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Atrial fibrillation
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Atrial fibrillation (AF or afib) is a cardiac arrhythmia (abnormal heart rhythm) that involves the two upper chambers (atria) of the heart. It can often be identified by taking a pulse and observing that the heartbeats don't occur at regular intervals, but a conclusive indication of AF is the absence of P waves on an electrocardiogram (ECG). AF is the most common arrhythmia; risk increases with age, with 8% of people over 80 having AF.
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Encyclopedia
Atrial fibrillation (AF or afib) is a cardiac arrhythmia (abnormal heart rhythm) that involves the two upper chambers (atria) of the heart. It can often be identified by taking a pulse and observing that the heartbeats don't occur at regular intervals, but a conclusive indication of AF is the absence of P waves on an electrocardiogram (ECG). AF is the most common arrhythmia; risk increases with age, with 8% of people over 80 having AF. In AF, the normal electrical impulses that are generated by the sinoatrial node are overwhelmed by disorganized electrical impulses that originate in the atria and pulmonary veins, leading to conduction of irregular impulses to the ventricles that generate the heartbeat. The result is an irregular heartbeat which may occur in episodes lasting from minutes to weeks, or it could occur all the time for years. The natural tendency of AF is to become a chronic condition. Chronic AF leads to a small increase in the risk of death.
Atrial fibrillation is often asymptomatic, and is not in itself generally life-threatening, but may result in palpitations, fainting, chest pain, or congestive heart failure. Patients with AF usually have a significantly increased risk of stroke (up to 7 times that of the general population). Stroke risk increases during AF because blood may pool and form clots in the poorly contracting atria and especially in the left atrial appendage (LAA). The level of increased risk of stroke depends on the number of additional risk factors. If the AF patient has none, the risk of stroke is similar to that of the general population. However, many patients do have additional risk factors and AF is a leading cause of stroke.
Atrial fibrillation may be treated with medications which either slow the heart rate or revert the heart rhythm back to normal. Synchronized electrical cardioversion may also be used to convert AF to a normal heart rhythm. Surgical and catheter-based therapies may also be used to prevent recurrence of AF in certain individuals. People with AF are often given anticoagulants such as warfarin to protect them from stroke.
Classification
The American College of Cardiology (ACC), American Heart Association (AHA), and the European Society of Cardiology (ESC) recommend in their guidelines the following classification system based on simplicity and clinical relevance.
| AF Category | Defining Characteristics |
|---|
| First detected | only one diagnosed episode | | Paroxysmal | recurrent episodes that self-terminate in less than 7 days | | Persistent | recurrent episodes that last more than 7 days | | Permanent | an ongoing long-term episode | |
All atrial fibrillation patients are initially in the category called first detected AF. These patients may or may not have had previous undetected episodes. If a first detected episode self-terminates in less than 7 days and then another episode begins later on, the case has moved into the category of paroxysmal AF. Although patients in this category have episodes lasting up to 7 days, in most cases of paroxysmal AF the episodes will self-terminate in less than 24 hours. If instead the episode lasts for more than 7 days, it is unlikely to self-terminate and it is called persistent AF. In this case, the episode may be terminated by cardioversion. If cardioversion is unsuccessful or it is not attempted, and the episode is ongoing for a long time (e.g. a year or more), the patient's AF is called permanent.
Episodes that last less than 30 seconds are not considered in this system. Also, this system does not apply to cases where the AF is a secondary condition that occurs in the setting of a primary condition that may be the cause of the AF.
Using this system, it's not always clear what an AF case should be called. For example, a case may fit into the paroxysmal AF category some of the time, while other times it may have the characteristics of persistent AF. One may be able to decide which category is more appropriate by determining which one occurs most often in the case under consideration.
In addition to the four categories in the above system, which are mainly defined by episode timing and termination, the ACC/AHA/ESC guidelines describe additional AF categories in terms of other characteristics of the patient.
- Lone atrial fibrillation (LAF) - absence of clinical or echocardiographic findings of other cardiovascular disease (including hypertension) or related pulmonary disease, and age under 60 years
- Nonvalvular AF - absence of rheumatic mitral valve disease, a prosthetic heart valve, or mitral valve repair
- Secondary AF - occurs in the setting of a primary condition which may be the cause of the AF, such as acute myocardial infarction, cardiac surgery, pericarditis, myocarditis, hyperthyroidism, pulmonary embolism, pneumonia, or other acute pulmonary disease
Signs and symptoms
Atrial fibrillation is usually accompanied by symptoms related to a rapid heart rate. Rapid and irregular heart rates may be perceived as palpitations, exercise intolerance, and occasionally produce angina (if the rate is faster and puts the heart under strain) and congestive symptoms of shortness of breath or edema. Sometimes the arrhythmia will be identified only with the onset of a stroke or a transient ischemic attack (TIA). It is not uncommon for a patient to first become aware of AF from a routine physical examination or ECG, as it may be asymptomatic in many cases.
As most cases of atrial fibrillation are secondary to other medical problems, the presence of chest pain or angina, symptoms of hyperthyroidism (an overactive thyroid gland) such as weight loss and diarrhea, and symptoms suggestive of lung disease would indicate an underlying cause. A previous history of stroke or TIA, as well as hypertension (high blood pressure), diabetes, heart failure and rheumatic fever, may indicate whether someone with AF is at a higher risk of complications.
Diagnosis
The evaluation of atrial fibrillation involves diagnosis, determination of the etiology of the arrhythmia, and classification of the arrhythmia. A minimal evaluation should be performed in all individuals with AF. This includes a history and physical examination, ECG, transthoracic echocardiogram, and routine bloodwork. Certain individuals may benefit from an extended evaluation which may include an evaluation of the heart rate response to exercise, exercise stress testing, a chest x-ray, trans-esophageal echocardiography, and other studies.
Screening
Screening for atrial fibrillation is not generally performed, although a study of routine pulse checks or ECGs during routine office visits found that the annual rate of detection of AF in elderly patients improved from 1.04% to 1.63%; selection of patients for prophylactic anticoagulation would improve stroke risk in that age category.
Routine primary care visit
This estimated sensitivity of the routine primary care visit is 64%. This low result probably reflects the pulse not being checked routinely or carefully.
Minimal evaluation
The minimal evaluation of atrial fibrillation should generally be performed in all individuals with AF. The goal of this evaluation is to determine the general treatment regimen for the individual. If results of the general evaluation warrant it, further studies may be then performed.
History and physical examination
The history of the individual's atrial fibrillation episodes is probably the most important part of the evaluation. Distinctions should be made between those who are entirely asymptomatic when they are in AF (in which case the AF is found as an incidental finding on an ECG or physical examination) and those who have gross and obvious symptoms due to AF and can pinpoint whenever they go into AF or revert to sinus rhythm.
Routine bloodwork
While many cases of AF have no definite cause, it may be the result of various other problems (see below). Hence, renal function and electrolytes are routinely determined, as well as thyroid-stimulating hormone (commonly suppressed in hyperthyroidism and of relevance if amiodarone is administered for treatment) and a blood count.
In acute-onset AF associated with chest pain, cardiac troponins or other markers of damage to the heart muscle may be ordered. Coagulation studies (INR/aPTT) are usually performed, as anticoagulant medication may be commenced.
Electrocardiogram
Atrial fibrillation is diagnosed on an electrocardiogram (ECG), an investigation performed routinely whenever irregular heart beat is suspected. Characteristic findings are the absence of P waves, with unorganized electrical activity in their place, and irregularity of R-R interval due to irregular conduction of impulses to the ventricles.
When ECGs are used for screening, the SAFE trial found that electronic software, primary care physicians and the combination of the two had the following sensitivities and specificities
- Interpreted by software: sensitivity = 83%, specificity = 99%
- Interpreted by a primary care physician: sensitivity = 80%, specificity = 92%
- Interpreted by a primary care physician with software: sensitivity = 92%, specificity = 91%
If paroxysmal AF is suspected but an ECG during an office visit only shows a regular rhythm, AF episodes may be detected and documented with the use of ambulatory Holter monitoring (e.g. for a day). If the episodes are too infrequent to be detected by Holter monitoring with reasonable probability, then the patient can be monitored for longer periods (e.g. a month) with an ambulatory event monitor.
Echocardiography
A non-invasive transthoracic echocardiogram (TTE) is generally performed in newly diagnosed AF, as well as if there is a major change in the patient's clinical state. This ultrasound-based scan of the heart may help identify valvular heart disease (which may greatly increase the risk of stroke), left and right atrial size (which indicates likelihood that AF may become permanent), left ventricular size and function, peak right ventricular pressure (pulmonary hypertension), presence of left ventricular hypertrophy and pericardial disease.
Significant enlargement of both the left and right atria is associated with long-standing atrial fibrillation and, if noted at the initial presentation of atrial fibrillation, suggests that the atrial fibrillation is likely to be of a longer duration than the individual's symptoms.
Extended evaluation
An extended evaluation is generally not necessary in most individuals with atrial fibrillation, and is only performed if abnormalities are noted in the limited evaluation, if a reversible cause of the atrial fibrillation is suggested, or if further evaluation may change the treatment course.
Chest X-ray
A chest X-ray is generally only performed if a pulmonary cause of atrial fibrillation is suggested, or if other cardiac conditions are suspected (particularly congestive heart failure.) This may reveal an underlying problem in the lungs or the blood vessels in the chest. In particular, if an underlying pneumonia is suggested, then treatment of the pneumonia may cause the atrial fibrillation to terminate on its own.
Transesophageal echocardiogram
A normal echocardiography (transthoracic or TTE) has a low sensitivity for identifying thrombi (blood clots) in the heart. If this is suspected - e.g. when planning urgent electrical cardioversion - a transesophageal echocardiogram (TEE) is preferred.
The TEE has much better visualization of the left atrial appendage than transthoracic echocardiography. This structure, located in the left atrium, is the place where thrombus most commonly is formed in the setting of atrial fibrillation or flutter. TEE has a very high sensitivity for locating thrombus in this area and can also detect sluggish bloodflow in this area that is suggestive of thrombus formation.
If no thrombus is seen on TEE, the incidence of stroke immediately after cardioversion is performed is very low.
Ambulatory holter monitoring
A holter monitor is a wearable ambulatory heart monitor that continuously monitors the heart rate and heart rhythm for a short duration, typically 24 hours. In individuals with symptoms of significant shortness of breath with exertion or palpitations on a regular basis, a holter monitor may be of benefit to determine if rapid heart rates (or unusually slow heart rates) during atrial fibrillation are the cause of the symptoms.
Exercise stress testing
Some individuals with atrial fibrillation do well with normal activity but develop shortness of breath with exertion. It may be unclear if the shortness of breath is due to a blunted heart rate response to exertion due to excessive AV node blocking agents, a very rapid heart rate during exertion, or due to other underlying conditions such as chronic lung disease or coronary ischemia. An exercise stress test will evaluate the individual's heart rate response to exertion and determine if the AV node blocking agents are contributing to the symptoms.
Etiology
AF is linked to several cardiac causes, but may occur in otherwise normal hearts. Known associations include:
- Hypertension (High blood pressure)
- Primary heart diseases including coronary artery disease, mitral stenosis (e.g. due to rheumatic heart disease or mitral valve prolapse), mitral regurgitation, hypertrophic cardiomyopathy (HCM), pericarditis, congenital heart disease, previous heart surgery
- Lung diseases (such as pneumonia, lung cancer, pulmonary embolism, sarcoidosis)
- Excessive alcohol consumption ("binge drinking" or "holiday heart syndrome"). Even otherwise healthy middle-aged women who consumed more than 2 drinks daily were 60% more likely to develop AF.
- Hyperthyroidism
- Carbon monoxide poisoning
- Dual-chamber pacemakers in the presence of normal atrioventricular conduction.
- A family history of AF may increases the risk of AF. A study of more than 2,200 AF patients found that 30 per cent had parents with AF. Various genetic mutations may be responsible.
Pathophysiology
Morphology
The primary pathologic change seen in atrial fibrillation is the progressive fibrosis of the atria. This fibrosis is primarily due to atrial dilation, however genetic causes and inflammation may have a cause in some individuals.
Dilation of the atria can be due to almost any structural abnormality of the heart that can cause a rise in the intra-cardiac pressures. This includes valvular heart disease (such as mitral stenosis, mitral regurgitation, and tricuspid regurgitation), hypertension, and congestive heart failure. Any inflammatory state that affects the heart can cause fibrosis of the atria. This is typically due to sarcoidosis but may also be due to autoimmune disorders that create autoantibodies against myosin heavy chains. Mutation of the lamin AC gene is also associated with fibrosis of the atria that can lead to atrial fibrillation.
Once dilation of the atria has occurred, this begins a chain of events that leads to the activation of the renin aldosterone angiotensin system (RAAS) and subsequent increase in matrix metaloproteinases and disintegrin, which leads to atrial remodeling and fibrosis, with loss of atrial muscle mass.
This process is not immediate, and experimental studies have revealed patchy atrial fibrosis may precede the occurrence of atrial fibrillation and may progress with prolonged durations of atrial fibrillation.
Fibrosis is not limited to the muscle mass of the atria, and may occur in the sinus node (SA node) and atrioventricular node (AV node), correlating with sick sinus syndrome. Prolonged episodes of atrial fibrillation have been shown to correlate with prolongation of the sinus node recovery time, suggesting that dysfunction of the SA node is progressive with prolonged episodes of atrial fibrillation.
Electrophysiology
The normal electrical conduction system of the heart allows the impulse that is generated by the sinoatrial node (SA node) of the heart to be propagated to and stimulate the myocardium (muscle of the heart). When the myocardium is stimulated, it contracts. It is the ordered stimulation of the myocardium that allows efficient contraction of the heart, thereby allowing blood to be pumped to the body.
In atrial fibrillation, the regular impulses produced by the sinus node to provide rhythmic contraction of the heart are overwhelmed by the rapid randomly generated electrical discharges produced by larger areas of atrial tissue, often localized to the pulmonary veins. It can be distinguished from atrial flutter, which is a more organized electrical circuit usually in the right atrium that produces characteristic saw-toothed p-waves on the ECG; in atrial flutter, the discharges circulate rapidly at a rate of 300 beats per minute (bpm) around the atrium; in AF, there is no regularity of this kind at all.
Thromboembolism
In atrial fibrillation, the lack of an organized atrial contraction can result in stagnant blood, especially in the left atrial appendage (LAA). Stagnant blood can result in a clot, which is called a thrombus while it is immobile at its place of origin. If the clot becomes mobile and is carried away by the blood circulation, it is called an embolus. When an embolus from the LAA leaves the heart, it proceeds through smaller and smaller arteries until it plugs one of them. A thromboembolism is said to occur when a thrombus that became an embolus plugs a blood vessel and prevents blood from flowing. The damage produced by an embolus is related to where the circulation takes it. An embolus that ends up in the brain produces the most feared complication of atrial fibrillation, namely stroke, while an embolus may also lodge in the mesenteric circulation (the circulation supplying the abdominal organs) or digit, producing organ-specific damage such as bowel ischemia or ischemia of the fingers or toes.
The LAA lies in close relation to the free wall of the left ventricle and thus the LAA's emptying and filling, which determines its degree of blood stagnation, may be significantly affected by left ventricular function.
Treatment
The main goals of treatment of atrial fibrillation are to prevent temporary circulatory instability and to prevent stroke. Rate and rhythm control are principally used to achieve the former, while anticoagulation may be required to decrease the risk of the latter. In emergencies, when circulatory collapse is imminent due to uncontrolled tachycardia, immediate cardioversion may be indicated.
The primary factors determining atrial fibrillation treatment are duration and evidence of hemodynamic instability. Cardioversion is indicated with new onset AF (for less than 48 hours) and with hemodynamic instability. If rate and rhythm control can not be maintained by medication or cardioversion, electrophysiological studies with pathway ablation may be required.
Anticoagulation
Most patients with AF are at increased risk of stroke. The possible exceptions are those with lone AF (LAF). A systematic review of risk factors for stroke in patients with nonvalvular atrial fibrillation concluded that a prior history of stroke or TIA is the most powerful risk factor for future stroke, followed by advancing age, hypertension, and diabetes. For patients with LAF, the risk of stroke is very low and is independent of whether the LAF was an isolated episode, paroxysmal, persistent, or permanent. The risk of systemic embolization (atrial clots migrating to other organs) depends strongly on whether there is an underlying structural problem with the heart (e.g. mitral stenosis) and on the presence of other risk factors, such as diabetes and high blood pressure. Finally, patients under 65 are much less likely to develop embolization compared with patients over 75. In young patients with few risk factors and no structural heart defect, the benefits of anticoagulation may be outweighed by the risks of hemorrhage (bleeding). Those at a low risk may benefit from mild (and low-risk) anticoagulation with aspirin (or clopidogrel in those who are allergic to aspirin). In contrast, those with a high risk of stroke derive most benefit from anticoagulant treatment with warfarin or similar drugs.
In the United Kingdom, the NICE guidelines recommend using a clinical prediction rule for this purpose. The CHADS2 score is the best validated clinical prediction rule for determining the risk of stroke (and therefore who should and should not be anticoagulated with warfarin); it assigns points (totaling 0-6) depending on the presence or absence of co-morbidities such as hypertension and diabetes. In a comparison of seven prediction rules, the best was CHADS2 which performed similarly to the SPAF and Framingham prediction rules.
To compensate for the increased risk of stroke, anticoagulants may be required. However, in the case of warfarin, if someone with AF has a yearly risk of stroke that is less than 2%, then the risks associated with taking warfarin outweigh the risk of getting a stroke from AF.
Atrial fibrillation in the context of mitral stenosis is associated with a seventeen-fold increase in stroke risk.
Acute anticoagulation
If anticoagulation is required urgently (e.g. for cardioversion), heparin or similar drugs achieve the required level of protection much quicker than warfarin, which will take several days to reach adequate levels.
In the initial stages after an embolic stroke, anticoagulation may be risky, as the damaged area of the brain is relatively prone to bleeding (hemorrhagic transformation). As a result, a clinical practice guideline by National Institute for Health and Clinical Excellence recommends that anticoagulation should begin two weeks after stroke if no hemorrhage occurred.
In cases of chronic stable atrial fibrillation without any other risk factors for thromboembolism, the Seventh American College of Chest Physicians (ACCP) Conference on Antithrombotic and Thrombolytic Therapy recommends initiating warfarin without heparin bridging. While there is a theoretical concern of causing a transient prothrombotic state with the initiation of warfarin, a study comparing the initiation of warfarin alone with warfarin and low molecular weight heparin shows no significant difference in the concentrations of endogenous anticoagulants or in markers of active clot formation.
Chronic anticoagulation
Among patients with "non-valvular" atrial fibrillation, anticoagulation with warfarin can reduce stroke by 60% while antiplatelet agents can reduce stroke by 20%. . There is evidence that aspirin and clopidogrel are effective when used together, but the combination is still inferior to warfarin.
Warfarin treatment requires frequent monitoring with a blood test called the international normalized ratio (INR); this determines whether the correct dose is being used. In atrial fibrillation, the usual target INR is between 2.0 and 3.0 (higher targets are used in patients with mechanical artificial heart valves, many of whom may also have atrial fibrillation). A high INR may indicate increased bleeding risk, while a low INR would indicate that there is insufficient protection from stroke.
An attempt was made to find a better method of implementing warfarin therapy without the inconvenience of regular monitoring and risk of intracranial hemorrhage. A combination of aspirin and fixed-dose warfarin (initial INR 1.2-1.5) was tried. Unfortunately, in a study of AF patients with additional risk factors for thromboembolism, the combination of aspirin and the lower dose of warfarin was significantly inferior to the standard adjusted-dose warfarin (INR 2.0-3.0), yet still had a similar risk of intracranial hemorrhage.
Elderly patients
The very elderly (patients aged 75 years or more) may benefit from anticoagulation provided that their anticoagulation does not increase hemorrhagic complications, which is a difficult goal. Patients aged 80 years or more may be especially susceptible to bleeding complications, with a rate of 13 bleeds per 100 person-years. This bleed rate would seem to preclude use of warfarin; however, a randomized controlled trial found benefit in treating patients 75 years or over with a number needed to treat of 50. Of note, this study had very low rate of hemorrhagic complications in the warfarin group.
Cardioversion
Cardioversion is a noninvasive conversion of an irregular heartbeat to a normal heartbeat using electrical or chemical means:
The main risk of cardioversion is systemic embolization of a thrombus (blood clot) from the previously fibrillating left atrium. Cardioversion should not be performed without adequate anticoagulation in patients with more than 48 hours of atrial fibrillation. Cardioversion may be performed in instances of AF lasting more than 48 hours if a transesophogeal echocardiogram (TEE) demonstrates no evidence of clot within the heart.
Whichever method of cardioversion is used, approximately 50% of patients relapse within one year, although the continued daily use of oral antiarrhythmic drugs may extend this period. The key risk factor for relapse is duration of AF, although other risk factors that have been identified include the presence of structural heart disease, and increasing age.
Rate control versus rhythm control using drugs
AF can cause disabling and annoying symptoms. Palpitations, angina, lassitude (weariness), and decreased exercise tolerance are related to rapid heart rate and inefficient cardiac output caused by AF. Furthermore, AF with a persistent rapid rate can cause a form of heart failure called tachycardia induced cardiomyopathy. This can significantly increase mortality and morbidity, which can be prevented by early and adequate treatment of the AF.
There are two ways to approach these symptoms using drugs: rate control and rhythm control. Rate control seeks to reduce the heart rate to one that is closer to normal, usually 60 to 100 bpm, without trying to convert to a regular rhythm. Rhythm control seeks to restore with cardioversion the regular heart rhythm and maintain it with drugs. 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.
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. AF is associated with a reduced quality of life, and while some studies indicate that rhythm control leads to a higher quality of life, the AFFIRM study did not find a difference.
A further study focused on rhythm control in patients with AF and simultaneous heart failure, based on the premise that AF confers a higher mortality risk in heart failure. In this setting, too, rhythm control offered no advantage compared to rate control.
In patients with a fast ventricular response, intravenous magnesium significantly increases the chances of successful rate and rhythm control in the urgent setting without significant side-effects.
Rate control
Rate control is achieved with medications that work by increasing the degree of block at the level of the AV node, effectively decreasing the number of impulses that conduct down into the ventricles. This can be done with:
In addition to these agents, amiodarone has some AV node blocking effects (particularly when administered intravenously), and can be used in individuals when other agents are contraindicated or ineffective (particularly due to hypotension).
Maintenance of sinus rhythm
The mainstay of maintaining sinus rhythm is the use of antiarrhythmic agents. Recently, other approaches have been developed that promise to decrease or eliminate the need for antiarrhythmic agents.
Antiarrhythmic agents
The anti-arrhythmic medications often used in either pharmacological cardioversion or in the prevention of relapse to AF 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.
Catheter ablation
In patients with AF where rate control drugs are ineffective and it is not possible to restore sinus rhythm using cardioversion, non-pharmacological alternatives are available. For example, to control rate it is possible to destroy the bundle of cells connecting the upper and lower chambers of the heart - the atrioventricular node - which regulates heart rate, and to implant a pacemaker instead. A more complex technique, which avoids the need for a pacemaker, involves ablating groups of cells near the pulmonary veins where atrial fibrillation is thought to originate, or creating more extensive lesions in an attempt to prevent atrial fibrillation from establishing itself.
Ablation is a technique that has shown some promise for cases of recurrent AF that are unresponsive to conventional treatments. Radiofrequency ablation (RFA) uses radiofrequency energy to destroy abnormal electrical pathways in heart tissue. Other energy sourses include laser, cryothermy and high intensity ultrasound. The energy emitting probe (electrode) is placed into the heart through a catheter inserted into veins in the groin or neck. Electrodes that can detect electrical activity from inside the heart are also inserted, and the electrophysiologist uses these to "map" an area of the heart in order to locate the abnormal electrical activity before eliminating the responsible tissue.
Most AF ablations consist of isolating the electrical pathways from the pulmonary veins (PV), which are located on the posterior wall of the left atrium. All other veins from the body (including neck and groin) lead to the right atrium, so in order to get to the left atrium the catheters must get across the atrial septum. This is done by piercing a small hole in the septal wall. This is called a transseptal approach. Once in the left atrium, the physician may perform Wide Area Circumferential Ablation (WACA) to electrically isolate the PVs from the left atrium.
Some more recent approaches to ablating AF is to target sites that are particularly disorganized in both atria as well as in the coronary sinus (CS). These sites are termed complex fractionated atrial electrogram (CFAE) sites.. It is believed by some that the CFAE sites are the cause of AF, or a combination of the PVs and CFAE sites are to blame. New techniques include the use of cryoablation (tissue freezing using a coolant which flows through the catheter), microwave ablation, where tissue is ablated by the microwave energy "cooking" the adjacent tissue, and high intensity focused ultrasound (HIFU), which destroys tissue by heating. This is an area of active research, especially with respect to the RF ablation technique and emphasis on isolating the pulmonary veins that enter into the left atrium.
Efficacy and risks of catheter ablation of atrial fibrillation are areas of active debate. A worldwide survey of the outcomes of 8745 ablation procedures demonstrated a 52% success rate (ranging from 14.5% to 76.5% among centers), with an additional 23.9% of patients becoming asymptomatic with addition of an antiarrhythmic medication. In 27.3% of patients, more than one procedure was required to attain these results. There was at least one major complication in 6% of patients. A thorough discussion of results of catheter ablation was published in 2007; it notes that results are widely variable, due in part to differences in technique, follow-up, definitions of success, use of antiarrhythmic therapy, and in experience and technical proficiency.
Cox maze
The Cox maze procedure is an open-heart surgical procedure intended to eliminate atrial fibrillation and was first performed in 1987. "Maze" refers to the series of incisions made in the atria, which are arranged in a maze-like pattern. The intention was to eliminate AF by using incisional scars to block abnormal electrical circuits (atrial macroreentry) that AF requires. This procedure required an extensive series of endocardial (from the inside of the heart) incisions through both atria, a median sternotomy (vertical incision through the breastbone) and cardiopulmonary bypass (heart-lung machine). A series of improvements were made, culminating in 1992 in the Cox maze III procedure, which is now considered to be the "gold standard" for effective surgical cure of AF. The Cox maze III is sometimes referred to as the "traditional maze", the "cut and sew maze", or simply the "maze".
Minimally invasive maze procedures
Minimaze procedures are minimally invasive versions of the original Cox maze procedure but without cardiac incisions. These procedures do not require a median sternotomy (vertical incision in the breastbone) or cardiopulmonary bypass (heart-lung machine). They use laser, cryothermy, radiofrequency, or acoustic energy to ablate atrial tissue near the pulmonary veins and make other required ablations to mimic the maze.
Minimally invasive surgical (endoscopic) maze procedures are now routinely conducted at hospitals around the US. This approach was developed in the early 2000s.
The Ex-Maze is a minimally invasive procedure, first reported in 2007, that also creates a lesion pattern across both atria epicardially on the beating heart. As with other procedures off-bypass, the surgeon can confirm that atrial fibrillation corrects to normal sinus rhythm during the procedure. Laparoscopic instruments are used to access the pericardium through the diaphragm. Like many heart-cauterizing instruments, the Ex-Maze device uses heat generated by a radiofrequency coil. The coil is inside a plastic tube that uses suction to maintain contact against the beating heart’s surface.
Epidemiology
Atrial fibrillation is the most common arrhythmia found in clinical practice. It also accounts for 1/3 of hospital admissions for cardiac rhythm disturbances, and the rate of admissions for AF has risen in recent years. Approximately 2.2 million individuals in the United States and 4.5 million in the European Union have AF.
The incidence of atrial fibrillation increases with age. The prevalence in individuals over the age of 80 is about 8%. In developed countries, the number of patients with atrial fibrillation is likely to increase during the next 50 years, due to the growing proportion of elderly individuals.
History
Because the diagnosis of atrial fibrillation requires measurement of the electrical activity of the heart, atrial fibrillation was not truly described until 1874, when Edmé Félix Alfred Vulpian observed the irregular atrial electrical behavior that he termed "fremissement fibrillaire" in dog hearts. In the mid-eighteenth century, Jean-Baptiste de Sénac made note of dilated, irritated atria in people with mitral stenosis. The irregular pulse associated with AF was first recorded in 1876 by Carl Wilhelm Hermann Nothnagel and termed "delirium cordis", stating that "[I]n this form of arrhythmia the heartbeats follow each other in complete irregularity. At the same time, the height and tension of the individual pulse waves are continuously changing". Correlation of delirium cordis with the loss of atrial contraction as reflected in the loss of a waves in the jugular venous pulse was made by Sir James MacKenzie in 1904. Willem Einthoven published the first ECG showing AF in 1906. The connection between the anatomic and electrical manifestations of AF and the irregular pulse of delirium cordis was made in 1909 by Carl Julius Rothberger, Heinrich Winterberg, and Sir Thomas Lewis.
See also
External links
- page on atrial fibrillation
- at patient.co.uk
- (evidence-based medicine) resource on atrial fibrillation
- [https://online.epocrates.com/u/29111/Chronic+atrial+fibrillation Atrial fibrillation] at [https://online.epocrates.com Epocrates Online]
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