Synchronized electrical cardioversion is a medical procedure designed to intentionally interrupt and reset these dangerously fast or irregular heart rhythms (tachyarrhythmias). It involves delivering a controlled, low-energy electrical shock to the chest wall to briefly depolarize the heart muscle, allowing the heart’s natural pacemaker to regain control and restore a normal rhythm. This intervention is specifically reserved for patients whose rapid heartbeat is causing them to become unstable, evidenced by symptoms like low blood pressure, chest pain, or altered mental status.
Understanding the Cardioversion Procedure
Unlike unsynchronized defibrillation, which delivers a shock immediately, synchronized cardioversion uses the heart monitor to detect the QRS complex, which represents the electrical activity of the ventricles contracting. The defibrillator is set to “synchronize” mode, ensuring the electrical shock is delivered a fraction of a second after the peak of the R-wave, the tallest spike in the QRS complex. This mechanism is the reason for the “synchronized” name and is a fundamental safety feature of the procedure.
The need for this precise timing is rooted in avoiding the heart’s “vulnerable period,” which corresponds to the T-wave on the electrocardiogram. The T-wave represents the repolarization, or resting, phase of the ventricles as they prepare for the next beat. Delivering an electrical shock during this vulnerable period can overexcite the heart muscle and inadvertently trigger a far more dangerous rhythm, specifically ventricular fibrillation (V-fib). Ventricular fibrillation is a chaotic, life-threatening rhythm that causes immediate cardiac arrest.
By synchronizing the shock to the R-wave, the procedure ensures the energy is delivered during the heart’s refractory period, a time when the heart muscle is less likely to be thrown into V-fib. Because the heart rhythm is organized enough to have a discernible QRS complex, a lower energy shock is typically effective in resetting the electrical circuit. This contrasts with defibrillation, which uses a higher-energy, unsynchronized shock, primarily for pulseless rhythms like V-fib where there is no organized electrical activity to time the shock against.
Specific Heart Rhythms Treated
Synchronized cardioversion is the treatment of choice for several types of unstable tachyarrhythmias that still maintain a pulse. These rhythms are characterized by an organized, but excessively rapid, electrical pattern that impairs the heart’s pumping ability. The presence of instability—such as persistent low blood pressure or signs of shock—is what immediately mandates this treatment over medication.
One of the most common rhythms treated is unstable Atrial Fibrillation (AFib) with a rapid ventricular response. In AFib, the upper chambers (atria) beat chaotically and rapidly, leading to an irregular and fast ventricular rate, often exceeding 150 beats per minute. This fast, irregular rate prevents the ventricles from filling completely, severely dropping the patient’s blood pressure and causing symptoms. Synchronized cardioversion aims to terminate the chaotic electrical activity in the atria, allowing the heart’s natural pacemaker to restore a regular and slower rhythm.
Another key rhythm is Atrial Flutter (A-flutter), which is an organized, rapid rhythm in the atria, typically around 300 beats per minute. While more organized than AFib, the rapid rate still compromises blood flow, leading to patient instability. Atrial flutter often requires lower energy levels than AFib for successful cardioversion.
The procedure is also used for unstable Monomorphic Ventricular Tachycardia (V-tach) when the patient still has a pulse. V-tach is a fast, regular rhythm originating in the ventricles, the heart’s main pumping chambers. Because the heart is beating so fast and inefficiently, it cannot supply the body with enough blood, causing the patient to become unstable. It is imperative that the patient has a pulse for this treatment, as a pulseless V-tach requires immediate, unsynchronized defibrillation.
Synchronized cardioversion can also be used for other unstable Supraventricular Tachycardias (SVT) where the electrical signal originates above the ventricles. These rhythms, which include Atrial Tachycardia, are often extremely fast and, if unresponsive to initial drug treatments, require the immediate electrical reset of cardioversion to stabilize the patient. The presence of a clear QRS complex in all these rhythms allows the device to properly synchronize the shock, ensuring a safe and effective delivery.
When This Treatment Is Not Used
The procedure is not used for patients with stable tachyarrhythmias who do not exhibit signs of hemodynamic compromise. For stable patients, a trial of antiarrhythmic medications is the preferred initial treatment. Only if medications fail should an elective, synchronized cardioversion be considered.
This treatment is not appropriate for pulseless rhythms like Ventricular Fibrillation (V-fib) or pulseless Ventricular Tachycardia. These rhythms represent cardiac arrest and require immediate, high-energy, unsynchronized defibrillation. Attempting to synchronize a shock in V-fib is impossible because there is no organized QRS complex for the machine to detect.
Certain organized rhythms are also unresponsive to this electrical therapy, such as Multifocal Atrial Tachycardia (MAT) and some forms of junctional tachycardia. These rhythms are driven by multiple or automatic electrical foci that often increase their rate further after an electrical shock, making the procedure counterproductive. Lastly, if the defibrillator cannot reliably sense the R-wave due to electrical interference or a very irregular rhythm, the synchronization feature must be turned off, and the procedure cannot be performed safely in synchronized mode.