Chapter 2 - Physiology and fluid dynamics Flashcards
Total energy made up of 3 components
1) Potential
2) Kinetic
3) Gravitational
Source of potential energy (2)
1) heart pump
2) vessel distension (elastin)
Kinetic energy depends on (2)
1) Density of blood
2) velocity of blood (squared)
Types of laminar flow (2)
1) Parabolic flow (bullet shape)
2) Plug flow - all travel at same velocity
Relationship of resistance to length and radius
R = 8nL / pir^4
n = viscosity L = length r = radius
Define inertial loss (4)
1) change in direction
2) change in velocity
3) deviation from laminar flow
4) loss of energy
Poiseuille’s Law
Q = P/R
Q = flow
Q = (P1 - P2) pi r^4 / 8nL
Law of conservation of mass on velocity and volume and cross-section relationship
V = Q/A
V = velocity Q = flow A = cross sectional area
Define eddy currents
small circular currents when streamline flow breaks
occurs with vortices
Reynolds number (Re)
Re = Vp2r / n
V = velocity p = density (constant) n = viscosity (constant) r = radius
Reynolds number limit before turbulent
2000
Bernoulli principle
velocity and pressure are inversely related
Define flow separation and where it occurs
Defn: pressure gradients in a vessel
1) intraluminal disease
2) bifurcation
3) turn point
Flow separation in systole and diastole
Systole: flow reversal
Diastole: stagnant, no movement/flow
Define steady flow
1) steady driving pressure
2) energy losses described by Poiseuille’s equation
Stages of pulsatile flow
1) Forward flow through periphery
2) Temporary flow reversal due to phase shifted negative pressure gradient and peripheral resistance
3) flow moves forward again
4) vessel recoil converting potential energy to kinetic energy
Flow reversal decreases in:
1) vasodilation (exercise, heat)
2) stenosis
Flow reversal increases in:
1) vasoconstriction
Low resistance flow vascular beds (6)
1) ICA
2) vertebral
3) renal
4) celiac
5) splenic
6) hepatic
High resistance flow vascular beds (6)
1) aorta
2) ECA
3) subclavian
4) iliac
5) extremity
6) fasting SMA
Response to vasoconsctriction and vasodilation in medium, small and minute/capillaries arteries
Vasoconstriction:
- medium and small = increase flow
- minute/capillaries = decrease flow
Vasodilatation
- medium and small = decrease flow
- minute/capillaries = increase flow
Effect of cardiac arrhythmia in assessing velocity
Difficult to accurately measure PSV
- take averages of few cycles (10)
- use velocity ratio instead
Effect of aortic stenosis in assessing velocity
- delay in systolic upstroke
- decreased PSV (underestimate stenosis)
Effect of aortic regurg/insufficiency in velocity waveform
1) pulsus bisferiens (double systolic peak)
2) diminished diastolic flow
3) reversed diastolic flow
Other factors that increase peak systolic velocity
1) high cardiac output
2) young physically fit adults
3) anemia
Low cardiac output on PSV waveform
1) lower velocity
2) rounded waveform
Effect of intra-aortic balloon pump on PSV waveform
1) double peak
2) underestimate PSV
Causes of increased systemic pulsatility in veins
1) upper extremilty and neck is normal
2) elevated right heart pressure (CVI, HF, tricuspid regurg/insufficiency, COPD, PH, CKD)
3) fluid overload (overhydration)
Effect of obstruction on flow proximal and distal
Proximal = high resistance signal Distal = low resistance monophasic pattern
Hemodynamic significant stenosis requires this much diameter or lumen reduction
75% cross-sectional reduction
50% diameter reduction
Effect of lesion in series
Worse than if single long lesion Disturbed flow pattern at exit of lesion - jet effect - turbulence - eddy formation
Effect of lesions in parallel
overall resistance less than resistance in individual stenosis
Non-narrowing factors that cause spectral broadening
1) Too large Doppler sample volume (sampling midstream and vessel wall together)
2) inappropriate doppler angle
3) doppler sample volume too close to wall
