to explain this correlation, examining the flow of CSF along the optic nerve sheath driven by a sudden large pressure perturbation in the brain. The model predicts that this perturbation triggers a propagating pressure wave along the nerve sheath towards the eye, and for sufficiently large perturbations this wave can steepen to form an elastic jump. We show that this pressure wave is reflected back towards the brain by the sclera, resulting in amplification of the CSF pressure of up to five times the perturbation pressure in some cases. We hypothesise that this rapid expansion of the sheath can lead to localised tissue bleeding. Furthermore, we show that this pressure perturbation constricts the blood vessels spanning the sheath, transmitting a pressure wave directly into the retinal circulation in the eye.
  

The Dispersing Characteristics of Antral Contraction Wave Flow in a Model Stomach

Experiments and numerical simulations are used to study the flow field and particle breakup due to mechanical stresses in a model human stomach. A simplified artificial stomach was constructed, consisting of a tube which was closed at one end to represent the antrum and the closed pylorus. Antral contraction waves were modeled by a moving hollow piston. The computational domain used in the simulations mirrored this setup. Experiments and simulations were performed for different fluid systems, relative occlusions and wave speeds. Velocity profiles, measured along the centerline using ultrasound Doppler, were used to validate the numerical simulations. The retropulsive jet and velocity were similar to those from simulations in complex 3D geometries of the stomach and with in-vivo measurements. Liquid drops of different sizes and initial locations were then tracked through the flows and their breakup behavior was recorded. The fluids were highly viscous and the interfacial tension was low to represent the conditions in the stomach. The strain rates and stresses along the various particle paths were determined from the simulations. Combining this particle tracking information with drop breakup behavior from the experiments, different breakup conditions and regimes were identified.