Higher Energy External Cardioversion for Refractory Atrial Fibrillation
abstract & commentary
Synopsis: A limited number of high-energy shocks can cardiovert many patients resistant to standard approaches with a low risk of complications. This is an alternative to intra-atrial cardioversion.
Source: Saliba W, et al. J Am Coll Cardiol 1999;34: 2031-2034.
Saliba and colleagues report a new technique for cardioversion of patients resistant to transthoracic cardioversion using standard approaches of up to 360 joules (J). Saliba et al linked two external defibrillators together so they could be synchronized to the patient’s electrocardiogram. Patches from each defibrillator were then placed in adjoining anteroposterior positions on the patient’s chest. Once it was seen that the defibrillators were synchronizing to the same portion of the electrocardiographic wave (QRS) complex, a single operator depressed the energy delivery button, simultaneously delivering a 720-J shock.
Over a three-year period, 55 patients at Saliba et al’s institution underwent a 720-J cardioversion attempt. The group included patients both with and without structural heart disease, but 85% were male. Only 12 of the patients had continuously been in atrial fibrillation for more than one year. All patients were anticoagulated with either warfarin (INR > 2) or heparin. Forty-eight (87%) patients were taking antiarrhythmic medications at the time of cardioversion. The antiarrhythmic medications included: amiodarone (41%), sotalol (20%), flecainide (20%), procainamide (4%), and disopyramide (2%).
Sinus rhythm was achieved in 46 of the 55 patients (84%). However, atrial fibrillation recurred in 28 of these patients, with 27 of the recurrences noted within 90 days.
There were no acute hemodynamic complications associated with the procedure. One patient developed transient post-shock bradycardia. Another patient developed right bundle branch block and later that night had an episode of torsades de pointe.
Saliba et al conclude that a limited number of high-energy shocks can cardiovert many patients resistant to standard approaches with a low risk of complications. They offer this as an alternative to intra-atrial cardioversion, a procedure that requires placement of intracardiac electrodes and may be hazardous in an anticoagulated patient.
Comment by John P. DiMarco, MD, PhD
About 15% of patients undergoing elective transthoracic cardioversion of atrial fibrillation are not restored to sinus rhythm using a protocol with energy levels between 200-360 J. This paper describes an approach that may be effective without the necessity for an invasive procedure.
The major reason why high-energy shocks have not been advocated is the potential risk of myocardial damage and arrhythmias associated with these shocks. In animal models, repeated high-energy shocks produce myocardial dysfunction and cellular damage. In humans, shocks well above the defibrillation threshold may cause excess bradycardia. However, failure to cardiovert atrial fibrillation is usually due to a high transthoracic impedance that prevents an adequate current from reaching the heart. This high impedance may be due to patient size, anatomical features, or electrode characteristics. Therefore, use of higher energy after an initial failure with standard settings is less likely to have severe consequences. If large surface area electrodes are carefully applied, skin irritation should also be minimized.
Other alternatives for converting atrial fibrillation in resistant patients are also available. Sotalol and ibutilide may lower the atrial defibrillation threshold. Oral and colleagues recently reported a 100% success rate if patients were pretreated with ibutilide before the cardioversion attempt (Oral H, et al. N Engl J Med 1999;340:1849-1854). If the cardioversion is unsuccessful due to early or immediate recurrence of atrial fibrillation, any antiarrhythmia may be helpful. Biphasic defibrillation waveforms lower ventricular defibrillation thresholds and will probably be more effective for atrial defibrillation as well. New defibrillator models with biphasic waveforms are now being produced. In the future, it may be possible to analyze the atrial fibrillation waveform after subtraction of ventricular events to allow optimal timing of the shock. If these noninvasive approaches fail, intra-atrial shock delivery is still an option.
The major limitation of all these techniques is the high late recurrence rate after successful cardioversion. No strategy for maintaining sinus rhythm is uniformly safe and highly effective. In many patients, a strategy of rate control and anticoagulation will eliminate symptoms and avoid the risks and inconvenience of cardioversions and antiarrhythmic drug therapy.
The frequency of converting atrial fibrillation to sinus rhythm may be increased by:
a. antiarrhythmic drugs.
b. higher energy shocks.
c. biphasic defibrillators.
d. All of the above