Atrial fibrillation. Classification

Transcription

1 Abhay Bajpai Edward Rowland Atrial fibrillation (AF) is the commonest cardiac arrhythmia. The incidence increases with age and affects 5% of UK population above the age of 65 yr and 10% above 75 yr. 12 In the United States, AF accounts for more than 35% of all admissions for cardiac arrhythmias. 3 Men are 1.5 times more likely to develop AF than women. AF is often associated with structural heart disease, but in many patients it can occur with no detectable disease (lone AF). Thromboembolic events and heart failure resulting from AF lead to significant morbidity, mortality and increased cost of management. Definition and electrocardiographic patterns AF is a supraventricular arrhythmia characterised by complete absence of coordinated atrial contractions. On the electrocardiogram there is consistent absence of P waves which are replaced by fibrillatory waves. AF is associated with an irregular and frequently rapid ventricular response if atrioventricular conduction is intact. Regular R-R intervals are possible in the presence of atrioventricular block or interference by ventricular or junctional tachycardia. 3 A wide QRS complex tachycardia that is rapid, irregular or sustained strongly suggests underlying bundle branch block or conduction over an accessory pathway (e.g. Wolf Parkinson White syndrome) especially if the ventricular rate is extremely rapid (over 200 beats min 1 ). AF can be commonly associated with other arrhythmias such as atrial flutter or atrial tachycardia. Atrial flutter is a more organised and regular form of atrial activation resulting in a saw-tooth pattern or flutter waves (f) on the electrocardiogram. Atrial flutter can arise during treatment of AF with antiarrhythmic drugs. Atrial flutter itself can degenerate into AF, can be triggered by AF or the pattern can alternate between AF and flutter. Other atrial arrhythmias can also trigger AF and are identified by the presence of P waves which are separated by an isoelectric baseline. Classification Classification of AF has always been controversial. 3 5 The current classification is based on two important elements: patterns of evolution of arrhythmia and the response to treatment. First onset AF is the first clinical presentation where the patient is still in AF and the episode has been present for less than 48 h. Paroxysmal AF is the occurrence of recurrent episodes that typically last minutes to hours, occasionally days, but eventually selfterminate. Persistent AF is present when arrhythmia is not self-terminating, but pharmacological or electrical cardioversion is required to restore sinus rhythm. AF is permanent when all attempts to restore sinus rhythm have been abandoned because of physician or patient decision, frequent recurrence, or inability to cardiovert the patient. Pathophysiology and mechanisms The mechanisms of AF are not fully clear, but at least three aspects seem to be important in its genesis and maintenance: There may be enhanced automaticity within sleeves of atrial tissue that extend into the pulmonary veins or vena caval junctions. 6 These foci can act as trigger points to generate multiple atrial ectopics leading to AF and their elimination by means of ablation may possibly provide a permanent cure, particularly in those with structurally normal hearts. In chronic AF, maintenance of arrhythmia is sustained by multiple re-entering and randomly circulating wavelets that collide and divide into daughter-wavelets thus maintaining the chaotic electrical state. 78 The longer the duration of AF, the more difficult it is to restore sinus rhythm and prevent recurrences. This is due to electrical and structural remodelling of atrial Key points Atrial fibrillation (AF) is the commonest cardiac arrhythmia; its incidence increases with age. Diabetes mellitus, hypertension and ventricular hypertrophy are commonly associated with non-valvular atrial fibrillation. Primary aims of management of AF are conversion to sinus rhythm, maintenance of sinus rhythm and prevention of thromboembolic complications. In elderly patients who are asymptomatic, adequate rate control of AF appears to offer the same benefits as rhythm control. Chronic AF carries a high risk of ischaemic stroke from thromboembolism; all patients at risk must receive adequate anticoagulation. Anticoagulation should be continued in patients with risk factors despite successful conversion to sinus rhythm. Abhay Bajpai Clinical Research Fellow Cardiovascular Sciences St George s University of London London UK Edward Rowland Consultant Cardiologist St George s Hospital London UK erowland@sgul.ac.uk (for correspondence) doi: /bjaceaccp/mkl051 Continuing Education in Anaesthesia, Critical Care & Pain Volume 6 Number ª The Board of Management and Trustees of the British Journal of Anaesthesia [2006]. All rights reserved. For Permissions, please journals.permissions@oxfordjournals.org 219

2 Table 1 Causes and risk factors for AF Acute causes Alcohol binge drinking Cardiac and non-cardiac surgery Myocarditis/pericarditis Pulmonary embolism Pulmonary hypertension Chest infections Hyperthyroidism Neurogenic AF tissue resulting in shortening of effective refractory periods, thereby maintaining and increasing the duration of AF. 910 Causes and risk factors As described above, the risk of developing AF increases with age. Whilst rheumatic valvular disease remains the most common cause for AF in developing countries, most patients develop AF on the basis of coronary artery disease and systemic hypertension. Clinically important causes and risk factors for AF are summarised in Table 1. Echocardiographic predictors include large atria, diminished ventricular function and increased left ventricular wall thickness. When AF occurs in normal hearts without signs of any demonstrable cardiovascular disease, it is termed lone atrial fibrillation Principles of management AF associated with cardiovascular disease Following myocardial infarction Hypertension, especially if left ventricular hypertrophy Valvular heart disease (often mitral) Congenital heart disease, mainly atrial septal defects (ASD) Sick sinus syndrome Diabetes mellitus Familial AF High vagal tone (nocturnal episodes) Identified in a small group of patients 11 High sympathetic tone (daytime episodes; associated with ischaemic heart disease, stress, excessive caffeine, alcohol) The management of AF still represents one of the major therapeutic challenges in medicine. Based on current evidence and guidelines, there are four main principles 3 : (i) Restoration of sinus rhythm by pharmacological or electrical means. (ii) Control of ventricular rate during paroxysmal or persistent AF, and chronically in those with permanent AF. (iii) Prevention of recurrence of paroxysmal or persistent AF following successful restoration of sinus rhythm. (iv) Prevention of thromboembolic phenomena. When a patient is seen in the emergency setting, the main priority is to control the fast ventricular response and, depending on the haemodynamic status of patient, this can be achieved either by urgently restoring sinus rhythm or by controlling the ventricular rate. Immediate electrical cardioversion is indicated in patients with a rapid ventricular rate who are either haemodynamically unstable or have evidence of acute myocardial ischaemia or heart failure that do not respond promptly to pharmacological measures. In less acute situations, pharmacological cardioversion can be attempted, thereby avoiding the requirement for general anaesthesia. Cardioversion by either means carries a risk of thromboembolism, particularly when the arrhythmia has been present for longer than 48 h; anticoagulation prophylaxis must be initiated before the procedure. In the long-run, it may become appropriate to accept the arrhythmia as permanent. Recent studies have pointed out that rate controlling the AF could be at least as effective as restoration of sinus rhythm in terms of symptom control and survival, particularly in stable patients aged approximately 60 years old. Restoration of sinus rhythm Direct current cardioversion Direct current cardioversion (DCC) involves a synchronised direct current electrical shock delivered across the chest wall. Sinus rhythm can be restored in a significant proportion of patients with success rates varying between 65% and 90%. The success of DCC appears to be greater with anterior-posterior positioning of paddles (sternum and left scapular) than with anterior-lateral (ventricular apex and right infraclavicular). Other factors include nature of heart disease, transthoracic resistance and output waveform (monophasic or biphasic). 13 Elective DCC is performed under adequate short-acting anaesthesia. In an emergency situation, the need for anaesthesia depends on the nature of the emergency and conscious level of the patient; it is reasonable to perform DCC under conscious sedation if the urgency of the situation is such that there can be no delay. In a recent report, elective DCC performed under conscious sedation was also shown to be safe and not associated with any intolerable discomfort to the patient. 12 Devices that deliver a monophasic waveform of current have been conventionally used for cardioversion. Initial shock energy of 200 J is recommended for cardioversion of AF using a monophasic device. The sequence of energy commonly used is: 200 J; 200 J; 300 J; and 360 J. 3 Biphasic machines achieve cardioversion at much lower energy levels and are increasingly replacing monophasic devices. It is safe to cardiovert patients with implanted devices such as permanent pacemaker or internal defibrillator provided the implanted device is interrogated immediately before and after cardioversion to assess any malfunction. The paddles used for cardioversion should be placed as far as possible from the implanted device, preferably in the anterior-posterior position. Brief arrhythmias can arise immediately following DCC. These are mainly ventricular and supraventricular premature beats, bradycardia and short periods of sinus arrest. Ventricular tachycardia or fibrillation can be precipitated in patients with hypokalaemia and digitalis intoxication. Patients with underlying conduction defects are at risk of developing profound bradycardia, complete heart block or asystolic periods following cardioversion. These patients are identified by having a slow ventricular response to AF in the absence of rate-reducing 220 Continuing Education in Anaesthesia, Critical Care & Pain Volume 6 Number

3 medications and facilities for temporary external or endocardial pacing must be made available prior to attempting cardioversion. Electrical cardioversion can also lead to transient ST segment elevation with a rise in blood concentrations of cardiac troponins and CK-MB, even without cardiac damage. The rate of relapse after DCC is high unless anti-arrhythmic drug therapy to maintain sinus rhythm is given concomitantly. However, prophylactic therapy to prevent recurrences following DCC should be considered individually for each patient. Pharmacological restoration of sinus rhythm Prior to considering any anti-arrhythmic therapy, it is important to realise that up to 60% of patients with recent onset AF spontaneously revert to sinus rhythm within 24 h to a few days. 13 Pharmacological cardioversion is considered in patients who are haemodynamically stable; it is often successful in AF of <48 h. Anti-arrhythmic drugs are classified according to the Vaughan Williams classification (Table 2). Current guidelines on the management of AF 3 recommend use of propafenone, flecainide, ibutilide or dofetilide as first choice for pharmacological cardioversion of AF of <7 days duration (conversion rates of up to 90%). 3 Flecainide and propafenone are safe in patients who do not have evidence of ischaemic heart disease or myocardial dysfunction. Dofetilide and ibutilide are relatively new class III agents and are most useful for cardioverting atrial flutter. However, there is an increased incidence of torsade de points or polymorphic ventricular tachycardia with use of these drugs. In clinical practice, amiodarone is a reasonable alternative to class Ic agents and is the drug of choice in patients with ventricular dysfunction and ischaemic heart disease. Amiodarone also has an added advantage of providing prompt rate control in addition to its anti-arrhythmic effect. This rate controlling effect (beta blockade and calcium channel blockade properties) is observed early following i.v. loading; the class III antiarrhythmic properties take effect at 8 24 h. Most of the toxicity of amiodarone is dose-dependent and related to chronic Table 2 Vaughan Williams Classification of Antiarrhythmic Drugs Class Action Drugs I Sodium channel blockade IA Prolong repolarisation Quinidine, procainamide, disopyramide IB Shorten repolarisation Lidocaine, mexiletine, tocainide, phenytoin IC Little effect on repolarisation Encainide, flecainide, propafenone II Beta-adrenergic blockade Propanolol, esmolol, acebutolol, l-sotalol III Prolong repolarisation (potassium channel Amiodarone, bretylium, d,l-sotalol, ibutilide blockade; other) IV Calcium channel blockade Verapamil, diltiazem, bepridil Miscellaneous Miscellaneous Actions Adenosine, digitalis, magnesium treatment; however, it should be used with caution in patients with acute ischaemia or myocardial dysfunction, as profound hypotension may be induced by i.v. or high-dose oral loading. There is emerging evidence that i.v. and oral amiodarone have different electrophysiological properties and it may be possible to administer i.v. amiodarone to cardiovert AF in patients who are already on chronic oral treatment. 14 Maintenance of sinus rhythm The relapse rate of AF following initial cardioversion is high (25 50% at 1 month). Maintenance of sinus rhythm after successful cardioversion is achieved by continuation of oral therapy with class I or III anti-arrhythmic drugs. Pre-treatment with these drugs also improves success rates of repeated DCC. Sotalol is useful for preventing recurrences of paroxysmal AF. However, it is vital to be aware of the pro-arrhythmic effects of these agents; they can prolong the QT interval and lead to dangerous ventricular arrhythmias. Thus, regular monitoring of corrected QT interval (QTc) and QRS duration is important during outpatient reviews. Renal insufficiency and electrolyte imbalance also predispose to polymorphic ventricular tachycardia. Rate control of atrial fibrillation It is necessary to control the ventricular rate in persistent AF. Also, rate control may be a preferred strategy in patients with paroxysmal AF who are stable at presentation, as 60% of these patients can spontaneously revert to sinus rhythm within 24 h. The aim of rate control in AF is to improve symptoms and prevent worsening of ventricular dysfunction. Recent evidence from randomised trials (AFFIRM, 15 PIAF, 16 RACE, 17 STAF 18 ) has shown that rate control is at least as effective as rhythm control in improving symptoms and functional capacity, particularly in those over 65 yr of age. What is considered adequate rate control remains controversial. Heart rate should neither be too high (leads to tachycardia induced cardiomyopathy) or too low (facilitates heart failure); it is considered to be controlled when the ventricular rate is bpm at rest and bpm during exercise. Rate control is achieved by drugs which predominantly affect conduction through the AV node. Commonly used agents are digoxin, beta-blockers (atenolol, propranolol, metoprolol, esmolol) and non-dihydropyridine calcium channel blockers (verapamil and diltiazem). In permanent AF, digoxin can usually provide adequate rate control. However, digoxin does not prevent excessive heart rates during exercise and combination therapy with beta-blockers or calcium antagonists may be necessary, especially in younger active individuals. In stable patients with acute AF, calcium antagonists or beta-blockers (oral or i.v.) are preferred to digoxin due to their rapid onset of action (3 7 min). Digoxin is the drug of choice for rate control in patients with heart failure, though there is emerging evidence in support of beta-blockers. Continuing Education in Anaesthesia, Critical Care & Pain Volume 6 Number

4 Table 3 Non-pharmacological management of AF Rhythm control Rate control Stroke prevention Device therapy Atrial pacing (single or multisite) Atrial defibrillators (stand-alone or with pacemaker function) Ablation therapy Operative (Maze procedure, Pulmonary vein isolation, His bundle ablation) Percutaneous transcatheter techniques (pulmonary vein isolation, radiofrequency ablation of triggers or substrate) Patients with sick sinus syndrome with AF and present with episodes of bradycardia, usually require the support of a permanent pacemaker to allow the use of rate controlling medications. Drugs acting on the AV node are contraindicated in patients with accessory conduction pathways (e.g. Wolf Parkinson White syndrome) as they can result in dangerously fast ventricular rates by increasing the conduction via the accessory pathway. In these situations amiodarone, flecainide or procainamide are drugs of choice. Prevention of thromboembolism Chronic AF is associated with a 3 7% annual risk of ischaemic stroke from thromboembolism. Guidelines 3 recommend administration of heparin prior to, or concurrently during, immediate electrical or pharmacological cardioversion. If AF has been present for longer than 48 h or the duration is unknown, warfarin should be given for 3 4 weeks following successful cardioversion. Patients admitted for elective cardioversion require adequate anticoagulation with warfarin 3 4 weeks before and after the procedure (INR 2 3). Those patients who cannot be anticoagulated due to contraindications prior to cardioversion should undergo transoesophageal echocardiographic examination to exclude the presence of thrombus. In chronic AF, the risks and benefits of antithrombotic therapy (aspirin, warfarin) must be considered in each individual patient. Chronic hypertension, age >65 yr, diabetes mellitus, previous ischaemic stroke, ventricular dysfunction and co-existent ischaemic or valvular heart disease are considered as high risk factors for thromboembolism in AF. All such high risk patients must receive warfarin unless contraindicated. In the absence of these factors (low risk patients) or when warfarin cannot be given, aspirin 300 mg daily is an alternative. Young patients with AF who do not have any clinical or echocardiographic evidence of heart disease ( lone AF ) are also at low risk of thromboembolism. Anticoagulation can be interrupted for a period of up to 1 week for surgical and diagnostic procedures that carry a risk of bleeding. However, these patients must receive heparin if they are at high risk of thromboembolism or have mechanical prosthetic valves. Transcatheter AV junctional ablation and permanent pacemaker implantation Radiofrequency transcatheter AV junction modification Principles of prevention of thromboembolism in patients with atrial flutter are the same as those for AF. New anticoagulants and thrombin inhibitors which do not require regular monitoring and blood tests are currently being compared with warfarin in various trials. Non-pharmacological management A wide variety of non-pharmacological approaches now exist for managing AF and provide rhythm or rate control when drug treatment has failed. Commonly used strategies are outlined in Table 3. Significant effort is currently being devoted to percutaneous catheter based ablation of the triggers of AF (atrial premature beats, monomorphic atrial tachycardias, atrial flutter). The vast majority (>90%) of these triggers are now known to arise from sleeves of atrial tissue with abnormal automaticity present within the pulmonary veins. The procedure involves isolation of pulmonary veins and radiofrequency ablation of the triggers of AF. Although the technique can provide long-term maintenance of sinus rhythm in a majority of patients, it can lead to systemic embolism and pulmonary vein stenosis, especially if multiple trigger areas are present. Methods using other energy sources, such a cryotherapy and ultrasound, are currently being evaluated and may minimise such complications. Management in special situations Cardiac surgery Percutaneous left atrial appendage transcatheter occlusion (PLAATO) The incidence of AF after cardiac surgery is high; 27 37% of patients undergoing coronary artery bypass grafting and 50% of those following valvular surgery will develop AF in the post-operative period. The majority of AF episodes occur within first 4 days of cardiac surgery with a peak incidence on the second post-operative day. Whilst it is still unclear why some patients develop AF post-operatively, certain factors have shown a statistical relationship with AF (Table 4). 19 There is clear evidence that C-reactive protein, a marker of inflammation, peaks on the second post-operative day coinciding with the peak incidence of AF. This suggests a unique role of 222 Continuing Education in Anaesthesia, Critical Care & Pain Volume 6 Number

5 Table 4 Factors related to development of post-operative AF Advanced age Males Previous AF Cardiac failure Hypertension Chronic obstructive airway disease Chronic renal failure Previous cardiac surgery inflammation during the post-operative period in about 40% of patients, particularly when it involves the pericardium or the heart muscle. 20 Post-operative AF is associated with increased morbidity and mortality, largely due to heart failure, stroke and prolonged hospital stay. Various therapies have been investigated for prophylaxis of post-operative AF Guidelines 3 recommend use of betablockers in patients undergoing cardiac surgery. Once a common practice to discontinue beta-blockers prior to surgery, these agents have now been shown to reduce the risk of post-operative AF by about 60%. No major differences exist between different beta-blockers (sotalol, metoprolol, propranolol) in preventing post-operative AF. Pre-operative use of amiodarone also appears to be equally effective. Haemodynamically unstable patients with AF should be cardioverted urgently, either by direct current or pharmacologically using amiodarone. Ibutilide is particularly useful in patients with atrial flutter. It is recommended that, following successful DCC, patients should receive oral amiodarone for 6 8 weeks. The vast majority of post-operative AF spontaneously reverts to sinus rhythm within 2 weeks. Thus, haemodynamically stable patients can be commenced on beta-blockers, provided these are not contraindicated, to achieve a rate control of <100 beats min 1. The treatment can be supplemented or substituted with i.v. digoxin and/or calcium channel blockers (verapamil or Diltiazem). All patients who develop AF post-operatively must be anticoagulated with heparin and warfarin as soon as it is deemed safe. Anticoagulation should be continued for 4 weeks after discontinuing amiodarone or documentation of spontaneous return of sinus rhythm. Left, right or bi-atrial pacing have also shown to reduce the occurrence of AF by 30 60%. Other therapies including use of magnesium and inflammatory agents have been incompletely evaluated. Acute myocardial infarction Prolonged P waves on ECG Atrial dilatation High left ventricular end-diastolic pressure Cardiomegaly on chest X-ray Right coronary artery grafting Prolonged bypass time Inadequate cardioprotection and hypothermia Intractable ischaemia or haemodynamic instability requires urgent electrical cardioversion. In patients with heart failure, i.v. digoxin or amiodarone is given to slow the ventricular response and improve left ventricular function. I.V. beta-blockers can be prescribed for rate control to patients who do not have evidence of clinical left ventricular dysfunction, bronchospasm or atrio-ventricular block. In the setting of acute myocardial ischaemia, administration of class Ic antiarrhythmic drugs can be harmful and these agents are best avoided. Pregnancy Digoxin, beta-blockers or calcium channel antagonists can be used for rate control of AF during pregnancy. Haemodynamically unstable patients should be electrically cardioverted. Current guidelines 3 recommend use of antithrombotic therapy (anticoagulant or aspirin) throughout the duration of pregnancy in all patients with AF, except those with lone AF. Oral anticoagulants carry a risk of teratogenicity and should be avoided if possible, especially in the first and third trimesters of pregnancy. Ventricular pre-excitation In patients with Wolf Parkinson White syndrome, AF can lead to extremely rapid ventricular rates. Unstable patients require urgent DCC to prevent ventricular fibrillation. I.V. flecainide or ibutilide should be given to restore sinus rhythm if there is no haemodynamic compromise. Drugs acting on the AV node (beta-blockers, digitalis glycosides, calcium channel antagonists, adenosine) are contraindicated since these can increase the conduction of electrical activity through the accessory pathway. Hyperthyroidism During the hyperthyroid state, beta-blockers are necessary to control the ventricular response, unless contraindicated when diltiazem or verapamil can be given. Anticoagulation is recommended if there are risk factors for stroke. Once patients are rendered euthyroid, antithrombotic prophylaxis is the same as for those without hyperthyroidism. Pulmonary disease It is once again important to realise that the vast majority of these patients will spontaneously revert to sinus rhythm and unless there is haemodynamic compromise, all that may be needed is rate control with adequate anticoagulation. Correction of hypoxaemia and acidosis are primary measures in patients who develop AF during acute pulmonary illness or exacerbation of obstructive airway disease. Calcium channel antagonists, verapamil or diltiazem, are preferred agents for rate control. Theophylline can sustain AF, whilst beta-blockers, sotalol, propafenone and adenosine are contraindicated in patients with bronchospastic lung disease. The long-term use of amiodarone can lead to pulmonary fibrosis and is best avoided. References 1. Lairikyengbam SK, Anderson MH, Davies AG. Present treatment options for atrial fibrillation. Postgrad Med Journal 2003; 79: Continuing Education in Anaesthesia, Critical Care & Pain Volume 6 Number

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