Hemodynamics: Blood Flow and Velocity

Hemodynamics

• Hemodynamics studies blood moving through the circulatory system.
• Doppler ultrasound detects and evaluates blood flow, including regurgitation and stenosis.

Flow

• Flow, also known as volume flow, is the volume of blood moving during a unit of time.

Velocity

• Velocity indicates the speed of a fluid moving between two locations.
• Velocity is measured in units of distance divided by time, such as cm/s or m/s.

Three Basic Forms of Flow

• Pulsatile flow
• Phasic flow
• Steady flow

Pulsatile Flow

• Pulsatile flow occurs when blood moves with variable velocity.
• Cardiac contraction and relaxation cause blood to accelerate and decelerate.
• Pulsatile flow is commonly found in the heart and arterial circulation.

Phasic Flow

• Phasic flow occurs when blood moves with a variable velocity.
• Blood accelerates and decelerates due to pressure changes in the abdominal and thoracic cavities during respiration, including inspiration and expiration.
• Phasic flow appears in the venous circulation.

Steady Flow

• Steady flow occurs when a fluid moves at a constant speed or velocity.
• Pressure, flow speeds, and flow patterns do not change with time.
• Steady flow occurs in the venous system when breathing is stopped momentarily.

Five Spatial Categories of Flow

• Plug
• Laminar
• Parabolic
• Disturbed
• Turbulent

Plug Flow

• In plug flow, the speed of all layers of fluid is constant.
• Blood moves as a single unit in plug flow

Laminar Flow

• Lamina means "layer".
• This flow has layers of fluid (streamlines) that slide over each other.
• Streamlines are straight and parallel to each other.
• Layers travel at individual speeds within laminar flow.
• Blood flows fastest at the center of the vessel.
• Flow speed decreases from the center to the walls.

Parabolic Flow

• A parabolic flow profile is a type of laminar flow profile.
• Flow speeds form a parabola.
• For parabolic flow, the average speed of the flow in a vessel is equal to ½ the maximum flow speed, which is found at the center.
• Average speed is 1/2 max. flow speed.
• Parabolic flow occurs when there is steady flow in a long straight tube.
• Blood vessels (arteries) are generally not long and straight.
• Parabolic flow is not usually seen in blood vessels.
• Nonparabolic Laminar flow is commonly seen.

Laminar Flow – Two Forms

• Plug flow occurs when all layers travel at the same velocity and happens at an arterial bifurcation.
• Parabolic flow occurs as blood moves distal to a bifurcation.

Disturbed Flow

• Disturbed flow occurs when parallel streamlines are disturbed from their straight form.
• Commonly seen at areas of bifurcation or stenosis.
• Disturbed flow is a form of laminar flow

Turbulent Flow

• Turbulent flow involves chaotic flow patterns.
• There are many different directions and speeds in turbulent flow.
• Blood may travel in circles called eddies.
• Overall, forward flow continues to occur with overall.
• Turbulent flow is associated with pathology, stenosis and murmurs.
• Turbulent flow may also occur with increasing speed of blood.

Factors Associated with Disturbed or Turbulent Flow

• Changes in flow velocity during the cardiac cycle.
• Changes in vessel dimension, i.e., diameter
• Change in vessel geometry, including curves, bifurcations, and branch vessels originating at acute angles.

Reynold's Number

• Reynold's Number is a unitless number.
• Predicts whether flow will be laminar or turbulent.
• Turbulence develops mainly due to changes in velocity and vessel diameter.
• An increase in velocity of blood flow, density of the blood, or diameter of vessel will raise Reynold's number.
• Reynold's number will be lower if viscosity (thickness) increases.
• At a Reynold’s number of <2000, flow tends to be laminar.
• At a Reynold’s number of >2000, flow tends to be turbulent.
• The critical Reynold's number is 2000.

Forms of Energy

• Pressure (a form of potential energy)
• Kinetic
• Gravitational

Pressure Energy

• Stored or potential energy.
• Has the ability to do work.
• The major form of energy in circulating blood.

Energy

• Energy includes many different physical forms, such as Kinetic energy and Potential energy.

Gravitational Energy

• A form of stored or potential energy.
• Associated with any elevated object.
• Also known as Hydrostatic pressure.
• Pressure exerted by a fluid at rest at a given point within the fluid, due to the force of gravity.

Total Energy

• Total energy = pressure + kinetic + gravitational
• Blood flows when total fluid energy at one location differs from total fluid energy at another location (energy gradient).

Energy Gradient (Difference)

• AKA pressure gradient.
• Blood flows from one point to another point when the total fluid energy is higher at one point than at another point.
• Pressure (energy) gradient is the difference in energy between point "A" and point "B.”

Pressure (Energy) Gradient

• Required in order for blood flow to occur.
• Blood will flow from higher to lower pressure (from an area of higher energy to lower energy).
• Greater pressure gradient, greater flow rate.

Energy Losses in Circulation

• Some energy is transmitted to blood by left ventricular contraction during cardiac systole.
• Energy is lost in the circulation in three ways: Viscous loss, Frictional loss, and Inertial loss

Viscous Energy Loss

• Viscosity is the thickness of a fluid.
• A fluid's viscosity and the resultant viscous energy loss in moving the fluid are directly proportional.
• Greater viscosity results in greater viscous energy loss.
• Measured in Poise.

Hematocrit

• Hematocrit is the percentage of RBC's in blood.
• Normally around 45%.
• Anemia results in reduced hematocrit, reducing viscosity and making it easier for blood to move.

Frictional Energy Loss

• Frictional energy loss occurs when flow energy is converted to heat as one object rubs against another.
• An example of this is blood sliding across vessel walls.

Inertial Energy Loss

• Inertial energy loss relates to the tendency of a fluid to resist changes in its velocity.
• A change in a fluid's speed (up or down) leads to a loss in the fluid’s energy.
• Occurs during three events: Pulsatile flow, Phasic flow, and Velocity changes
• Pulsatile flow is found in the arterial circulation.
• Phasic flow is found in the venous circulation.
• Velocity changes are 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.

Volumetric Flow Rate

• The amount of blood flowing through a certain point during a certain amount of time.
• Usually expressed in mL per minute or second.
• Volumetric flow rate is determined by: Pressure Gradient and Resistance to flow

Resistance to Flow

• Determined by: Fluid viscosity, Tube length and Tube radius.
• If fluid (blood) viscosity or tube (vessel) length increases, flow resistance increases.
• If tube radius (vessel diameter) increases, flow resistance decreases.

Pressure-Flow Relationships

• 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.
• Pressure gradient = flow x resistance.
• This formula is similar to Ohm's Law, which is used to describe relationships between voltage, current, and resistance in an electrical system.

Poiseuille's Law and Equation

• Defines the relationship between pressure, volume flow & resistance.
• Variables: Q = volume flow, P1-P2 = pressure gradient, r = radius (2 x radius=diameter), η = viscosity, and l = length.
• If pressure difference increases, flow rate increases.
• If diameter (radius) increases, flow rate increases.
• If length increases, flow rate decreases.
• If viscosity increases, flow rate decreases.

Poiseuille and Resistance

• Poiseuille's equation can be broken down into two resistance equations: R = 8ηl/πr^4 or R = Pressure/Flow

Poiseuille's Law and Equation(stenosis)

• Decreasing the diameter by 50% corresponds to a 75% decrease in area.
• Decreased distal flow is likely to occur and is called a hemodynamically significant or critical stenosis.

The Bernoulli Principle

• When a fluid flows without a change in velocity from one point to another, the total energy content remains constant.
• In reality, there is always some energy "lost" or is transferred to a different form.
• In the vascular system, energy is almost all dissipated in the form of heat because of friction.
• The total energy in the vascular system is a balance between potential energy (pressure) and kinetic energy (velocity).
• If velocity of blood goes up, there must be a pressure decrease.

Bernoulli Effect Pressure/Velocity Relationship

• A conservation of energy.
• Energy is not destroyed, rather transformed to another form.
• The Bernoulli effect is a drop in pressure associated with high flow speed at a stenosis.
• Pressure energy decreases while flow energy increases.

Continuity Rule

• Blood is not created or destroyed as it passes through an area of stenosis.
• Therefore, volumetric flow rate must remain constant for the three regions: Proximally, At, and Distally

Venous Hemodynamics

• Veins have thin walls and are collapsible.
• Normal function: Low pressure and are partially filled and partially expanded.
• Typical resistance: Normally veins are low resistance vessels.

Transmural Pressure

• Transmural pressure is the pressure acting to expand the veins.
• Typical increases in venous volume the pressure increases only slightly.
• As venous volume increases and veins become more circular in shape the pressure increases slightly higher.
• The vein become maximially filled the pressure increases greatly, with the veins being stretched to or beyond their maximum dimension.

Venous Pressure-Volume Relationships

• Transmural pressure determines venous shape and volume.
• High transmural pressure results in venous dilatation (round shape).
• Low transmural pressure results in venous collapse (dumbbell shape).

Breathing and Venous Flow

• During respiration the diaphragm moves up and down.
• This movement alternately changes the pressure in two fixed cavities: Thoracic cavity (above the diaphragm) and Abdominal cavity (below the diaphragm)

Inspiration and Blood Flow

• With inspiration, the diaphragm moves downward.
• The intrathoracic cavity volume increases, and pressure within the cavity decreases, increasing venous return from the abdomen to the heart.
• The intraabdominal cavity volume decreases, and pressure within the cavity increases, decreasing venous flow from the lower extremities to the abdomen.

Expiration and Blood Flow

• With expiration, the diaphragm moves upward.
• The intrathoracic cavity volume decreases, and pressure within the cavity increases, decreasing venous return from the abdomen to the heart.
• The intraabdominal cavity volume increases, and pressure within the cavity decreases, increasing venous outflow from the lower extremities to the abdomen.