Blood thinners and atrial fibrillation

Welcome to the blog section of my website.  I will try to update this regularly with topics of interest to my patients.  Please be aware that this information is of a general nature and is NOT a substitute for the advice of a physician or other health care provider.  I welcome your comments and feedback.

For my first section, I wanted to address the use of blood thinners in atrial fibrillation.  Atrial fibrillation is the single most common reason a patient comes to see me, even more than coronary disease (“heart attacks”).  One in four people over age 40 will develop atrial fibrillation in their lifetime.  It is one cause of an “irregular heartbeat.”  The most serious potential serious consequence is the risk of stroke, which occurs because the atria (the chambers at the top of your heart) do not contract properly in this condition.  Everybody has some risk of stroke based on their age, blood pressure and other risk factors.  The presence of atrial fibrillation raises this risk approximately SIX TIMES from what it would be otherwise.  Strokes from atrial fibrillation are also typically more substantial even then most strokes, with a higher percentage of these patients suffering a bedridden state, severe neurologic disability, or death.

Because of this we doctors tend to recommend blood thinners as part of the treatment for this condition.  Aspirin is one of the most important drugs for people with blocked arteries and heart attacks.  However it is less effective at preventing strokes in atrial fibrillation patients.  Taking aspirin for this purpose, stroke reduction in atrial fibrillation, has been shown to reduce stroke about 15-20%.  However when you realize that atrial fibrillation increases your stroke risk by 6x, there is clearly significant residual risk left over.  While some low risk patients may therefore be able to control their risk with aspirin, a more effective blood thinner is necessary for most patients.

The blood thinner that has been used for a long time is warfarin.  Yes, this is a purified form of “rat poison,” but no, we doctors are not trying to poison you!  This drug works at the level of the liver to inhibit formation of proteins that cause the blood to clot.  We follow the effect of this drug by a test called the INR (international normalized ratio).  Without drugs the normal INR is 1 to 1.2.  We have excellent data that shows if the INR is between 2 and 3 in a patient with atrial fibrillation, the risk of stroke in minimized, and the bleeding risk is also minimized, only modestly higher than normal.  The stroke risk reduction from warfarin is greater than 60%, and in many cases is very close to what the risk would be if atrial fibrillation were not present at all.

The largest problem with warfarin is that it is a difficult drug to dose.  Many other medications are also cleared from the body through the same mechanism in the liver, so changes in medication commonly affect the amount of warfarin that is required.  Dietary changes, particularly green leafy vegetables, are also important.  For this reason the medication is a hassle for a lot of patients and their family doctors, requiring  frequent blood tests and dose adjustments.  A new blood thinner has been desired for some time.

Several new blood thinners are in development.  One is already on the market, dabigatran (brand name Pradax).  It is a pill you take twice a day that works differently to prevent clotting.  It does not have much in the way of significant drug interactions, and dosing is very reliable, unless you have severe kidney impairment.  Therefore no regular blood testing is required and dietary modifications are not required.  It has been well studied, in one large trial with over 18,000 patients, and found to be more effective than warfarin at reducing strokes, with a similar risk of bleeding.  The only side effect was dyspepsia (upset stomach) in a small percentage of patients, that was not serious.  It is a very useful new product and I think will have a significant clinical role.

I use both warfarin and dabigatran in regular clinical practice with good results.  Many people have done well on warfarin.  I am also using the new drug dabigatran with a lot of success.  The one side effect of dabigatran is cost!  It is not yet covered by MSP and costs about $3.50 per day.  Many third party drug plans pay most or all of this cost.  Most patients on the drug like it and I have not had to stop it in anyone because of stomach upset.

Despite usually explaining things very well to patients I encounter some who are resistant to going on anticoagulation with either warfarin or dabigatran.  They are generally afraid of bleeding risk.  There is lots of data to show stroke risk in atrial fibrillation patients dramatically exceeds the bleeding risk for the vast majority of patients, and therefore that these blood thinners should be used in most patients (there are always individual exceptions).  I usually try to spend my time and go over the rationale for the stronger blood thinners with mixed results for these patients.  I have seen many patients with stroke due to atrial fibrillation and profound neurologic disability and I try my best to prevent this in my patients.

What do you think?  Do physicians do a good job of explaining the rationale for these blood thinners?  For drug therapy in general?  What would you think if you were in this situation?

John Vyselaar, MD, FRCPC