Flashcards in Arterial Hemodynamics Deck (21)
What dictates the volume of blood leaving the heart?
Blood Pressure and Peripheral Resistance
Maintenance of energy in the movement of fluid. Explains relationship between velocity and pressure at a stenosis.
Increase velocity= decrease pressure
How long does it take laminar flow to be reestablished after disturbed?
Approx 3 cm
What is flow separation and when does it occur?
Occurs with sudden widening of vessel. Fluid layers separate to fill in new space creating mixed blood flow pattern with flow reversal along the wall. Contains boundary layer separation or transition zone= where lamina reach zero V
(Red in Middle)
Causes of disturbed flow?
Bifurcations and branches: increase angle of vessel= increase disturbed flow
Curved vessels: Helical flow pattern where fluid flows faster on outside and may appear reversed on inside. Faster fluid in centre moves outward and slower fluid replaces it in middle (Blue in middle)
What must happen to the dP in order to preserve blood flow in the body when there is an increase in resistance?
It must increase.
Q= dP/ R
What is flow rate (Q)?
Amount of fluid traveling past a pt in a given time
Area x velocity x time
Defines relationship between P, Q, and R of fluid flowing through cylinder tube.
Q= pie x dP x r^4 / 8Ln
N= viscosity (measured in poise)
(If an entire vessel was narrowed, the flow speed would be reduced- however in a short segment the opposite is true)
What is Reynolds number?
>/= 2000 (Reynolds number- when turbulence occurs)
Re= avg flow x density x diameter / viscosity
Where does turbulence occur?
Chaotic flow when fluid is exiting a tight spot and entering an enlarged space
- space filled in by eddies & whirls
What do you see in a waveform prox to a stenosis
- increased pulsatility
- decreased PSV
- thumping if sever
What do you see in a waveform at a stenosis
- PSV increases as occlusion level increased- up to 80% occlusion where it starts to decrease. 50% occlusion= 2X PSV
- EDV markedly increased with >70% stenosis
What do you see in a waveform distal to a stenosis?
- Immediately distal = post stenotic turbulence (max seen within 1 cm and possible bruit)
- Tardus parvus distal
Define levels of pulsatility?
High= narrow systolic peak, flow reversal in early diastole, little or absent late diastolic flow
Moderate= tall, sharp peak and little diastolic flow
Low= broad systolic peak and forward flow through diastole
Formulas for R.I, P.I, and S/D ratio
R.I= PSV- EDV/ EDV
P.I= PSV- lowest point/ Mean
S/D ratio= PSV/ EDV
- method used to measure Doppler shift based on wave interference
- sum of transmitted and received frequencies
What is the Doppler Formula?
dF= 2Fo x coso x v / C
How can you compensate for aliasing (exceeding the nyquist limit= 1/2PRF)
1) Move the baseline
2) Increase the angle of insonation
3) Increase scale (PRF)
4) Lower freq probe
5) CW probe
Fast Fourier Transformation (FFT)
- dissects complex beat freq into component waves
Produces colour Doppler (mean flow V)