Chapter 18 Hemodynamics

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Chapter 18Hemodynamics
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Hemodynamics

• The study of blood moving through the circulatory
  system.
• Flow
• AKA volume flow the volume of blood moving
  during a unit of time
• Units L/m, ml/s i.e. 5 L/m 5000 ml/m 83
  ml/s
• Velocity
• Indicates the speed of a fluid moving from one
  location to another
• Units any distance divided by a unit of time
• cm/s, m/s

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Flow Three Forms

• Pulsatile flow
• Occurs when blood moves with a variable velocity
• Due to cardiac contraction and relaxation blood
  accelerates and decelerates
• Commonly appears in the arterial circulation

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Flow Three Forms

• Phasic flow
• Occurs when blood moves with a variable velocity
• Blood accelerates and decelerates in response to
  changes in pressure in the abdominal and thoracic
  cavities during respiration, i.e. inspiration
  expiration
• Appears in the venous circulation

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Flow Three Forms

• Steady flow
• Occurs when a fluid moves at a constant speed
  or velocity
• i.e. water through a garden hose
• Occurs in the venous system when breathing
  is stopped or in veins distal to a more
  proximal venous obstruction, e.g. in the common
  femoral vein due to an iliac vein obstruction

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Laminar Flow

• Laminar Flow
• Exists when flow streamlines are aligned
  parallel
• Lamina means layer
• Layers generally travel at individual speeds

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Laminar Flow Two Forms

• Plug flow
• Occurs when all of the layers are traveling at
  the same velocity
• Occurs at an arterial bifurcation
• Parabolic flow
• Occurs as blood moves distal to a bifurcation

PLUG FLOW
PARABOLIC FLOW
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Turbulent Flow
Turbulent, chaotic flow
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Turbulent Flow

• Chaotic flow patterns
• Many different directions speeds
• Associated with pathology, i.e. stenosis
  (narrowing) of a cardiac valve or arterial
  segment
• Eddy currents flow vortices are set up
• Converts flow (kinetic) energy into another
  energy form, i.e. pressure energy
• Bernoulli Principle Conservation of energy
• Associated with a murmur or a bruit due to
  tissue vibration
• Thrill, a palpable murmur or bruit

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Nonlaminar Flow

• Factors associated with nonlaminar flow
• Changes in flow velocity during the cardiac cycle
• Changes in vessel dimension, i.e., diameter
• Change in vessel geometry
• Curves
• Bifurcations
• Branch vessels originating at acute angles

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Laminar Flow Dissipation of Energy

• Blood flows in concentric layers, laminae,
  laminar flow, non-disturbed
• Velocity is different in each layer with lowest
  velocity noted closest to the vessel wall ?
  parabolic flow
• Energy dissipates, primarily in the form of heat
  due to friction, as it moves toward the periphery
• Frictional energy losses are lower in large
  vessels as opposed to smaller vessels

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Reynolds Number

• Reynolds
• V Velocity
• ? density of the fluid
• r radius of the tube or vessel
• ? viscosity of the fluid
• A unitless number
• Predicts whether flow will be laminar or
  turbulent
• Turbulence develops mainly due to changes in
  velocity and vessel diameter
• At a Reynolds of lt2000 flow tends to be
  laminar
• At a Reynolds of gt2000 flow tends to be
  turbulent

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Energy Gradient

• Blood flows from one point to another point when
  an the total fluid energy at one point differs
  from the total fluid energy at another point.
• Energy Gradient
• The difference in energy between point A
  point B.
• Forms
• Pressure (potential) the largest part of total
  energy
• Kinetic
• Gravitational

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Pressure Energy

• Stored or potential energy
• Has the ability to do work
• The major form of energy in circulating blood
• Pressure energy provides flow by overcoming
  resistance
• Example
• Water behind a dam
• Can of whipped cream

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Kinetic Energy

• Associated with movement
• Determined by two factors
• The objects mass
• Kinetic energy of the object is directly
  proportional to the mass of the object
• The speed at which the object moves
• Two objects traveling at the same speed but
  having different masses will have different
  kinetic energy, e.g. a ping-pong ball and a golf
  ball
• Example
• Water flowing over a dam

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Gravitational Energy

• A form of stored or potential energy
• Associated any elevated object
• Example
• Water flowing over a dam has height
• Downhill skier

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Energy Losses in the Circulation

• Energy is imparted to blood by left ventricular
  contraction during cardiac systole
• Energy is lost in the circulation in three ways
• Viscous loss
• Frictional loss
• Inertial loss

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Viscous Energy Loss

• Viscosity the thickness of a fluid
• A fluids viscosity and the resultant viscous
  energy loss in moving the fluid are directly
  proportional
• Greater viscosity ? Greater viscous energy loss
• Measured in Poise
• Hematocrit is the percentage of RBCs in blood.
• Normally 45
• Anemia ? reduced hematocrit, a reduced viscosity
  which makes moving the blood easier

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Frictional Energy Loss

• Occur when flow energy is converted to heat as
  one object rubs against another
• Example
• Blood sliding across vessel walls
• Blood moving in laminae
• Moves faster at mid stream with each layer moving
  slower, moving from center stream to the vessel
  wall

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Inertial Energy Loss

• Relates the tendency of a fluid to resist changes
  in its velocity.
• A change in a fluids speed, up or down, leads to
  a loss in the fluids energy.
• Occurs during three events
• Pulsatile flow
• Found in the arterial circulation
• Phasic flow
• Found in the venous circulation
• Velocity changes
• Found at a vessel narrowing (stenosis)
• Velocity is maximum at the most severely narrowed
  segment
• Velocity decreases distal to the stenosis as the
  vessel segment expands

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Stenosis

• Stenosis a narrowing in the lumen of a vessel.
• Effects
• Change in flow direction
• Increased velocity as vessel narrows
• Turbulence in the post-stenotic region. Chaotic
  flow with eddy currents and vortices
• Pressure gradient
• Conversion of pulsatile flow to steady flow

Stenosis in a vessel
US of a stenotic vessel (CCA)
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Pressure-Flow Relationships

• Pressure gradient flow x resistance
• Pressure gradient is directly proportional to
  flow and resistance
• Pressure gradient increases or decreases when
  flow or resistance increase or decrease
  respectively
• Flow pressure gradient / resistance
• Flow is directly proportional to the pressure
  gradient and inversely proportional to the
  resistance
• Flow increases if the pressure gradient increases
  or the resistance decreases
• Resistance pressure gradient / flow
• Resistance is directly proportional to the
  pressure gradient and inversely proportional to
  flow
• Resistance increases if the pressure gradient
  increases or the flow decreases

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Ohms Law

• Applies to the movement of electricity through a
  wire
• Analogous to the vascular system where the
  pressure gradient (?P) (Q)flow x (R)resistance

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Venous Hemodynamics

• Veins
• Thin-wall collapsible
• Normal function
• Low pressure
• Partially filled partially expanded
• Typical resistance
• Normally veins are low resistance vessels

Cross-section of a vein.
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Venous Hemodynamics

• Increased flow during exercise
• Cross-sectional shape changes from flattened
  hourglass shape to oval and finally to round
• Accommodates a large volume increase with very
  little increase in pressure
• Venous dilatation
• Decrease in the resistance to flow, facilitating
  an increase in outflow toward the heart
• Empty rapidly, returning to semi-collapsed state
• Relatively small changes in pressure under normal
  conditions as the veins change in shape with
  changes in volume

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Venous Pressure-Volume Relationships

• Venous shape volume
• Determined by the pressure acting to expand the
  veins
• Known as transmural pressure which is equal to
• Intraluminal pressure minus tissue pressure
• High transmural pressure ? venous dilatation
  (round shape)
• Low transmural pressure ? venous collapse
  (dumbbell shape)
• Transmural pressure
• Increases only slightly with typical increases in
  venous volume
• Increases slightly higher as venous volume
  increases the veins become more circular in
  shape
• Increase greatly as the veins become maximally
  filled with the veins being stretched to or
  beyond their maximum dimension

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Hydrostatic Pressure

• Pressure related to the weight of the blood
  pressing on a vessel measured at a height
  above or below heart level
• Units of pressure mmHg
• Measured pressure
  circulatory pressure hydrostatic pressure

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Hydrostatic Pressure - Standing

• In the erect individual, hydrostatic pressure
  will change 1.0 mmHg for every 1.36 cm or 22
  mmHg for every 12 inches above or below the
  reference level (the heart)
• In the erect individual, pressures taken at
  the level of the heart will accurately represent
  a persons true blood pressure
• In the erect individual, measurements taken below
  the level of the heart will be erroneously
  high
• In the erect individual, measurements taken above
  the level of the heart will be erroneously
  low

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Hydrostatic Pressure Supine

• The Supine Individual
• Hydrostatic pressure is zero
• Pressure measured anywhere in the in the body
  will be representative of true circulatory
  pressure

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Breathing Venous Flow

• During respiration the diaphragm moves up down
• This movement alternately changes the
  pressure in two fixed cavities
• Thoracic cavity
• Above the diaphragm
• Abdominal cavity
• Below the diaphragm
• This is correct in both the
  supine erect individual

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Inspiration - Supine

• Diaphragm moves downward
• Intrathoracic cavity volume increases pressure
  within the cavity decreases
• Venous return to the heart increases
• Intraabdominal cavity volume decreases pressure
  within the cavity increases
• Venous outflow from the lower extremities
  decreases

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Expiration - Supine

• Diaphragm moves upward
• Intrathoracic cavity volume decreases pressure
  within the cavity increases
• Venous return to the heart decreases
• Intraabdominal cavity volume increases pressure
  within the cavity decreases
• Venous outflow from the lower extremities
  increases

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Flow

• Movement of fluid between two points requires
• A pathway for the fluid to flow
• A difference in energy levels between two points

• Volume of flow depends on the net energy
  difference between the two points, which is
  affected by
• losses due to movement of fluid, i.e. friction
• resistance within the pathway, inverse
  relationship

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Kinetic Energy

• The ability of blood to do work as a result of
  its velocity
• Small compared to pressure energy
• Proportional to
• Density of blood which is normally stable
• Square of its velocity

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Dissipation of Energy

• Normally blood moves in layers, concentric
  laminae
• Each layer flows with different velocity
• Layers in the center have greatest velocity
• Rate of velocity change is greatest near the
  walls
• Loss of energy is primarily due to friction
  between the layers
• Smaller the vessel, greater the energy losses

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Bernoulli PrinciplePressure/Velocity Relationship

• Bernoulli Principle conservation of energy,
  energy is not destroyed, it is transformed to
  another form
• Bernoulli Equation
• Modified Bernoulli equation

V2 .5 m/s (500 cm/s) ?P (mmHg) 4 x .52 ?P 4
x .25 ?P 1 mmHg
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Bernoulli Principle

• Pressure Velocity Inverse relationship, i.e.,
  ?V ??Pr, ?V ? ?Pr
• Occurs at stenoses, changes in vessel geometry
  and abrupt changes in direction

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Poiseuilles Law Equation

• Defines the relationship between pressure, volume
  flow resistance

Q volume flow P1 P2 pressure gradient r
radius ? viscosity l length
Volume Flow (Q) changes primarily due to pressure
gradients (P1 P2) and changes in
vessel radius (r) Doubling the radius ? 16 fold
increase in flow Decrease the radius by 50 ? 95
decrease in flow
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Poiseuille Resistance

• Poiseuilles equation can be broken down into two
  resistance equations.