Heart, Stroke | March 29, 2016 | Author: The Super Pharmacist
Normally, electrical impulses move from across the smaller, upper chambers of the heart (i.e. atria) to the lower, larger chambers of the heart (i.e. ventricles). This electrical stimulation causes the chambers of the heart to contract in sequence, which generates blood pressure and pumps blood throughout the body.
In atrial fibrillation, the electrical impulses in the atria are abnormal. As a result, the chambers of the heart do not contract sequentially. This uncoordinated beating of the heart causes many short-term and long-term consequences.
The ventricles are responsible for generating most of the blood pressure in the body. The right ventricle pumps blood to the lungs, while the left ventricle is responsible for pumping blood to the rest of the body. The atria provide a small, but important addition to blood pressure, called the “atrial kick.”
During atrial fibrillation, the atrial kick is lost. Loss of the atrial kick reduces cardiac output by 20 to 30% in otherwise healthy individuals and by even greater amounts in people with heart disease. As a result, people with atrial fibrillation may experience a drop in blood pressure (i.e. hypotension). This drop in blood pressure may cause people to become lightheaded or dizzy, or they may lose consciousness (i.e. pass out) because of lack of blood flow to the brain. Indeed, the most serious short-term complication of atrial fibrillation is reduced blood flow to the brain, the kidney, and other important organs (i.e. hemodynamic instability).
Abnormal electrical conduction in atrial fibrillation prevents the atria from properly squeezing blood into the ventricles. This not only reduces cardiac output and blood pressure, but it also causes blood to stagnate in the atria. Stagnant blood tends to form blood clots, which is a major concern in atrial fibrillation. Indeed, the most common serious complication of atrial fibrillation is arterial thromboembolism i.e. a blood clot within an artery. Arterial blood clots could break loose and travel through the bloodstream to the lungs or to the brain, where they may cause pulmonary embolism or stroke, respectively. Therefore, people with chronic atrial fibrillation may need to take anticoagulants to prevent blood clot formation in the atria.
Antithrombotic therapy with an anticoagulant drug, such as warfarin, substantially reduces the risk of an arterial blood clot in patients with atrial fibrillation.There is a trade-off between the risk of clotting and the risk of bleeding, however. Treatment with warfarin increases a patient's risk of having a clinically significant episode of bleeding, such as gastrointestinal tract hemorrhage or hemorrhagic stroke. Newer anticoagulant medications are associated with less risk of serious bleeding, but the risk of bleeding with these newer medications is still greater than for someone who is not on anticoagulant treatment.Therefore, one must balance the risk of blood clots with the risk of bleeding.
When determining whether to use anticoagulant medications to prevent blood clots in patients with atrial fibrillation, physicians use a means of calculating risk called the CHA2DS2-VASc score. CHA2DS2-VASc is actually an acronym that corresponds to eight risk factors for complications arising from atrial fibrillation. Patients are assigned one point for each risk factor except for age greater than 75 years or a history of stroke, transient ischemic attack, or thromboembolism, which are each assigned two points. The maximum number of points that can be assigned is nine.
1. Stroke/transient ischemic attack/thromboembolism (2 points)
2. Congestive heart failure
3. Hypertension (i.e. high blood pressure
4. Age ≥75 years (2 points)
5. Diabetes mellitus
6. Vascular disease (prior myocardial infarction, peripheral arterial disease, etc.)
7. Age 65 to 74 years
8. Sex category (Female)
For most patients with atrial fibrillation, a CHA2DS2-VASc score of two or greater is considered high risk, and the risk of a stroke from a blood clot outweighs the risk of serious bleeding. As such, people in this risk category are treated with an oral anticoagulant in most cases.
A CHA2DS2-VASc score of zero is considered low risk and is not normally treated with oral anticoagulants.
People with atrial fibrillation who have a CHA2DS2-VASc score of one are neither high risk nor low risk of ischemic stroke. The decision whether to use anticoagulants in these individuals is much more complex. Patients with a CHA2DS2-VASc score of one may or may not be treated with oral anticoagulant medications. The decision whether or not to use oral anticoagulants is made on an individual basis by a cardiologist and the affected patient. Unfortunately, anticoagulants are unlikely to help most patients with a CHA2DS2-VASc score of one.
Historically, warfarin was the only reliable oral anti-coagulation method for patients with atrial fibrillation.
As patients who have been prescribed warfarin well know, warfarin interacts with virtually all other medications and many foods. Consequently, finding the proper dose of warfarin is highly individualised. It usually requires patients to have frequent blood testing to determine their personal level of anticoagulation. Initially, blood tests may be required as often as once per week.
Fortunately, a number of oral anticoagulant medications are now available that do not require frequent blood testing. The active ingredient for these medications are apixaban, dabigatran, edoxaban, and rivaroxaban. Moreover, the risk of bleeding associated with these newer medications is less than it is for warfarin. Unfortunately, the cost of these newer medications can be considerably higher than it is for warfarin, which may be a concern for some patients with atrial fibrillation.
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