The pathogenesis is as we have been talking
about due to artherosclerosis. What happens
is artherosclerosis develops slowly narrowing
the blood vessel. It can also develop because
the atherosclerotic plaque ruptures and a
blood clot forms and pieces of blood clot
can embolize down into the smaller arteries
of the leg causing gangrene or the blood clot
can come from further up in the arterial system.
It can come from a blood clot over an atherosclerotic
plaque in the aorta. It can come from a blood
clot actually in the heart. It can come when
the patient has a heart attack. A blood clot
may form inside the heart and embolize and
travel down and cause a sudden arterial occlusion
particularly in an area that has already been
narrowed by atherosclerosis. The arterial
emboli as then I have said once more can occur
in the left atrium from atrial fibrillation
and the carditis or infection in the heart
valves, sometimes will have blood clot overlying
the infected valve that can embolize and of
course artificial heart valves can form a
blood clot that can embolize as well.
Artherosclerosis in the aorta and other major
arteries are very common source of ruptured
plaque with the blood clots that embolises
and of course these emboli don’t have to
go to the legs. They can also go to the carotid
arteries and cause a stroke.
Atrial fibrillation, let's take a few minutes
to talk about it because atrial fibrillation
is extremely common. Remember what atrial
fibrillation is. The heart beats in a nice
normal fashion like this papump, papump, papump.
In atrial fibrillation, the atrium no longer
is working in a coordinated fashion but in
a very irregular way like this…and passing
many many electrical impulses down into the
ventricle only some of which gets through.
So instead of a nice regular pulse pum, pum,
pum you have this pum, papapum, pumpapapum,
papapup… very irregular. When that happens
in the atrium, the disorganized atrial activity
increases the likelihood that a blood clot
will form in the atrium and if it breaks off
and embolises and goes to the brain it can
cause a stroke. In fact 15% to 20% of all
strokes in the United States are due to embolization
from a blood clot in the left atrium that is the
result of atrial fibrillation. AF increases
the risk of stroke by 4 to 5 fold. It's an
independent risk factor for ischemic stroke
severity and recurrence. In other words because
the arteries have not been prepared with collateral
circulation when the embolus arrives, usually
a very healthy piece of the brain is deprived
of blood flow and dies forming a stroke.
So the stroke risk unfortunately occurs even
if people don’t have any symptoms from their
atrial fibrillation or even if they only occasionally
have atrial fibrillation and go back and forth
between the normal rhythm and atrial fibrillation.
And so the term often used by the doctors
is cardioembolic stroke. That is, it is an
embolism that comes from the heart passes
out into the circulation and unfortunately
goes to the brain. Now why would it go to
the brain? Why doesn’t it go elsewhere?
Why doesn’t it go to the kidneys or elsewhere?
Sometimes it does, but it turns out that 25%,
fully a quarter of the blood flow that is
coming out of the heart goes to the brain.
So the brain gets a lot of the blood flow
so it also has a much higher chance of getting
the blood clot that is coming out of the heart.
Now you can also have strokes because of artherosclerosis,
the narrowing in the blood vessels that we
talked about before that can lead to a heart
attack. The atherosclerotic plaque can rupture,
a blood clot can form and the artery can be
completely occluded, or the plaque can rupture,
a blood clot forms and a chunk of the blood
clot embolises up to the brain not from the
left atrium in this case but from a ruptured
atherosclerotic plaque in the carotid artery.
So narrowing or constriction of the carotid
arteries is a major cause of strokes and about
25% of all strokes are caused by carotid artery
artherosclerosis. As I said it can be due to
embolization or to complete occlusion relating
to rupture of one of these atherosclerotic
plaques. Detection of the asymptomatic carotid
plaque is very poor by physical exam. You
might hear a bruit, that is you might hear
a sound over the artery, reflecting turbulent
blood flow in the carotid artery but very
often you don’t hear anything so the only
way to make this diagnosis is usually with
a non-invasive test that is with an ultrasound
test that shows you the atherosclerotic plaque.
So peripheral artery disease whether it is