Synchronized cardioversion (SCV) is a medical procedure that uses a controlled, low-energy electrical shock to restore a normal heart rhythm. It treats specific types of fast heart rhythms where the heart’s electrical system is organized but firing too rapidly. This electrical jolt momentarily resets the heart’s electrical activity, allowing its natural pacemaker to take over and resume a healthy, regular beat. The precise timing of the electrical discharge is the defining feature, making it a safe and effective method for managing certain rhythm disorders.
Understanding When Cardioversion is Necessary
Synchronized cardioversion is used for tachycardias—fast heart rhythms that maintain an organized electrical pattern. This distinguishes it from disorganized rhythms, such as ventricular fibrillation, which require immediate, unsynchronized defibrillation. SCV is typically performed when a patient with a fast rhythm shows signs of instability, including low blood pressure, chest pain, or altered mental status, or when medications have failed.
The procedure commonly treats atrial fibrillation, atrial flutter, and supraventricular tachycardia (SVT). A key factor is the presence of a measurable QRS complex on the electrocardiogram, which represents the electrical activity of the heart’s lower chambers. This organized activity allows the device to “synchronize” the shock, a safety feature impossible in a chaotic, pulseless rhythm.
The Science of Electrical Timing
The mechanism of synchronized cardioversion depends entirely on the precise timing of the electrical energy delivery. When set to synchronized mode, the defibrillator actively monitors the heart rhythm to detect the R-wave, the tall peak of the QRS complex. The machine automatically delays the electrical discharge to deliver the shock within milliseconds immediately following the R-wave.
This timing safeguards against the dangerous R-on-T phenomenon. The T-wave represents the heart’s vulnerable repolarization period, where cells recover their electrical charge. If a shock lands during the T-wave, it could inadvertently trigger ventricular fibrillation, a fatal, chaotic rhythm. Synchronizing the shock to the R-wave ensures the electrical reset happens at the safest moment, momentarily stunning all heart cells. This stops the abnormal circuit and allows the natural pacemaker (sinus node) to restart the rhythm correctly.
What Happens During the Procedure
Patient preparation begins with establishing intravenous (IV) access for medication administration and continuous monitoring of heart rhythm and vital signs. Since the electrical shock can be painful, the patient receives conscious sedation or general anesthesia to ensure comfort and immobility. Medications such as propofol, etomidate, or midazolam are used to achieve a deep, short-acting state of unconsciousness.
Once sedation is achieved, electrode pads are placed on the chest, typically in an anteroposterior (front and back) position for effective energy delivery. The medical team presses the “sync” button on the defibrillator and verifies the machine correctly marks the R-wave on the monitor. The device is then charged to a specific energy level, which varies based on the arrhythmia being treated; energy typically starts lower for rhythms like atrial flutter.
The procedure culminates with a verbal warning and the clinician delivering the electrical shock. The clinician must maintain pressure on the delivery button until the synchronized discharge occurs. The patient does not feel the shock due to sedation, and the medical team watches the monitor closely to confirm conversion back to a regular sinus rhythm.
Recovery and Potential Adverse Effects
After successful SCV, the heart immediately resumes a normal sinus rhythm, and the patient is monitored while the effects of sedation wear off. Recovery usually involves spending an hour or more in a recovery area, and the patient is typically discharged home the same day, requiring a driver. A common, minor side effect is temporary skin irritation or redness where the electrode pads were placed, which may feel like mild soreness.
If the procedure fails, the medical team may attempt a repeat shock at a higher energy level or pursue other treatments. The most significant risk, although rare, is stroke, which can occur if the shock dislodges a pre-existing blood clot from the heart. To mitigate this, patients with a rhythm lasting longer than 48 hours must take blood-thinning medication for several weeks before and after the procedure. Rarely, the electrical shock can cause the heart rhythm to worsen.