Balloon dilation catheters are often used to quantify the physiological state of peristaltic activity in tubular organs and comment on their ability to propel fluid which is important for healthy human function. To fully understand this system's behavior, we analyzed the effect of a solitary peristaltic wave on a fluid-filled elastic tube with closed ends. A reduced order model that predicts the resulting tube wall deformations, flow velocities and pressure variations is presented. This simplified model is compared with detailed fluid-structure 3D immersed boundary simulations of peristaltic pumping in tube walls made of hyperelastic material. The major dynamics observed in the 3D simulations were also displayed by our 1D model under laminar flow conditions. Using the 1D model, several pumping regimes were investigated and presented in the form of a regime map that summarizes the system's response for a range of physiological conditions. Finally, the amount of workdone during a peristaltic event in this configuration was defined and quantified. The variation of elastic energy and work done during pumping was found to have a unique signature for each regime. An extension of the 1D model is applied to enhance patient data collected by the device and find the work done for a typical esophageal peristaltic wave. This detailed characterization of the system's behavior aids in better interpreting the clinical data obtained from dilation catheters. Additionally, the pumping capacity of the esophagus can be quantified for comparative studies between disease groups.

The immersed boundary method is a model of fluid-structure interaction that describes a structure, or a collection of structures, immersed in fluid. This formulation uses Eulerian coordinates for the momentum, incompressibility, and viscosity of the fluidstructure system and Lagrangian coordinates for the structural deformations and resultant forces. Integral transforms with delta function kernels connect the two frames. In the continuum equations, both the fluid and the structure are typically modeled as incompressible. Upon discretization, however, the structure's incompressibility is only approximately maintained. To obtain a robust method under large structural deformations, we introduce a volumetric energy in the solid region that stabilizes the formulation and improves the accuracy of the numerical scheme. This volumetric energy is added by decomposing the strain energy into isochoric and dilatational components, as in standard solid mechanics formulations. Using standard quasi-static solid mechanics benchmarks, we study the performance of the proposed stabilized method with various choices of the finite element basis employed for the structural discretization.

The esophagogastric junction (EGJ) is located at the distal end of the esophagus and acts as a valve allowing swallowed food to enter the stomach and preventing acid reflux. Irregular weakening or stiffening of the EGJ muscles results in changes to its opening and closing patterns which can progress into esophageal disorders. Therefore, understanding the physics of the opening and closing cycle of the EGJ can provide mechanistic insights into its function and can help identify the underlying conditions that cause its dysfunction. Using clinical functional lumen imaging probe (FLIP) data, we plotted the pressure-cross-sectional area loops at the EGJ location and distinguished two major loop types – a pressure dominant loop (PDL) and a tone dominant loop (TDL). In this study, we aimed to identify the key characteristics that define each loop type and determine what causes the inversion from one loop to another. To do so, the clinical observations are reproduced using 1D simulations of flow inside a FLIP device located in the esophagus, and the work done by the EGJ wall over time is calculated. This work is decomposed into active and passive components, which reveal the competing mechanisms that dictate the loop type. These mechanisms are esophagus stiffness, fluid viscosity, and the EGJ relaxation pattern.

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Background: The goal of this study was to conceptualize and compute measures of "mechanical work" done by the esophagus using data generated during functional lumen imaging probe (FLIP) panometry and compare work done during secondary peristalsis among patients and controls. Methods: 85 individuals were evaluated with a 16 cm FLIP during sedated endoscopy, including controls (n=14), achalasia subtypes I, II and III (n=15, each), GERD (n=13), EoE (n=9) and SSc (n=5). The FLIP catheter was positioned to have its distal segment straddling the EGJ during stepwise distension. Two metrics of work were assessed: "active work" (bag volumes ≤ 40 mL where contractility generates changes in lumen area) and "work capacity" (bag volumes ≥ 60 mL when contractility cannot alter the lumen area). Results: Controls showed median (IQR) of 7.3 (3.6-9.2) mJ of active work and 268.6 (225.2-332.3) mJ of work capacity. All achalasia subtypes, GERD, and SSc showed lower active work done than controls (p≤0.003). Achalasia subtypes I, II, GERD, and SSc had lower work capacity compared to controls (p<0.001, 0.004, 0.04, and 0.001 respectively). Work capacity was similar between controls, achalasia type III and EoE. Conclusions Mechanical work of the esophagus differs between healthy controls and patient groups with achalasia, EoE, SSc and GERD. Further studies are needed to fully explore the utility of this approach, but these work metrics would be valuable for device design (artificial esophagus), to measure the efficacy of peristalsis, to gauge the physiological state of the esophagus, and comment on its pumping effectiveness.