Quiz 2 Flashcards
how much of the cardiac output supplies the brain?
15%
how much of the flow from the CCA enterws the normal ICA?
80%
brain has a high or low metabolic rate?
high-little circulaory reserve
where does the brain store oxygen or glucose?
Brain has no significant oxygen or glucose stores
what is the brain dependant on?
the vascular system
short episodes of interuppted cerebral flow can bring on what?
symtoms of cereral dysfuction
how long does it take for cellular death to occur?
3-8 minutes
how does blood flow with resistance?
the path of least resistance
what is resistance affected by?
length and width
does length or width have the greater effect?
long narrow vessel has the greatest resistance and a short wide has the least resistance
is length or width more limiting becuase of Poiseuille’s Law?
width
what does adequate arterieal perfusion rely on?
- systemic blood pressure
- cardiac output
- blood volume
If circulation is compromised by atherosclerotic disease _______________________
compensation may be insufficient
what does circulation compromised by atherosclerotic disease lead to?
regional or diffuse hypoxia or anoxia
what has a vital role in arterial occlusion?
collateral circulation
what testing is available for collaterization?
- duplex US
- angiography
- MRA (MRI angiography)
- CTA (CT angiography)
when evaluating the symtoms of cerebrovascular insufficiency what must clinicians be aware of?
extent or lack of collaterization
do collaterals develop fast or slow?
slowly often over a period of years
do patients reveal symtoms of cerebrovascualr disease with total occlusion of ICA?
no
why does collateral circulation develop?
becuase of a change in the pressure gradient between the vessels, caused by a stenosis
what happens to pressure, distal to a stenosis?
drop in pressure
when may blood vessels abandon their normal flow routes to take advantage of the new,lower pressure route?
if the pressure drops enough to attract flow from a vessel situated further down the line
(subclavian steal syndrome)
what is opening of collateral pathways dependant on?
patient age and time sequence of occlusion
in older individuals, collateral pathways may already be _____________
hypoplastic or atherosclerotic
when do collateral pathways have a better chance of developing?
with slow evolving atherosclerotic occlusion
when are collaterals not able to adapt?
not able to adapt rapidly enough to sudden occlusions such as emboli
where does our body develop alternate routes for blood flow?
each hemisphere of the brain (these are called collaterals)
what do the most common collateral pathways involve?
the circle of willis
what are the 3 major collateral routes?
- other side of the circle of willis
- the posterior cerebral circulation
- the external carotid artery branches
how many patients have a complete circle of willis (no varients)
50%
what does a totally occluded left ICA result in?
decrease in flow to the left hemisphere of the brain
describe the pathway of blood to the left from the other side of the circle?
Right CCA to right ICA to right ACA,across the anterior communicating artery ,then retrograde down the left ACA to the left MCA to perfuse the left hemisphere
describe the pathway from the posterior circulation?
Vertebral artery flow enters the basilar artery,to the left posterior cerebral artery,across the left posterior communicating artery to the distal left ICA and finally to the left MCA
describe the pathway from the ECA?
The left ECA to an ECA branch(superficial temporal,maxillary or facial),retrograde down an ICA branch(supraorbital,nasal or frontal) to continue retrograde flow down the ophthalmic artery,into the carotid siphon and finally to the MCA to perfuse the left side of the brain
what is the second most importnat source of collateral flow?
next to the circle of willis, the anastomosis between branches of the ECA and ICA via the orbital and ophthalmic arteries
what is the only branch of the ICA to give off branches that leave the intracranial space?
ophthalmis artery
what branches are apart of the peri-orbital circulation?
frontal and supraorbital arteries and the nasal artery
what do the peri-orbital arteries of the ICA communicate with?
branches of the superficial temporal artery as well as the facial artery
what sites of communication become important in cases of severe obstruction or occlusion of the ICA?
periorbital branches of the ICA communication with the superficial temporal artery and facial artery
collateral branches:
- lacrimal
- supraorbital
- anterior and posterior ethmoidal
- medial paplebral
- frontal
- dorsal nasal artery
what happens when occlusion occurs in the ICA?
flow in the opthlamic artery becomes retrograde in order to supply the brain
what is #1 less important collateral possibilities?
the occiptal branch of the ECA communicating with the atlantic branch of the vertebral artery
what is the #2 less important collateral possibilities?
the deep cervical branch of the subclavian artery communicating with the more proximal branches of the vertebral artery
what is the #3 less important collateral possibilites?
the “rete mirabile”consisting of a network of transdural arteries which may anastomose across the subdural space with the tiny arteries covering the surface of the brain
what happens with CCA occlusion?
flow reverses in the ECA and flows into the ICA in the opposite direction toward the brain
what system is included in the duplex exam of the carotids?
vertebral artery disease
what diagnostic information is most essential with vertebrals?
presence or absence of flow and direction of flow
how much of cerebral blood flow does the vertebral basilar system provide?
20-30%
in the case of vertebral occlusion near its origin flow is ______________-
shunted to the thyrocervical and costocervical trunks and compensatory enlargment of the opposite vertebral artery occurs
where do the vertebral arise from?
subclavian arteries
where in the cervical spine does the verbetral coarse?
C6-C2
what do the vertebral arteries form?
the basilar artery
what is a normal varient with the vertebrals?
left vertebral artery may arise directly from the aortic arch-6% of patients
how does the basilar artery connect to the circle of willis?
via the PCAA
symtoms and sign of posterior circulation ischemia
slide 6 collateral 2
what is the flow patterns of the vertebrals?
low resistant-similar to the ICA
what do the velocities range in the vertebrals?
20-60 cm/s
below ___________may indicate vertebral insufficiency due to prox or dist occlusion
20 cm/s
what may higher than normal velocities indicate?
stenosis
what may high volume flow states indicate?
a compensatory flow pattern as seen in collateral flow
what may high volume flow states be seen in some patients?
may be hypoplastic or absent on some patients
how often is right vertebral imaged?
80% of the time
how often is the left vertebral imaged?
60% of the time
what is the least likely level of disease in vertebrals?
mid portion (seen with least difficulty)
what vertebral artery is more commonly larger?
left
________ vertebrals are hypoplastic
7-10%
what doppler parameters are set for vertebrals?
low flow states
what flow direction should the vertebrals be?
antgrade (toward the head)
why do we look at spectral in the vertebrals?
rule out SSS
what is the normal flow pattern in the vertebrals?
- low resistant
- window not always present
what are the velocites in the vertebrals?
40-60 cm/s
what does a high resistant vertebral flow pattern with no diastolic flow imply?
- distal VA stenosis or occlusion
- hypoplastic VA
what are the S/S of a stenosis in the vertebrals?
dizziness and unsteady walking
what is indicative of vertebral disease?
increased compensatory flow-PSV completely reversed flow
describe the appearance of subclavian steal syndrome?
Lt SCA origon occlusion or high grade stenosis
-flow is reversed in ipsilateral vertebral artery
when does SSS result?
when the short low resistant path becomes a high reistant path due to narrowing
when may compensatory flow be seen in the vertebral artery?
presence of ipsilateral carotid occlusive disease
what shows the vertebrals ability to maintain flow via circle of Willis to cerebrum?
increased peak velocities
what is subclavian steal syndrome?
hemodynamically significant stenosis or occlusion in proximal subclavian arteries can result in the vertebral acting as a collateral pathway
where is SSS more commonly seen?
on the left side
what is the sonographic appearance of SSS?
-high PSV >500cm/s
-loss of normal triphasic signal
-color aliasing
-decreased ipsilateral brachial artery systolic BP
-40 mmHg difference between arms
damped flow distally (monophasic)
-reversed flow ipsilateral VA
why does flow reverse in the VA for SSS?
to supply the ipsilateral arm
impending steal
incomplete reversal of signal
what will happen when the VA has not completely reversed?
exercising the arm will completely reverse the signal below the baseline