Defibrillation and Cardioversion

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.

  Cardioversion is a synchronized administration of shock during the R waves or QRS complex of a cardiac cycle.

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

Indications for defibrillation include the following:

• Pulseless ventricular tachycardia (VT)
• Ventricular fibrillation (VF)
• 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 fibrillation
• 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)
• Sedation

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