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Prof. Bulent Gorenek,
While the indications for cardioversion in atrial fibrillation are widely communicated in the literature, the procedure itself is seldom described. Here, we will review basic principles and techniques of direct current and internal cardioversion, from patient preparation to energy waveforms requirements and paddle positioning, as well as complications, implantable devices and pretreatment with antiarrhythmic drugs.
Direct current cardioversion is one of the most effective means of converting atrial fibrillation into sinus rhythm. Medical cardioversion is one alternative, however direct current cardioversion has the highest overall success rate (1,2). In all, patients who require cardioversion will often undergo direct current cardioversion rather than pharmacologic conversion because of its:
Cardioversion should be performed with the patient in a fasting state, under adequate general anaesthesia. The anaesthetic agent must 1) provide analgesia and sedation, and 2) cause the least cardiovascular compromise while allowing for rapid recovery (10-12). Oxygen saturation and electrolytes should be normal and anticoagulation status monitored. Drug levels, such as digoxin and antiarrhythmic agents, should be within the therapeutic range. Digoxin shouldn't be withheld unless there is a suggestion of digitalis excess or toxicity (11). A baseline 12-lead electrocardiogram should be recorded, and venous cannulation should be secured. A pacing catheter may be placed prophylactically in the right ventricle if sick sinus syndrome is suspected. For back-up, external pacing pads may be used for both cardioversion and for prophylaxis should asystole or bradycardia ensue. Overnight hospitalisation is seldom required (10).
Biphasic Waveforms superior to Monophasic Waveforms
Whether cardioversion will succeed depends on whether delivery of current flow through the heart is adequate or not (13). The major determinants of current delivery are 1) the nature of the shock waveform (mono or biphasic) 2) level of delivered energy. In terms of shock waveform, both monophasic and biphasic waveforms are used. Currently, most evidence favors the use of biphasic external defibrillators due to their categorically lower energy requirements and greater efficacy (14,15). Indeed, in a study of 912 patients with AF and atrial flutter, restoration of sinus rhythm was higher in patients who had received biphasic waveforms (94 versus 84 percent for monophasic waveforms) and with lower cumulative energy (199 J versus 554 J) (16). Biphasic waveforms may be of special interest in patients who have failed to revert with the use of monophasic waveforms (17). Furthermore, fewer shocks are required, thereby potentially reducing procedure times and thus, requirements for intravenous sedation. A lower incidence of skin burns (18) and less skeletal muscle damage (19) have also been reported. Additionally biphasic waveforms result in fewer post-shock arrhythmias, and a shorter period of myocardial stunning (20).
The amount of energy needed for initial attempts of DCC has been controversial. Once satisfactory synchronisation is obtained, sedation or anaesthesia is initiated, and a shock is delivered. After shock delivery, if conversion is unsuccessful, higher energy repeat direct current cardioversion is attempted. This can be repeated until the arrhythmia terminates or the decision is made to abandon direct current cardioversion. Using a monophasic waveform, the energy required will often be >200 joules (21), with possibly more being required in obese patients and long-standing atrial fibrillation (22,23). However, biphasic devices have been shown more effective in two randomised clinical trials and to require less energy delivery than monophasic devices (19,24). Higher biphasic energies as a first direct cardioversion shock for patients who are overweight or have a higher transthoracic impedance might be recommended. The Best-AF trial demonstrated that, when biphasic waveforms were used, there was a significant increase in the first-shock success for direct current cardioversion if the initial energy selected was 200 J rather than 100 J (25). In patients who were overweight or obese, first-shock success was significantly greater if a higher-energy shock was selected. However, in patients with a normal or low body mass index there was no difference in the first-shock success regardless of whether 100 J or 200 J was used. The initial energy may be lower for cardioversion of atrial flutter (25). In a review including 985 cardioversions in 840 patients with atrial flutter, the median energy level for successful cardioversion was 50 joules with a biphasic defibrillator and 200 joules with a monophasic defibrillator (26).
Lown et al. recommended an anteroposterior electrode configuration over anteroanterior positioning (27,28) however the two current conventional orientations that are commonly used for electrode placement are anteroposterior and anterolateral. A number of studies have examined the effect one over the other and showed that less energy is required and higher success rates are achieved when using the anteroposterior position (20-32). Some reports, nevertheless, have failed to confirm these findings (33,34). In some patients one position and not the other, may be effective; thus, it has been suggested that if initial shocks are unsuccessful in terminating the arrhythmia, the electrodes should be relocated and cardioversion repeated (31). Thus when cardioversion fails, shocks can be repeated at highest energy until the arrhythmia terminates or a decision is made to abandon direct current cardioversion. Repositioning the paddles should also be done in case of failure. Furthermore, the double-paddle technique is another alternative as well as pharmacologic facilitation of cardioversion: in one study, patients who had AF and had failed 360-J monophasic cardioversion were loaded with amiodarone orally. If repeat 360-J monophasic cardioversion persisted in failing, the patients underwent the double-paddle technique: two monophasic defibrillators were used with two sets of paddles for each patient; each defibrillator was set for a synchronous shock at the maximum output of 360 J; they then were discharged simultaneously, resulting in successful conversion of 13 out of 15 patients (35).
Internal cardioversion is performed with the patient under conscious sedation or general anaesthesia. Because of the potential risk of bleeding, warfarin therapy is usually withheld in view of the procedure and resumed afterward. Temporary anticoagulation before and after the procedure can be accomplished with heparin. Internal cardioversion is indicated:
Despite such significant advantages, the spread of this new methodology in clinical practice has been limited by the need of a laboratory of electrophysiology with fluoroscopy and of specific technical competence for lead positioning, either in the coronary sinus or in the left pulmonary artery. Simplification of the procedure is obviously very important for the future of internal cardioversion, particularly because of the recent improvement in success rates of external cardioversion using biphasic shocks, higher energy shocks or pre-treatment with drugs before cardioversion (24,42,43).
Despite its widespread clinical use, controversy surrounds the electrophysiologic mechanisms by which direct current cardioversion terminates atrial fibrillation involving multiple microreentrant circuits. Most investigators agree that defibrillation occurs when a certain amount of current density reaches the myocardium. However, it is unclear what amount of current density is needed and what energy setting is necessary to achieve homogeneous current density. Complications associated with direct current cardioversion are mainly risks related to general anaesthesia, thromboembolic events and postcardioversion arrhythmias. Overall however, risk is low in patients who are selected adequately (44) - only a 1–2% risk of thromboembolic events (45-47). Thromboembolic events, nevertheless, are more likely to occur in patients with atrial fibrillation who have not been anticoagulated prior to cardioversion. Patients with a previous embolism do not have an increased risk of embolisation if anticoagulation is adequate (48). The estimated incidence of thromboembolism varies, but in a large non-randomised series that included 437 patients, embolism occurred in 5.3 % of non-anticoagulated patients compared to 0.8 % of those receiving anticoagulation (49). Many kinds of arrhythmias, especially ventricular and supraventricular premature beats, bradycardia, sinus arrest, may arise following cardioversion and commonly subside spontaneously (50). More dangerous arrhythmias, such as ventricular tachycardia and fibrillation, may arise in the face of hypokalemia, digitalis intoxication, or inadequate synchronisation (51,52).
Patients with Implantable Devices
Patients who have implanted permanent pacemakers or cardioverter-defibrillators can undergo external cardioversion with minimal risk to their devices and themselves, provided appropriate precautions are taken. Devices are typically implanted anteriorly, so the electrode paddle should be at least 8 cm from the pacemaker battery and an anteroposterior paddle position is recommended (2,53,54). Elective cardioversion should be begun with low-energies in order to avoid damage to the pacemaker circuitry and the electrode-myocardial interface. After cardioversion, the pacemaker should be interrogated and evaluated to ensure normal pacemaker function (2).
Pretreatment with Antiarrhythmic DrugsPretreatment or repeat treatment with antiarrhythmic drugs such as ibutilide, amiodarone, sotalol, propafenone or flecainide increases the likelihood of restoration of sinus rhythm and helps prevent recurrent atrial fibrillation (2). Enhanced efficacy may involve decreasing the energy required to achieve cardioversion, prolonging atrial refractory periods, and suppressing atrial ectopy that may cause early recurrence of atrial fibrillation (55-55). Antiarrhythmic medications may be initiated out of hospital or in hospital immediately prior to direct-current cardioversion.
Direct current cardioversion is an effective means of restoring sinus rhythm in patients with atrial fibrillation - improving patient outcomes is thus usually in our hands. Attention to proper technique for direct current cardioversion has the power to optimise efficacy.
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Prof. Bulent-Gorenek, Ankara, Turkey.Authors' disclosures: None declared.