C8: Hemodynamics Flashcards
define hemodynamics
the study of blood flow in the circulatory system, it refers to both blood and power
which muscles in the body are referred to as the venous heart
calf muscles
what are the 4 items that blood is composed of
- plasma (55%) - made of water and protein
- RBCs (45%)
- WBCs (~1%)
- platelets (~1%)
what does the term hematocrit refer to
the % of blood volume that is composed of cells
define density
the mass per unit volume or the resistance of an object to accelerate
define viscosity
resistance to flow
what vessels control vascular resistance of blood flow
arterioles
is the largest amount of blood located in which vessels in the body?
veins
how is the pressure difference that is need for blood to flow created in the body
by the heart or gravity
how are pressure and flow rate related
directly
formula for pressure gradient
P gradient = P1 - P2 / Length
basic formula for volume flow rate
V flow rate (Q) = delta Pressure / Resistance
what is Poiseuille’s resistance formula for a long straight tube
R = 8 x length x viscosity / pie x (r)^4
…w/ radius being the most dominant factor
formula for volume flow rate for a long straight tube only (when considering resistance)
Q = delta pressure x pie x (d)^4 / 128 x length x viscosity
if the size of a vessel is constant, will a thin or large vessel produce slow flow
thinner = slower
but if theres a stenosis, then thinner means faster velocity
describe plug flow, where is it seen?
flow pattern that has almost all RBCs travelling at the same velocity…. will have a thin envelope
-seen at the opening of larger vessels like the AO
describe laminar parabolic flow, where is it seen?
flow pattern where the RBCs move in concentric layers w/ the fastest velocity at the centre… will have fairly thin envelope and clear window
-most common flow pattern in normal arteries
describe jet flow, where is it seen?
flow pattern seen at areas of significant reduction in vessel diameter… e.g. stenosis
describe disturbed flow, where is it seen?
flow pattern that natural occurs at bifurcations, curves or tapering of vessels…. not all layers of blood move in laminar fashion
- may see spectral broadening and/or reversal
- eg carotid bulb
describe turbulent flow, where is it seen?
flow pattern that is seen just past a stenosis
-flow velocities and direction vary greatly.
what type of spectral waveform would you get w/ turbulent flow
is turbulent flow normal?
- filled in spectral tracing since there are many velocities
- not normal except near the heart
what does a Reynolds number (RN) predict
can predict onset of turbulence in a vessel
having an RN greater than what value indicates turbulence?
2000
what is the continuity rule?
what does it explain
it says: in the presence of a stenosis, the volume of flow must remain constant proximal to, at, and distal to the stenosis
it explains why we get a ‘jet’ when we have a stenosis… the velocity of blood must increase in a stenosis in order to keep the volume flow constant
if the diameter of a stenosis is 1/2 of the vessel, what happens to the velocity of blood flow in this area
the velocity will double to maintain volume flow rate
whats the relationship b/w pressure and velocity
inversely related
when is a stenosis considered hemodynamically significant
if it causes a reduction of 50% or > in diameter of the vessel
total fluid energy is composed of what 2 factors
potential energy (PE) kinetic energy
formula for PE
intravascular pressure plus gravitational potential energy
how is intravascular pressure produced
by the contraction of the heart, hydrostatic pressure and static filing pressure
describe the changes of pressure experienced at a stenosis
at a stenosis theres a pressure drop in order to accommodate the acceleration of blood through the stenosis… after the stenosis blood decelerates
describe the changes of PE experienced at a stenosis
PE is converted to KE when it enters the stenosis and then is converted pack to PE after the stenosis
what is the simplified Bernoulli’s equation
change in pressure = 4 (v2)^2
what is tradus parvus
why does it occur
the drop in acceleration time and peak systolic velocity of arterial flow after a stenosis…..
…. the stenosis detracts from the pressure gradient that was created by the heart
does tardus parvus indicate proximal or distal stenosis
proximal
what type of spectral trace will you get if theres a stenosis distally
you’ll have a smaller peak velocity and you’ll lose diastole because blood cant get through the stenosis as easily
what is the windkessel effect
why does it occur?
the continued forward flow in the AO during diastole… occurs as a result of the elasticity of the vessel walls contracting back to their original shape
why isnt there any backwards flow in the AO during diastole
the AO valve has closed
which phenomenon is responsible for creating the dicrotic notch
windkessel effect
why does flow reversal in arteries happen
the pressure wave travels along the walls of the vessel faster than the blood flowing in the vessel…. the wave reaches the end of the arterial system and causes the wave to be reflected backwards….. this causes the vessel wall to expand and contract against the flow of blood causing reversal
reversal also occurs due to the increased pressure down stream and the heart relaxing
what is a subclavian steal?
occurs when a stenosis is located at the origin of the vertebral artery… the stenosis causes a decrease in pressure which allows blood to instead flow retrogradely and into the subclavian artery rather than the vertebral…
… this ‘steal’ occurs in peak systole when the velocity is the highest and causes the other vertebral artery to have to work much harder and deliver the majority of the blood to the brain
