Flow simulations of rectal evacuation: towards a quantitative evaluation from video defaecography

Abstract

Mechanistic understanding of anorectal (patho)physiology is missing to improve the medical care of patients suffering from defaecation disorders. Our objective is to show that complex fluid dynamics modelling of video defaecography may open new perspectives in the diagnosis of defaecation disorders. Based on standard X-ray video defaecographies, we developed a bi-dimensional patient-specific simulation of the expulsion of soft materials, the faeces, by the rectum. The model quantified velocity, pressure and stress fields during the defaecation of a neostool with soft stool-like rheology for patients showing normal and pathological defaecatory function. In normal defaecation, the proximal–distal pressure gradient resulted from both the anorectal junction which formed a converging channel and the anal canal. The flow of the neostool through these anatomical parts was dominated by its shear-thinning viscous properties, rather than its yield stress. Consequently, the evacuation flow rate was significantly affected by variations in pressure applied by the rectum, and much less by the geometry of the anorectal junction. Lastly, we simulated impaired defaecations in the absence of obvious obstructive phenomena. Comparison with normal defaecation allowed us to discuss critical elements which should lead to effective medical management.

Simulated Increase in Monoarticular Hip Muscle Strength Reduces the First Peak of Knee Compression Forces During Walking

“…The smoothed position of boundary nodes extracted from the video and their velocity were used as boundary conditions along the moving wall of the rectum. The set of equations was solved by lattice-Boltzmann methods for yield stress fluids and moving boundary conditions, see details in Ahmad et al 2022. We computed flow, pressure and stress fields and also the stress applied in the normal direction by the fluid over the boundaries of the rectum, i.e.…”

Editorial: theme issue on complex rheology in biological systems

“…For this theme issue, we gratefully received contributions from across the physics and life sciences with interests in biorheology ranging in length scale from the rheological properties of intracellular biomolecular networks [ 1 , 10 ] to the scale of the direct extracellular environment [ 3 , 7 , 10 , 12 , 13 , 16 ], tissues [ 3 – 5 ] and even entire organs that actively exert forces onto non-Newtonian fluids [ 6 ]. The mechanical properties at the cellular level are discussed in relationship to cancer [ 2 ], as well as in relationship to the transport of red blood cells in disordered porous environments, be it in vascular networks or in microfluidic devices.…”

“…While the properties of the fluids and structures are often poorly understood, the level of complexity increases still further when the fluids and structures interact. Such flow–structure interaction is important in achieving quantitative evaluation from video defaecography for diagnostics [ 6 ], and is also important for understanding how blood flows through microvessels and vascular tissues [ 5 ]. Our understanding of flow can be improved experimentally by controlling a rigid flow geometry in microfluidic devices [ 5 ].…”

“…Vascular biology (e.g. haematology) is clearly a field where rheology is vital [ 5 ], but other key rheological control problems emerge in digestive [ 6 ] and reproductive biology [ 7 ]. Other biological flows contain rheologically induced structural or phase transitions, and the understanding of how biological fluids and soft solids flow and deform is a key scientific area within this collaboration of physical and life sciences (blood [ 8 ], the cytosol, silk protein solutions [ 9 ], saliva, mucus [ 10 ], synovial fluid, biofilms [ 11 – 13 ], tissue buckling [ 14 ], bacterial rheotaxis [ 15 ] and E. coli bacteria swimming in media with liquid crystalline order [ 16 ] are just some examples [ 17 ]).…”