Hemodynamics - from textbook Flashcards
Where is kinetic energy and pressure greatest? Where is it the lowest?
Highest in arteriole
Lowest in venous
Potential energy is (2)
Kinetic energy is (1)
Potential = gravity and blood pressure Kinetic = blood flow^2
Describe laminar flow
Layers of blood sliding over each other
Lowest velocity at the walls
Highest velocity in the centre
Mean velocity is 1/2 max velocity
Type of flow with and just distal to stenosis
Flattened/plug flow
Post stenotic turbulence is a diagnostic feature observed in post stenotic zone and is typically associated with a
Bruit
Poiseuilles equation for velocity
V =
Numerator: pressure gradient * radius^2
Denom: 4lengthviscosity
Poiseuille’s equation for flow
Q =
Numerator: pi * pressure gradient * radius^4
Denom: 8lengthviscosity
What occurs when theres a bifurcation or the margin of a stenotic jet (2)
Eddies
Reversal of flow on one side of the vessel
The systolic pressure in the ankle is usually _____ than brachial pressure in normal patients
Ankle systolic is greater than brachial pressure
What does an ankle-brachial index of 0.9 mean?
Ankle pressure is less than brachial (it should be more)
Recall: systolic pressure increases distally (systolic amplification) because of increased stiffness of peripheral artery branches and presence of reflected waves
It indicates obstructive disease in the lower extremity
How does systolic pressure in toes compare to systolic in ankle?
What is the normal toe-brachial cutoff?
What is abnormal?
Systolic pressure in toes is less than the systolic pressure in ankles, therefore the normal cutoff for toe-brachial would be 1.0 or lower.
An index cutoff of 0.8 then is used for the presence of obstructive arterial disease
In the aorta, how much of an area and diameter reduction of a stenosis must occur before there is a drop in distal pressure and blood flow
What is this definition called?
How is this different from smaller vessels like the iliacs, carotids, renals and femoral arteries?
critical stenosis
Aorta:
70% diameter narrowing
90% cross section area reduction
Carotids/smaller:
50% diameter
75% cross sectional area
What does the waveform look like distal to a stenosis? (3)
In the case of minimal stenosis, will this be detected? How can you detect very mild stenosis? (2)
Damping of pressure waveform, prolonged time to peak, increased width at half the amplitude (of a normal limb)
In the case of very mild stenosis, distal changes may not be evident when the patient is at rest. Therefore, exercise or induction of hyperemia is used. Enhanced blood flow through the stenosis results in increased energy loss due to frictional forces and leads to a detectable decrease in pressure distal to the lesion.
A dicrotic notch distal to a stenosis is a good or bad finding? Why (3)
Good finding - means the prox lesion is not critical.
In severe stenosis, disappearance of reversal of flow.
This is because there is resistance to reverse flow created by the stenotic lesion, there’s a decrease in peripheral resistance as a result of relative ischemia, and dampening of the pressure wave
Phasic changes of venous flow reflect changes in (2)
Right atrium pressure
Intrathoracic pressure with respiration
