You are in: eMedicine Specialties > Cardiology > Electrophysiology Procedures Synchronized Electrical CardioversionArticle Last Updated: Jul 18, 2006AUTHOR AND EDITOR INFORMATIONSection 1 of 8
  Author: Vijai V Chauhan, MD, Assistant Professor, Department of Surgery, Division of Emergency Medicine, St Louis University School of Medicine Vijai V Chauhan is a member of the following medical societies: Society for Academic Emergency Medicine Coauthor(s): Antonella Quattromani, MD, Medical Director of Electrophysiology, Associate Professor of Medicine, Department of Medicine, Division of Cardiology, St Louis University Hospital Editors: Russell F Kelly, MD, Program Director, Assistant Professor, Department of Internal Medicine, Division of Cardiology, Cook County Hospital, Rush Medical College; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; Steven J Compton, MD, FACC, FACP, Director of Cardiac Electrophysiology, Alaska Heart Institute, Providence and Alaska Regional Hospitals; Amer Suleman, MD, Consultant in Electrophysiology and Cardiovascular Medicine, Department of Internal Medicine, Division of Cardiology, Medical City Dallas Hospital; Leonard Ganz, MD, Associate Professor of Medicine, Temple University School of Medicine; Cardiac Electrophysiologist, Children's Hospital of Pittsburgh, University of Pittsburgh Medical Cent, West Penn Hospital Author and Editor Disclosure Synonyms and related keywords: synchronized electrical cardioversion, defibrillation, ventricular dysrhythmia, atrial fibrillation, ventricular fibrillation, synchronized direct current shocks, VF, paroxysmal supraventricular tachycardia, PSVT, ventricular tachycardia, VT, defibrillators, implanted cardioverter/defibrillators, ICDs, transesophageal echocardiography, TEE, radiofrequency catheter ablation HISTORY AND BACKGROUNDSection 2 of 8
  In 1775, Abildgaard performed experiments using electricity to stun and revive animals. Beck was the first to successfully use defibrillation on humans to treat ventricular dysrhythmia in 1947. In 1962, Lown evaluated the use of electrical cardioversion in the treatment of atrial fibrillation to a sinus rhythm using synchronized direct current shocks. Cardioversion refers to an electrical energy discharge that is synchronized with the large R or S wave of the QRS complex. Synchronization in the early part of the QRS complex avoids energy delivery in the early phase of repolarization when ventricular fibrillation (VF) can be easily induced. Defibrillation refers to an unsynchronized discharge of energy and is only recommended for VF. For excellent patient education resources, visit eMedicine's Heart Center. Also, see eMedicine's patient education articles Atrial Fibrillation and Supraventricular Tachycardia. BASIC PRINCIPLESSection 3 of 8
Transient delivery of electrical current causes a momentary depolarization of most cardiac cells. This allows the sinus node to resume normal pacemaker activity. In the presence of reentrant-induced dysrhythmia, such as paroxysmal supraventricular tachycardia (PSVT) and ventricular tachycardia (VT), electrical cardioversion interrupts the self-perpetuating circuit and restores a sinus rhythm. Electrical cardioversion is much less effective in treating arrhythmia caused by increased automaticity (eg, digitalis-induced tachycardia, catecholamine-induced arrhythmia). Two types of defibrillators are in use today. Most use a monophasic sinusoidal waveform (positive sine wave) for external cardioversion and defibrillation, although a recently introduced biphasic truncated waveform defibrillator is now available. Since the development of implanted cardioverter/defibrillators (ICDs), use of biphasic waves has shown that less energy is required to convert an arrhythmia to a normal sinus rhythm. In 1997, a low-energy, impedance-compensating biphasic waveform was evaluated for atrial and ventricular arrhythmia management. This defibrillator automatically adjusts to the patient's transthoracic impedance. EXTERNAL CARDIOVERSIONSection 4 of 8
Indications In an urgent or emergent setting, any patient with reentrant tachycardia having an either narrow or wide QRS complex (ventricular rate >150) who is unstable (eg, chest pain, pulmonary edema, lightheadedness, hypotension) should be immediately treated with synchronized electrical cardioversion. Synchronized electrical cardioversion may be used to treat stable VT that does not respond to a trial of intravenous medications. In hemodynamically stable patients, synchronized electrical cardioversion is also used to electively restore sinus rhythm in atrial fibrillation, atrial flutter, or other supraventricular tachycardia (SVT). Contraindications Contraindications include patients with known digitalis toxicity–associated tachydysrhythmia, patients with sinus tachycardia caused by various clinical conditions, or patients with multifocal atrial tachycardia. Technique Advanced cardiac life support (ACLS) guidelines should be followed as indicated. The key components in preparing the patient are intravenous access, airway management equipment, sedative drugs, and a converter/defibrillator monitoring device. The patient should be adequately sedated with a short-acting agent such as midazolam. In addition, an opioid analgesic, such as fentanyl, is commonly used. Reversal agents, such as flumazenil and naloxone, should be available. The cardioverter should be placed in the synchronized mode, which permits a search for a large R or S wave. The cardioverter automatically discharges an electric current that lasts less than 4 milliseconds and avoids the vulnerable period of cardiac repolarization when VF can be induced. The operator should be aware of this brief delay as the cardioverter searches for a large positive or negative deflection. If deflections are too small, the physician can change the leads or place them closer to the patient's chest or heart. If the patient develops VF, always turn off synchronization to avoid delay in energy delivery. Two options exist for placement of paddles on the chest wall. First is the anterolateral position in which a single paddle is placed on the left fourth or fifth intercostal space on the midaxillary line; the other paddle is placed just to the right of the sternal edge on the second or third intercostal space. The second option is the anteroposterior position. A single paddle is placed to the right of the sternum, as above, and the other paddle is placed between the tip of the left scapula and the spine. Since the skin can conduct away a significant portion of the current, conductive gel or pre-gelled pads are commonly used to ensure good contact. Under ideal circumstances, only 10-30% of the total current reaches the heart. The paddles should be placed firmly against the chest wall to avoid arcing and skin burns. Always be aware of the risk of any staff member touching the patient or the stretcher, bed, or other equipment in which the patient is in contact. Pacemakers and ICDs should be at least 10 cm from direct contact with paddles and should eventually be interrogated for any malfunction after cardioversion. The anteroposterior approach is preferred in patients with implantable devices in order to avoid shunting current to the implantable device and damaging the system. Energy requirements for atrial fibrillation are 100-200 J initially and 360 J for subsequent shocks. A study showed good response to higher energy shocks of 720 J for the treatment of refractory atrial fibrillation (Saliba, 1999). Biphasic shocks require a typical energy level of 75 J for correction of atrial fibrillation. Cardioversion of atrial fibrillation secondary to hyperthyroidism is 90% successful. Only 25% of patients with atrial fibrillation caused by severe mitral regurgitation are successfully treated, and half revert in the first 6 months. Atrial flutter and PSVT require less energy: 50 J initially, then 100 J if needed. Cardioversion of VT involves shocks of 50-100 J initially, then 200 J if unsuccessful. Complications Complications may affect patients or health care workers. Injury incidence is 1 case per 1700 shocks for paramedics in the field. The patient may become hypoxic or hypoventilate from sedation. Most burns from shocks are superficial partial-thickness burns, but a few are deep. Cardiac complications include dysrhythmia, hypotension, and pulmonary edema. Inducible arrhythmias include bradycardia, atrioventricular (AV) block, asystole, VT, and VF. In patients with acute coronary syndromes or acute myocardial infarction, bradycardia or AV blocks can be induced, and they may need an external or internal pacemaker. VT and VF commonly occur in patients with prior similar history. Postcardioversion VF consists of 2 types. The first type occurs immediately after a shock and is related to improper synchronization. This type of VF readily responds to defibrillation (unsynchronized countershock). The second type is related to digitalis toxicity and manifests within a few minutes of cardioversion. Initially, it can be a junctional or paroxysmal atrial tachycardia, then VF, which can be difficult to convert to a sinus rhythm. TREATMENT OF A SPECIFIC ARRHYTHMIASection 5 of 8
Atrial fibrillation If the patient is clinically unstable, emergent cardioversion is recommended. Stable patients should have their ventricular rate controlled, and most should be anticoagulated with intravenous heparin and started on warfarin for stroke prevention because of a high risk of thromboembolism. If a high degree of certainty exists that atrial fibrillation is of less than 48 hours' duration, then a patient can proceed to cardioversion. If the arrhythmia is of uncertain duration or of confirmed duration longer than 48 hours, then the patient can proceed to transesophageal echocardiography (TEE) for the evaluation of a thrombus in atrium or appendage (a suggestion of smoke is considered positive by most authorities). If TEE findings are negative, the patient can proceed to elective cardioversion. Otherwise, patients should be anticoagulated for 3 weeks before cardioversion with a repeat TEE. All patients should be anticoagulated with warfarin for 4 weeks after cardioversion because mechanical function of the atrium lags by up to 7 days after restoring sinus rhythm. If the foregoing treatment fails, patients can be managed with medical treatment alone, repeat cardioversion after antiarrhythmic (eg, ibutilide) treatment, ablation therapy, or atrial defibrillation. Other SVTs At present, recurrent atrial flutter is usually permanently cured by radiofrequency catheter ablation. If the patient is unstable, then cardioversion can be used. Anticoagulation is recommended if external cardioversion is used in the treatment of atrial flutter, but there is a low risk of an atrial thrombus. Other patients with SVT rarely require external cardioversion unless they are unstable. Ventricular tachycardia Patients who do not respond to intravenous medications in treating stable monomorphic VT associated with acute coronary syndrome or acute myocardial infarction should be initially treated with 50- to 100-J synchronized shocks. If no response to low-energy shock is noted, then a 200-J shock should be administered, followed by 300- and 360-J shocks as needed. In unstable VT, unsynchronized shocks should be delivered. Biphasic defibrillators do not require escalating energy, but 3 sequential shocks of 150 J should be used. SPECIAL CONDITIONSSection 6 of 8
In pediatric patients with PSVT or VT who are not hemodynamically stable, an initial synchronized shock of 0.5 J/kg is recommended. In subsequent attempts, the energy is increased. During pregnancy, recommendations for other adults are applicable. INTERNAL CARDIOVERSIONSection 7 of 8
The success of internal defibrillation with low-energy shocks to treat VF and VT resulted in further studies of internal cardioversion for the treatment of atrial fibrillation. Indications Internal cardioversion for atrial fibrillation is used in patients who are resistant to external cardioversion or inadvertently induced during an electrophysiologic study. Cardioversion should occur before placement of an atrial defibrillator. Technique The patient should receive anticoagulation as for external cardioversion, although they should be withheld for safe venous puncture. Various techniques are available; the following is a commonly used procedure. Three temporary catheters are inserted in the venous system and positioned under fluoroscopic guidance. Two catheters of large surface area are used for shock delivery, and a third quadripolar catheter is used for R-wave synchronization and temporary ventricular postshock pacing. The first defibrillation catheter is advanced into the distal coronary sinus; the second is positioned in the right atrium appendix or the lateral wall of the right atrium. These catheters are connected to an external defibrillator that delivers biphasic shocks. The quadripolar catheter is placed in the apex of the right ventricle and is also connected to an external pacemaker. A right atrium-to-coronary sinus cardioversion vector was successfully used with mean of energy of 5.6 +/- 4.7 J (0.4-35) in one study (Levy, 1997). REFERENCESSection 8 of 8
Synchronized Electrical Cardioversion excerpt Article Last Updated: Jul 18, 2006 |
