Education and debate

ABC of Atrial Fibrillation: CARDIOVERSION OF ATRIAL FIBRILLATION

Cardioversion from atrial fibrillation to sinus rhythm should be considered for suitable patients as an alternative to leaving the patient in a cardiac arrhythmia and treating with drugs. The potential benefits of a return to sinus rhythm are an improvement in wellbeing and exercise capacity; the avoidance of potentially dangerous drug treatment; and a possible reduction in thromboembolic risk.

Suitability for cardioversion
* Recent onset atrial fibrillation
* No structural heart disease, such as mitral valve disease, poor left
  ventricular function, dilated left atrium
* Successful treatment of any precipitating cause of atrial fibrillation--for
  example, thyrotoxicosis, chest infection

Electrical cardioversion

Electrical cardioversion works by permitting uniform repolarisation and restoring ordered conduction. After the initial asystolic period, the sinoatrial node rapidly resumes its role as cardiac pacemaker, permitting synchronised atrial electrical activity.

External electrical cardioversion with a synchronised direct current shock can be effective in restoring sinus rhythm. The effectiveness of the procedure can range from 20-90% as the procedure is highly influenced by the underlying aetiology--the highest success rates are seen in patients with atrial fibrillation secondary to hyperthyroidism, while the lowest rates are seen in patients with severe mitral regurgitation.

Methods of cardioversion
Electrical
* Synchronised external direct current shock
* Transoesophageal
* Internal
Pharmacological
* Class I drugs
* Class III drugs

Attention to proper technique for external cardioversion will substantially improve efficacy. The energy requirement and success of external cardioversion are also directly related to the duration of atrial fibrillation, size of the f (fibrillation) waves, and the presence of mitral valve disease (especially if there has been previous valve surgery). Many other factors, however, predict refractoriness to successful cardioversion or unsuccessful maintenance of sinus rhythm.

Transoesophageal cardioversion and internal cardioversion are alternative methods of performing electrical cardioversion, but these are used less often, except in specialist centres.

Pharmacological cardioversion and antiarrhythmic treatment

An alternative to electrical cardioversion is pharmacological cardioversion, especially in patients with atrial fibrillation of recent onset. Drugs that are usually used to maintain sinus rhythm after electrical cardioversion are also effective for pharmacological cardioversion.

In general, class I and III
antiarrhythmic drugs are the
most useful agents for
pharmacological cardioversion

Class I agents

The most commonly used class I drugs are quinidine, flecainide, and propafenone. Quinidine is particularly effective in cardioversion of patients with atrial fibrillation and in maintaining sinus rhythm, but side effects can occur in a fifth of patients, two thirds of whom have to stop taking the drug.

Flecainide and propafenone have a rate of successful cardioversion of 25-55% when given orally. Several studies of flecainide have shown it to have a 92% success rate if given intravenously and to reduce significantly the recurrence of the arrhythmia. The drug has no rate limiting properties, however, and has been reported to cause adverse effects in 74% of patients. Propafenone, another class Ic compound, may be more useful than flecainide in view of its inherent rate limiting properties (as a ß blocker), permitting potentially greater ventricular rate control.

Amiodarone

Amiodarone, a class III antiarrhythmic drug, has been shown to be highly effective in the cardioversion of atrial fibrillation, even in previously refractory cases and in maintaining sinus rhythm. An intravenous infusion of amiodarone acts relatively rapidly, restoring sinus rhythm in up to three quarters of cases, making it as effective as electrical cardioversion. In cases of resistant atrial fibrillation, a four week loading of amiodarone (600 mg/day) before cardioversion and a low dose (on average 200 mg/day) maintenance regime after successful cardioversion was effective in achieving cardioversion and sustaining sinus rhythm.

Other drugs

Although occasionally effective in converting atrial fibrillation to sinus rhythm, verapamil has a much lower success rate of conversion than the class I and III antiarrhythmics. Digoxin is no better than placebo for restoring sinus rhythm. Nor does evidence exist that digoxin is effective as prophylaxis against recurring atrial fibrillation after cardioversion. In addition, in patients with recurrent atrial fibrillation, paroxysms of atrial fibrillation occur more frequently, at a faster rate, and for longer in patients receiving digoxin.

Procedure for cardioversion
Preparation
* Arrange admission for monitoring with
electrocardiography--for example, to a
coronary care unit
* Ensure electrolytes (especially potassium)
are normal
* Ensure anticoagulation is adequate, with an
international normalised ratio of 2.0 to 3.0
* If the patient usually takes digoxin and has
no evidence of digoxin toxicity the drug may
be taken up to the day before cardioversion; if
digoxin toxicity is present then serum digoxin
concentrations should be checked and
cardioversion delayed
Pharmacological cardioversion
* Start infusion of drug--for example,
flecainide, amiodarone--under continuous
monitoring with electrocardiography
Electrical cardioversion
* Arrange for patient to be fasted
* Give short general anaesthetic to eliminate
discomfort associated with the transthoracic
shock
* Give synchronised direct current shock:
start at 100 J, with intermediate "step-ups,"
eventually to 360 J
* After the procedure monitor the patient for
at least one hour to ensure stability of rhythm
and blood pressure

Thromboembolism, antithrombotic treatment, and cardioversion

Peripheral emboli have been estimated to complicate external cardioversion in 1-3% of cases. Thromboembolism after pharmacological cardioversion probably has similar rates.

The importance of prophylactic anticoagulation in patients with chronic atrial fibrillation is now established. The role of prophylactic anticoagulation to prevent thromboembolism after cardioversion for patients in atrial fibrillation has been clinically examined in several large series, which established that prior anticoagulant treatment was beneficial in attempted cardioversion. The thromboembolic events that occurred in patients who had received anticoagulants were noted predominantly in the first week after cardioversion, suggesting that this is a high risk period.

The dose of warfarin should be adjusted to maintain an international normalised ratio of 2.0 to 3.0, although in patients at high risk of embolism--for example, those with previous thromboembolism or with mechanical prosthetic heart valves--the target is 3.0 to 4.5.

Mechanisms and factors contributing to
thromboembolism
Mechanical
* Sudden resumption of mechanical atrial systole may result in the
embolisation of any pre-existing clot, which is dislodged by the mechanical
effect of a change in cardiac rhythm during cardioversion
* The return of atrial systole and effective atrial contraction after
cardioversion may take up to three weeks
* Cardioversion may promote the formation of new thrombi due to
transient atrial dysfunction ("stunning")
Duration of atrial fibrillation
* A recently formed, poorly adherent thrombus is more likely to dislodge
at the time of cardioversion
* The relationship between duration of atrial fibrillation and
thromboembolism is affected by haemodynamic status, atrial size,
underlying atrial pathology, and effectiveness of anticoagulation
Left atrial size
* Formation of thrombi is more likely in dilated left atria
Abnormalities in haemorheological function and prothrombotic markers
* Clotting factor levels
* Atrial natriuretic peptide, leading to haemoconcentration, and a raised
packed cell volume

Management after cardioversion

After successful cardioversion to sinus rhythm it is important to continue with oral anticoagulants. The routine (and optimal) use of antiarrhythmic drugs before and after cardioversion, however, remains controversial. The use of these drugs after cardioversion is to maintain sinus rhythm and prevent recurrence of arrhythmia.

Recommendations of American Association of Chest
Physicians for anticoagulation before and after
cardioversion
* Warfarin for three weeks before non-emergency cardioversion of atrial
  fibrillation of >24-48 hours' duration
* Warfarin for four weeks after cardioversion
* Intravenous heparin followed by warfarin if cardioversion cannot be
  postponed for three weeks
* Anticoagulants may not be needed for atrial fibrillation of < 2 days'
  duration or for cardioversion of supraventricular tachycardia.
  Consideration should be given to managing atrial flutter similarly to
  atrial fibrillation

Current practice favours maintaining oral anticoagulation after cardioversion. The risk of embolism probably continues even after successful cardioversion as atrial mechanical function may not be restored for several weeks. The optimal duration of anticoagulation is still unclear, although the American Association of Chest Physicians has drawn up recommendations, including the suggestion that warfarin should be continued for four weeks after cardioversion. In patients with a high risk of recurrent atrial fibrillation, however, it may be prudent to continue anticoagulation for longer than four weeks.

Predictors of refractoriness to
cardioversion or unsuccessful
maintenance of sinus rhythm
* Age >50 years
* Arrhythmia for >1 year
* Hypertension
* Structural heart disease, including poor
  cardiac function, valvar disease, previous
  mitral valve surgery, and other organic
  heart disease
* No correctable precipitating factor--for
  example, thyroid disease, infection

Without antiarrhythmic drugs there is a high risk of relapse of atrial fibrillation, with the proportion of patients remaining in sinus rhythm ranging from 69% at one month to 58% at six months, 23% at one year, and 16% at two years. These drugs are most useful during the three months after successful cardioversion. If patients have an identifiable cause of atrial fibrillation that has been corrected--for example, thyrotoxicosis--antiarrhythmic treatment for three months may be sufficient. If patients, however, have no obvious acute precipitating factors and adverse features for recurrence of atrial fibrillation are present then treatment with antiarrhythmics should be continued for a longer period.

Changes after cardioversion of atrial fibrillation
Short term
* Hypotension and bradycardia--bradycardia more common in patients
with the sick sinus syndrome and after acute myocardial infarction
* Arrhythmias, usually due to either inadequate synchronisation or
digoxin toxicity; ventricular arrhythmias after cardioversion are less
common but more serious (ventricular fibrillation is the most common, in
about 1% of cases, but is usually reverted by repeat shock)
* Premature beats and conduction disturbances (first degree or
  second degree atrioventricular block) are also common
* Small rises in creatine kinase concentration may occur with electrical
cardioversion, usually from skeletal muscle, and myocardial damage is
unlikely
* Raised transient ST segment after cardioversion, usually associated with
previous pericardiotomy, age, and diminished long term maintenance of
sinus rhythm
Long term
* Reduction in left atrial size
* Improvement in ventricular function and also in some cases cardiac
output and exercise or functional capacity due to a combination of a
reduction in heart rate and the restoration of atrial systole

Prognosis after cardioversion

A long previous duration of arrhythmia, previous episodes of atrial fibrillation, and age >50 years predict unsuccessful maintenance of sinus rhythm and reversion to atrial fibrillation. In addition, the presence of coronary artery disease, hypertension, and other organic disease--such as mitral valve disease, aortic stenosis, and cardiomyopathy--are detrimental to maintaining normal sinus rhythm.

Recent studies suggest that left atrial size does not influence the outcome after cardioversion but that the duration of atrial fibrillation is the most important predictor for outcome. Even with a dilated left atrium, long term sinus rhythm (about 80% at 12 months) is possible with the use of antiarrhythmic drugs.

View larger version (121K): [in this window] [in a new window]   Patients in sinus rhythm at time of cardioversion and at one month and six months of follow up according to duration of atrial fibrillation.

The duration of the arrhythmia seems to be the most important factor influencing prognosis after cardioversion. Twice as many patients who have had atrial fibrillation for less than three months remain in sinus rhythm as patients who have had atrial fibrillation for more than 12 months.

Key references
ACP/ACC/AHA Task Force Statement. Clinical competence in elective direct
current (DC0 cardioversion. J Am Coll Cardiol 1993;22:336-9
Clark A, Cotter L. Practical procedures--DC cardioversion. Br J Hosp Med
1991;46:114-5
Laupacis A, Albers GW, Dunn JE, Dunn MI, Feinberg W, Jacobson AK.
Antithrombotic therapy in atrial fibrillation. Chest 1995;108(supp10:352-9S
Lip GYH. Cardioversion of atrial fibrillation. Postgrad Med J 1995;71:457-65

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• van der Velden, H. M.W, Jongsma, H. J (2002). Cardiac gap junctions and connexins: their role in atrial fibrillation and potential as therapeutic targets. Cardiovasc Res 54: 270-279 [Abstract] [Full text]  
• Gallagher, M. M., Guo, X.-H., Poloniecki, J. D., Guan Yap, Y., Ward, D., Camm, A. J. (2001). Initial energy setting, outcome and efficiency in direct current cardioversion of atrial fibrillation and flutter. J Am Coll Cardiol 38: 1498-1504 [Abstract] [Full text]  
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