During defibrillation, a randomly timed high-voltage electric current is discharged across two electrodes placed on the chest of a patient in cardiac arrest. The purpose of defibrillation is to simultaneously depolarize a large critical mass of myocardium. As a result, nearly all ventricular myocytes will enter their absolute refractory periods, when no action potentials can be generated. Successful defibrillation means that the reentry focus underlying the ventricular dysrhythmia is now either quiescent or eliminated. At this point, the pacemaker with the highest automaticity (such as the sinus or atrioventricular nodes) will take over control of ventricular pacing and contraction with a proper sequence of depolarization and repolarization.
Successful defibrillation occurs when ventricular fibrillation (VF) has terminated for at least 5 seconds following the shock. It is still considered shock success even if the postshock rhythm is nonperfusing, such as asystole, or if hemodynamics remain unstable. The definition of successful defibrillation is independent of resuscitation measures such as return of spontaneous circulation, survival to hospital discharge, and neurologic outcome.
A number of variables can affect the likelihood of terminating VF via an electrical current. Time is perhaps the most important. The probability of successful defibrillation decreases, the longer the patient remains in a pulseless dysrhythmia. Higher success rates have been noted if the underlying cause is ischemic in nature, such as an acute myocardial infarction. Nonischemic causes of cardiac arrest (such as tamponade, tension pneumothorax, pulmonary embolus, hypovolemia, hypoxemia, acidosis, and electrolyte abnormalities) have lower defibrillation success rates. Measures to decrease the transthoracic impedance against an electric current can also improve the chance of successful defibrillation. These methods include applying firm pressure (at least 25 lb) on the paddles, using proper paddle sizes and conductive gel, defibrillating during end-expiration, and using “stacked” shock strengths with a higher frequency.
VF and pulseless ventricular tachycardia (VT) are the primary indications for electrical defibrillation. These dysrhythmias are rarely spontaneously reversible and will often deteriorate into asystole if the underlying reentry circuit is not eliminated. Rapid defibrillation is absolutely essential to restore spontaneous circulation promptly and to achieve the best possible neurologic outcome. For pulseless VT, whether monomorphic or polymorphic, the shock must be “unsynchronized” to achieve proper defibrillation, as opposed to electrical cardioversion.
Contraindications to defibrillation include pulseless electrical activity (PEA) and asystole, the two major “nonshockable” cardiac arrest rhythms. A patient with VT who has a pulse and a stable perfusing rhythm should not receive defibrillation. A patient with VT who becomes unstable with evidence of decreased cardiac output should receive synchronized cardioversion. Defibrillation should not be performed if there is any danger to the rescuer or patient. For instance, excessive moisture on the patient’s chest could lead to improper current distribution, or a patient lying in a wet environment could increase the risk of electrical injury to the bystanders.