What is the difference between cardioversion and defibrillation? Synchronized cardioversion involves the delivery of a low-energy shock which is timed or synchronized to be delivered at a specific point in the QRS complex (see the image below). A synchronized shock is delivered at this precise moment to avoid causing or inducing ventricular fibrillation.6 In this example, atrial fibrillation (AF) was converted to normal sinus rhythm (NSR) by the delivery of the electrical shock synchronized to the R wave on the QRS complex. The vulnerable T wave was avoided, which could convert this rhythm to ventricular fibrillation if the shock was delivered at that moment.
The reason a shock must be synchronized is that the cardiac cycle has both a vulnerable and a refractory period. The refractory period occurs during the QRS complex (see the image below).
The T wave is considered the vulnerable period, especially the middle and second half of the T wave. By timing the shock to be delivered during the QRS complex, electrical stimulation is avoided during the vulnerable period, which reduces the risk of inducing ventricular fibrillation.
Defibrillation involves the delivery of a high-energy shock without the need to time the shock to the unstable rhythm (see the image below). In this example, the delivered shock is not synchronized with the ECG because the rhythm is unstable and there is no apparent QRS complex or T wave to avoid. The rhythm was converted to NSR after the shock was delivered.
Both defibrillation and synchronized cardioversion impose a therapeutic dose of electrical energy on the myocardium. Defibrillation is used to treat certain types of arrhythmias (ventricular fibrillation and pulseless ventricular tachycardia). Synchronized cardioversion is used to treat other arrhythmias, including atrial fibrillation (AF), atrial flutter, and stable ventricular tachycardia when medications have failed to convert the rhythm, or when the patient is becoming unstable and the rhythm must be immediately terminated.
AF is well recognized to be the most common dysrhythmia worldwide, and its prevalence is estimated to increase in the coming years, resulting in an increase of admission to the emergency department for symptoms related to this cardiac rhythm abnormality.7 Transthoracic electrical cardioversion remains the preferred method for restoring sinus rhythm in patients with AF.3 Direct current cardioversion (DCCV) is still rooted within clinical guidelines as an important management option for AF especially in those with symptomatic and persistent AF.4
Though advances such as the advent of biphasic waveform shocks and increasingly refined patient selection for rhythm control strategies have improved electrical cardioversion success rates for AF, potentially higher risk options, including pretreatment with antiarrhythmic agents and delivering higher energy shocks, are often required or contemplated for those in whom the procedure fails.8
Before performing synchronized cardioversion, it is essential that appropriate precautions and steps be taken to increase the likelihood of a positive outcome. The first step is to identify the patient’s rhythm on the monitor. Take time to obtain a 12-lead EKG if the patient is stable and there is any doubt about the patient’s rhythm. Since cardioversion is painful, the patient will need to be properly sedated with intravenous medication. There is a wide diversity regarding the medication strategy to facilitate cardioversion, including hypnotic agents, sedative agents, and additional analgesics. The agent of choice is determined by many factors including the duration of sedation and recovery, the desired depth of sedation, the likelihood of undesirable cardiorespiratory effects, or recall of the procedure.5
Emergency equipment should be made ready, such as a suction device and bag-mask device in case manual ventilation is needed. Additional airway management equipment should be easily accessible, such as an oral airway and intubation equipment. Some hospitals also have sedation reversal medications on standby to be used if needed.
If necessary, the patient’s chest hair should be shaved at the site where the electrodes will be placed. Follow hospital policy regarding placing the patient on supplemental oxygen prior to the procedure. Some facilities may also use a CO2 monitor during conscious sedation for cardioversion. If the patient is not placed on oxygen, it should be ready if required.
After an IV has been started, administration of appropriate sedation medications should be given before the synchronized cardioversion takes place. Electrodes should be placed below the clavicle on the right side of the chest and about two inches below the mid-axillary line beside the nipple on the left side.
The “SYNC” button on the defibrillator machine should be pressed. Review the rhythm strip to ensure the R wave is being marked and sensed by the machine. This will ensure the shock is delivered at the appropriate time on the QRS complex, and not depolarizing on the T wave. Select the appropriate level of energy and follow standard precautions to “clear” the patient before delivering a shock. Expect a slight delay in the delivery of the shock as the machine times its delivery to the QRS complex.
After the shock, reassess the patient’s rhythm. If the patient has not converted and a second shock is indicated, you will again need to push the SYNC button, as the machine will default to defibrillation mode. Follow the cardioversion procedure detailed above. Continue to monitor the patient’s level of consciousness and vital signs. The patient should be closely monitored until he/she is awake and vital signs are stable.
In some instances, it may be necessary to make a few adjustments if something is not working as it should during a cardioversion. For example, a marker may not appear above the QRS complex because the machine is not sensing it. If this occurs, adjusting the amplitude (or gain) is advised. Meticulous procedural technique to maximize electrical cardioversion success is, therefore, imperative as evidenced by efforts to identify optimal electrode placement, initial energy settings, and modifiable chest wall electrode interface characteristics.1 The goal of such efforts is ultimately to increase the current delivered to the myocardium in order to depolarize the critical mass required for cardioversion by altering one or both of its major determinants: the operator-selected defibrillator energy level and the transthoracic impedance (TTI).10 Though many contributors to TTI are unmodifiable patient characteristics, applying force to paddle electrodes has been shown to reduce TTI by improving electrical contact at the electrode/skin interface and by decreasing thoracic volume.8
Although synchronized cardioversions are often performed without complications, they can occur. Cardiac complications can include hypotension and dysrhythmia, including ventricular fibrillation or asystole. In some cases, a patient may not be adequately breathing on their own and may need to be manually ventilated. That’s why emergency equipment should always be available.