C3&4 Hemodynamics and Doppler Flashcards
what are the 2 primary functions of the venous sys?
Return blood from capillaries to the heart
Act as a reservoir to maintain homeostasis
what % of blood is in veins?
80%
what is needed for blood to flow?
pressure or energy gradient (flow from high press to low)
when does blood lose press after it leaves the heart?
as it flows through high resistance vessels
what is the press at the level of the venules and capillaries?
15mmHg
whats hydrostatic press?
and how is it measured?
the weight of a column of blood or gravitational pressure, measured from the heart to a point of press
whats the primary factor in determining intravascular pressure? and how does this fact effect venous return?
hydrostatic press
impedes venous return
whats the hydrostatic pressure in a supine and standing position?
supine: 15mmHg because all body parts on the same level as the RA
standing: 110mmHg at ankles because gravity resists blood flow from the legs to the heart
(veins in leg dilate and blood pools)
whats the press at the RA?
0mmHg
which part of the body is pressure highest?
lower part
if you raise your arms above your head, what happens to hydrostatic press? and what type of value will it have
it lowers and becomes negative (because hydrostatic pressure is relative to the RA)
what causes hypotension and what does hypotension cause?
standing in one position for too long
you will faint
whats the hydrostatic press when your walking?
25mmHg
what are the mechanisms for venous return?
cardiac function respiration muscle contraction press gradient valves compliance
what type of pressure is responsible for the expand/contract on veins?
the intramural/intraluminal and interstitial pressure
what does compliance permit?
large increase in venous flow without significant increase in venous pressure
how does higher volumes and pressures effect compliance?
they decrease it
which have more compliance, veins or arteries?
veins
arteries have greater elasticity
whats transmural press and does it effect the shaped of the vein?
difference between intraluminal (inner) and interstitial (outer) pressures
yes, determines the cross sectional shape of the vein
whats intraluminal pressure (ILP)?
press within the vein (will increase will higher blood volume)
higher ILP = rounder vein
how are intraluminal and transmural press related?
directly
whats interstitial pressure (ISP)?
press outside of the vein
higher ISP = collapsed vein
describe the effects of high vs los transmural press
high: vein wall will become circular and distend
low: vein wall collapses and is elliptical shaped
how does valsalva maneuver effect the venous system? and flow?
it increases the press in both the upper and lower extremity veins
flow should stop or diminish in upper and lower body (flat waveform)
NO REVERSAL
what creates dynamic pressure?
contraction of heart
4 parts of venous wave
- atrial systole
- systolic wave
- atrial diastole
- diastolic wave
when are the two periods of increased venous flow during the cardiac cycle?
ventricular systole When AV (MV and TV) valves open
describe what happens to press and volume in the atria during ventricular systole
volume increases
press decreases
this lowers venous pressure because theres increased flow into the RA
does respiration have a greater effect on the upper or lower extremities?
lower
because upper is closer to heart (its more pulsatile)
does respiration have the same effect on venous flow when standing?
no, it has less
does expiration increase of decrease venous flow?
expiration increases venous flow
how does muscle contraction effect venous press?
it decreases it
where does flow most often occur when the muscles are relaxed?
in perforators from the superficial to deep veins
whats the venous heart?
calf muscle
which calf muscle are the most efficient of the pumps?
gastroc and soleus muscles
how much venous blood is ejected during 1 contraction?
40-60% of the veins volume (generates 200mmHg)
what is ambulatory venous hypertension and what causes it?
what does it cause?
abnormally high venous press when standing, caused by dysfunctional valves
Cause PRIMARY varicose veins and edema
does muscles contraction help blood return to the heart with ambulatory venous hypertension?
no, it expels blood in all different directions
which vessel is a common site for an UE valve?
jugular vein
where is the venous valve sinus?
what is its size compared to the vein?
what pathology is comply found here?
the are between the valve leaflet and the intimal wall
wider than the vein
thrombosis due to stagnent flow
are valves more commonly found in superficial or deep veins?
superficial
which vessel has the greatest # of valves
GSV (then LSV and calf veins)… # lower as you move up the leg
whats the Doppler effect?
-perceived change in frequency due to motion of the source or the observer
(motion of the source is the motion of the RBCs)
-the difference between transmitted ad received frequency
how does the echo frequency change if the RBCs are moving towards the transducer?
echo frequency is larger (antegrade flow, + flow)
how does the echo frequency change if the RBCs are moving away from the transducer?
echo frequency is smaller (retrograde flow, - flow)
what are the 3 methods we use to detect and analyze the doppler shifts?
color flow
spectral flow
audible sound
whats the doppler shift equation?
Doppler shift freq = 2(operating freq) x velocity x cos0/c
OR
Doppler shift freq = returned freq - operating freq
whats the most influential factor that effects the doppler shift freq?
angle of insonation
whats the ideal angle of intonation and what do we use?
ideal: 0
used: 60, its reproducible and has a low margin of error
why is an angle of 90 bad?
cos 90 is zero so no doppler shift can be detected
what happens when we increase the angle?
larger margin of error and changes the velocity reading significantly
is angle important when assessing venous flow? do we use velocities?
no and no
differences between CW and PW?
CW:
- 2 elements, one rings and one listens constantly
- no image
- no specific sampling depth
- no aliasing
PW:
- 1 element, fires and then listens
- image
- can specify depth and region
- aliasing occurs
what factor limits out ability to sample at higher velocities?
PRF
what causes aliasing?
when PRF is too low…. we aren’t sampling fast enough
whats the nyquist limit?
1/2 of the PRF, if exceeded, aliasing will occur because red blood cell velocities are travelling faster than our ability to sample
what is the fast Fourier transformation
separates out the individual frequencies from the doppler shift frequencies and then displays them as a spectral trace
in what 3 axises are the spectral trace displayed?
x: time
y: velocity
z: power/brightness
what does the z axis depend on?
the number of RBCs passing through the sample area
what the function of quadrature detection?
it processes the signal as a + or – value depending on the direction of flow relative to the transducer
what are the 6 components of a waveform?
- peak systole
- diacritic notch - early diastole flow reversal signifies closing of AO valve
- end diastole - lowest point
- envelope - white line that shows the various frequencies passing through the sample, brightness determined by # of RBCs
- window - clear area below envelope
- spectral broadening - thickness of white line
definition of pulsatility (for arteries)
is pulsatility qualitative or quantitative?
relationship of peak to minimum velocities over the mean velocity of the cycle
can be both
definition of resistance (for arteries)
relationship of peak systolic velocity to end diastolic velocity
describe high and low resistance waveforms
low: will always have forward flow through diastole
high: has reversal of flow during diastole
what structures have low resistance flow?
organs we cant live without
brain, liver, kidneys
how are pulsatility and resistance related?
directly
describe laminar parabolic flow
what type of spectral waveform does it produce?
blood moves in concentric layers with the fastest velocity in the centre and the slowest in the periphery
…. profiles include parabolic and plug/blunt
very thin envelope
whats the avg velocity of laminar parabolic flow?
1/2 the maximum velocity
what factors effect laminar flow?
velocity
diameter of vessel
curves, branches
describe disturbed pattern flow
what causes it and is it considered normal?
forward flow with a variety of velocities and directions
caused by friction and E loss
considered normal
describe turbulent flow
what causes it and is it considered normal?
no linear flow with many different velocities and directions
caused by a stenosis
abnormal except near the heart
if spectral gains are too high what can occur?
false spectral broadening, background noise or mirror image
with antegrade flow, if the baseline is too high, what can this cause?
aliasing
whats the purpose of a wall filter?
eliminates low level echos
what are the 2 types of color?
colour doppler and power doppler
what does power doppler measure and what does it depend on?
what are its advantages and disadvantages?
- measures the strength of the signal rather than doppler shift
- based of # of RBCs
Pros: no aliasing more sensitive not angle dependant less blooming
Cons:
slow frame rate
cant determine direction
what is power doppler commonly used to asses?
small vessels
tissue flow
slow flow
how do stationary reflectors and moving reflectors appear with colour doppler?
stationary: grey scale
moving: colour
what does colour doppler represent?
mean frequency shift
…. also, movement of RBCs based on direction and velocity
whats autocorrelation?
process that uses 6-20 pulses per scan line to give information about received echos
how does the lightness of the colour relate to frequency?
lighter: higher freq and vice versa
what info does colour doppler provide?
provides info about: direction Mean frequency power or amplitude variance
is colour dopp qualitative or quantitative?
qualitative
how does colour effect PRF and frame rate?
lowers PRF and frame rate which degrades the 2D image
does height of the colour box effect frame rate?
no, only width (narrower= higher frame rate)
what are the 3 types of maps?
shifting hue: different colours represent different frequencies (most common)
changing shade (saturation): the same colour saturated with white… more white saturation = higher frequency
variance maps: ability to tag certain frequencies
how are speed of flow and PRF related?
directly
faster flow= higher PRF
what occurs if PRF is too high?
range ambiguity
flow will not be detected and vessel will fill poorly
where is the baseline found for colour doppler
in the middle of the colour map, can be changed to accommodate more red or blue
whats another term for power doppler?
angio
list the common colour artifacts and describe them
mirror image artifact: when a false duplication of a structure occurs
blooming/bleeding
colour flash: occurs when adjacent motion causes colour to flash outside of the vessel
aliasing: PRF is too low
visible bruit: soft tissue vibration near an area of high flow
(indirect sign of severe blockage)
what would happen to the received frequency if the transmitted frequency was halved?
received frequency would also be halved…. because the received frequency is directly proportional to the transmitted frequency
what process generates the Color Doppler image?
autocorrelation
if we want a high frame rate when scanning with colour doppler, should your scan line density be high or low?
LOW
what type of flow (low, med or high resistance) will have a waveform thats removed from the baseline?
low resistance
Why does colour Doppler degrade the 2D image?
Because it lowers the PRF
In what vessels do mirror image artifacts commonly occur?
Subclavian artery, due to the lungs
What is threshold with reference to colour gains?
The gain level just before blooming occurs
How does depth effect our ability to detect flow?
It impedes it
When using colour Doppler, which area is the area that contains the scan lines?
The colour box
Valves in perforating veins direct flow in which direction?
Superficial to deep veins
At rest with no muscle activity, what is the main function of veins?
Act as venous reservoirs
Why might some reversal happen at the venae cava/atrial junction?
Because there’s no valves between the IVC/SVC and the R atria
What is capacitance?
The veins ability to adapt to changes in blood volume
When you increase or decrease the colour scale, what are you actually changing?
PRF
What’s the function of colour priority?
Let’s you choose if you want to give priority to colour or grey scale
As you increase the angle of insonation how does it effect the Doppler shift frequency
Makes it smaller
