Patrice Flaud

Accurate modelling of unsteady flows in collapsible tubes

The context of this paper is the development of a general and efficient numerical haemodynamic tool to help clinicians and researchers in understanding of physiological flow phenomena. We propose an accurate one-dimensional Runge–Kutta discontinuous Galerkin (RK-DG) method coupled with lumped parameter models for the boundary conditions. The suggested model has already been successfully applied to haemodynamics in arteries and is now extended for the flow in collapsible tubes such as veins. The main difference with cardiovascular simulations is that the flow may become supercritical and elastic jumps may appear with the numerical consequence that scheme may not remain monotone if no limiting procedure is introduced. We show that our second-order RK-DG method equipped with an approximate Roes Riemann solver and a slope-limiting procedure allows us to capture elastic jumps accurately. Moreover, this paper demonstrates that the complex physics associated with such flows is more accurately modelled than with traditional methods such as finite difference methods or finite volumes. We present various benchmark problems that show the flexibility and applicability of the numerical method. Our solutions are compared with analytical solutions when they are available and with solutions obtained using other numerical methods. Finally, to illustrate the clinical interest, we study the emptying process in a calf vein squeezed by contracting skeletal muscle in a normal and pathological subject. We compare our results with experimental simulations and discuss the sensitivity to parameters of our model.

Flushing of intravascular access devices (IVADs) - Efficacy of pulsed and continuous infusions

Purpose To compare, under controlled conditions similar to practical situations, the efficacy of pulsed and continuous infusion for flushing IVADs. For both of them different flow rates, flushing volumes, and times were tested. Methods The PU catheter lumens were filled with a mixture of fibronectin and bovine albumin to simulate physiologic protein depot. Flushing was performed with normal saline. An adapted pump controlled the flow rates. Efficacy was measured by the amount of albumin recovered from the lumen of the tested devices. Flow rate, volumes, and times tested were based upon values reported and/or measured in nursing practices. We compared: (A) single 10 mL bolus and 6 flushing times, (B) continuous infusion of 500 mL in 24 hours, (C) 10 successive boluses 1 mL flushed in 0.5 s each and 6 different time intervals between each bolus. Statistics were performed using the Mann and Whitney U test. Results In group A, the maximum percentage of recovered protein (79.1% was achieved with the 10 mL bolus flushed in 2.5. In group B, 77% of protein was recovered at 24h. In group C: maximum efficacy (90 +/- 3%) was obtained when the time interval between 2 boluses was 0.4 s. Conclusions We conclude that hydrodynamics has a determinant effect on the efficacy and that the adjunction of an intermittent component in the flow increases it. Flow type and the time interval between 2 boluses are the 2 critical variables.

Active Catheters for Neuroradiology

Surgeons performing endovascular interventions have high expectations with regard to the 15 improvement of their operating tools and, more specifically, of their catheters. Active catheters, in which the tip moves actively using shape memory alloy (SMA) actuators, constitute a promising approach. In this article, we review existing SMA-based active catheters present in the literature. We analyze their performances regarding the requirements imparted to neuroradiology. Then, we propose a new analytical model for predicting the 20 thermo-mechanical behavior of steerable catheters actuated through SMA wires. Particularly, we give an expression for the maximal achievable bending angle of the catheter tip. These results are finally applied to the design of single-use small-diameter active catheters especially devoted to neuroradiology. In particular, we present a 3.3-Fr catheter suited for navigating into the Willis’ polygon and for accurate positioning into aneurysmal cavities. 25

Wheezing recognition algorithm using recordings of respiratory sounds at the mouth in a pediatric population

BACKGROUND Respiratory diseases in children are a common reason for physician visits. A diagnostic difficulty arises when parents hear wheezing that is no longer present during the medical consultation. Thus, an outpatient objective tool for recognition of wheezing is of clinical value. METHOD We developed a wheezing recognition algorithm from recorded respiratory sounds with a Smartphone placed near the mouth. A total of 186 recordings were obtained in a pediatric emergency department, mostly in toddlers (mean age 20 months). After exclusion of recordings with artefacts and those with a single clinical operator auscultation, 95 recordings with the agreement of two operators on auscultation diagnosis (27 with wheezing and 68 without) were subjected to a two phase algorithm (signal analysis and pattern classifier using machine learning algorithms) to classify records. RESULTS The best performance (71.4% sensitivity and 88.9% specificity) was observed with a Support Vector Machine-based algorithm. We further tested the algorithm over a set of 39 recordings having a single operator and found a fair agreement (kappa=0.28, CI95% [0.12, 0.45]) between the algorithm and the operator. CONCLUSIONS The main advantage of such an algorithm is its use in contact-free sound recording, thus valuable in the pediatric population.

Homothety ratio of airway diameters and site of airway resistance in healthy and COPD subjects.

Our objective was to evaluate whether a decrease in the homothety ratio (h: diameter of child/parent bronchus, constant over generations) explains the shift in airway resistance toward periphery in chronic obstructive pulmonary disease (COPD). Using a validated computational model of fluid motion, we determined that reduced values of h (<0.76) were associated with a shift in resistance toward periphery. The calculated luminal diameters of terminal bronchioles using normal h (0.80-0.85) or reduced h (0.70-0.75) fitted well with measured micro-CT values obtained by McDonough et al. (N. Engl. J. Med., 2011; 365:1567-75) in control and COPD patients, respectively. A semi-analytic formula of resistance using tracheal dimensions and h was developed, and using experimental data (tracheal area and h from patients [Bokov et al., Respir. Physiol. Neurobiol., 2010; 173:1-10]), we verified the agreement between measured and calculated resistance (r=0.42). In conclusion, the remodeling process of COPD may reduce h and explain the shift in resistance toward lung periphery.

Carotid circumferential wall stress homeostasis in early remodeling: Theoretical approach and clinical application

To assess the influence of cardiovascular risk factors on arterial wall growth and the remodeling process.

Implementing Boundary Conditions in Simulations of Arterial Flows

Computational hemodynamic models of the cardiovascular system are often limited to finite segments of the system and therefore need well-controlled inlet and outlet boundary conditions. Classical boundary conditions are measured total pressure or flow rate imposed at the inlet and impedances of RLR, RLC, or LR filters at the outlet. We present a new approach based on an unidirectional propagative approach (UPA) to model the inlet/outlet boundary conditions on the axisymmetric Navier-Stokes equations. This condition is equivalent to a nonreflecting boundary condition in a fluid-structure interaction model of an axisymmetric artery. First we compare the UPA to the best impedance filter (RLC). Second, we apply this approach to a physiological situation, i.e., the presence of a stented segment into a coronary artery. In that case a reflection index is defined which quantifies the amount of pressure waves reflected upon the singularity.

Thermal and hydrodynamic modelling of active catheters for interventional radiology

Interventional radiologists desire to improve their operating tools such as catheters. Active catheters in which the tip is moved using shape memory alloy actuators activated using the Joule effect present a promising approach for easier navigation in the small vessels. However, the increase in temperature caused by this Joule effect must be controlled in order to prevent damage to blood cells and tissues. This paper is devoted to the simulation and experimental validation of a fluid-thermal model of an active catheter prototype. Comparisons between computer-predicted and experimentally measured temperatures are presented for both experiments in air and water at 37°C. Good agreement between the computational and experimental results is found, demonstrating the validity of the developed computer model. These comparisons enable us to highlight some important issues in the modelling process and to determine the optimal current for the activation of the catheter.

Preliminary Study of Pulsed-Electromagnetic Fields Effects on Endothelial (HUVEC) Cell Secretions—Modulation of the Thrombo-Hemorrhagic Balance

We investigated the role of low-amplitude magnetic pulse with low repetition frequency superimposed on the environmental electromagnetic field (EMF) on the secretion of anti-aggregant (Prostacyclin or PGI2) and pro-aggregant (Thromboxane A2) agents in endothelial cells of the human umbilical cord vein (HUVEC). We established that magnetic pulse exposure modulates both PGI2 and TXA2. These modulations depend on the frequency, width of the pulse, and intensity of the magnetic field. Moreover, we corroborated previous results obtained with an endothelial cell line (EaHy-926), concerning the increased thrombo-embolic risk for the 1 Hz frequency.

New method for estimating arterial pulse wave velocity at single site

Abstract The clinical importance of measuring local pulse wave velocity (PWV), has encouraged researchers to develop several local methods to estimate it. In this work, we proposed a new method, the sum-of-squares method , that allows the estimations of PWV by using simultaneous measurements of blood pressure (P) and arterial diameter (D) at single-location. Pulse waveforms generated by: (1) two-dimensional (2D) fluid-structure interaction simulation (FSI) in a compliant tube, (2) one-dimensional (1D) model of 55 larger human systemic arteries and (3) experimental data were used to validate the new formula and evaluate several classical methods. The performance of the proposed method was assessed by comparing its results to theoretical PWV calculated from the parameters of the model and/or to PWV estimated by several classical methods. It was found that values of PWV obtained by the developed method are in good agreement with theoretical ones and with those calculated by PA-loop and D2P-loop. The difference between the PWV calculated by and PA-loop does not exceed 1% when data from simulations are used, 3% when in vitro data are used and 5% when in vivo data are used. In addition, this study suggests that estimated PWV from arterial pressure and diameter waveforms provide correct values while methods that require flow rate (Q) and velocity (U) overestimate or underestimate PWV.