IS HEART DISEASE ALL DUE TO BLOOD CLOTS?

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http://www.redflagsweekly.com/kendrick/2003_mar21.html

 

 

IS HEART DISEASE ALL DUE TO BLOOD CLOTS?

 

By Malcolm Kendrick MbChB, MRCGP (email - malcolm )

 

Up to now I have resisted writing about this area, as blood clotting is a

mind-boggling and complicated area of human physiology. In the end, however,

if you want to understand coronary heart disease (CHD), you cannot ignore

the role of the humble blood clot.

 

For it is now accepted by everyone involved in CHD research that the final

event, the thing that kills you with CHD, is the formation of a blood clot

on top of an atherosclerotic plaque. If the blood clot is big enough to

completely block a critical artery, in a critical area, you will die.

 

In the last few years medics have become increasingly expert at trying to

clear these potentially fatal clots. Aspirin is the first line of defence,

then the clot busters streptokinase, or tissue plasminogen activator (tPA)

are used.

 

Increasingly, cardiologists get to work with thin wires and balloons, and

stents, to remove the clot, prize the artery apart, and stick a metal

framework to keep the artery open after unblocking it. New drugs have been

developed to keep the artery patent. This is all great stuff, and many

thousands of people who used to die are now being saved.

 

So there is no argument from anyone about the final event in CHD. It's a

blood clot. It is also recognised that blood clots develop over

atherosclerotic plaques on quite a regular basis without causing any

symptoms at all, presumably because they are not big enough to fully block

the artery.

 

However, in these silent episodes, once the blood clot stabilises it adds to

the plaque size, and can lead to greater narrowing of the artery. In this

way, repeated blood clots forming over an area of existing plaque cause

atherosclerotic plaques to 'grow'. And if you look at plaques closely, you

can - in many plaques - clearly see bands, with each band indicating an

episode of plaque growth.

 

This is all agreed upon by almost everyone. And if you were a simple soul,

like me, you might argue that if plaques grow, and eventually kill you due

to clots forming on the artery wall, could this not be how they start in the

first place? Are atherosclerotic plaques not, in fact, just the remnants of

repeated blood clots, which are 'drawn in' to the artery wall, in time

turning into a form of scar tissue?

 

If you did think this, you wouldn't be the first. This hypothesis was

initially proposed by Karl Von Rokitansky in 1852. Although supporters of

Rudolf Virchow may argue that he said it first. Unfortunately, therefore, I

can hardly claim that this idea is either new, or mine.

 

Can it really be that simple? Surely there must be something wrong with the

hypothesis that atherosclerotic plaques are the remnants of repeated blood

clots? Where does this idea break down? I could say, don't ask me, I happen

to believe it's true. But I will attempt to be a little more objective than

this.

 

The key point of objection is that, whilst you can see how blood clots can

form over a 'damaged' artery wall, it is very difficult to see how they form

over a healthy artery wall. After all, a critical function, perhaps the

critical function of the lining of the artery (the endothelium) is to

prevent blood clots from forming. So how can this process actually start? A

good point from my learned friend.

 

But I put it to you, members of the jury, that every 'factor' that has been

identified as increasing the risk of CHD, has clearly identifiable

pro-coagulant activity. Equally, every factor that has been identified as

reducing the risk of CHD has clearly identifiable anti-coagulant activity.

Which, I would vouchsafe, is pretty heavyweight proof.

 

Is this really true?

 

Well, yes. But you have to understand that there are three interconnected

factors at play here that can cause a clot to form over the artery wall.

Factor one, is 'damage' to the endothelium. Once damaged, the endothelium

stops acting as a non-stick anti-coagulant surface. Indeed, if the

endothelium is stripped away, it exposes the middle layer of the artery, the

media, to the blood, and the media releases the most powerful pro-clotting

factor known to man: Factor VII, the 'extrinsic' factor.

 

The second factor is how pro-coagulant the blood is itself. There are a

multitude of clotting factors in the blood. Some of which you may have heard

of, such as factor XIII - the one that is missing in haemophiliacs; some of

which you probably haven't heard of e.g. Von Willibrand factor. Increase a

pro-coagulant factor, and you increase the chance of clots forming.

 

The third factor is the structure of the blood clot itself. Some clots are

wobbly and weak; others are very tough, and difficult to break up. For

example, incorporated into all blood clots is a substance called

plasminogen. This is an enzyme which, when activated, chops the clot into

pieces. (Which is why tPA - tissue plasminogen activator - is given to

people having a heart attack) However, if you have a high level of

Plasminogen activator inhibitor - 1 (PAI - 1) in the blood, plasminogen is

less effective at breaking the clot up.

 

So, you have to look at three basic factors:

 

The anti-coagulant status of the endothelium

The pro-coagulant state of the blood

The relative 'toughness' of the blood-clot once it is formed

 

Bearing this in mind, I think it is interesting to run through a few factors

known to alter the risk of CHD, and see how they fit:

 

Smoking:

 

Smoking creates free-radicals in the blood, these reduce nitric oxide (NO)

synthesis in the endothelium, and NO is the single most powerful

anti-coagulant factor in the body. Smoking also has pro-coagulant effects in

the blood; it raises fibrinogen levels. It also has endothelium damaging

effects. So, if you want to avoid CHD. STOP SMOKING!

 

Drinking:

 

Ethanol, in moderate doses, reduces free-radical synthesis, reduces clotting

factors, such as fibrinogen, and reduces the blood clot toughness. However,

excess alcohol consumption creates rebound platelet stickiness (platelets

are hugely important in blood-clotting). Moderate drinking protects against

CHD, heavy drinking is a risk factor.

 

Diabetes:

 

A high blood sugar level leads to increased free-radical synthesis, see

above. A high blood sugar, independent of its effects on NO synthesis, also

causes endothelial 'damage.'

 

Haemophilia:

 

Not surprisingly, haemophilia reduces blood coagulability. Haemophilia also

reduces the absolute risk of CHD by 80%.

 

Statins:

 

Statins have strong anti-coagulant effects, they stabilise plaques and

increase NO synthesis.

 

Aspirin:

 

Aspirin reduces the stickiness of platelets (see alcohol). Platelet

aggregation is the first step in blood clotting.

 

Omega-3 fatty acids

 

Omega-3 fatty acids have strong anti-coagulant effects in the blood

 

Stress

 

Physical, or psychological stress causes the release of the stress hormones:

cortisol, adrenaline, growth hormone and glucagon. These hormones all

increase blood coagulabiltiy, raise the blood sugar level (see above), and

'damage' the endothelium.

 

Raised blood pressure

 

I am a little ambivalent about this risk factor. I am unconvinced that a

raised blood pressure really is a 'cause' of plaque development. However, it

is possible to see how high pressure, and turbulent blood flow, could strip

away a layer of endothelium, exposing the blood to the media, and thus

factor VII, thus stimulating a blood clot to form. It is certainly true that

plaques don't form in low pressure blood vessels (e.g. veins).

 

However, the clinical trials on blood pressure lowering are very

unconvincing when it comes to a correlation between the degree of blood

pressure lowering and the prevention of CHD.

 

HDL

 

HDL has strong anti-coagulant effects

 

LDL (Oxidised LDL)

 

This is a complex pathway. When platelets start to stick together, they

release free radicals. Free radicals oxidise LDL. Oxidised LDL is a powerful

blood clotting factor. LDL is also incorporated into the blood clot as it

forms, and provides a 'lipid' surface (along with VLDL) for the construction

of fibrin. Fibrin is the hugely strong protein strand that binds a clot

together and makes it 'tough.'

 

Frankly, I think that's enough. If you wish to, it is possible to link every

single factor known to have an impact on CHD rates to one of three effects:

endothelial damage, blood coagulation, or toughness of the clot. If you

don't affect any of these three things, then you have no effect on CHD

rates. If you manage to impact all three, then it's time to increase the

life insurance. (On a positive note you won't need a big pension fund).

 

So, if it's that simple, then why have you never heard of this before? Now

that is another story altogether. But if enough people think I am making all

of this up, then I will provide a series of references from prestigious

journals to support every single fact that I have presented.

 

To my mind, the answer as to the underlying cause(s) of CHD is not only 'out

there,' it has been staring everyone in the face for the past fifty years.

Perhaps it is too obvious for anyone to see it.