arterial flow characteristics

laminar flow

• concentric layers of flow each with a slight difference in velocity

• center layers have the highest velocity

parabolic flow

identified in medium sized vessels and the layers of flow have a narrower range of velocities than laminar flow

plug flow/blunt flow

• identified during systole in larger vessels (aorta) and all layers of flow are moving at relatively the same speed

• also seen at arterial branch origins and proximal to a stenosis

arterial waveforms are determined by:

where the artery arises and what it is feeding

systole

cardiac contraction pushes blood forward through the arterial system

diastole

• the heart relaxes, refills with deoxygenated blood from the body on the right and oxygenated blood from the lungs on the left

• forward flow continues in low resistance arteries

pulsatility

continuous variation in flow

cardiac pulsatility

flow in the arteries normally pulsate and some veins (ex: hepatic veins) also demonstrate cardiac pulsatility due to location adjacent to continuous cardiac contractions

arterial pulsatility

• related to the number of changes in flow direction during one cardiac cycle

• primary factor that determines the resistance characteristics of the arterial signal displayed by PW doppler

what happens to the pulsatility in arteries when there is vasoconstriction in the extremities?

it leads to increased pulsatility in arteries

what happens to the pulsatility in arteries when there is vasodilation and stenosis in the extremities?

it leads to decreases pulsatility in arteries

monophasic

• antegrade flow continues through the entire cardiac cycle

• ex: vessels that feed low resistance vascular beds

biphasic

• flow during systole is antegrade and some flow is reversed during diastole

• ex: vessels feeding medium-high resistance vascular beds

triphasic

flow during systole is antegrade and during diastole initially some flow is reversed, followed by a small amount of forward flow in end diastole

antegrade

moving forward in the normal direction of blood flow

retrograde

backward flow or filling, or against the normal direction of flow

turbulence

• disrupted flow caused by a stenosis, tortuosity or bifurcation

• appears at the exit point of a stenosis

murmur

• abnormal blood flow sound in the heart

• usually from valvular regurgitation or stenosis

bruit

• abnormal blood flow sound in a blood vessel

• can be due to stenosis

• also seen with vessel branching or tortuosity

thrill

• abnormal blood flow sensation in a blood vessel → vibration

• can be due to stenosis

• also seen with pseudoaneurysm and in NORMAL hemodialysis grafts

anatomic causes of turbulence in blood vessels:

• bifurcation/branching

• tortuous vessel course

• kinking of vessel

• coiling of vessel

• eccentric change in vessel course

acquired causes of turbulence in blood vessels:

• atherosclerosis formation

• stent placement

• bypass graft placement

• myointimal hyperplasia

• aneurysm formation

stenosis

narrowing of a vessel lumen

factors that determine the hemodynamic significance of a stenosis include:

• shape and degree of stenosis

• diameter of the stenosis

• length of the stenosis

• contour of the stenosis → smooth vs rough

• distal peripheral resistance

• pressure gradient

• presence/absence of collaterals

• compounding factors from other diseases

stenosis in a series

• multiple areas of stenosis in the same vessel will increase the resistance to flow more than a single stenotic area

• velocity and stenosis can be underestimated at the second stenosis because of the drop in velocity after the first stenosis

stenosis in parallel

• stenosis in different vessels coursing in the same direction → collaterals

• has a lesser effect on resistance to flow than series stenoses

what percentage diameter and percentage area stenosis is considered hemodynamically significant in most arteries?

• 50% diameter stenosis

• 75% area stenosis

stenosis and resistance:

stenosis leads to increased resistance proximal to the obstruction

stenosis and velocity:

• stenosis causes increased peak systolic and end diastolic velocities at the site of stenosis, due to the body trying to maintain blood flow volume

• increased velocities can also be seen immediately distal to the stenotic area

what is the most common reason for underestimation of arterial stenosis?

improper sample volume location

where should the cursor be placed when evaluating a stenosis?

• it should be placed initially at the narrowest point of the stenosis

• it is important to move the doppler cursor through the stenotic area and out the other side to find the highest velocity

distal to the stenosis, vasodilation occurs in the capillaries causing:

• lower velocity

• low resistance flow with an increase in antegrade diastolic flow as the vascular beds try to “encourage” more flow distally

turbulent, swirling blood is identified where in the stenosis?

• distal to a focal stenosis

• AKA eddy currents or vortices

significant stenosis leads to:

• proximal to the stenosis, flow velocity will be dampened with increased resistance

• increased velocity as the blood flows through and escapes the stenotic area

• increases velocities can also be detected immediately prox and dist to the stenotic area

• post stenotic turbulence seen dist to the stenosis

• drop in pressure at the stenosis = greater the velocity at the stenosis

• drop in flow volume at stenosis

• decreased resistance and velocity distal to stenosis

arteries carrying blood to organs normally exhibit:

low resistance waveforms with antegrade flow throughout the cardiac cycle

arteries carrying blood to extremities/muscles normally exhibit:

high resistance waveforms with reversed flow during diastole

anytime a monophasic waveform is seen in an extremity artery, it is considered:

abnormal

anytime a biphasic/triphasic waveform is seen in an artery supplying an organ, it is considered:

abnormal

how does vasoconstriction affect resistance and phasicity?

it leads to increased resistance to flow in vessels and can change a monophasic waveform to biphasic

how does vasodilation affect resistance and phasicity?

it leads to decreased resistance to flow in the vessels and can change a triphasic waveform to monophasic

how does clenching the ipsilateral fist affect the upper extremity doppler evaluation?

it can increase distal resistance and pulsatility in the proximal arteries

how does releasing the clenched ipsilateral fist affect the upper extremity doppler evaluation?

the distal resistance drops significantly, pulsatility is reduced and flow increases to the hand

factors that reduce systolic velocity in arteries:

• CHF

• coronary artery disease

• diastolic dysfunction

• significant aortic stenosis

• significant mitral stenosis

• dilated cardiomyopathy

• pericarditis

• increased blood viscosity → polycythemia vera, sickle cell anemia, leukemia, smoking

• cold exam room

• reduced heart rate

• hypocalcemia

factors that elevate systolic velocity in arteries:

• compensatory heart beats with arrhythmias

• volume overload in normal ventricles → liver disease, renal disease, pregnancy, obesity

• systemic HTN

• significant aortic regurgitation

• hypertrophic cardiomyopathy

• reduced blood viscosity → thrombocytopenia, chronic anemia

• warm exam room

• increased heart rate

• hypercalcemia

spectral broadening

widening of the spectral waveform with filling in of the spectral window

bandwidth

the difference between the highest and lowest frequencies in the pulse

causes of spectral broadening include:

• stenosis

• tortuosity

• improper sample location closer to the wall

• increased sample size

• bifurcations

what type of bandwidth does laminar flow have?

has a narrow bandwidth of reflected signals because the blood cells are all moving at relatively the same speed which allows for the display of a spectral window on the tracing

what type of bandwidth does turbulent flow have?

has a wide bandwidth because the blood cells are moving at different velocities which causes the spectral window to fill in

post stenotic turbulence/eddy currets/vortices

• flow spreads out into the larger vessel area distal to stenotic area

• wide range of flow velocities

• causes aliasing on color doppler and spectral broadening on PW doppler

acceleration time (AT)

• measured to differentiate inflow from outflow disease

• time from onset of systole to point of maximum systolic peak

what can be adjusted on the ultrasound machine to make the measurement of the AT easier?

increased sweep speed makes the AT measurement easier due to wider waveforms on the doppler display

stenosis and AT:

• AT increases in extremities as stenosis increases

• it takes a longer amount of time to reach peak systole due to narrowed lumen

• AT increases distal to a significant stenosis due to decreased distal resistance

what happens to the AT in bilateral extremity arteries when cardiac output is decreased?

the AT increases

AT in lower resistance vessels vs higher resistance vessels:

in normal lower resistance vessels like the ICA and cerebral vessels, the AT in increased when compared to a higher resistance vessel, like the extremity vessels

resistive index (RI)

• measurement of vascular resistance

• compares the difference between the systolic and diastolic velocities to the max velocity of the vessel segment

• used to evaluate tissue resistance to blood flow

• used to demonstrate changes in resistance caused by a stenosis/occlusion

if there is no diastolic flow (EDV 0 cm/s), what will the RI be?

1.0

how is resistance affected by a distal occlusion?

it will cause increased resistance prox to the obstruction = high RI

pulsatility index (PI)

• degree of resistance in a vessel

• measures the variability in blood velocity during the cardiac cycle

• compares the difference between the systolic and diastolic velocities to the avg velocity of the vessel segment

• used to distinguish prox disease from dist disease

• higher in extremities then organs, normally

what is the normal PI of the CFA?

>5.0

what is the normal PI of the popliteal artery?

>8.0

organs usually have PI values of:

<1.5

the brain usually has PI value of:

<1.0

damping factor

• describes the attenuation of the doppler signal with a stenosis

• compares the PI proximal to the stenosis to the PI distal to the stenosis

• a significant stenosis has a higher damping factor than a mild stenosis

effects of exercise on arterial flow:

• exercise should increase flow and the pressure gradient in normal arteries

• in normal pts, ankle and brachial pressures after exercise will be the same as the resting pressures or mildly increased

when exercise is performed in a patient with stenosis, what happens to flow?

• the muscles demand more flow but the arterioles cannot reduce the resistance any further

• the reduced inflow due to increased demand causes the pressure at the ankles to drop

in pts with arterial disease, what happens to the ankle and brachial pressures when they perform exercise?

• the ankle pressures drop while the brachial pressures stay the same

• the drop in ankle pressure leads to a reduced ABI calculation

• ischemic symptoms occur in the muscles due to to the reduced flow to the region

collateral formation

• numerous, smaller vessels that provide an alternative pathway to perfuse an organ/extremity

• blood vessels that dilate in response to increased flow in an alternate pathway due to stenosis or obstruction in the primary pathway

when does collateral flow occur?

• smaller potential collateral vessels normally have higher resistance flow than their native vessels, due to decreased vessel diameter and increased length

• when the resistance in the larger vessel exceeds that in the smaller collateral vessels, flow enters the collateral pathways

• when the the smaller vessels have lower resistance than the primary pathway that has the stenosis/obstruction

collaterals can mask symptoms that would normally be caused by:

the stenosis of the primary pathway

collaterals are only associated with:

chronic disease and is NOT seen with acute obstruction because they have no time to form the pathway

collaterals have limited ability for:

vasodilation and do not respond the same as native arteries do to exercise