Preventing Stroke in Atrial Fibrillation
Atrial fibrillation is known to be associated with an increased risk
of a stroke. There are a number of risk factors that particularly
increase this risk of stroke. The scoring system to assess the risk
of stroke is called CHADS2. The risk factors included in this score
include high blood pressure, or hypertension, diabetes mellitus,
congestive heart failure, age 75 or greater, or prior stroke. For
each of the first four risk factors, one point is given, and there are
two points given for a stroke or transient ischemic attack (TIA).
The most recent guideline for prevention of stroke in atrial
fibrillation utilizes this scoring system. High blood pressure, or
hypertension, is included as a score even if the patient’s blood
pressure is currently normal with treatment. Similarly, diabetes,
whether treated with medications or with insulin, is included in this
risk factor. There is no specific distinction made with respect to
diet-regulated high blood pressure or diabetes. The risk factor of
prior stroke is not defined to be necessarily due to atrial
fibrillation, but includes any previous cause of stroke. Congestive
heart failure is usually diagnosed with the occurrence of symptoms of
fluid overload, or shortness of breath. While age of 75 is used,
there is general recognition that the risk of stroke increases per
decade, particularly 65 years of age or greater, but the risk is
considerably higher at 75 years of age or greater.
It is known that patients with no risk factors—none of these risk
factors—are at a low risk of stroke. Depending on the patient’s age,
the risk of stroke still is higher than others without atrial
fibrillation. With one risk factor, the risk of stroke rises
significantly above that of a patient without any risk factors.
However, the risk of stroke with two or more points markedly increases
that risk.
The use of medications to prevent stroke in atrial fibrillation has
evolved somewhat over time. The mainstays of treatment still,
however, include warfarin, known by its trade name, Coumadin, or
aspirin, and are still the two most commonly used agents. Warfarin
and aspirin act differently in the way in which they prevent blood
clots. Warfarin needs to be adjusted according to the results of a
blood test. The accepted blood test is based on the International
Normalized Ratio, or INR. This is a standard across laboratories
worldwide, which assures the ability to compare laboratory results.
The value of INR depends partly on the patient’s history and prior
stroke, particularly, if the patient has had a previous stroke on
anticoagulation. The most commonly used range for atrial fibrillation
is 2.0 to 3.0 of the INR. The INR has been known to fluctuate to a
great degree over time, modified by vitamin K injections as well as
factors that affect its metabolism. Warfarin is affected by vitamin K
injection. Vitamin K is present in many green leafy vegetables and
therefore taking any meal, for example, of spinach, would reduce the
INR significantly.
There are a number of factors that affect metabolism of warfarin by
the liver. One of the most common effects on the liver is alcohol
ingestion. Alcohol affects metabolism of warfarin, and can markedly
affect the INR.
There are a number of precautions that patients should take when
they are on warfarin. Close follow-up of the INR is important, since
the risk of bleeding goes up significantly as the INR increased above
3.0 or 3.5. Bleeding risks can be divided into spontaneous or related
to trauma or intervention. Sometimes bleeding appears to be truly
spontaneous. This may include internal bleeding, such as bleeding
from a stomach ulcer, or from the intestine. Usually when this occurs
without an excessively high INR, there usually is an underlying
abnormality in the gastrointestinal tract. On the other hand,
bleeding at extremely high INR levels may not as clearly represent an
underlying abnormality. This is particularly true for bleeding in the
urine, which may occur on warfarin and does not as frequently
represent underlying cause. In most cases, however, physicians will
investigate as to whether there is an underlying abnormality that may
lead to an increased risk of bleeding on warfarin.
Perhaps the greatest risk of bleeding with warfarin is with trauma.
Even a relatively innocent bump of one’s knee or elbow can lead to
significant bleeding, and sometimes bleeding into the joints. One of
the most serious forms of bleeding is bleeding in the brain, called
intracranial hemorrhage. This can occur spontaneously, but is
particularly of concern if there is any head trauma such as due to a
fall or otherwise hitting one’s head. Some cases of such bleeding,
which may result in a condition called a subdural hematoma, may not be
clearly identified initially. It may present as decreased mental
function or overall decreased mental status, as well as a prolonged
headache, particularly in a patient without a history of headaches.
As a result, many physicians will obtain a CT scan of the head if
there has been head trauma in a patient on warfarin, even though the
symptoms may be fairly minimal.
Warfarin is available both as a generic or as the trade name,
Coumadin. There may be some variation from preparation to preparation
on its effects on the INR.
Some people on Warfarin prefer to check their INR values
themselves. There are currently services available which allow one to
obtain a home-based system, called a Point of Care System, that allows
the patient with a pinprick to measure their INR without needing to go
to a laboratory. Some physicians suggest that a comparative value be
used, that a patient have an INR obtained through a laboratory at the
same time that the Point of Care System is used. Some patients find
this to be a convenience.
Aspirin also may be used to prevent stroke in atrial fibrillation.
There are a number of studies which describe aspirin’s role, and have
demonstrated its utility. The exact dose of aspirin that is used
varies from study to study, including 81 to 325 mg a day. There is no
evidence that a dose higher than 325 mg a day is necessary. Many
physicians recommend taking the enteric, or stomach-coated preparation
of aspirin, to decrease the likelihood of irritation of the stomach
called gastritis, which may lead to internal bleeding. Aspirin,
unlike warfarin, acts on the platelets. The effect on the platelets
lasts approximately one week, if taken as expected.
Plavix, or clopidogrel, has been used in a number of settings, such
as after a heart artery stent. However, it is not routinely used to
reduce stroke in atrial fibrillation.
Some patients, however, may have an indication to be on Plavix, such
as a heart stent, but also may be recommended to be on warfarin.
Being on multiple agents that affect different parts of the blood
clotting process will increase the risk of stroke. However, many
patients are on such combinations, and it is worthwhile to ask one’s
physician directly whether multiple agents should be used.
The use of non-anticoagulant medications while on aspirin or
warfarin can also be problematic. For example, the use of
nonsteroidal anti-inflammatory agents such as Advil and Indocin, seems
also to potentiate or increase the likelihood of bleeding on warfarin
or aspirin.
New agents to prevent the risk of stroke in atrial fibrillation are
actively being developed and tested. Dabigatran has been approved as
an anticoagulant. Its role in reducing stroke in atrial fibrillation
has also been demonstrated. It is as effective as warfarin, but does
not require the blood tests such as the INR to be obtained. Instead,
a fixed dose is prescribed for each patient. Dabigatran also does not
have any significant interactions with food or other medications.
Medications that cause bleeding, however, will still need to be used
with caution. Dabigatran is taken twice daily.
Patients often inquire what would need to be done if in fact they
developed an acute bleeding problem that results in an emergency while
being on an anticoagulant. In most cases, it is possible to reverse
the effects.
Learn more about stroke.