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Atrial fibrillation (AF or AFib) is a cardiac arrhythmia (an abnormality of heart rate or rhythm) originating in the atria. Abnormal electrical impulses in the atria cause the ventricles to contract erratically. AF is the most common cardiac arrhythmia. If rapid, it may compromise blood flow and cause fainting, orthostatic hypotension (low blood pressure on standing up) or low blood pressure. In addition, it predisposes to thrombosis and embolism to the brain, being a prime risk factor for stroke.
Signs and symptoms
Atrial fibrillation may be asymptomatic, and be discovered as a chance finding during a health checkup. Often, there are symptoms.
- Related to low blood pressure: fainting, orthostatic hypotension (low blood pressure on standing up) or low blood pressure. Angina may be present.
- Related to ineffective circulation: heart failure
- Related to the formation of blood clots: stroke, transient ischemic attacks (TIAs)
A type of AF, termed paroxysmal, is when the arrhythmia occurs on an unexpected, intermittent basis. These patients tend to have less heart damage than those with chronic (sustained) AF.
AF is an intermediate-phase disease, meaning that the condition usually develops in response to damage to the heart, or by changes in the cardiovascular system. Causes include:
- Untreated arterial hypertension
- Abnormal mitral valve function (e.g. due to rheumatic heart disease )
- Heart surgery
- Coronary heart disease
- Excessive alcohol consumption ("binge drinking")
In turn, untreated AF can precipitate further damage to the heart by weakening the muscles of the ventricles. This weakened condition of the ventricles is termed cardiomyopathy, which in turn leads to an end-stage condition called heart failure.
In atrial fibrillation, the sinus node does not produce the regular impulses necessary for the rhythmic contraction of the heart. Instead, all tissue of the atrium discharges spontaneously, randomly generating an electrical impulse that is powerful enough to lead to conduction of the bundle of His and contraction of the ventricles. It can be distinguished from atrial flutter, which is a similar but more coordinated electrical activity of the atria which leads to more organised discharges.
Often, the rhythm produced is more rapid (over 100 but ususally less than 160 beats per minute) than normal cardiac activity. The result is tachycardia (fast heart rate), which is termed a supraventricular tachycardia (SVT) due to its origin. If the rate is particularly fast (rapid AF), the ventricle cannot maintain cardiac output, leading to a mild degree of cardiogenic shock with pulmonary edema and hypotension. Protracted AF predisposes to congestive heart failure (CHF).
Due to the unorganised blood flow in the atrium there is a higher degree of turbulence then usual. This predisposes to the formation of thrombi (blood clots), which may embolise - in theory to any organ but most prominenently to the brain. The result is a cerebrovascular accident (stroke).
Rate and rhythm control
AF can cause disabling and annoying symptoms. Palpitations, angina, lassitude, and decreased exercise tolerance are related to rapid heart rate and inefficient cardiac output caused by AF. There are two ways to approach these symptoms: rate control and rhythm control. Rate control treatments seek to reduce the heart rate to normal, usually 60 to 100 beats per minute. Rhythm control seeks to restore the normal heart rhythm, called normal sinus rhythm. Studies suggest that rhythm control is mainly a concern in newly diagnosed AF, while rate control is more important in the chronic phase. Rate control with anticoagulation is as effective a treatment as rhythm control in long term mortality studies, the AFFIRM Trial.
AF can induce a reduction in left ventricular ejection fraction, called cardiomyopathy. This can significantly increase mortality and morbidity. Controlling the rate and possibly the rhythm can improve mortality and morbidity.
Rate control methods include:
- beta blockers (e.g. sotalol)
- calcium channel blockers (e.g. verapamil)
- adenosine (in acute setting)
These medications work by slowing the generation of impulses from the atria and the conduction of those impulse from the atria to the ventricles.
Rhythm control methods include: -
- electrical DC cardioversion – an external defibrillator, applied to the chest while the patient is sedated, works in principal by a sudden electrical shock to the heart, which can induce a normal rhythm. This procedure is moderately successful in the short term, but most patients will relapse within one year. The key risk factor for relapse is duration of AF.
- radiofrequency ablation (RFA) – this procedure commonly uses radiofrequency energy to destroy abnormal electrical pathways in heart tissue. The energy emitting probe (electrode) is placed into the heart through a catheter. The practitioner first "maps" an area of the heart to locate the abnormal electrical activity before the responsible tissue is eliminated. Ablation is a newer technique and has shown some promise for cases unresponsive to conventional treatments. New techniques include the use of cryoablation (tissue freezing using a coolant which flows through the catheter), and microwave ablation, where tissue is ablated by the microwave energy "cooking" the adjacent tissue.
These anti-arrhythmic medications alter the flux of ions in heart tissue, making them less excitable, setting the stage for spontaneous and durable cardioversion. These medications are often used in concert with electrical cardioversion. However, the AFFIRM study showed no difference in risk of stroke in patients who have converted to a normal rhythm with anti-arrhythmic treatment, compared to those who have only rate control.2.
This is an area of active research, especially with respect to the RF ablation technique.
In confirmed AF, anticoagulant treatment is a crucial way to prevent stroke. Treatment of AF patients over age 60 with warfarin (also known as Coumadin®) results in a significant reduction in the subsequent risk of stroke. Patients under age 60 who have any structural heart disease (ie: valvular heart disease, ejection fraction <= 35%, history of heart attack) also benefit from warfarin. Patients under age 60 who do not have structural heart disease do not require warfarin, and can be treated with aspirin1. Other guidelines are also used. The new anticoagulant ximelagatran has been shown to prevent stroke with equal efficacy as warfarin, without the difficult monitoring process associated with warfarin.
- Fuster V, Ryden LE, Asinger RW, Cannom DS, Crijns HJ, Frye RL, Halperin JL, Kay GN, Klein WW, Levy S, McNamara RL, Prystowsky EN, Wann LS, Wyse DG, Gibbons RJ, Antman EM, Alpert JS, Faxon DP, Fuster V, Gregoratos G, Hiratzka LF, Jacobs AK, Russell RO, Smith SC, Klein WW, Alonso-Garcia A, Blomstrom-Lundqvist C, De Backer G, Flather M, Hradec J, Oto A, Parkhomenko A, Silber S, Torbicki A; American College of Cardiology/American Heart Association/European Society of Cardiology Board. ACC/AHA/ESC guidelines for the management of patients with atrial fibrillation: executive summary. A Report of the American College of Cardiology/ American Heart Association Task Force on Practice Guidelines and the European Society of Cardiology Committee for Practice Guidelines and Policy Conferences (Committee to Develop Guidelines for the Management of Patients With Atrial Fibrillation): developed in Collaboration With the North American Society of Pacing and Electrophysiology. J Am Coll Cardiol 2001;38:1231-66. ACC/AHA/ESC Fulltext. PMID 11583910.
- Note 2: Wyse DG, Waldo AL, DiMarco JP, Domanski MJ, Rosenberg Y, Schron EB, Kellen JC, Greene HL, Mickel MC, Dalquist JE, Corley SD; Atrial Fibrillation Follow-up Investigation of Rhythm Management (AFFIRM) Investigators. A comparison of rate control and rhythm control in patients with atrial fibrillation. N Engl J Med 2002;347:1825-33. PMID 12466506.
- Note 3: Long term management of Atrial Fibrillation
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