[Ultrasound Physics] HEMODYNAMICS

[Contents]

 

1. Characteristics of blood

2. Flow types (Laminar)

3. Flow types (Turbulent)

4. Terms to Know

1. Characteristics of blood

1) Density = Mass/unit volume [g/ml]

- Blood is dense than water

- Density ↑, Propagating Speed ↓(stiffness ↑, propagating speed ↑)

 

2) Viscosity

- Resistance to flow by fluid in motion.

 

3) Flow Volume Rate

- Rate at which a certain amount of blood is moving [L/min]

- A pressure difference is needed for flow to occur (No pressure change = No flow)

- Flow Volume Rate

= pressure difference / flow resistance

= Average Flow x Area of tube

(Volume flow = mean velocity x area of vessel)

- Pressure Difference↑, Flow Rate↑

- Flow Resistance↑, Flow Rate↓

 

4) Flow Resistance

- Depends on the viscosity of a fluid, the tube length, and radius.

: ↑Viscosity, ↑Flow Resistance, Flow Rate↓

: ↑Length, ↑ Flow Resistance, Flow Rate↓

: ↑Radius, ↓ Flow Resistance, Flow Rate↑

 

5) 3 Categories of Blood Flow

- Pulsatile flow

: Movement with variable velocity

: Blood accelerates & decelerates from cardiac contraction (arterial circulation)

- Phasic flow

: Movement with variable velocity.

: Blood accelerated and decelerates due to respiration (Venous Circulation)

- Steady flow

: Fluid movement at a constant speed/velocity

: Brief moment when hold breath

2. Flow types (Laminar)

 

1) Laminar Flow = Smooth (normal physiological states), silent flow

 

 

- flow streamlines are aligned & parallel

- Layers traveling at individual speeds

- < 1500 Reynolds #

 

2) 3 forms of laminar flow [★]

 

 

 - Plug Flow

: Uniform flow traveling at a constant speed

: laminar. individual layers with parallel aligned streamlines

: ex) Large vessels, Entrance of a vessel

- Parabolic Flow (bullet)

 

 

: Layers have individual speeds

: Velocity is the greatest in the center of the lumen (center = fast)

: Velocity is minimum at vessel walls (sides = slow)

: ex) Smaller vessels

- Disturbed Flow (between Laminar & Turbulent)

: Flow is altered from straight line but remains going forward

 (not turbulent because still forward motion)

: Cells move in different directions

: 예) Stenosis, Bifurcation

: 1600-2000 Reynolds #

3. Flow types (Turbulent)

1) Turbulent flow (pathology) [★]

- Chaotic flow patterns. Flow in many different directions

- Affected by velocity

- No streamlines

- Hurricane like swirling = Vortex / Whirling, circular = Eddy Current

: Little to no forward motion

- Not in normal physiological state= Pathology & elevated blood velocity

: Usually seen downstream from a significant stenosis “post-stenotic turbulence”

· Flow in a stenosis is greater than proximal and distal to it.

               : pressure is reduced distal/downstream to stenosis

- Converts flow energy into other forms of energy

: Sound vibration = murmur/bruit

: Tissue vibration = thrill

- bruit

               : seen as bright echoes near the zero-baseline located underneath the systolic peak

- > 2000 Reynolds # (turbulent flow can be predicted) [★★]

- Spectral broadening: ass/w turbulent flow [★★★]

 

 

 

2) Reynolds Number

- Predicts the onset of turbulent flow

- Reynolds # = Avg. flow speed x tube diameter x density / Viscosity [★★]

: Flow Speed ↑, Reynolds #↑

: Tube diameter ↑, Reynolds #↑

: Density↑, Reynolds #↑

: Viscosity↑, Reynolds #↓

4. Terms to Know

 

1) Pressure Energy

- A form of potential energy

- A major form of energy for circulating blood

: A pressure difference (pressure gradient) is needed for blood flow

- Blood gets Energy through Contraction of heart (systole)

- Energy loss

: Inertia

- Energy lost due to change in speed

- Stenosis

- energy loss is greatest in tortuous vessel with multiple obstructions

: Due to friction

- Energy turned into heat (like absorption in sound waves)

- Blood sliding across vessel walls causes friction

: Due to Viscosity (thickness)

- ↑ hematocrit (RBC’s) =↑ Viscosity

- Anemia = ↓ Viscosity

 

2) Flow in stenosis

 

 

 

- Reversed flow in doppler

- d/t pressure drop caused by high-grade proximal stenosis

 

3) Continuity Rule “volumetric flow rate” “stenosis” [★]

- flow rate must remain constant throughout a vessel (all regions)

               : proximal, at, and distal to stenosis

- Volumetric flow rate is constant

: Blood/fluid is neither created nor destroyed as it flows through vessel/tube

- Flow speed is increased with smaller diameter (stenosis)

 

4) Poiseuille’s Law “average flow speed” “entire vessel”

- Poiseuille’s equation: Flow rate = πPR⁴ / 8VL [★]

               P = Pressure difference

               R = radius

               V = viscosity

               L = length

- ↑ pressure difference → ↑ flow rate

- Flow speed decreases with smaller vessel diameters

 

5) Bernoulli Effect “Pressure” “stenosis” [★]

- Decreased pressure in high flow regions

- Pressure is less within a stenosis

: Pressure is greater proximal & distal to the stenosis.

 

6) Stenosis

- Effects of stenosis

: Change in flow direction

: Increased velocity as vessel narrows (continuity rule)

: Turbulence downstream from stenosis

: Pressure gradient across stenosis

: Loss of pulsatility

- 50% diameter stenosis ≈ 75% area stenosis

- 75% diameter stenosis ≈ 90% area stenosis

 

Reference

 

* Davies Ultrasound Physics review

* https://sites.google.com/site/lindadmsportfolio/ultrasound-physics/

* https://sites.google.com/site/nataljasultrasoundphysics/

* https://sites.google.com/site/ektasphysicseportfolio/doppler