Controlling the Electrical Chaos of Atrial Fibrillation
By Peter Ott, MD, associate professor of medicine, Division of Cardiology, University of Arizona College of Medicine - Tucson
Atrial fibrillation, the most common heart rhythm abnormality, affects about 3 million individuals in the United States, with the number projected to increase as our population ages. Half of these patients will be 80 years or older.
In atrial fibrillation, the heart rate tends to be fast and irregular; thus the patient often complains about heart palpitations, racing or skipping sensations. These patients experience three types of episodes:
• Paroxysmal atrial fibrillation: episodes recur and typically stop spontaneously within hours to days.
• Persistent atrial fibrillation: episodes do not terminate spontaneously, but medical therapy can revert the rhythm to normal.
• Permanent atrial fibrillation: a chronic, irregular heart rhythm that cannot be reverted back to a normal rhythm, but if associated with a fast heart beat, the rate can be controlled with medication.
The Electric Heart: A Brief Lesson in Anatomy and Physiology
The heart consists of four chambers: two upper chambers (right and left atrium) and two lower chambers (right and left ventricle). These chambers rely on the heart’s own electrical system to stimulate muscle contractions to pump blood through the body. The heart’s electrical system includes a “spark plug” (sinus node), located at the top of the right upper chamber (atrium) that initiates the heart beat by sending an electrical impulse which travels through both the right and left atria. This electrical impulse eventually conducts through a connector cable (AV node), exciting the lower chambers (ventricles), which then contract and expel blood.
In atrial fibrillation, the atrium is in a state of electrical chaos with several hundred impulses exciting the upper chambers in random pattern. Thus the upper chambers can no longer contract in a coordinated fashion. These impulses also jostle to travel through the AV node and stimulate the lower chambers, typically in a fast and irregular fashion.
Symptoms: With the fast and irregular heart rate, the patient often complains about palpitations, racing or skipping sensations. Since the heartbeat is no longer coordinated, the patient may have fatigue, lightheadedness, chest discomfort or shortness of breath. Note that these symptoms are not necessarily specific to atrial fibrillation and can occur with many other conditions. The uncoordinated upper chamber’s contractions increase the risk of blood clots and stroke. (More on this later.)
Diagnosis: An electrocardiogram (ECG) records the electrical activity of the heart (both upper and lower chambers) and exhibits characteristic changes when atrial fibrillation is present. Sometimes a long-term ECG recording is required to capture the event. This might be a 24-hour ECG or an “event monitor” that is worn for days.
Causes: Atrial fibrillation increases with age, affecting approximately 10 to 15 percent of individuals 80 years or older. These patients often have other conditions such as high blood pressure, heart failure or heart-valve disease. On rare occasion, an overactive thyroid gland (hyperthyroidism) or excessive alcohol consumption can precipitate periods of atrial fibrillation. Other triggers may be an acute illness, such as a severe infection or major surgery, in particular cardiac surgery. In those situations the condition often resolves spontaneously. Recently, obesity and obstructive sleep apnea have been recognized as new risk factors. Research has disclosed genetic defects that predispose families to atrial fibrillation.
- About one-fifth of all strokes occurring in the United States (approximately 500,000 per year) are related to atrial fibrillation. These are potentially preventable with blood-thinning medicines, such as warfarin (Coumadin anticoagulation). A new medicine that does not require blood-test monitoring has been approved recently as an anti-coagulant, but unfortunately is very expensive.
Strokes related to atrial fibrillation tend to be more devastating and lethal than those related to other conditions. The average annual stroke risk in atrial fibrillation patients is approximately 5 percent, but may range between less than 1 percent and 15 percent, depending on several risk factors. Doctors consider the risk factors of age greater than 75 years, hypertension, diabetes, heart disease and prior stroke to determine the need for warfarin, aspirin therapy or other therapy. Anticoagulation therapy reduces the risk of stroke by approximately 75 to 80 percent. The small risk of bleeding complications requires close monitoring of the blood test INR (international normalized ratio), with a goal of a value generally between two and three. Only in extremely selected and fairly healthy individuals is full-dose aspirin sufficient to prevent a stroke. Warfarin is superior to the combination of aspirin and Plavix (clopidogrel) to prevent stroke, but the combination sometimes is needed in patients who have atrial fibrillation and a drug-eluding stent (DES).
- Electrical cardioversion or cardioversion with intravenous medication are therapies to revert atrial fibrillation to a normal rhythm. However, neither of these strategies will be able to maintain a normal rhythm for the long haul. Therefore, medications that suppress atrial fibrillation and maintain a normal rhythm may be prescribed.
- Another option is to use medications to prevent fast heart rates in the lower chambers while accepting atrial fibrillation in the upper chambers.
The best individual strategy requires careful evaluation by the treating physician. None of these medications can “cure” atrial fibrillation and it is likely to return—hopefully at a lesser frequency and with less severe symptoms.
Recent advances have been made in catheter ablation, a therapy that cauterizes or burns out regions inside the heart that are responsible for initiating and maintaining atrial fibrillation. In highly selective patients, typically younger age, normal heart, brief duration of atrial fibrillation, this approach may be able to control the condition in up to 70 to 80 percent of cases. This procedure is fairly complex and has a small risk of complications including stroke, bleeding and damage to the heart.
Cardiac pacemakers are sometimes used with medication to manage atrial fibrillation, particularly if these medications result in slowing the heart rate too much, but are needed to keep the heart rate from beating too fast.
In selected patients catheter ablation (or cauterizing) of the AV node is performed. These patients all require cardiac pacing before such therapy. This procedure eliminates fast and irregular heart rates, often making medications unnecessary. Anticoagulation medication (Coumadin) is still required to prevent strokes.
Prevention: Since atrial fibrillation is strongly linked to hypertension and heart disease, every effort should be made to avoid these conditions by healthy lifestyle choices, smoking cessation, regular exercise and only modest alcohol use. Once hypertension or heart disease is present, aggressive medical therapy is mandatory.
Helpful resources are available from the Heart Rhythm Society http://www.hrsonline.org/PatientInfo/
Dr. Ott is associate professor of medicine and the Peter Ott, MD, Endowed Chair in Electrophysiology.
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