Electric Cardioversion of Atrial Fibrillation

Abstract

Electrical cardioversion (EC) has been performed for atrial fibrillation (AF) for over 40 years. EC is safe, effective and reliable method for aborting AF especially in unstable situations. Numerous technical and patient characteristics contribute to the success of EC. Recently various developments in this field and newer antiarrhythmic agents have lead to substantial evolution of this procedure. We review the current clinical applications, and techniques.

Introduction

Electrical cardioversion is a safe and effective procedure for the management of patients with tachyarrhythmias. First used for the termination of life threatening ventricular arrhythmias, EC for atrial fibrillation was met with considerable controversy in the 1960s. The procedure was widely adopted once its safety and efficacy was established over 40 years ago ¹ In recent years electrical cardioversion techniques and applications have significantly evolved. The purpose of this review is to present contemporary and practical aspects of EC for atrial fibrillation.

Indications for EC in AF

AF is the most frequent electrically converted arrhythmia. Indications for urgent EC in patients with AF include active myocardial ischemia, significant hypotension, acute heart failure and the presence of accessory pathways with preexcitation. Elective cardioversion for AF may be performed as part of a long term rhythm control strategy- especially for symptomatic paroxysmal AF. Figure 1 outlines the broad principles guiding acute or persistent AF EC in clinical practice.

Figure 1.

Algorithm outlining principles of electrical cardioversion (EC) in acute and persistent atrial fibrillation (AF). CHF denotes congestive heart failure, AMI acute myocardial infarction, CC chemical cardioversion using antiarrhythmic agents, TEE transesophageal echocardiography, INR international normalized ratio.

Predictors of Cardioversion Success: Technical Factors

The major factors which determine success with EC include transthoracic impendence (TTI), electrode position, type of waveform used in addition to patient specific factors.

Transthoracic impedance (TTI) is resistance to flow of current between the two chest electrodes. Higher impedance yields lower delivered currents. The thoracic cage shunts 82% of the transthoracic current, while 14% is shunted by the lungs and only 4% of the delivered current eventually passes through the heart. The average adult human impedance is approximately 70 to 80 ohms (Ω). The determinants of TTI include: energy selected, electrode size, paddle-skin coupling material, number and time interval of previous shocks, phase of ventilation, distance between electrodes (size of the chest), and paddle electrode pressure.

The larger the electrode the lower is the impedance. As the electrode size increases the current density for a given energy level decreases. Therefore oversized electrodes may provide insufficient current density, thus reducing the likelihood of successful cardioversion or defibrillation. Conversely, undersized electrodes may cause myocardial injury from exposure to excessive current density. There is no firm data regarding optimal paddle size for cardioversion of AF, but a diameter of 8–12 cm is generally recommended.²

TTI decreases with firm pressure on electrodes and with the use of electrolyte-impregnated pads. Repeated direct current counter shocks result in progressively decreased transthoracic apparent impedance, a relationship which is dependent on the time interval between countershocks. In one study it was demonstrated that this decrease was considerably greater at 3-minute intervals when compared to 15-second and 1-minute intervals.³ Also as air is a poor conductor of electricity, TTI decreases during expiration.

Electrode Position

Optimal electrode position is essential for the delivery of sufficient current and uniform current density to depolarize the critical mass. The anteroposterior (AP) position has been found to provide a more uniform current density through the atria than the anterolateral (AL) position. While some studies concluded that the AP electrode position is superior to the AL, others have found no difference in success rate. Thus we should consider an alternative arrangement if the initial position proves unsuccessful. One recent study suggests AP may be the preferred position as mean energy requirements and CPK levels were lower.²

Electrode Type

Hand-held paddles achieve a lower TTI when compared to self-adhesive pads at the same energy level due to the firmer application onto the patient. This likely translates into improved transmyocardial current delivery. In a study by Kirchhof et al, hand-held paddle electrodes yielded higher success rates during EC for AF.⁴

Biphasic vs. Monophasic Defibrillators

For decades defibrillators delivered current in one direction, and were termed monophasic. See Figure 2. The main limitation of the monophasic waveform (MPW) is that it does not compensate for TTI. Thus, at any given energy level, patients with different TTIs would receive different transmyocardial currents. Patients receiving higher currents were at risk for myocardial injury and post-shock cardiac dysfunction whereas patients receiving lower currents were at risk for defibrillation failure.

Figure 2.

Comparison of biphasic truncated exponential waveform vs monophasic damped sine waveform.

CHEST by Tang W. Copyright 2001 by AM COLLEGE OF CHEST PHYSICIANS. Reproduced with permission of AM COLLEGE OF CHEST PHYSICIANS in the format Journal via Copyright Clearance Center.

Biphasic waveforms (BPW) for external defibrillators were FDA approved in 1996. These devices reverse polarity 5–10 milliseconds after the initial discharge. The device also measures each patient’s TTI and compensates for variation. Depending on the type of BPW used, the compensation for different TTI occurs by either modifying the percentage of first phase of the electric shock’s duration or by modifying the resistor network within the device. Because of its ability to compensate for TTI, BPWs are able to defibrillate more effectively and at lower energies than MPW.⁵ For cardioversion of AF, several studies have demonstrated that BPW have greater first shock efficacy, require fewer total shocks, and impose lower energy requirements. For these reasons the most recent ACC/AHA/ESC guidelines consider BPW as the standard modality for AF EC.²

Cardioversion Energy

The optimal initial energy for cardioversion of AF is still a matter of debate. Traditionally a step up approach has been used with an initial energy of 100 Joules (J). If the initial shock fails, the energy would be increased in 100J increments up to a maximum of 400J. Multiple studies however have shown that using low initial energy may not be successful for persistent AF.

A study by Gallagher et al concluded that conventional protocols using MPW starting at 100J can often restore sinus rhythm if AF duration is < 30days. In AF of longer duration success was unlikely with energy < 200J. The study recommended an initial energy ≥200J for patients with AF > 30days and ≥300J if AF > 180 days and in heavy persons.⁶ Current ACC/AHA/ESC guidelines recommend an initial energy of ≥200J when using MPW and also 200J for BPW EC.²

There is also debate on the number of shocks that maybe administered before labelling AF as refractory to EC. Currently there are no firm data to determine the number of shocks that can be safely delivered during EC.

Special Situations

Digitalised Patients

Digitalis toxicity is a contraindication for EC because of the increased risk of developing malignant ventricular tachyarrhythmias². It is crucial to exclude any clinical or ECG signs of toxicity. Patients receiving digoxin without clinical evidence of digitalis toxicity are at low risk for serious post cardioversion ventricular arrhythmias, even when serum digoxin levels are modestly elevated.⁷

Patients with Pacemakers or Defibrillators

Pacemakers and defibrillator circuits are designed to be protected from external electrical surges. Programmed data can be altered by these discharges and electricity propagating through the pacemaker’s electrodes can cause endocardial injury with resultant loss of ventricular capture. To avoid such complications it is recommended to place the external defibrillator electrodes in AP position in order for it to be as far as possible from the pacemaker or defibrillator generator.² Moreover, to ensure appropriate function, the implanted device should be interrogated and, if necessary, reprogrammed before and after cardioversion.²

Predictors of Cardioversion Success: Clinical Features

Predicting the outcome of cardioversion for AF was the goal of many studies. Many potential predictors were suggested including duration of AF, patient’s age, underlying heart disease and left atrial size. Restoration and maintenance of sinus rhythm is more likely in patients with AF of short duration.⁸ Different studies looked into different time intervals between onset of arrhythmia and defibrillation. Based on those studies it’s believed that long term maintenance of sinus rhythm after EC is less likely in patients with AF of more than one year duration.⁹ Older age is associated with higher recurrence rate of AF following EC.¹⁰ Underlying hypertensive heart disease or rheumatic heart disease decrease the likelihood of restoring and maintaining sinus rhythm following EC for AF.¹¹ The effect of other factors such as left ventricular function and presence of coronary artery disease on cardioversion outcome is still debated. Conflicting evidence exists in the literature on the effect of left atrial size on the immediate and long term outcome of EC for AF. Some studies suggest that increased left atrial size predicts high recurrence rate.¹⁰ Other studies concluded that the size of left atrium carries no prognostic significance.¹²

What if the Initial Shocks Fail?

The immediate success rate of external cardioversion (EC) varies between 70 and 99%. This is believed to be because of considerable difference in patient characteristics and the different type of waveforms used in different studies.² Patients who have failed conventional cardioversion may convert to sinus rhythm with other therapeutic options like pharmacologic facilitation of EC, high-energy cardioversion and internal cardioversion.

Pharmacologic Facilitation

Antiarrhythmic medications increase the success of EC probably through prolongation of atrial refractory periods, suppression of atrial ectopy and lowering of cardioversion threshold. Both amiodarone and ibutilide increase the success rate of EC as well as suppress early recurrences.¹³ Vernakalant hydrochloride was recently demonstrated in a phase 3 trials to be effective in rapid conversion of short-duration AF and is well tolerated.¹⁴ Current ACC/AHA/ESC guidelines recommend treatment with pharmacologic agents in patients who fail to respond to direct current cardioversion and in those who develop immediate or subacute recurrence. The guidelines considered such pre-treatment optional in patients who develop late recurrence or those undergoing initial EC.²

High Energy Cardioversion

High energy cardioversion using a quadruple pad approach was also suggested. In one study, patients who failed EC using the conventional approach underwent cardioversion with quadruple electrode position (two in anteroposterior and two in apical posterior position). 74% of the patients who failed the conventional approach were successfully cardioverted with the quadruple approach. Overall, EC success rate increased from 48% using the conventional approach to 89% using the quadruple pads approach.¹⁵

Internal Cardioversion

Internal cardioversion (IC) is another technique that can be employed in patients who fail external cardioversion. It involves delivering a shock between two electrodes where both may be internal or one internal and a back plate. Studies have demonstrated that this technique is safe despite requiring transfemoral venous access for placement of the internal lead in the right atrium. Internal cardioversion (93%) was found to be more effective in terminating AF compared to external EC (79%).¹⁶ Bradycardia may require ventricular pacing after IC. Low cardiac output from ventricular stunning may also occur. Because of the potential complications of this procedure and the associated cost, external EC remains first line in AF termination. In general, IC may be considered in patients with AF refractory to EC, obese patients, patient with severe COPD and patient with implanted pacemakers or defibrillators.¹⁷

Recurrences of AF After Electric Cardioversion

Most recurrences of AF occur within the first month after direct-current cardioversion.² In a study by Tieleman et al, 61 patients were followed for one month following successful EC with daily evaluation of their heart rhythm. During the period of follow up 57% developed recurrence of AF, 63% of those recurred within the first 5 days.¹⁸ Wijfells documented that artificial maintenance of AF leads to a marked shortening of atrial effective refractory period and an increase in rate, inducibility and stability of AF. All these changes were completely reversible within 1 week of sinus rhythm. Hobbs demonstrated that changes in atrial electrophysiology associated with AF in humans are reversible after cardioversion and that the extent of this reversal is dependent on the duration of sinus rhythm after cardioversion.

Animal studies also suggest that intracellular calcium is important in the electrical remodeling of the atria in AF. The use of calcium channel blockers during AF was a significant predictor of maintenance of sinus rhythm after EC of AF.¹⁸ Verapamil in combination with propafenone prior to EC was associated with reduced incidence of early recurrence of AF (ERAF) compared to propafenone only. Different antiarrhythmic medications were shown to decrease the likelihood of ERAF. Amiodarone use post EC was shown to increase the likelihood of maintaining sinus rhythm and reduced supraventricular ectopic activity that may trigger recurrent AF. The addition of enalapril to amiodarone may decrease the rate of ERAF after EC compared to amiodarone alone. Propafenone, quinidine and metoprolol XL were also shown to decrease the risk of ERAF.

As for late recurrence, the appropriateness of using antiarrhythmic medication to prevent these recurrences is questionable. One metanalysis evaluated long term quinidine use to prevent recurrence. It found quinidine more effective than no antiarrhythmic therapy in suppressing recurrences of AF but was associated with a trend towards increased total mortality.¹⁹ The use of methylprednisone tapper dose over period of 5 months was found to decrease likelihood of recurrence for AF.²⁰

Complications of External Electric Cardioversion

Pain and Skin Burns

This develops in about 20–25% of patient. Pain experienced is related to the total energy and number of shocks delivered. To reduce burns, operators should apply optimal paddle force equally to both paddles. The paddles should be applied such that even contact with the skin occurs along their edges. Burns may also be minimized by starting with lower energy shocks.

Arrhythmias

Various benign arrhythmias like ventricular and supraventricular premature beats, bradycardia, and short periods of sinus arrest occur after EC and subside spontaneously. More dangerous arrhythmias, such as ventricular tachycardia and fibrillation, may be precipitated in patients with hypokalemia or digitalis toxicity. Bradyarrhythmias following DC cardioversion for atrial tachyarrhythmia are uncommon. A case series reported severe bradycardia in patients undergoing cardioversion for AF in the setting of acute inferior myocardial infarction. Interestingly this bradycardia did not respond to IV atropine, and temporary pacing was required.²¹

Electrocardiographic Changes

ST segment and T wave changes can occur after EC. In one study, 19% of patient undergoing elective cardioversion for AF and atrial flutter developed transient ST segment elevation. Serum cardiac markers in those patients did not reveal evidence of myocardial damage. Conversion rates and long term maintenance of sinus rhythm was lower in patients who developed ST segment elevation. Ben-Dov et al reported two cases of simultaneous echocardiography during post cardioversion ST segment elevation. They noted normal ventricular function and no wall motion abnormalities, suggesting these ECG changes are benign.²²

Myocardial Injury

Early studies suggest that DC counter shock may precipitate some degree of myocardial necrosis, especially following intense and repetitive shocks. These conclusions are mainly based on elevations in CK and CK-MB following shocks. Subsequent studies utilizing Troponin I and T levels as markers for myocardial necrosis report no increase after EC. The increases in CK and CK-MB in prior studies was likely secondary to skeletal muscle injury rather than myocardial injury.

Thromboembolization

In a large nonrandomized series the estimated incidence of thromboembolism in patients undergoing EC for AF was 5.3 percent of non-anticoagulated patients compared to 0.8 percent of those receiving anticoagulation. The conventional approach for anticoagulation for patients with AF more than 48 hours is to anticoagulate three weeks prior to scheduled cardioversion and four weeks afterwards. Anticoagulation can be initiated with unfractionated heparin followed by an oral anticoagulant or enoxaparin may be used although there is no direct data in AF.

Over the past few years transesophageal echocardiography (TEE) guided EC evolved as a reasonable alternative to EC following conventional anticoagulation. The use of this approach was proposed as a means to safely expedite cardioversion by excluding atrial thrombi. The ACUTE trial was a multicenter, randomized clinical trial that randomized 1,222 patients with AF of more than two days duration into either TEE guided anticoagulation group or conventional anticoagulation group. At 6 month follow up, there was no difference in composite embolic events, all cause mortality or occurrence of normal sinus rhythm between the two groups. The bleeding rate was significantly lower in TEE group. It was concluded that TEE guided anticoagulation strategy is a clinically effective alternative to conventional anticoagulation strategy.²³

The routine use of TEE for elective EC for AF raised concerns about its economic burden. Economic analysis from ACUTE trial data gathered at eight weeks follow up found the cumulative costs were 24% higher in the conventional group, primarily due to bleeding complications. The analysis concluded that TEE-guided strategy is an economically feasible approach compared with the conventional strategy. Other authors however argue the costs of TEE guided anticoagulation are modestly higher than the costs of conventional anticoagulation. We believe the decision whether to use TEE guided anticoagulation versus conventional approach should be tailored to each patient, taking into consideration the patient’s preference and bleeding risk and provider expertise in performing TEE.

Conclusion

Electrical cardioversion is a safe, effective and reliable method of converting AF especially in unstable patients. Advances in EC and effective pharmacological agents are likely to further improve success in attaining and maintain sinus rhythm in a wider spectrum of AF patients. Established guidelines are available, but given the variability in clinical presentation, comorbidity, compliance and logistical differences, AF will require individualized therapy to optimize success.

Biography

Maen Nusair, MD, Greg C. Flaker, MD, and Anand Chockalingam, MD, are at the University of Missouri Hospital and Clinics, Division of Cardiovascular Medicine. Dr. Chockalingam is also in the Cardiology Section at the Harry S Truman VA Medical Center in Columbia.

Contact: nusairm@health.missouri.edu