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Preventing stroke in atrial fibrillation

A new technique helps to predict the risk of stroke, and helps doctors decide when to anticoagulate

By DrRich

It has long been known that patients who have chronic, persistent, or frequent episodes of atrial fibrillation have an increased risk of stroke.  (Click here for a review of atrial fibrillation and its treatment.)  

The increased risk comes from the fact that, during atrial fibrillation, the heart's atria are not squeezing effectively.  Consequently, the blood tends to "pool" in the atria - and whenever blood flow is disrupted, blood clotting can occur.  Clots that form in the atria can break loose eventually, and if the clots travel through the arteries to the brain, a stroke results.

For patients who have atrial fibrillation, the risk of stroke depends on their age, and on other medical conditions they may have.  Because the risk of stroke depends on several factors in addition to the presence of atrial fibrillation, deciding whether anticoagulation with Coumadin (a blood thinner) is warranted has been a relatively complicated decision.

Recently, however, researchers from the Washington University of Medicine in St. Louis have devised a simple model to help doctors and patients decide whether anticoagulation therapy is warranted.  The model - called the CHADS model - assigns a score from 0 to 6, based on the patient's age and other medical conditions.  The number of points "awarded" is given here:

 

Medical condition

# points

prior stroke

2

congestive heart failure

1

high blood pressure

1

diabetes

1

age 75 or older

1

 

From this point system, a CHADS score from 0 - 6 is computed for each patient with atrial fibrillation.  Next, the CHADS score is used to estimate the yearly risk of stroke as follows:

 

CHADS Score

Yearly risk of stroke

0

1.9%

1

2.8%

2

4.0%

3

5.9%

4

8.5%

5

12.5%

6

18.2%

 

How should the CHADS score be used?

In deciding whether or not to place patients with atrial fibrillation on anti-coagulation, the CHADS score helps to predict how high the risk is of not using anticoagulation.  From the above table, for every 100 patients with atrial fibrillation whose CHADS score is 0, about 2 per year will have a stroke.  For every 100 patients with atrial fibrillation whose CHADS score is 6, about 18 per year will have a stroke. (By way of  comparison, for every 100 people with no atrial fibrillation, about 1 per year will have a stroke.) 

Treatment with Coumadin reduces the risk of stroke in atrial fibrillation by about 2/3 (66%,) and treatment with aspirin reduces the risk by 1/4 (25%).  The risk that anticoagulation will produce a major bleeding episode varies depending on the patient's medical problems and the care with which the protime or INR (i.e., the "thinness" of the blood) is monitored, but generally is less than 1 - 2% per year.

Based on all this information, the developers of the CHADS model recommend strongly considering therapy with Coumadin for anybody whose CHADS score is 1 or higher.  Certainly, most patients with CHADS scores of 2 or higher stand a strong chance of benefitting from anticoagulation.

Precautions in using the CHADS model

The CHADS model was developed by studying the records of 1733 Medicare beneficiaries aged 65 to 95, and in this age group the model appears very accurate at predicting stroke risk.

How well the CHADS model works for younger patients is not entirely clear, however, since the data was gathered for elderly patients.  The model is not valid at all for individuals with the valve disease called mitral stenosis - these patients have a very high risk of stroke, and virtually all of them should take anticoagulation therapy.

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