Cardioversion and defibrillation
Cardioversion and defibrillation are treatments for tachydysrhythmias. They are used to deliver an electrical current to depolarize a critical mass of myocardial cells. When the cells repolarize, the sinus node is usually able to recapture its role as the heart’s pacemaker. One major difference between cardioversion and defibrillation has to do with the timing of the delivery of electrical current. Another major difference concerns the circumstance: defibrillation is usually performed as an emergency treatment, whereas cardioversion is usually, but not always, a planned procedure. Electrical current may be delivered through paddles or conductor pads. Both paddles may be placed on the front of the chest (Fig. 27-29), which is the standard paddle placement, or one paddle may be placed on the front of the chest and the other connected to an adapter with a long handle and placed under the patient’s back, which is called an anteroposterior placement (Fig. 27-30).
When using paddles, apply the appropriate conductant between the paddles and the patient’s skin. Do not substitute any other type of conductant, such as ultrasound gel.
Defibrillation is used in emergency situations as the treatment of choice for ventricular fibrillation and pulseless VT. Defibrillation depolarizes a critical mass of myocardial cells at once; when they repolarize, the sinus node usually recaptures its role as the pacemaker. The electrical voltage required to defibrillate the heart is usually greater than that required for cardioversion. If three defibrillations of increasing voltage have been unsuccessful, cardiopulmonary resuscitation is initiated and advanced life support treatments are begun. The use of epinephrine or vasopressin may make it easier to convert the dysrhythmia to a normal rhythm with defibrillation. These drugs may also increase cerebral and coronary artery blood flow. After the medication is administered and 1 minute of cardiopulmonary resuscitation is performed, defibrillation is again administered. Antiarrhythmic medications such as amiodarone (Cordarone, Pacerone), lidocaine (Xylocaine), magnesium, or procainamide (Pronestyl) are given if ventricular dysrhythmia persists. This treatment continues until a stable rhythm resumes or until it is determined that the patient cannot be revived.
Cardioversion involves the delivery of a “timed” electrical current to terminate a tachydysrhythmia. In cardioversion, the defibrillator is set to synchronize with the ECG on a cardiac monitor so that the electrical impulse discharges during ventricular depolarization (QRS complex). Because there may be a short delay until recognition of the QRS, the discharge buttons must be held down until the shock has been delivered. The synchronization prevents the discharge from occurring during the vulnerable period of repolarization (T wave), which could result in VT or ventricular fibrillation. When the synchronizer is on, no electrical current will be delivered if the defibrillator does not discern a QRS complex. Sometimes the lead and the electrodes must be changed for the monitor to recognize the patient’s QRS complex. If the cardioversion is elective, anticoagulation for a few weeks before cardioversion may be indicated. Digoxin is usually withheld for 48 hours before cardioversion to ensure the resumption of sinus rhythm with normal conduction. The patient is instructed not to eat or drink for at least 8 hours before the procedure. Gel-covered paddles or conductor pads are positioned front and back (anteroposteriorly) for cardioversion. Before cardio-