(See also Overview of Arrhythmias.)
Atrial fibrillation has been attributed to multiple wavelets with chaotic reentry within the atria. However, in many cases, firing of an ectopic focus within venous structures adjacent to the atria (usually the pulmonary veins) is responsible for initiation and perhaps maintenance of atrial fibrillation. In atrial fibrillation, the atria do not contract, and the atrioventricular (AV) conduction system is bombarded with many electrical stimuli, causing inconsistent impulse transmission and an irregularly irregular ventricular rate, which is usually in the tachycardia rate range.
Atrial fibrillation is one of the most common arrhythmias, affecting about 2.3 million adults in the US. Men and whites are more likely to have atrial fibrillation than women and blacks. Prevalence increases with age; almost 10% of people > 80 years are affected. atrial fibrillation tends to occur in patients with a heart disorder.
The absent atrial contractions predispose to thrombus formation; annual risk of cerebrovascular embolic events is about 7%. Risk of stroke is higher in older patients and in patients with a rheumatic valvular disorder, mechanical heart valve, hyperthyroidism, hypertension, diabetes, left ventricular systolic dysfunction, or previous thromboembolic events. Systemic emboli can also cause malfunction or necrosis of other organs (eg, heart, kidneys, gastrointestinal tract, eyes) or a limb.
Atrial fibrillation also may impair cardiac output; loss of atrial contraction can lower cardiac output at normal heart rate by about 10%. Such a decrease is usually well tolerated except when the ventricular rate becomes too fast (eg, > 140 beats/minute), or when patients have borderline or low cardiac output to begin with. In such cases, heart failure may develop.
The most common causes of atrial fibrillation are
Less common causes of atrial fibrillation include
Atrial septal defects and other congenital heart defects
Lone atrial fibrillation is atrial fibrillation without an identifiable cause in patients < 60 years.
Paroxysmal atrial fibrillation is atrial fibrillation that typically lasts < 1week and that converts spontaneously to normal sinus rhythm. Episodes may recur.
Persistent atrial fibrillation is continuous atrial fibrillation that lasts > 1 week or requires an intervention to terminate.
Long-standing persistent atrial fibrillation lasts > 1 year, but there is still the possibility of restoring sinus rhythm.
Permanent atrial fibrillation cannot be converted to sinus rhythm (the term also includes patients for whom a decision has been made not to attempt conversion to sinus rhythm). The longer atrial fibrillation is present, the less likely is spontaneous conversion and the more difficult is cardioversion because of atrial remodeling (rapid atrial rate-induced changes in atrial electrophysiology that are dominated by a decrease in atrial refractoriness and may also include increase in spatial dispersion of atrial refractoriness, slowed atrial conduction velocity, or both).
Atrial fibrillation is often asymptomatic, but many patients have palpitations, vague chest discomfort, or symptoms of heart failure (eg, weakness, light-headedness, dyspnea), particularly when the ventricular rate is very rapid (often 140 to 160 beats/minute). Patients may also present with symptoms and signs of acute stroke or of other organ damage due to systemic emboli.
The pulse is irregularly irregular with loss of a waves in the jugular venous pulse. A pulse deficit (the apical ventricular rate is faster than the rate palpated at the wrist) may be present because left ventricular stroke volume is not always sufficient to produce a peripheral pressure wave at fast ventricular rates.
Diagnosis of atrial fibrillation is by ECG (see figure Atrial fibrillation). Findings include
Other irregular rhythms may resemble atrial fibrillation on ECG but can be distinguished by the presence of discrete P or flutter waves, which can sometimes be made more visible with vagal maneuvers. Muscle tremor or electrical interference may resemble f waves, but the underlying rhythm is regular. Atrial fibrillation may also cause a phenomenon that mimics ventricular extrasystoles or ventricular tachycardia (Ashman phenomenon). This phenomenon typically occurs when a short R-R interval follows a long R-R interval; the longer interval lengthens the refractory period of the infra-Hisian conduction system, and the subsequent QRS complex(es) are conducted aberrantly, typically with right bundle branch morphology.
Echocardiography and thyroid function tests are important in the initial evaluation. Echocardiography is done to assess structural heart defects (eg, left atrial enlargement, left ventricular wall motion abnormalities suggesting past or present ischemia, valvular disorders, cardiomyopathy) and to identify additional risk factors for stroke (eg, atrial blood stasis or thrombus, complex aortic plaque). Atrial thrombi are more likely in the atrial appendages, where they are best detected by transesophageal rather than transthoracic echocardiography.
If a significant underlying disorder is suspected, patients with new-onset atrial fibrillation may benefit from hospitalization, but those with recurrent episodes do not require hospitalization unless other symptoms suggest the need for it. Once causes have been managed, treatment of atrial fibrillation focuses on ventricular rate control, rhythm control, and prevention of thromboembolism.
Patients with atrial fibrillation of any duration require rate control (typically to < 100 beats/minute at rest) to control symptoms and prevent tachycardia-induced cardiomyopathy.
For acute paroxysms of rapid rate (eg, 140 to 160 beats/minute), IV AV node blockers are used (for doses, see table Antiarrhythmic Drugs). CAUTION: AV node blockers should not be used in patients with Wolff-Parkinson-White syndrome when an accessory AV pathway is involved (indicated by wide QRS duration); these drugs increase frequency of conduction via the bypass tract, possibly causing ventricular fibrillation. Beta-blockers (eg, metoprolol, esmolol) are preferred if excess catecholamines are suspected (eg, in thyroid disorders, exercise-triggered cases). Nondihydropyridine calcium channel blockers (eg, verapamil, diltiazem) are also effective. Digoxin is the least effective but may be preferred if heart failure is present. These drugs may be used orally for long-term rate control. When beta-blockers, nondihydropyridine calcium channel blockers, and digoxin—separately or in combination—are ineffective, amiodarone may be required.
Pearls & Pitfalls
In patients with heart failure or other hemodynamic compromise directly attributable to new-onset atrial fibrillation, restoration of normal sinus rhythm is indicated to improve cardiac output. In other cases, conversion of atrial fibrillation to normal sinus rhythm is optimal, but the antiarrhythmic drugs that are capable of doing so (class Ia, Ic, III) have a risk of adverse effects and may increase mortality. Conversion to sinus rhythm does not eliminate the need for chronic anticoagulation.
For acute conversion, synchronized cardioversion or drugs can be used. Before conversion is attempted, the ventricular rate should be controlled to < 120 beats/minute, and, many patients should be anticoagulated (for criteria and methods, see Prevention of thromboembolism during rhythm control). If atrial fibrillation has been present > 48 hours, patients should typically be given an oral anticoagulant (conversion, regardless of method used, increases risk of thromboembolism). Anticoagulation should be maintained for > 3 weeks before conversion or can be given for a shorter time before conversion if transesophageal echocardiography (TEE) does not show left atrial thrombus. Anticoagulation should be continued for at least 4 weeks after cardioversion. Many patients need chronic anticoagulation (see Long-term measures to prevent thromboembolism).
Synchronized cardioversion (100 joules, followed by 200 and 360 joules as needed) converts atrial fibrillation to normal sinus rhythm in 75 to 90% of patients, although recurrence rate is high. Efficacy and maintenance of sinus rhythm after the procedure is improved with use of class Ia, Ic, or III antiarrhythmic drugs 24 to 48 hours before the procedure. Cardioversion is more effective in patients with shorter duration of atrial fibrillation, lone atrial fibrillation, or atrial fibrillation with a reversible cause; it is less effective when the left atrium is enlarged (> 5 cm), atrial appendage flow is low, or a significant underlying structural heart disorder is present.
Drugs for conversion of atrial fibrillation to sinus rhythm include class Ia (procainamide, quinidine, disopyramide), Ic (flecainide, propafenone), and III (amiodarone, dofetilide, dronedarone, ibutilide, sotalol, vernakalant) antiarrhythmics (see table Antiarrhythmic Drugs). All are effective in about 50 to 60% of patients, but adverse effects differ. These drugs should not be used until rate has been controlled by a beta-blocker or nondihydropyridine calcium channel blocker. The converting drugs with oral formulations are also used for long-term maintenance of sinus rhythm (with or without previous cardioversion). Choice depends on patient tolerance. However, for paroxysmal AF that occurs only or almost only at rest or during sleep when vagal tone is high, drugs with vagolytic effects (eg, disopyramide) may be particularly effective. Exercise-induced AF may be better prevented with a beta-blocker.
For certain patients with recurrent paroxysmal atrial fibrillation who also can identify its onset by symptoms, some clinicians provide a single oral loading dose of flecainide (300 mg for patients ≥ 70 kg, otherwise 200 mg) or propafenone (600 mg for patients ≥ 70 kg, otherwise 450 mg) that patients carry and self-administer when palpitations develop (“pill-in-the-pocket” approach). This approach must be limited to patients who have no sinoatrial or AV node dysfunction, bundle branch block, QT prolongation, Brugada syndrome, or structural heart disease. Its hazard (estimated at 1%) is the possibility of converting atrial fibrillation to a slowish atrial flutter that conducts 1:1 in the 200 to 240 beat/minute range. This potential complication can be reduced in frequency by coadministration of an AV nodal suppressing drug (eg, a beta-blocker or a nondihydropyridine calcium antagonist).
Angiotensin-converting enzyme (ACE) inhibitors, angiotensin II receptor blockers (ARBs), and aldosterone blockers may attenuate the myocardial fibrosis that provides a substrate for atrial fibrillation in patients with heart failure, but the role of these drugs in routine atrial fibrillation treatment has yet to be defined.
Patients, particularly those in whom the current episode of atrial fibrillation has been present > 48 hours, have a high risk of thromboembolism for several weeks after pharmacologic or direct current cardioversion. If the onset of the current episode of atrial fibrillation is not clearly within 48 hours, the patient should be anticoagulated for 3 weeks before and at least 4 weeks after cardioversion regardless of the patient's predicted risk of a thromboembolic event (class I recommendation).
Alternatively, therapeutic anticoagulation is started, transesophageal echocardiography (TEE) is done, and, if no left atrial or left atrial appendage clot is seen, cardioversion may be done, followed by at least 4 weeks of anticoagulation therapy (class IIa recommendation).
If urgent cardioversion is required because of hemodynamic compromise, cardioversion is done and anticoagulation is started as soon as is practical and continued for at least 4 weeks.
If the onset of the current episode of atrial fibrillation is clearly within 48 hours, cardioversion may be done without prior anticoagulation if the patient has nonvalvular atrial fibrillation and is not at high risk of a thromboembolic event. After cardioversion, therapeutic anticoagulation is given for 4 weeks (class I recommendation); however, anticoagulation may not be necessary in patients at low risk of a thromboembolic event (class IIb recommendation).
After 4 weeks of postconversion anticoagulation therapy, some patients require long-term anticoagulation (see below) .
For patients who do not respond to or cannot take rate-controlling drugs, ablation of the AV node may be done to cause complete heart block; insertion of a permanent pacemaker is then necessary. Ablation of only one AV nodal pathway (AV node modification) reduces the number of atrial impulses reaching the ventricles and eliminates the need for a pacemaker, but this approach is considered less effective than complete ablation and is rarely used.
Ablation procedures that isolate the pulmonary veins from the left atrium can prevent atrial fibrillation without causing AV block. In comparison to other ablation procedures, pulmonary vein isolation has a lower success rate (60 to 80%) and a higher complication rate (1 to 5%). Accordingly, this procedure is often reserved for the best candidates—younger patients with drug-resistant AF who have no significant structural heart disease.
Long-term measures to prevent thromboembolism are taken for certain patients with atrial fibrillation during long-term treatment depending on their estimated risk of stroke vs risk of bleeding.
Patients with rheumatic mitral stenosis and patients with mechanical artificial heart valves are considered to be at high risk of a thromboembolic event as are patients with nonvalvular atrial fibrillation who have additional risk factors. The additional risk factors are identified by the CHA2DS2-VASc score (see table CHA2DS2-VASc Score).
The guidelines for antithrombotic therapy in atrial fibrillation differ in different regions. The current guidelines in the United States are as follows:
Long-term oral anticoagulant therapy is recommended for patients with rheumatic mitral stenosis, mechanical artificial heart valve, and for nonvalvular atrial fibrillation patients with a CHA2DS2-VASc score of ≥ 2 (level I recommendation).
No antithrombotic therapy is recommended for patients with nonvalvular atrial fibrillation and a CHA2DS2-VASc score of 0 (level IIa recommendation).
No antithrombotic therapy, aspirin therapy, or oral anticoagulant therapy is recommended for patients with nonvalvular atrial fibrillation and a CHA2DS2-VASc score of 1 (level IIb recommendation).
Patients with atrial fibrillation and a mechanical heart valve(s) are treated with warfarin.
Patients with atrial fibrillation and significant mitral stenosis are treated with warfarin.
For patients with nonvalvular atrial fibrillation who are to be treated with an oral anticoagulant, a class I indication is given for warfarin with a target international normalized ratio (INR) of 2.0 to 3.0 (level of evidence A), apixaban (level of evidence B), dabigatran (level of evidence B), edoxaban (level of evidence B), and rivaroxaban (level of evidence B).
These general guidelines are altered in patients with more than moderate renal impairment.
The left atrial appendage may be surgically ligated or closed with a transcatheter device when appropriate antithrombotic therapy is absolutely contraindicated.
An individual patient's risk of bleeding may be estimated with any of a number of prognostic tools of which the most commonly used is HAS-BLED (see table HAS-BLED Tool for Predicting Risk of Bleeding in Patients With Atrial Fibrillation). The HAS-BLED score serves best in identifying conditions that, if modified, reduce bleeding risk rather than in identifying patients with a higher risk of bleeding who should not receive anticoagulation
HAS-BLED Tool for Predicting Risk of Bleeding in Patients With Atrial Fibrillation
Atrial fibrillation is an irregularly irregular atrial rhythm that may be episodic or continuous; paroxysms of tachycardia may occur.
QRS complexes are typically narrow; a wide complex may occur with intraventricular conduction defects or Wolff-Parkinson-White syndrome.
Patients should have electrocardiography, echocardiography, and thyroid function testing.
Heart rate is controlled typically to < 100 beats/minute at rest; first-line drugs include beta-blockers and nondihydropyridine calcium channel blockers (eg, verapamil, diltiazem).
Restoration of sinus rhythm is not as important as rate control and does not eliminate the need for anticoagulation but may help patients with continuing symptoms or hemodynamic compromise (eg, heart failure); synchronized cardioversion or drugs can be used.
Anticoagulation is usually necessary before cardioversion.
Long-term oral anticoagulation to prevent stroke is required for patients with risk factors for thromboembolism
January CT, Wann S, Alpert JS, et al: 2014 ACC/AHA/HRS Guideline for the management of patients with atrial fibrillation: a report of the American College of Cardiology/American Heart Association Task Force of Practice Guidelines and the Heart Rhythm Society. Circulation130:2071-2104, 2014.