Synchronized cardioversion is a time-sensitive medical procedure that uses a precisely timed electrical current to reset the heart’s rhythm. This intervention is reserved for patients experiencing an abnormally fast heart rate, known as a tachyarrhythmia, that is causing the patient to become medically unstable. The key distinction for this procedure is that the patient must still have a detectable pulse, differentiating it from other electrical therapies used during cardiac arrest. The goal is to momentarily depolarize the majority of the heart muscle cells, allowing the heart’s natural pacemaker, the sinus node, to resume control of the electrical activity. This planned electrical discharge is a rapid and effective method when fast rhythms are compromising blood flow.
Heart Rhythms Treated by Synchronized Cardioversion
Synchronized cardioversion is indicated when a rapid heart rhythm is organized enough to detect a pulse, but is causing the patient to become hemodynamically unstable. Unstable signs include acute symptoms such as chest pain, lightheadedness, shortness of breath, altered mental status, or dangerously low blood pressure. The procedure is the preferred treatment for several specific rhythms.
One common rhythm treated is Atrial Fibrillation (A-Fib), where the upper chambers of the heart beat chaotically and rapidly, leading to poor coordination with the lower chambers. Similarly, Atrial Flutter, which involves a fast electrical circuit in the atria, often requires this synchronized shock to restore normal sinus rhythm. These supraventricular tachycardias (SVT) are often too fast for medications to work quickly enough in an unstable patient.
The procedure is also indicated for certain forms of Ventricular Tachycardia (V-Tach) when the patient still has a pulse. V-Tach originates in the lower chambers (ventricles) and is characterized by rapid, wide electrical complexes on an electrocardiogram. It is a dangerous rhythm because it can quickly transition into pulseless cardiac arrest.
Timing the Electrical Shock
The “synchronized” aspect of the procedure dictates the exact moment the electrical current is delivered to the heart. The defibrillator device is placed in a special mode that detects the heart’s electrical cycle, specifically targeting the R-wave of the QRS complex. The QRS complex represents the electrical activity associated with the contraction of the ventricles.
The shock is deliberately timed to avoid the T-wave, which immediately follows the R-wave on the electrocardiogram. The T-wave corresponds to the period when the ventricles are recovering their electrical charge, known as repolarization. This phase includes a brief window called the “vulnerable period.”
If an electrical shock were to land during this vulnerable period, it could trigger a lethal rhythm called Ventricular Fibrillation (V-Fib). This dangerous phenomenon is sometimes referred to as the R-on-T phenomenon. The synchronization feature ensures the shock is delivered during the heart’s absolute refractory period, maximizing the chance of a successful rhythm conversion while avoiding the induction of a life-threatening complication.
When Defibrillation is the Right Choice
While synchronized cardioversion is used for unstable patients who still have a pulse, defibrillation is reserved for patients in cardiac arrest who are pulseless. Defibrillation, which is unsynchronized, delivers an immediate, high-energy electrical shock without regard to the heart’s electrical cycle. This immediate delivery is necessary because the heart’s electrical activity is either completely chaotic or absent.
The two main rhythms requiring defibrillation are Ventricular Fibrillation (V-Fib) and pulseless Ventricular Tachycardia. In V-Fib, the ventricles are merely quivering with disorganized electrical signals, making it impossible for the heart to pump blood effectively. In these pulseless states, there is no organized electrical complex for a machine to synchronize to. The immediate, high-energy shock is intended to instantly stop all electrical activity, giving the natural pacemaker a chance to restart with a normal rhythm.