Fluid flow and measurement

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  • Clinical Applications, especially as it relates to ANAESTHESIA
  • ∆ = pronounced “delta”
  • Velocity gradient is equal to the difference between velocities of different fluid molecules divided by the distance between molecules
  • These masks, also termed high air flow oxygen enrichment (HAFOE) devices, provide a constant and predictable inspired oxygen concentration irrespective of the patient's ventilatory pattern. This is achieved by supplying the mask with oxygen and air at a high total flow rate.
  • gradient. This results in water being drawn up through the tube and broken into a fine spray as it comes in contact with the high-speed gas jet


  • 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