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- 1. Introduction Definition of Flow Types of Flow Factors Affecting Flow Clinical Applications Conclusion
- 2. A fluid is a state of matter (or matter- in- transition) in which its molecules move freely and do not bear a constant relationship in space to other molecules Thus it has the ability to take up the shape of its container
- 3. Fluids are Liquid e.g. blood, i.v. infusions Gas e.g. O2 , N2O Vapour (transition from liquid to gas) e.g. N2O (under compression in cylinder), volatile inhalational agents (halothane, isoflurane, etc) Sublimate (transition from solid to gas bypassing liquid state) Dry ice (solid CO2), iodine
- 4. Flow is defined as the quantity of fluid (gas, liquid, vapour or sublimate) that passes a point per unit time A simple equation to represent this is: Flow (F) = Quantity (Q) Time (t) Flow is sometimes written as ∆Q (rate of change of a quantity)
- 5. There are two types of flow: Laminar flow Turbulent flow
- 6. Smooth, steady and orderly flow of fluid in a tube All the fluid molecules move in a straight line Therefore they move in parallel layers or laminae with no disruption between the layers Velocity of flow is greatest in the axial stream (centre of the tube). It becomes progressively slower as the layers move to the periphery Axial stream velocity is twice the mean flow velocity Velocity of the layer in contact with the wall is virtually zero
- 7. Diagrammatic representation of laminar flow
- 8. Fluid does not move in orderly manner The fluid molecules become more disorganized They form swirls and eddies as they move down the pressure gradient in haphazard manner There is increased resistance to flow as the eddy currents interfere with each other Therefore greater energy is required for a given flow rate, compared to when the flow is laminar
- 9. Diagrammatic representation of turbulent flow
- 10. Pressure: flow is directly proportional to the pressure difference across the tube Q ∞ ∆P Radius: flow is directly proportional to the fourth power of the radius (or diameter) of the tube Q ∞ r4, or Q ∞ d4 Length: flow is inversely proportional to the length of the tube Q ∞ 1/l Viscosity: flow is inversely proportional to the viscosity of the fluid Q ∞ 1/η
- 11. The relationship between pressure and flow is linear within certain limits As velocity increases, a critical point (or critical velocity) is reached where flow changes from laminar to turbulent Beyond this point, flow is proportional to the square root of pressure gradient
- 12. This number is calculated from an equation that incorporates the factors that determine the critical point Reynolds’ number = vρr or vρd η η v = velocity of fluid flow ρ = density of fluid r = radius of tube d = diameter of tube η = viscosity of fluid Reynolds number does not have any associated unit It is a dimensionless number
- 13. if Reynolds’ number exceeds 2000, flow is likely to be turbulent a Reynolds’ number of less than 2000 is usually associated with laminar flow
- 14. Viscosity (η) is the property of a fluid that causes it to resist flow It is a measure of the frictional forces acting between the layers of fluid as it flows along the tube η = force x velocity gradient area Unit of viscosity is pascal second (Pa s)
- 15. Viscosity of a liquid decreases with increased temperature, while viscosity of a gas increases with increased temperature From Hagen-Poiseuille equation, the more viscous a fluid is the lesser the flow. This however applies to laminar flow and not turbulent flow, where flow is dependent on the density of the fluid
- 16. Density (ρ) is defined as mass per unit volume Unit of density is kilogram per meter cube (kgm-3) Density is an important factor of fluid in turbulent flow through a tube, in which flow is inversely proportional to square root of density
- 17. In a tube, the length of the fluid pathway is greater than the diameter diameter length In an orifice, the diameter of the fluid pathway is greater than the length diameter length
- 18. As the diameter of a tube increases, the Reynolds number increases. Eventually if the diameter of the tube increases enough, it will exceed the length of the tube. We then call this an orifice Flow through a tube is laminar and hence dependent on viscosity (provided that the critical velocity is not exceeded) If the flow is through an orifice it is turbulent and dependent on density
- 19. The flow rate of a fluid through an orifice is dependent upon: the square root of the pressure difference across the orifice the square of the diameter of the orifice the density of the fluid (flow through an orifice inevitably involves some degree of turbulence)
- 20. There are two types Variable orifice (fixed pressure change) flowmeters e.g Rotameter, peak flowmeter Variable pressure change (fixed orifice) flowmeters e.g. Bourdon gauge, pneumotacograph
- 21. At low flows, the bobbin is near the bottom of the tube and the gap between the bobbin and wall of the flowmeter acts like a tube (diameter < length) Gas flow is laminar and hence the viscosity of the gas is important As flow rate increases, the bobbin rises up the flowmeter and the gap increases until it eventually acts like an orifice (diameter > length) At this point the density of the gas affects its flow
- 22. This useful clinical instrument is capable of measuring flow rates up to 1000 L per min Air flow causes a vane to rotate or a piston to move against the constant force of a light spring This opens orifices which permit air to escape The vane or piston rapidly attains a maximum position in response to the peak expiratory flow It is held in this position by a ratchet The reading is obtained from a mechanical pointer which is attached to the vane or piston
- 23. Accurate results demand good technique These devices must be held horizontally to minimize the effects of gravity on the position of the moving parts The patient must be encouraged to exhale as rapidly as possible
- 24. Bourdon gauge is used to sense the pressure change across an orifice and is calibrated to the gas flow rate It uses a coiled tube which uncoils as pressure increases A system of cogs converts uncoiling of the coil into clockwise movement of the needle over a calibrated scale These rugged meters are not affected by changes in position and are useful for metering the flow from gas cylinders at high ambient pressure
- 25. Measures flow rate by sensing the pressure change across a small but laminar resistance Uses differential manometer that senses the true lateral pressure exerted by the gas on each side of the resistance element and transduce them to a continuous electrical output It is a sensitive instrument with a rapid response to changing gas flow It is used widely for clinical measurement of gas flows in respiratory and anaesthetic practice However, practical application requires frequent calibration and correction or compensation for differences in temperature, humidity, gas composition and pressure changes during mechanical ventilation.
- 26. Resistance to breathing is much greater when an endotracheal tube of small diameter is used Flow is significantly reduced in proportion to the fourth power of the diameter changing the tube from an 8mm to a 4mm may reduce flow by up to sixteen-fold Therefore the work of breathing is significantly increased Over time, a spontaneously breathing patient becomes exhausted and soon becomes hypercapnic due to reduced respiration
- 27. In anaesthetic breathing systems, the following can cause turbulent flow, making the work of breathing greater › a sudden change in diameter of tubing › irregularity of the wall › acute angles at connections › Unnecessary long circuits Thus, breathing tubes should possess smooth internal surfaces, gradual bends and no constrictions They should be of as large a diameter and as short a length as possible
- 28. Heliox is a mixture of 21% oxygen and 79% helium Helium is an inert gas that is much less dense than nitrogen (79% of air) Heliox much less dense than air In patients with upper airway obstruction, flow is turbulent and dependent on the density of the gas passing through it Therefore for a given patient effort, there will be a greater flow of heliox (density = 0.16) than air (density = 1.0) or oxygen alone (density = 1.3) However, heliox contains 21% oxygen – it may be of lesser benefit in hypoxic patient
- 29. Humidification, in addition to its other benefits, makes inspired gas less dense This may be of benefit by reducing the work of breathing
- 30. For a given fluid, with the same pressure applied to it, flow is greater through a shorter, wider cannula Thus they are preferred in resuscitation
- 31. Flow is principally laminar There is a possibility of turbulence at the junction of the vessels or where vessels are constricted by outside pressure Here turbulence results in a bruit which is heard on auscultation
- 32. As fluid passes through a constriction, there is an increase in velocity of the fluid Beyond the constriction, velocity decreases to the initial value At point A, the energy in the fluid is both potential and kinetic At point B the amount of kinetic energy is much greater because of the increased velocity As the total energy state must remain constant, potential energy is reduced at point B and this is reflected by a reduction in pressure
- 33. In the Venturi tube, the pressure is least at the site of maximum constriction Subatmospheric pressure may be induced distal to the constriction by gradual opening of the tube beyond the constriction
- 34. describes a phenomenon whereby gas flow through a tube with two Venturis tends to cling either to one side of the tube or to the other used in anaesthetic ventilators (fluidic ventilators), as the application of a small pressure distal to the restriction may enable gas flow to be switched from one side to another

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