Cardioversion is synchronized to the heart's electrical activity and used in patients with a pulse, while defibrillation is unsynchronized and used in cardiac arrest patients without a pulse.
Cardioversion and defibrillation are both medical procedures that use electrical shocks to restore a normal heart rhythm. While they both deliver electricity to the heart, their application, timing, and target conditions differ significantly. Understanding these differences is crucial in emergency and cardiac care settings.
The primary distinction lies in the patient's condition and the timing of the electrical shock. Defibrillation is an emergency procedure performed on patients experiencing cardiac arrest due to a chaotic, life-threatening heart rhythm, such as ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT). In these situations, the heart's electrical activity is completely disorganized, and the goal is to stop all electrical activity momentarily to allow the heart's natural pacemaker to reset to a normal sinus rhythm. The shock delivered during defibrillation is unsynchronized; it is delivered immediately upon button press.
In contrast, synchronized cardioversion is typically performed on patients who are hemodynamically unstable but still have a pulse. As stated in the reference, unlike defibrillation, which is used in cardiac arrest patients, synchronized cardioversion is performed on patients that still have a pulse but are hemodynamically unstable. This procedure is used to treat specific unstable, abnormally fast heart rhythms, including hemodynamically unstable ventricular and supraventricular rhythms like unstable ventricular tachycardia with a pulse, atrial fibrillation (AFib), atrial flutter (AFlutter), or supraventricular tachycardia (SVT) that doesn't respond to other treatments. The key difference here is the synchronization. The device delivering the shock detects the heart's electrical activity (specifically the R-wave on the ECG) and delivers the electrical impulse at a precise moment in the cardiac cycle. This synchronization prevents the shock from being delivered during the vulnerable repolarization phase (T-wave), which could potentially trigger a more dangerous rhythm like ventricular fibrillation.
Key Differences Summarized
| Feature | Defibrillation | Synchronized Cardioversion |
|---|---|---|
| Pulse | No pulse (cardiac arrest) | Pulse present |
| Timing | Unsynchronized | Synchronized to the R-wave on ECG |
| Indication | Chaotic, life-threatening rhythms causing cardiac arrest (VF, pulseless VT) | Unstable, fast rhythms with a pulse (VT with pulse, unstable AFib, AFlutter, SVT) |
| Patient Status | Cardiac arrest | Hemodynamically unstable (but awake or sedated if elective) |
| Energy Level | Typically higher (e.g., 120-200 J biphasic, 360 J monophasic) | Typically lower initially (e.g., 50-100 J for AFib/AFlutter, higher for VT) |
| Procedure | Emergency, immediate | Emergency (for unstable patients) or elective (for stable, persistent arrhythmias) |
Understanding Each Procedure
Defibrillation
Defibrillation is a critical, potentially life-saving intervention for specific types of cardiac arrest. When the heart enters ventricular fibrillation or pulseless ventricular tachycardia, its lower chambers quiver instead of pumping blood, leading to immediate circulatory collapse. A defibrillator delivers a strong, unsynchronized electrical shock designed to momentarily stop the chaotic electrical activity. This allows the heart's natural pacemaker to hopefully resume a normal, effective rhythm. Because there is no organized electrical activity to synchronize to, the shock is delivered as soon as the device is charged and the operator presses the shock button.
Synchronized Cardioversion
Synchronized cardioversion is used when the heart is beating too fast but still has a somewhat organized electrical pattern and the patient has a pulse but is unstable (e.g., experiencing chest pain, shortness of breath, low blood pressure, or altered mental status due to the rapid rhythm). It is used to treat both hemodynamically unstable ventricular and supraventricular rhythms. The synchronized delivery of the shock is crucial. The defibrillator is set to "sync" mode, which detects the heart's R-wave (representing ventricular depolarization). The device waits for the next R-wave and delivers the shock a few milliseconds after it. This timing avoids the T-wave, preventing the induction of ventricular fibrillation. While used in emergencies for unstable rhythms, synchronized cardioversion can also be performed electively (after preparation and sedation) for stable but persistent arrhythmias like atrial fibrillation or flutter that are not responding to medications.
Practical Insights
- Safety First: Always ensure safety of personnel before delivering a shock. Clear the patient and bed.
- Defibrillator Modes: Most modern defibrillators have both "defib" (unsynchronized) and "sync" (synchronized) modes. Selecting the correct mode is vital.
- Pad Placement: Proper placement of defibrillator pads or paddles (anterolateral or anterior-posterior) is essential for current flow through the heart.
- Energy Settings: The appropriate energy level depends on the specific device (monophasic vs. biphasic), the patient's condition, and the rhythm being treated.
In summary, the core difference lies in the presence or absence of a pulse and the timing of the electrical shock (synchronized vs. unsynchronized), dictated by the underlying heart rhythm and the patient's clinical stability.
