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Blood clots: The good, the bad, and the deadly

Those arising from atherosclerosis and atrial fibrillation can be very dangerous.

When you poke yourself on a thorn while gardening or get a paper cut at the office, your body marshals the forces needed to stop the flow of blood and repair the damage. If it weren’t for the blood’s ability to clot (form a thrombus, in medicalese), even these minor scrapes of daily living could cause us to bleed uncontrollably. These healing clots also form inside the body at sites of blood vessel injuries. Normally, when the clot’s job is done, it dissolves away.

But sometimes clots form in places where they do more harm than good — like in the arteries that supply the heart or the brain, or in the veins of the legs. Clots inside the body can also be quite mobile, traveling through the bloodstream from place to place. For example, a clot formed in a leg vein can travel to the lungs, and one arising in the heart can end up in the brain. Such silent migrations can have deadly consequences.

Plaque rupture and clot formation

illustration of artery showing formation of blood clot

A plaque that ruptures in an artery triggers clot formation. First, platelets rush to the scene and clump together. Then a cascade of biochemical reactions creates a fibrin mesh, which traps circulating red blood cells in its net. The plaque and the clot can obstruct the vessel and completely halt blood flow, leading to a heart attack or stroke.

Three-part process

Whether at the surface of the skin or deep within the body, clotting is a complex, highly orchestrated process. It can be broken down into three basic parts:

Formation of a platelet plug. Platelets circulating in the blood are the first responders to an injury. They stick together and release chemicals that trigger the next phase.

The clotting cascade. In a chemical chain reaction, the injury itself and platelet activity switch various molecules from inactive to active states. This yields a netlike mesh made of fibrin, a protein that reinforces the platelet plug and snares passing red blood cells.

Clot dissolution. When the danger has passed, a reverse cascade dissolves the clot. This process involves the friction of blood flow, the removal of pro-clotting chemicals by the liver, and the activation of chemicals that produce a clot-dissolving enzyme called plasmin.

Clot busters may be safe after “wake-up” strokes

If doctors determine that a stroke is caused by a blood clot, they often administer a powerful clot-busting drug (see main article) — as long as it can be given within four-and-a-half hours of the time symptoms started.

But what if you wake up after a night’s sleep with stroke symptoms — as an estimated one-quarter of stroke sufferers do? No one knows when the “event” began, and in those uncertain situations, doctors are advised not to use clot busters for fear the drugs will cause excessive bleeding.

However, research presented in February 2012 at an American Stroke Association conference indicates that clot busters can be safe and effective when given to people with wake-up strokes, if their symptoms upon arrival at the emergency room and subsequent CT images are similar to those of people treated within the four-and-a-half-hour window.

Researchers reached this tentative conclusion by comparing two groups of stroke victims treated with the clot buster alteplase — 68 people who had wake-up strokes and more than 300 others who knew when stroke symptoms began and who received the clot buster within the recommended time window. After three months, the rates of recovery, death, and excessive bleeding were the same in both groups.

The four-and-a-half-hour guideline will stay in effect at least until published randomized studies confirm these findings. And, as interesting as these preliminary results may be, the most important things you should know about stroke are its sudden warning signs:

  • numbness or weakness in the face, arms, or legs

  • trouble seeing, speaking, or understanding speech

  • loss of balance or coordination

  • severe headache with no obvious cause.

Another very important thing is to call an ambulance and get to the hospital fast if you have any of these signs.

Dangerous clots from plaque

Clots can become dangerously explosive when they arise from atherosclerosis. This disease process starts with an injury to the blood vessel’s delicate inner lining. The damage accelerates as white blood cells, particles of cholesterol-carrying low-density lipoprotein (LDL), and inflammatory chemicals form a gooey pool. Eventually, a thin cap forms over the mess, which is now called a plaque.

If the cap splits open, the body responds as it would to a cut in the skin, and clotting begins. If the break is small and heals quickly, the episode passes harmlessly. But if the rupture is extensive, the resulting clot can completely block the artery (see illustration). When this happens in an artery supplying the heart, it causes a heart attack. When a clot forms in an artery feeding the brain, or when one forms elsewhere and travels to the brain, it can cause an ischemic stroke, the most common kind of stroke.

Plaque-initiated clots can also form in the abdominal aorta, the main blood pipeline to the lower part of the body, and in the arteries supplying the legs and feet. Clots not related to plaque can also form in places where blood settles abnormally. For example, people with persistent atrial fibrillation — rapid and uncoordinated beating of the heart’s upper chambers — are at increased risk of clots because blood stagnates in the chambers. Adding injury to insult, clots generated by atrial fibrillation can travel to the brain and lead to a stroke.

Some drugs for preventing and dissolving clots

Medication type

Generic name

Brand name

Antiplatelet drugs

aspirin

ticlopidine*

Ticlid

clopidogrel*

Plavix

prasugrel

Effient

abciximab

ReoPro

eptifibatide

Integrilin

tirofiban

Aggrastat

dipyridamole*

Permole, Persantine

cilostazol*

Pletal

ticagrelor

Brilinta

Anticoagulant drugs

warfarin*

Coumadin, Jantoven

fondaparinux*

Arixtra

rivaroxaban

Xarelto

heparins*

various

dabigatran

Pradaxa

bivalirudin

Angiomax

Thrombolytic (clot-busting) drugs

alteplase

Activase

reteplase

Retavase

streptokinase

Streptase

tenecteplase

TNKase

*Asterisks indicate prescription drugs that are available as generics.

Drug war against clots

Because clots can cause so much internal havoc, researchers are constantly looking for ways to prevent platelets from sticking together and to interrupt the clotting cascade at one or several of its stages. Two classes of drugs that accomplish those objectives — termed antiplatelets and anticoagulants, respectively — are frequently (but not accurately) called blood thinners. If you and your doctor want to reduce your chances of developing an unwanted clot, you’ll probably discuss these two types of medications (see table).

Clot-busting drugs (thrombolytics) are used in emergency situations to quickly dissolve problematic clots. They are laboratory formulations of chemicals the body produces naturally to dissolve clots, and they’re usually delivered directly into a vein under close medical supervision. Most other anticlotting medications — except for a few injectable ones — are available in pill form.

Remember that any one of these anticlotting drugs — even aspirin — can cause excessive bleeding, so if you are taking one, explicitly follow instructions for use and watch for signs of abnormal bleeding, such as bruising and swelling (without physical injury) or blood in the stool.

It goes without saying that living healthfully is your best strategy to avoid the invisible internal insults that kick off the clotting process. If you already have cardiovascular disease or compelling risk factors for it, you may already be taking one or more of these anticlotting drugs. If you’re not, ask your doctor whether you should be.

Posted by: Dr.Health

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