Hemodynamics Flashcards
What is velocity?
rate of displacement over time (distance per unit time)
What is flow?
volume per unit time
What is the equation for velocity in a conduit of varing CSA?
v = Q/A
For a given constant flow, how does velocity vary with respect to CSA?
inversely proportional
What does flow depend on?
the pressure gradient of the fluid and the dimensions of the entire CV system
How do velocity and area relate?
the ratio of the cell’s velocity past one cross section relative to that past cross section depends on the inverse ratio of the respective areas
V1/V2 = A1/A2
What does the velocity area rule (V1/V2 = A1/A2) pertain regardless of?
whether a given CSA applies to a system that consists of a single large tube or to a system made up of several smaller tubes in parallel
How is lateral/static pressure measured?
measured with tubes tangential to the direction of flow
How is total pressure measured?
three pitot tubes face upstream
What does dynamic pressure reflect?
the kinetic evergy of the flowing fluid
What is the equation for the dynamic compoenent of total pressure?
Pdyn = pv^2/2
p - density
pressure change -> velocity change. -> CSA change -> narrowing/widening of vessel
What does the area/volume of fluid in a tube determine?
lateral/static pressure
less volume (narrower tube) = decrease in static pressure
What changes when CSA changes resutling in a velocity change?
alters the dynamic component of pressure
What results in lateral and dynamic pressure when a vessel narrows?
pressure gradient difference
increase in velocity
dynamic component will increase
lateral component will decrease
How does peak velocity change in the ascending vs descending aorta?
In the descending thoracic aorta, the peak velocity is substantially less than in the ascending aorta
* lesser velocities have been recorded in more distal arterial sites
At what site does the dynamic pressure component attain higher values?
site of constriction it may attain substantial values
* most locations, it is a negligible fraction regardless of the orientation of the pressure probe
How does aortic stenosis affect dynamic and lateral pressure?
the entire output of LV is ejected through a narrow aortic valve orfice (calcified)
The high flow velocity is associated with a large kinetic energy -> the lateral pressure is correspondingly reduced (dynamic is increased)
What lateral pressure is recorded when two probes are both in the LV?
same pressures recorded
What lateral pressure is recorded when one probe is in the LV and the other is at the aortic valve?
The lateral pressure during ejection is much less than in the ventricular cavity
- increased velocity of flow in narrow valve orifice
What lateral pressure is recorded when one probe is in the LV (close to the aortic valve) and the other is in the aorta?
The pressure difference was even more pronounced
- substantial energy was lost through friction (viscosity) as blood flowed rapidly through the narrow aortic valve
What is Poiseuille’s law?
for steady laminar flow of a Newtonian fluid through a cylindrical tube flow (Q):
- varies directly to pressure difference
- varies directly to the fourth power of the radius of the tube
- varies inversely to the length of the tube
- varies inversely to the viscosity of the tube
What is the equation of Poiseuille’s law?
Q = pie(Pi-Po)r^4/8nl
What are characteristics of laminar flow?
when the thin layer of fluid in contact with the wall adheres to the wall and is motionless
the layer of fluid just central to the external laminate must shear against this motionless layer and moves slowly at a finite velocity
more central layers move progressively more rapidly
maximal velocity is at the center of the stream (and equal to twice the mean velocity of flow across the entire CSA of the tube)
How do fluid elements progress through laminar flow?
they remain in one lamina (streamline) as the fluid progresses longitudinally along the tube
What are characteristics of turbulent flow?
development of irregular motions of fluid elements in the flow of fluid along the tube
How do fluid elements progress through turbulent flow?
they do not remain confined to definite lamina, but rapid (radical) mixing occurs
What type of flow requires greater pressure to force fluid?
turbulent
* to produce a given flow, the heart must do considerably more work to generate a given flow if turbulence develops
What is the pressure drop (gradient) about proportional to in turbulent flow?
the square of the flow rate
What is the pressure drop (gradient) about proportional to in laminar flow?
the first power of flow rate
What does Reynolds number predict and represent?
whether flow will be laminar or turbulent under given conditions
represents the ratio of inertial to viscous forces
What is the equation for Reynolds number?
Nr = pDv/n
p - density
D - diameter
v - mean velocity
n - viscosity
What does a Nr of 2000 predict?
laminar flow
* low number = laminar
What does a Nr greater than 3000 predict?
turbulent flow
* high number = turbulent
What markers predispose turbulent flow in a vessel?
large diameter, high velocity, and low viscosity
What is turbulence usually accompanied by in the CV system?
audible vibrations (murmurs)
In severe anemia whar are functional cardiac murmurs usually caused by?
the decreased velocity of blood in anemia
the high flow velocities associated with high cardiac output (usually prevail)
What can cause murmurs in CV system (other than turbulence)?
abrupt changes in dimensions (CSA)
What type of flow are bloot clots more likely to develop in?
turbulent
What is one of the problems with the use of artificial valves in surgical treatment of valvular heart disease?
thrombi may occur in association with the prosthetic valve
they may dislodge and occlude a crucial artery (primary prevention - blood thinners)
* important to design valves to avert turbulence
What does force exerted parallel to a surface cause?
a shearing stress on the liquid below and produces a differential motion of each layer of liquid relative to the adjacent layers
at the bottom (wall) the flowing liwuid exerts stress on the surface in contact with the liquid
What chemical event does the mechanical event of pushing on endothelial cells transduce too?
dilation of the vessel
What is the relationship of flow rate and shear stress?
The greater the flow rate, = greater shear stress the liquid will exert on the walls
What does blood flow in large arteries tend to do to the wall?
to pull the endothelial lining along with it
force is proportional to shear rate of layers of blood very close to the wall
What is the typical shear stress in veins and arteries?
veins - 1-6 dynes/cm2
arteries - 10-70 dynes/cm2
What are the biochemical effects of laminar flow and high shear stress?
can protect against atherosclerosis by reducing synthesis of atherogenic genes and increasing production of atheroprotective genes
What are the biochemical effects of turbulent flow?
likely to increase synthesis of genes that promote inflammation (cause atherosclerotic plaque)
What are the common sites of turbulent flow?
bifurcations and curvatures of vessels
How might physiological shear stress be atheroprotective?
by causing a decrease in angiotensin type 1 receptors on the surface of endothelial cells
What tends to occur in hypertension to the subendothelial layers?
they tend to degenerate locally, and small regions of endothelium may lose their normal support
What is a dissecting aneurysm?
viscous drag on wall causes a tear between a normally supported region and an unsupported region
blood may then flow from the vessel lumen through the rift in the lining and dissect between the various artery layers
* extremely serioius
Where are dissecting aneurysms most common?
in the proximal portion of the aorta
