Synchronized cardioversion is a controlled medical procedure used to restore a regular heart rhythm when a patient is experiencing a rapid, unstable, but organized rhythm. The heart’s natural electrical system can sometimes become chaotic, causing the heart to beat too quickly, which impairs its ability to pump blood effectively. This procedure involves applying a brief, high-energy electrical shock to the chest wall to interrupt the heart’s faulty electrical circuit. The energy delivered in this shock is precisely measured in Joules (J). The goal is to momentarily stop all electrical activity, allowing the heart’s natural pacemaker to reset and resume a normal, steady rhythm.
Synchronized Cardioversion: Defining the Procedure and Energy
Synchronized cardioversion involves delivering a measured dose of electricity specifically timed to the patient’s existing heart rhythm. The electricity acts as a global reset switch, temporarily depolarizing heart muscle cells to stop the rapid, abnormal rhythm. This procedure targets unstable but organized electrical activity, such as ventricular tachycardia with a pulse or certain supraventricular tachycardias.
The procedure is distinct from defibrillation, which delivers an unsynchronized, higher-energy shock for completely disorganized rhythms like ventricular fibrillation. Because the patient is conscious, the procedure requires procedural sedation to ensure comfort and safety. Synchronization relies on the machine detecting the heart’s electrical activity, specifically the R-wave of the QRS complex, allowing for a lower, more controlled energy delivery.
Standardized Energy Protocols for Common Rhythms
The energy required for successful cardioversion depends highly on the specific heart rhythm being treated. Clinical guidelines provide starting energy recommendations, which use stepwise escalation if the initial shock fails. These recommendations assume the use of modern biphasic defibrillators, which require lower energy settings than older monophasic devices.
For Atrial Fibrillation (AFib), the initial biphasic setting usually ranges from 120 to 200 Joules, though some guidelines suggest 100 Joules for rhythms of very short duration. If the first shock is unsuccessful, the energy is increased for subsequent attempts. Long-standing AFib often requires a higher initial energy setting because the tissue may be more resistant to electrical conversion.
Atrial Flutter (AFlutter) and other narrow-complex supraventricular tachycardias require significantly lower energy levels. For AFlutter, the typical starting dose is often 50 Joules with a biphasic device. If 50 Joules is unsuccessful, the energy is generally increased to 100 Joules for the next attempt.
Ventricular Tachycardia (VTach) that is monomorphic and occurs while the patient still has a pulse generally requires an initial energy of 100 Joules. If the rhythm is not corrected, the energy level is increased in increments, such as to 150 Joules, then 200 Joules, up to the device’s maximum capacity.
Why Joule Levels Must Be Adjusted
Clinical protocols provide standardized starting points, but the actual energy dose must often be adjusted based on patient and equipment variables. The treating clinician must evaluate factors that affect the flow of electricity across the chest.
A major factor influencing the necessary energy level is the patient’s chest impedance, which is the resistance to the electrical current passing through the body. Patients with a larger body size may have higher impedance, requiring a greater initial energy setting for the electricity to effectively reach the heart. Poor technique, such as insufficient pressure on the electrode pads or incorrect placement, can also increase impedance and necessitate a higher dose.
The type of defibrillator used is another variable; modern biphasic waveform technology is more energy-efficient than older monophasic technology. Biphasic devices deliver current in two directions, requiring lower Joules for successful conversion. If a lower-energy shock fails, the energy must be escalated for subsequent attempts to overcome the resistance of a refractory rhythm.
Ensuring Efficacy: The Mechanics of Synchronization
The synchronization feature ensures the shock is delivered at the safest possible moment in the cardiac cycle. When engaged, the defibrillator monitors the electrocardiogram (ECG) to detect the R-wave, which represents the start of ventricular contraction. The machine then discharges the electrical current precisely on or just after the peak of this R-wave.
This timing ensures the electrical shock avoids the T-wave, which represents the vulnerable period of ventricular repolarization. Delivering a shock during the T-wave, known as the R-on-T phenomenon, can trigger a chaotic, lethal rhythm called ventricular fibrillation. By carefully timing the shock to the R-wave, the synchronization mechanism minimizes the risk of inducing this life-threatening complication.