Defibrillation is nonsynchronized random administration of shock during a cardiac cycle. In 1956, alternating current (AC) defibrillation was first introduced to treat ventricular fibrillation in humans.  Later in 1962, direct current (DC) defibrillation was introduced.  See the video below.
Cardioversion is a synchronized administration of shock during the R waves or QRS complex of a cardiac cycle. See the video below.
During defibrillation and cardioversion, electrical current travels from the negative to the positive electrode by traversing myocardium. It causes all of the heart cells to contract simultaneously. This interrupts and terminates abnormal electrical rhythm. This, in turn, allows the sinus node to resume normal pacemaker activity.
Indications for defibrillation include the following:
Pulseless ventricular tachycardia (VT)
Cardiac arrest due to or resulting in VF
Indications for electrical cardioversion include the following:
Supraventricular tachycardia (atrioventricular nodal reentrant tachycardia [AVNRT] and atrioventricular reentrant tachycardia [AVRT])
Atrial flutter (types I and II)
Ventricular tachycardia with pulse
Any patient with reentrant tachycardia with narrow or wide QRS complex (ventricular rate >150 bpm) who is unstable (eg, ischemic chest pain, acute pulmonary edema, hypotension, acute altered mental status, signs of shock)
Contraindications include the following:
Dysrhythmias due to enhanced automaticity, such as in digitalis toxicity and catecholamine-induced arrhythmia
Multifocal atrial tachycardia
For dysrhythmias due to enhanced automaticity such as in digitalis toxicity and catecholamine-induced arrhythmia, a homogeneous depolarization state already exists. Therefore, cardioversion is not only ineffective but is also associated with a higher incidence of postshock ventricular tachycardia/ventricular fibrillation (VT/VF).
Defibrillation is an emergent maneuver and, when necessary, should be promptly performed in conjunction with or prior to administration of induction or sedative agents.
Cardioversion is almost always performed under induction or sedation (short-acting agent such as midazolam). The only exceptions are if the patient is hemodynamically unstable or if cardiovascular collapse is imminent. For more information, see Procedural Sedation.
Equipment includes the following:
Defibrillators (automated external defibrillators [AEDs], semiautomated AEDs, standard defibrillators with monitors)
Paddle or adhesive patch
Conductive gel or paste
ECG monitor with recorder
Emergency pacing equipment
Blood pressure cuff (automatic or manual)
Oxygen saturation monitor
Code Cart with ACLS (Advanced Cardiovascular Life Support) medications
The use of hand-held paddle electrodes may be more effective than self-adhesive patch electrodes. The success rates are slightly higher for patients assigned to paddled electrodes because these hand-held electrodes improve electrode-to-skin contact and reduce the transthoracic impedance. 
Paddle placement on the chest wall has 2 conventional positions: anterolateral and anteroposterior.
In the anterolateral position, a single paddle is placed on the left fourth or fifth intercostal space on the midaxillary line. The second paddle is placed just to the right of the sternal edge on the second or third intercostal space.
In 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. An anteroposterior electrode position is more effective than the anterolateral position for external cardioversion of persistent atrial fibrillation. [3, 4, 5] The anteroposterior approach is also preferred in patients with implantable devices, to avoid shunting current to the implantable device and damaging its system.
Emergent application, which may be life-saving, and elective cardioversion should be used cautiously, with attention to patient selection and proper techniques. Repetitive, futile attempts at direct current cardioversion should be avoided.
Advanced cardiovascular life support (ACLS) measures should be instituted in preparing the patient, such as obtaining intravenous access and preparing airway management equipment, sedative drugs, and a monitoring device.
For elective procedures, prepare as follows:
Nil per os (NPO) for 8 hours prior to the procedure
Stop digoxin 48 hours prior to the procedure
Continue medications on the morning of the procedure under the direction of the physician
After the procedure, do not drive, operate machinery, or sign important documents for 24 hours and/or until sedation has worn off
There is no patient preparation for emergency procedures.
Defibrillation and cardioversion are demonstrated in the videos below.
Monophasic versus biphasic waveforms
Defibrillators can deliver energy in various waveforms that are broadly characterized as monophasic or biphasic.
Monophasic defibrillation delivers a charge in only one direction. Biphasic defibrillation delivers a charge in one direction for half of the shock and in the electrically opposite direction for the second half.
Newer defibrillators deliver energy in biphasic waveforms. Biphasic waveform defibrillators deliver a more consistent magnitude of current. They tend to successfully terminate arrhythmias at lower energies than monophasic waveform defibrillators. [6, 7, 8]
Energy selection for defibrillation or cardioversion
In 2010, the American Heart Association issued guidelines for initial energy requirements for monophasic and biphasic waveforms. 
Atrial fibrillation energy requirements are as follows:
200 Joules for monophasic devices
120-200 Joules for biphasic devices
Atrial flutter energy requirements are as follows:
100 Joules for monophasic devices
50-100 Joules for biphasic devices
Ventricular tachycardia with pulse energy requirements are as follows:
200 Joules for monophasic devices
100 Joules for biphasic devices
Ventricular fibrillation or pulseless ventricular tachycardia energy requirements are as follows:
360 Joules for monomorphic devices
120-200 Joules for biphasic devices
See the images below.
The most common complications are harmless arrhythmias, such as atrial, ventricular, and junctional premature beats.
Serious complications include ventricular fibrillation (VF) resulting from high amounts of electrical energy, digitalis toxicity, severe heart disease, or improper synchronization of the shock with the R wave. [10, 11]
Thromboembolization is associated with cardioversion in 1-3% of patients, especially in patients with atrial fibrillation who have not been anticoagulated prior to cardioversion. Current American College of Cardiology (ACC)/American Heart Association (AHA) guidelines recommend to anticoagulate for 3-4 weeks before and after cardioversion. The presence of an intracardiac thrombus should be excluded using transesophageal echocardiography prior to cardioversion if therapeutic anticoagulation has not been achieved.
Myocardial necrosis can result from high-energy shocks. ST segment elevation can be seen immediately and usually lasts for 1-2 minutes. ST segment elevation that lasts longer than 2 minutes usually indicates myocardial injury unrelated to the shock.
Myocardial dysfunction is due to an absence of cardiac output and coronary blood flow during arrest, resulting in ischemia. Myocardial dysfunction due to stunning may reverse within first 24-48 hours. Left ventricle function evaluation should be delayed for 48 hours after arrest. 
Pulmonary edema is a rare complication of cardioversion. It is probably due to transient left atrial standstill and left ventricular systolic dysfunction. It is more common in atrial fibrillation due to valvular heart disease or left ventricular systolic dysfunction.
Painful skin burns can occur after cardioversion or defibrillation; they are moderate to severe in 20-25% of patients. They most likely are due to improper technique and electrode placement. It occurs less with use of biphasic waveform defibrillators and use of gel-based pads. Prophylactic use of steroid cream or topical ibuprofen reduces pain and inflammation. [13, 14]
Allergic reaction to sedation medication is a potential complication.
Types of Cardioversion
Antiarrhythmic medications are used to alter flow of electrical activity through the heart. Based on the clinical situation, chemical cardioversion can be performed in the hospital in monitored setting or in an outpatient setting.
This is also known as direct current (DC) cardioversion. Electrical shock is synchronized (perfectly timed) to convert an abnormal rhythm to a normal sinus rhythm. DC cardioversion is performed in the hospital in a monitored setting.
Internal cardioversion is performed less frequently nowadays, owing to the presence of biphasic waveform defibrillators and intravenous ibutilide. It is performed if external cardioversion fails. It is performed using preexisting implantable cardioverter defibrillators (ICDs), epicardial wires during surgery, or internal paddles applied to the epicardium in the presence of sternotomy wires. Advantages of internal cardioversion are that this technique avoids the risk of a skin irritation from external shock. Disadvantages are that it may damage ICD systems, it consumes the battery of the ICD, and it does not always convert atrial arrhythmias.
Cardioversion in patients with digitalis toxicity
Digoxin overdose or toxicity can present with any type of tachyarrhythmias or bradyarrhythmias. Cardioversion in the setting of digoxin toxicity is a relative contraindication. Digitalis sensitizes the heart to the electrical stimulus. Prior to cardioversion, electrolytes should be normalized. Cardioversion may cause additional arrhythmias, especially ventricular fibrillation.
Cardioversion in patients with permanent pacemakers/ICDs
Cardioversion in patients with permanent pacemaker/ICD should be performed with extra care. Improper technique may damage the device, lead system, or myocardial tissue, resulting in device malfunction. The electrode paddle or patch should be at least 12 cm from the pulse generator and anteroposterior paddle position. [15, 16] The lowest amount of energy should be used during cardioversion, based on the patient’s clinical condition. After cardioversion, the pacemaker/ICD should be interrogated to ensure normal function of the device.
Cardioversion during pregnancy
Cardioversion can be performed safely in pregnant women. The fetal heart rate should be monitored during the procedure using fetal monitoring techniques.