Although, atrial fibrillation (AF) is the most common arrhythmia occurring in general population, incomplete knowledge of the etiology and complexity of the previously known causes of atrial fibrillation makes it still one of the most difficult arrhythmias to treat. Factors responsible for the occurrence of atrial fibrillation and necessary for its perpetuation were presented by a well-known French electrophysiologist Coumel in the form of a triangle (fig. 1) (1). Removal or modification of many described factors allows for an arrhythmia elimination, and if this is not possible, a lot of targets for the use of drug therapy can be found.

First attempts to found an effective drug preventing the occurrence of atrial fibrillation date back to the first description of arrhythmia in animals presented by William Harvey in 1628 and to the observations, made in subsequent years, which revealed that the incidence of irregular pulse in patients is associated with unfavourable prognosis. In 1785 the beneficial effects of digitalis in patients with heart failure were described, and a quinidine (2), which was introduced for the treatment of AF in the nineteenth century, is considered to be the first typical antiarrhythmic drug. In subsequent years, next antiarrhythmic drugs were synthesized and introduced into clinical practice and their classification made in 1969 by Vaughan Williams after a few modifications it is still used today (tab. 1) (3).

For the present, there is no perfect drug, allowing for prevention of atrial fibrillation occurrence and for effective maintenance of sinus rhythm. List of antiarrhythmic drugs recommended for use in patients with atrial fibrillation rather shortens than lengthens. This particularly concerns IA class drugs (quinidine and disopyramidum), which are characterized by potential pro-arrhythmic effect (risk of QT prolongation and the occurrence of torsade de pointes type ventricular tachycardia) and IC class, contributing by their pro-arrhythmic potential especially in patients with heart failure, to unfavorable prognosis, as it was demonstrated in the CAST study published in 1989 and evaluating the efficacy of moricizine, encainide and flecainide in patients after myocardial infarction, with reduced ejection fraction and ventricular extrasystoles (4).

In currently available guidelines of the European Society of Cardiology concerning the treatment of patients with atrial fibrillation, published in 2010 and updated in 2012, antiarrhythmic drugs were divided into drugs used to restore sinus rhythm and drugs used for long – term sinus rhythm maintaining (5, 6).

In patients with atrial fibrillation of recent onset (< 48 hours), for whom the strategy for maintaining sinus rhythm was chosen or for whom other medical indications for the restoration of sinus rhythm exist (for example, clinical symptoms persisting despite adequate control of ventricular rate), it is possible to carry out pharmacological cardioversion by administration of antiarrhythmic drug as an intravenous bolus. According to the current ESC guidelines in the first class of recommendations for patients in whom pharmacological cardioversion is preferred and there is no structural heart disease, an intravenously flecainide, propafenone, ibutilide or vernakalant should be used and in patients with structural heart disease an amiodarone should be used. ESC experts emphasize the need for continous medical supervision and electrocardiogram monitoring during and directly after intravenous drug administration because of the risk of ventricular arrhythmias, sinus arrest or atrioventricular block. In selected patients with atrial fibrillation of recent onset and without significant structural heart disease, according to the ESC guidelines, an outpatient pharmacological cardioversion by oral administration of high single dose of flecainide or propafenone (IIa class of recommendations) should be considered. The “pill-in-the-pocket” approach and pharmacological outpatient self-cardioversion is possible, if its safety was previously demonstrated in the hospital. It should be offered to patients suffering from severe symptoms of AF, with infrequent reccurence of arrhythmia and after the careful consideration all the indications and contraindications for the use of these drugs. Both flecainide and propafenone belong to Vaughan Williams Ic class and act by inhibiting the intracellular fast sodium current and slowing the conduction. Furthermore, flecainide inhibits the opening of potassium channels, particularly fast current component K (I_Kr), extending the action potential duration in atria and ventricles cells. In contrast, within the Purkinje fibers, flecainide shortens the action potential duration due to an inhibition of the Na channel. Recent studies suggest that flecainide also blocks the opening of ryanodine receptor (RyR), thus reducing the spontaneous calcium release from the sarcoplasmic reticulum, which can potentially cause late depolarization and triggered activity (7). Due to weak blocking of beta – adrenergic receptors propafenone should not be administered to patients with severe obstructive lung disease. IC class drugs are discouraged in patients with significant structural heart disease, as they may prolong the QT interval and widen QRS complexes, creating the risk of ventricular proarrhythmia, as well as unintentionally accelerate the ventricular fibrillation due to conversion of atrial fibrillation to atrial flutter with ventricular conduction 1:1. Described percentage of this type of pro-arrhythmic action of flecainide vary from 3.5 to 5.0% (8). In order to reduce the risk of rapid conduction of supraventricular arrhythmia to ventricles, along with propafenone or flecainide there should be administered drugs that inhibit atrioventricular conduction (for example, beta-blockers, verapamil or diltiazem). However, in one study it was demonstrated that propafenone and flecainide can be safely taken in outpatient cardioversion (1/569 episode of converting to atrial flutter with rapid ventricular rhythm), and their effectiveness is quite high (94%). The return of sinus rhythm after oral administration of propafenone (450-600 mg) and flecainide (200-300 mg) was observed after 2-6 hours (9). In a recently performed large randomized FLEC-SL study the efficacy of oral flecainide therapy for prevention of atrial fibrillation recurrence was proved. It has been shown that a short-term (4 weeks) treatment with flecainide after the conversion to sinus rhythm in patients with persistent atrial fibrillation is not worse than the long-term treatment (6 months), and the drug is more efficient than placebo (10).

Vernakalant is a drug, which has been used to pharmacological cardioversion in patients with atrial fibrillation of recent onset (IA class of recommendations), patients with atrial fibrillation with a duration time < 7 days and with moderate structural heart disease or < 3 days after cardiac surgery (IIb class of recommendation, level of evidence B). The drug acts mainly in the atria, inhibiting several ion channels and leading to a prolongation of refraction and the slowing of conduction in the atria. The influence of vernakalant on conduction in the ventricles is limited. The efficacy of this drug was evaluated in several clinical studies. In ACT study (I-IV, a medium-sized randomized placebo-controlled studies) it was demonstrated the superiority of vernakalant in comparison with placebo in restoring sinus rhythm in patients with atrial fibrillation lasting < 7 days. In the CRAFT study the efficacy of different vernakalant doses restoring sinus rhythm were evaluated, showing the percentage of conversion of AF to sinus rhythm of 61% with a vernakalant dosage of 2 mg/kg for the first dose + 3 mg/kg in the next dose. In ACT, AVRO and Scene2 studies an infusion of 1 mg/kg within 10 minutes was administered, and then a bolus of 2 mg/kg, if AF was present 15 minutes after the first infusion. In the majority of patients (75-82%), sinus rhythm returned after the first dose (11-13). In a direct comparison (AVRO study) vernakalant more effectively than amiodarone restored sinus rhythm after 90 minutes (51.7 vs 5.2%; p < 0.0001) and 4 hours after infusion (54.4 vs 22.6%; p < 0.0001).

In the subgroups of patients with cardiovascular diseases (coronary heart disease, including myocardial infarction or hypertension) the efficacy of the drug was similar to the control group (45.7 vs 47.3%), and the incidence of adverse events (blood pressure decrease, bradycardia or ventricular arrhythmia) did not increase significantly, with the exception of patients with heart failure, which were less likely to benefit from the drug. As was proved, vernakalant is ineffective in restoring sinus rhythm in patients with atrial fibrillation lasting more than 7 days and in patients with atrial flutter (14, 15). Among the significant adverse events observed in patients, who were treated with vernakalant, compared with patients receiving placebo, the most commonly observed (5-7%) were transient hypotension, bradycardia, which, however, did not cause the need of drug withdrawal and short nsVT episodes, were the most commonly observed (7.3 vs 1.6% in the placebo group). Adverse effects more often occured in patients with the heart failure, but in spite of QTc prolongation (from 20 to 25 msec), there was no arrhythmia of torsade de pointes type. Mentioning the above-cited clinical studies, ESC experts in the current guidelines have formulated recommendations to use vernakalant, as safe and effective agent in patients with minimal or moderate heart disease, but a special caution was recommended in patients with heart failure I and II NYHA class, due to the increased risk of hypotension and ventricular arrhythmias. Contraindications to the use of vernakalant involve: systolic blood pressure < 100 mmHg, acute coronary syndrome within the previous 30 days, severe heart failure in III and IV NYHA class and significant aortic stenosis.

In patients with AF of recent onset and structural heart disease the only drug that can be administered for pharmacological cardioversion is amiodarone (IA class of recommendations). Sinus rhythm after intravenous injection of amiodarone was able to restore within 24 hours in 80-90% of patients in comparison with return of sinus rhythm in 40-60% of subjects treated with placebo. Amiodarone does not act effectively in a short period of time after administration. According to the guidelines, for pharmacological cardioversion there should not be used drugs ineffective in converting recent onset AF to sinus rhythm in patients with atrial fibrillation. ESC experts included to this group: digoxin, verapamil, metoprolol and other beta-blockers and ajmaline (III class of recommendations).

In Polish conditions, only propafenone and amiodarone are the drugs commonly available in clinical practice. In many centers the drug commonly used to restore sinus rhythm is phenazolinum, which does not have place in the current ESC guidelines. Phenazolinum belongs to antihistamines (H1-receptor antagonist) with anti-allergic effect. Because of the anticholinergic and cell membrane stabilizing effects, phenazolinum is included to Vaughan Williams class IA drugs (similar to quinidine). The agent prolongs the duration and decreases the amplitude of the action potential and the resting potential of phase 4. The efficacy of phenazolinum in restoring sinus rhythm is estimated at 52-70% in comparison with the complete effectiveness of pharmacological cardioversion of approx. 62% (16). In the absence of a recommendation for the drug in current guidelines, phenazolinum use in restoring sinus rhythm should be treated with adequate reserve, albeit soon there will be published the results of a randomized study on the effectiveness of phenazolinum that may affect the justification for the increased use of the drug.

Initiation of treatment aimed at maintaining sinus rhythm should be justified by the presence of persistent symptoms associated with atrial fibrillation episodes in patient. The updated guidelines on atrial fibrillation, taking into account the results of meta-analyzes and review papers confirming the efficacy of antiarrhythmic therapy, as well as emerging reports associated with adverse effects (17-19), underline the leading role of the principle of “safety first”. Therefore, it was considered as a valuable observation that in some patients, for example in those with increased risk connected with treatment, a short-term antiarrhythmic drug therapy (4 weeks after cardioversion) can be used. It is also emphasized that the choice of the antiarrhythmic drug should be based more on the predicted safety of its use, rather than on the expected efficacy, although it is known that the probability of maintaining sinus rhythm by using the antiarrhythmic drug increases about 2 times. In a large meta-analysis of studies comparing antiarrhythmic drugs with placebo, no treatment or administration of heart rate controlling drugs, it was confirmed that drugs being both sodium antagonists with rapid (quinidine, disopyramide) and slow (propafenone, flecainide) binding kinetics, potassium channels inhibitors (dofetilide) and influencing on potassium channels and beta-adrenergic receptors (sotalol), as well as drugs affecting many ion channels with simultaneous inhibition of the sympathetic nervous system (amiodarone) significantly reduce the rate of recurrence of atrial fibrillation (20).