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Flow through a blood vessel is determined by:
- 1. Pressure difference
- 2. Vascular resistance
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Vessel diameter increase as _____ increases.
Pressure
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Veins or arteries, which are more distensible?
Veins are 6-10 x as distensible as arteries
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Arteries
- Transport blood to tissues
- High pressure
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Arterioles
- Last, smallest branches of the arterial system
- Act as "control valves" for blood flow into capillaries
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Capillaries
Exchange between blood and interstital spaces
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Vasodilitation occurs when?
- During exercise
- After a meal
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Vasoconstriction occurs when?
- In a resting state
- With fasting
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Venules
Collect blood from capillaries and connect to larger veins
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Veins
- Blood returned to heart
- Body contians many more veins than all other parts of the vascular system
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AO/ICV
Have the smallest cross-section area and therefore the highest velocity
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Relationship of velocity to area
Velocity is inverse to area
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Energy derived from the cardiac pump
Potential/pressure energy
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Potential energy
- Pressure/stored energy
- Derived from the cardiac pump
- Distends vessels and stores energy
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Kinetic energy
- Energy of motion
- KE < PE due to distensibility of vessels
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Kinetic energy can increase due to:
- Increase in flow>> i.e. exercise
- Stenosis (increased velocity)
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Movement of fluid between 2 points requires:
- Difference in energy
- Difference in pressure
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Example of low resistance
- Organs
- Low resistance = greater energy difference = greater flow
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Example of high resistance
- Resting muscle
- High resistance = less flow
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Pouseuille's law
Defines relationship between pressure, volume flow, and resistance
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The longer the vessel the _____ the resistance.
- Higher
- High pressure is required to maintain flow
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The shorter the vessel the ____ the flow volume.
Higher
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What has the most significant impact on flow rate?
- Radius of the vessel
- Smaller the radius, the higher the resistance
-
Smaller the radius, the _____ the resistance.
Higher (and higher pressure)
-
Plug flow
- Flat profile
- Seen in stenotic area
- Same velocity throughout vessel (slower flow at wall)
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Laminar flow
- Most common flow pattern
- Seen during systole in smaller arteries
- Flow in center is highest and gradually tapers as it reaches the wall of the vessel
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Disturbed flow is signified by?
Spectral broadening
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Mathematical number to determine when flow turbulence will occur?
Reynold's number
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Reynolds number for when turbulance will occur
2000 Re or higher
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Harmonic resonance produced by flow turbulence transmitted into vessel walls is known as
audible bruit
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Where is normal disturbed flow visualized throughout the cardiac cycle?
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Significant stenosis
70% blockage
-
Tardus parvus
Dampened wave forn distal to a stenosis
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Monophasic flow
- Low resistance, continuous and steady throughout S and D
- Seen in normal arteries that supply low resistance vascular beds -liver, spleen, brain, kidneys
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Triphasic flow
- Highest resistance to flow
- Antegrade/forward flow during systole
- Retrograde/reverse flow during diastole
- Seen in distal AO/iliac arteries
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Cosine of the doppler angle is ______ly related to the doppler angle.
- Inversely related
- Angle of 0 = cosine 1.00
- Angle of 90 = cosine 0
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Increase transmit frequency and _____ the size of the Doppler shift
Increase
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Decrease velocity of flow and _____ the size of the Doppler shift.
Decrease
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Increase the Doppler angle and _____ the size of the Doppler shift.
Decrease
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Decrease the Doppler angle and ______ the size of the Doppler shift.
Increase
-
Cosine = 0
- 90 degree angle
- No Doppler shift
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Cosine = 1.00
- 0 degree angle
- Largest Doppler shift
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With spectral analysis the frequency is displayed on which axis?
Vertical y axis
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What is displayed on the X axis of spectral analysis?
Time
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Amplitude or power is displayed as what on spectral analysis?
Brightness
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FFT
- Fast fourier time
- Current computer program for analyzing and displaying spectral representation
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How can aliasing be eliminated?
- Increase PRF (increased nyquist limit)
- Shift baseline
- Increase size of angle
- Decrease transmit frequency
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Main disadvantage of pulsed Doppler is?
Aliasing
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Nyquist Limit
- 1/2 the PRF
- Exceeding this produces aliasing
-
PRF control
Changed the number of pulsed emitted per second
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Primary advantage of CW Doppler
No velocity limit/ no aliasing
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Main disadvantage of CW Doppler
- Large fixed sample volume
- POOR RANGE RESOLUTION
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Color Doppler
- Qualitative, NOT quantitative information
- Will produce aliasing
- Good range resolution
- BART
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How to eliminate color aliasing
- Increase PRF (scale setting)
- Decrease sample volume depth
- Use larger Doppler angle
- Lower transmit frequency
- Go from triplex(color, image, spectra) to duplex (color, image)
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Power Doppler
- Encodes amplitude of blood cells
- Provides no info on direction of flow
- NO aliasing and less angle dependant
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Components of pulsed Doppler instrumentation:
- Pulser
- Gated detector
- Spectral analysis
- Memory
- Display
- Speakers
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Length of pulses produced by pulser
5-30 cycles
-
The time period for acquisition of Doppler signal is determined by the _____.
Size of the gate
-
Phase shift means
Motion is present
-
Rhythmic cycles or beats produced when waves of different frequencies produce interference are added together and termed
Beat frequency
-
Minimum number of cycles per pulse is?
4
-
Number of cycles required for fast flow
6-10
-
Number of cycles needed for gray scale
2-3 cycles
-
Number of cycles needed for slow flow
20-30
-
Typical sampling rate
25,600 times per second
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High pass filter
- Rejects all frequencies below the threshold vaule
- Usually 100Hz
- Removes wall thump but not slow flow
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Low pass filter
- Eliminates high frequency noise
- Improper adjustment can eliminate high frequency (fast) flow
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One speed estimate has a _____ pulse minimum.
Three
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More than 3 pulses are required in order to:
- Improve accuracy of the estimate
- Determine the variance
- Improve detection of slow flow
-
Typical ensemble length range
10-20 pulse
-
Increasing ensemble length will ___ frame rate.
Decrease
-
Ensemble length
- AKA pulse packet size
- The number of pulses required to produce one color scan line
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