Computer simulations reveal complex distribution of haemodynamic forces in a mouse retina model of angiogenesis.

There is currently limited understanding of the role played by haemodynamic forces on the processes governing vascular development. One of many obstacles to be overcome is being able to measure those forces, at the required resolution level, on vessels only a few micrometres thick. In this paper, we present an in silico method for the computation of the haemodynamic forces experienced by murine retinal vasculature (a widely used vascular development animal model) beyond what is measurable experimentally. Our results show that it is possible to reconstruct high-resolution three-dimensional geometrical models directly from samples of retinal vasculature and that the lattice-Boltzmann algorithm can be used to obtain accurate estimates of the haemodynamics in these domains. We generate flow models from samples obtained at postnatal days (P) 5 and 6. Our simulations show important differences between the flow patterns recovered in both cases, including observations of regression occurring in areas where wall shear stress (WSS) gradients exist. We propose two possible mechanisms to account for the observed increase in velocity and WSS between P5 and P6: (i) the measured reduction in typical vessel diameter between both time points and (ii) the reduction in network density triggered by the pruning process. The methodology developed herein is applicable to other biomedical domains where microvasculature can be imaged but experimental flow measurements are unavailable or difficult to obtain

Laparoscopic management of appendicitis and symptomatic cholelithiasis during pregnancy

BACKGROUND: Laparoscopic surgery during pregnancy is a challenging procedure that most surgeons are reluctant to perform. The objective of this study was to evaluate whether laparoscopic appendectomy and cholecystectomy is safe in pregnant women. The management of these situations remains controversial. We report a single center study describing the successful management of 16 patients during pregnancy. METHODS: More than 3,356 laparoscopic procedures were performed in our institutions between May 1990 and June 2005. Sixteen of these patients were operated on in the second and third trimester between 22 and 32 weeks of estimated gestational age. We performed 11 laparoscopic appendectomies and 5 laparoscopic cholecystectomies. We also reviewed the management and operative technique used in these patients. RESULTS: In this study, the laparoscopic appendectomy or cholecystectomy was performed successfully in all patients. Three patients were in their second trimester, weeks 22, 23, and 25, and 13 were in the third trimester, weeks 27 (three patients), 28 (five patients), 31 (three patients), and 32 (two patients). No maternal or fetal morbidity occurred. Open laparoscopy was performed safely in all patients and all patients delivered healthy babies. CONCLUSION: From our experience laparoscopic management of appendicitis and biliary colic during pregnancy is safe, however the second trimester is preferable for laparoscopic cholecystectomy. Pregnancy is not a contraindication to the laparoscopic approach to appendicitis or symptomatic cholelithiasis. We believe that laparoscopic operations, when performed by experienced surgeons, are safe and even preferable for the mother and the fetu

Human blood plasma in capillary-size flow: Revealing hidden elasticity and scale dependence

The dynamical mechanical analysis of blood generally uses models inspired by conventional flows, assuming scale-independent homogeneous flows and without considering fluid-surface boundary interactions. The present experimental study highlights the pertinence of a non-conventional approach to identify dynamic properties of human blood plasma. A finite shear elastic response (solid-like property) is identified in nearly static conditions, which also depends on the scale (being reinforced at small scales). Therefore, blood plasma is a scale dependent viscoelastic solid that flows over a weak stress threshold. This finding opens new routes for medical diagnosis and device fabrication

Альтернативное представление о крови и плазме крови в сосудистой системе

International audienceПриведены результаты экспериментального исследования свойств крови. Для того чтобы обеспечить в эксперименте граничные условия, близкие к условиям, имеющим место в потоке крови, были выполнены измерения вязкоупругих характеристик на поверхности волокнистого слоя толщиной 10 нм, состоящего из белков внеклеточного матрикса. Установлено, что на кривой зависимости модуля сдвига от круговой частоты для натуральной цельной крови имеется плато. Показано, что в медленном потоке плазмы крови вблизи стенки сосуда может образоваться стационарный слой плазмы, способствующий увеличению функциональной толщины поверхностных слоев в сосудах. Напряжение сдвига, рассчитанное по скорости сдвига на стенке и вязкости крови, является неточным, если не учитывается наличие этого промежуточного слоя

Alternative view on blood and blood plasma in the vasculature

International audienceResults of an experimental study of blood properties are reported. To approach the boundary conditions of blood biophysics in a real flow, the viscoelastic measurements are carried out on surfaces coated with a 10-nm thick fibrous layer composed of extracellular matrix protein. For native whole blood, a plateau of the shear elastic modulus as a function of frequency is observed. The immediate consequence of the shear elasticity is that it is necessary to exceed the stress threshold so that blood can flow. This elastic threshold depends on the boundary conditions

Blood rheology on modified rheometer surfaces

International audienceBackground: Rheology of a bulk sample is a collective phenomenon based on the force transmission through the sample. Conventional rheometry uses steel plates, however, steel is not a physiological contact surface for blood. Blood viscosity and shear moduli might be underestimated in these tests, because the force transmission through the gap is strongly influenced by the contact of the sample to the test surfaces. It was shown that shear moduli are higher when high wetting substrates are used (E. Kume et al.; L. Noirez; U. Windberger et al.).Method: To test blood under near-physiological condition, we modified the test plates by coatings with (1) endothelial cells (HUVECs) grown on a collagen hydrogel, (2) collagen hydrogel without HUVECs, (3) glass that was the substrate for the hydrogel coating, and (4) steel. We tested blood and plasma from healthy human volunteers in oscillating shear flow to assess the blood behavior at near-equilibrium conditions.Results: A linear elastic property was identified by a G´-plateau in both coatings and on glass, but not if both surfaces were made of steel. The highest linear G´-value was obtained on glass and hydrogel surfaces followed by HUVECs, and steel. Yield points were assessed by extrapolating the end of the G´-plateau to the applied shear stress, and revealed the highest values for glass and hydrogel coating, followed by HUVEC seeding. An extended yielding was observed for the modified surfaces, which culminated in a second linear phase until the shear elasticity was finally lost at shear stresses of over 300 mPa. With steel surfaces, elasticity was lost already at 5mPa, demonstrating that the viscoelastic response highly depends on the substrate interaction.Our data indicate that a modification of the boundary surface has deep consequences for flow. To model the flow through biomedical devices, controlled interfacial conditions that reflect the planned application are key parameters

The Electrical Impedance of Pulsatile Blood Flowing Through Rigid Tubes: An Experimental Investigation

Small fluctuations present in an Impedance Cardiogram are often dismissed as noise, but may be due to unknown physiological origins. One such origin suggested in literature is the impedance variation induced by changes in red blood cell orientation during pulsatile blood flow. This study investigated the relationship between the impedance, velocity and acceleration of blood as it pulses during the cardiac cycle. This was achieved experimentally by pumping blood through rigid tubes in a mock circulatory system while measuring the impedance and velocity of the blood. Analysis of collected data confirms that impedance responds to changes in both velocity and acceleration. During acceleration, impedance and velocity are linearly related. However, during deceleration, it was found that the relationship between impedance and velocity is non linear. As velocity increases, the relationship becomes linear with a reducing slope. This indicates that for the same change in acceleration at low velocities, the impedance response is significantly larger than at higher velocities. Experimental data demonstrating these trends is presented for varied pulse rates (20 – 100 beats per minute), stroke volumes (20 – 60 ml) and systolic/diastolic ratios (50/50 – 30/70)