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Diagnostics of Functional Abnormalities in the Microcirculation System Using Laser Doppler Flowmetry
Published in Andrey V. Dunaev, Valery V. Tuchin, Biomedical Photonics for Diabetes Research, 2023
Irina A. Mizeva, Elena V. Potapova, Elena V. Zharkikh
Arterioles have an increased contractile capacity. The Poiseuille equation states that the flow rate in the tube is proportional to the 4th degree of the radius of the tube section for an ideal fluid, and therefore changes in vasoconstriction and vasodilation substantially affect the blood flow. The largest pressure drop occurs in arterioles, for example in skeletal muscles, which are responsible for 50%–60% of the changes in vascular pressure and, as a result, for the resistivity of the vascular bed [31]. The capillary link usually accounts for no more than 15% of the pressure drop because of the changes in the number of functioning capillaries.
Nature of Flow of a Liquid
Published in Wilmer W Nichols, Michael F O'Rourke, Elazer R Edelman, Charalambos Vlachopoulos, McDonald's Blood Flow in Arteries, 2022
To obtain the volume of flow, it is necessary to determine the volume of the paraboloid that has this parabola as its profile. That is, we must determine the volume of the solid of revolution of this parabola. This volume is or, substituting the value for V (eqn 3.6), This is commonly referred to as the Poiseuille equation. It is important to note that volume flow is directly related to the constant π/8 and the fourth power of the radius; therefore, flow increases exponentially with increases in internal radius.
Blood Flow Mechanics
Published in Michel R. Labrosse, Cardiovascular Mechanics, 2018
The assumption of blood flow circulating in rigid pipes is commonly used for the analysis of the cardiovascular system. In particular, to estimate the shear rate at the wall and the maximum and average blood velocities, the pressure–flow relationship, commonly called the Poiseuille equation, is used. The Poiseuille equation can be used to estimate the resistance to flow in blood vessels.
Haemodialysis catheters – a review of design and function
Published in Expert Review of Medical Devices, 2022
The most popular lumen shape for contemporary devices is the ‘double-D’ (DD) design, in which two adjoining semi-circular lumens are divided by a septum. DD lumens have the theoretic drawback of greater intraluminal resistance and non-laminar flow compared with OO lumens of equal size. However, a DD design permits use of lumens of greater diameter, producing exponentially higher flow with less pressure as represented by the Poiseuille equation. Given lumens of the same gauge, the outer perimeter of an OO catheter is notably larger than a DD catheter because of less relative internal space occupied by lumen. A smaller perimeter is thought to make implantation safer and easier, while also conferring a lower risk of central vein thrombosis or stenosis. The DD design therefore strikes a balance between catheter dimensions and the internal blood volume-surface ratio.
XEN Implant for Glaucoma Treatment: A Review of the Literature
Published in Seminars in Ophthalmology, 2019
Aikaterini Chatzara, Irini Chronopoulou, George Theodossiadis, Panagiotis Theodossiadis, Irini Chatziralli
The XEN gel stent or XEN implant (Allergan, Dublin, Ireland) bypasses the normal route of aqueous humor outflow and reduces IOP though subconjunctival filtration. It is a biocompatible gelatin stent cross-linked with glutaraldehyde, which is inserted ab interno from the anterior chamber into the subconjunctival space and does not cause foreign body reaction while sustains its potency, as it is supported by human and non-human preclinical testing.12,13 There are three models, each 6 mm in length, but with different inner diameters of 140, 63 and 45 μm. The different diameters of the stents are designed in adherence to Hagen‒Poiseuille equation, so as to regulate accordingly outflow resistance.12 Assuming laminar flow of a non-compressible fluid, the outflow resistance and therefore pressure differential increases linearly in relation to the length of the tube and decreases to the fourth power of the lumen radius. A longer and thinner tube will provide more resistance to flow than a shorter and wider tube. This equation was applied as the principle of XEN implant.14 Specifically, it is known that aqueous humor production occurs at a rate of 2‒3 μL/min. In order to prevent hypotony, a device would need to create approximately 5 mm Hg of steady-state pressure at this rate, which can be thought of as the pressure difference between the two ends of a tube. If the length of the tube is set at 6 mm, a lumen of 45 μm would provide a steady-state pressure of approximately 6‒8 mm Hg at 2‒2.5 μL/min as calculated by the Hagen‒Poiseuille equation.14 As far as flow rate is concerned, at 5 mm Hg, the flow rate through this implant was measured at 1.2 μL/min. This is less than that of aqueous humor production and therefore theoretically protects against hypotony at average aqueous humor production.14
Assessment of early paediatric airway sequelae after using cuffed or uncuffed endotracheal tubes with ultrasound and flexible endoscopy
Published in Egyptian Journal of Anaesthesia, 2023
Mohammed Nassef Elsayed, Ahmed Mohammed Al-Attar, Emad Eldin Abd Elmonem Arida, Aliaa Rabia Abd Elaziz
In the present research, an obvious difference between the two groups as regard the inner diameter of the TT is demonstrated. Consecutively and according to Hagen – Poiseuille equation for laminar flow, it is expected that the work of breathing is higher in the CTT group than the UTT group. However, in the setting of general anaesthesia, the patient is already on a controlled mechanical ventilation which in turn alleviates the work of breathing resulting from the small inner diameter of the endotracheal tube [27].