Synchronized Cardioversion: Procedure, Joules, and Sedation

Synchronized cardioversion is one of those procedures that many physicians understand in theory, but where uncertainties regularly arise during practical execution. When exactly do I press the shock button? What energy do I select for atrial fibrillation, and what for ventricular tachycardia? How deeply do I sedate – and with what? These questions arise not only for those early in their careers, but also for experienced healthcare professionals who rarely perform this procedure. This article guides you step by step through the entire process – from indication assessment through sedation to energy selection and the most common mistakes you should avoid.

Basic Principle: Synchronization Versus Defibrillation

The crucial difference between synchronized cardioversion and defibrillation lies in the timing of energy delivery. During synchronized cardioversion, the defibrillator detects the R-wave on the ECG and delivers the shock precisely during the refractory period of the heart – that is, at a time when the myocardium is not vulnerable. This minimizes the risk of inducing ventricular fibrillation through the energy delivery.

With defibrillation, on the other hand, the shock is delivered asynchronously, meaning independently of the cardiac rhythm. This is correct for ventricular fibrillation and pulseless ventricular tachycardia, because there is no organized electrical activity to synchronize with.

Remember: Synchronized cardioversion is intended for unstable tachycardias with a pulse. Defibrillation is reserved for pulseless rhythms (ventricular fibrillation, pulseless VT).

Indications: When Do You Perform Synchronized Cardioversion?

The AHA guidelines define clear indications for synchronized cardioversion. The fundamental prerequisite is always: the patient has a palpable pulse and shows signs of hemodynamic instability.

Signs of Hemodynamic Instability

• Hypotension (systolic blood pressure < 90 mmHg or clinically significant drop)
• Acute altered mental status (confusion, somnolence, syncope)
• Signs of acute heart failure (pulmonary edema, dyspnea)
• Anginal chest pain as an expression of myocardial ischemia

Rhythms That Are Treated with Synchronized Cardioversion

• Atrial fibrillation with hemodynamic instability
• Atrial flutter with hemodynamic instability
• Supraventricular tachycardia (SVT) that is unresponsive to vagal maneuvers and adenosine, and is unstable
• Monomorphic ventricular tachycardia with a pulse and signs of instability

Important Exception: Polymorphic VT

Polymorphic ventricular tachycardia (e.g., Torsades de Pointes) is not treated with synchronized cardioversion but is defibrillated like ventricular fibrillation. The reason: R-waves in polymorphic VT are so variable in morphology and amplitude that the defibrillator cannot reliably synchronize. The device "waits" for a recognizable R-wave that may never come – valuable time is lost. Therefore, always treat polymorphic VT with unsynchronized defibrillation at maximum energy.

Preparation: Before You Deliver the Shock

Structured preparation is critical for the success and safety of cardioversion. The following checklist is based on clinical practice:

Personnel and Equipment Requirements

• Monitoring: 12-lead ECG (if time permits), at minimum a clean 3- or 5-lead ECG via the defibrillator
• Oxygen supply and suction equipment ready
• Intubation equipment within reach (in case of airway complications during sedation)
• IV access – ideally two peripheral lines
• Emergency medications: epinephrine, atropine, amiodarone ready
• Defibrillator with functioning adhesive electrode pads or paddles
• At least one additional person (sedation requires monitoring)

Electrode Position

Use self-adhesive defibrillation electrode pads. These offer several advantages over paddles: even skin contact, hands free for other tasks, and the ability to monitor simultaneously through the pads.

The standard position is anterolateral:

• Right electrode: below the right clavicle, parasternal
• Left electrode: left midaxillary line, at the level of V6 (below the nipple in men, laterally beneath the breast in women)

Alternatively, you can choose an anteroposterior placement (sternum anteriorly, between the shoulder blades posteriorly). This position may offer advantages in obese patients or in atrial fibrillation, as more myocardial mass lies within the current pathway.

Important: Remove medication patches (e.g., nitroglycerin patches, fentanyl patches) in the area of electrode placement. The metallic foil can cause sparks and burns. Dense chest hair can also impair skin contact – shave quickly in an emergency or press the pads firmly in place.

Sedation: Analgesia and Anxiolysis

Synchronized cardioversion in awake patients is painful and anxiety-inducing. Adequate sedation is therefore mandatory – except in the case of unconsciousness or immediately life-threatening instability, where the shock cannot be delayed.

Goal of Sedation

You aim for procedural sedation: the patient should be unconscious during shock delivery but arousable immediately afterward. Spontaneous breathing should be maintained if possible.

Medication Selection

The choice of sedative depends on the clinical situation, hemodynamic stability, and your experience. The following agents are most commonly used:

Propofol:

• Dose: 0.5–1 mg/kg IV as a bolus, slowly titrated
• Advantages: rapid onset (30–40 seconds), short duration of action (5–10 minutes), pleasant emergence
• Disadvantages: can cause significant hypotension and respiratory depression – use with caution in already unstable patients
• Particularly suitable for patients who are not yet severely hemodynamically compromised

Midazolam:

• Dose: 0.02–0.05 mg/kg IV (typically 1–3 mg in adults), titrated
• Advantages: less hemodynamic impact than propofol, reversible with flumazenil
• Disadvantages: slower onset, longer duration of action, less reliable amnesia at low doses
• Well suited for hemodynamically borderline patients

Etomidate:

• Dose: 0.1–0.15 mg/kg IV
• Advantages: minimal hemodynamic compromise, rapid onset
• Disadvantages: myoclonus, adrenal suppression (rarely clinically relevant with a single dose)
• Good choice in significant hypotension

Ketamine:

• Dose: 0.5–1 mg/kg IV
• Advantages: circulatory stability (sympathomimetic effect), preserved protective reflexes, analgesic component
• Disadvantages: hypersalivation, emergence phenomena (reducible with a low dose of a benzodiazepine)
• Particularly suitable in severe hypotension

Practical Tips for Sedation

• Titrate! Do not administer the entire dose as a rapid bolus; instead, titrate in small increments until the desired depth of sedation is reached.
• Assess depth of sedation through verbal and tactile stimulation before shock delivery.
• Keep the airway clear – have a jaw thrust maneuver, oropharyngeal airway, or nasopharyngeal airway ready.
• Continuously monitor oxygen saturation via pulse oximetry.
• Ensure you have a bag-valve-mask within reach.

Energy Selection: Joule Settings by Rhythm

Energy selection depends on the underlying rhythm and the type of defibrillator used (biphasic vs. monophasic). Today's standard biphasic defibrillators operate at lower energies with higher conversion rates.

Recommended Energies (Biphasic)

Rhythm	Starting Energy	Escalation
Atrial fibrillation	120–200 J (manufacturer-dependent)	Increase stepwise
Atrial flutter	50–100 J	Increase stepwise
SVT	50–100 J	Increase stepwise
Monomorphic VT	100 J	Increase stepwise

Notes on Energy Selection

• Atrial flutter and SVT frequently respond to low energies (50 J). It makes sense to start low and escalate if unsuccessful.
• Atrial fibrillation generally requires higher energies than atrial flutter, because the disorganized atrial activation is more difficult to terminate.
• Read the manufacturer's specifications for your defibrillator – the recommended starting energy may vary by device.
• With monophasic defibrillators (rarely in use today), higher energies are generally used: 200 J for atrial fibrillation, 50–100 J for other rhythms.

Step-by-Step Procedure

Here is the structured sequence for a synchronized cardioversion as you would perform it in practice:

1. Verify the indication: Unstable tachycardia with a pulse? Is a synchronizable rhythm present?
2. Brief the team: Clear role assignment – who sedates, who cardioverts, who manages the airway.
3. Establish monitoring: ECG (via defibrillator pads or cables), pulse oximetry, blood pressure measurement.
4. Prepare the defibrillator: Turn on the device, position pads correctly.
5. Activate SYNC mode: Press the SYNC button. Markers must appear on the R-waves on the monitor (usually as small arrows, dots, or triangles). Verify that the marker captures every R-wave.
6. Select the energy: Set the joule level according to the rhythm.
7. Perform sedation: Administer the sedative in a titrated fashion, verify depth of sedation.
8. Safety check: "Attention – everyone clear of the patient!" Visually confirm that no one is touching the patient or the bed. Remove the oxygen source from the patient (fire risk from sparking).
9. Deliver the shock: Press the charge button, then press and hold the shock button. With synchronized cardioversion, a brief delay may occur as the device waits for the next R-wave. Do not let go!
10. Assess the result: Analyze the rhythm on the monitor. Has the cardioversion converted the rhythm?
11. If unsuccessful: Increase energy, recheck the SYNC mode (many defibrillators automatically deactivate Sync mode after each shock!), and cardiovert again.
12. Post-procedure care: Continue monitoring, observe for residual sedation effects, obtain a 12-lead ECG, treat the underlying cause of the arrhythmia.

Common Mistakes and How to Avoid Them

Mistake 1: SYNC Mode Not Activated or Inadvertently Deactivated

This is the classic and potentially most dangerous mistake. Many defibrillators automatically reset the SYNC mode to the default (asynchronous) mode after each shock. If you do not reactivate SYNC before a second attempt, you deliver an unsynchronized shock – with the risk of inducing ventricular fibrillation.

Solution: Verify the SYNC mode and R-wave markers on the monitor before every shock.

Mistake 2: Releasing the Shock Button Too Early

During synchronized cardioversion, the device waits for the next R-wave. There is a brief but noticeable delay between pressing the button and shock delivery. Inexperienced operators release the button because they think the device has malfunctioned.

Solution: Hold the shock button down until the shock has actually been delivered. Inform the team in advance about the possible delay.

Mistake 3: Poor ECG Quality

If the ECG signal is noisy, shows artifacts, or the R-wave amplitude is too low, the defibrillator cannot synchronize reliably. The sync markers appear irregularly or on T-waves – the latter would be extremely dangerous.

Solution: Select the lead with the highest R-wave amplitude. Improve electrode contact. Visually verify the sync markers. If in doubt, switch leads.

Mistake 4: Inadequate Sedation

Too little sedation results in an awake, traumatized patient who consciously experiences the shock. Too deep sedation leads to respiratory depression and hemodynamic deterioration.

Solution: Titrate sedation. Check the response to verbal stimulation. Be prepared for airway management.

Mistake 5: Cardioversion for Sinus Tachycardia

Sinus tachycardia is not an indication for cardioversion – it is a physiological response to a stimulus (hypovolemia, pain, sepsis, fever). Treatment consists of addressing the underlying cause.

Solution: Analyze the rhythm carefully. When in doubt, obtain a 12-lead ECG. Sinus tachycardia shows P-waves before every QRS complex, a rate usually < 150/min, and a gradual increase in rate.

Mistake 6: Synchronized Cardioversion for Polymorphic VT

As described above – polymorphic VT is treated with unsynchronized defibrillation. Attempting synchronization here risks a fatal delay.

Special Situation: Elective Cardioversion

For elective cardioversion (e.g., in hemodynamically stable atrial fibrillation), additional considerations apply:

• Verify anticoagulation: If atrial fibrillation has been present for > 48 hours or the duration is uncertain, either a transesophageal echocardiography (TEE) to rule out intracardiac thrombi or at least three weeks of effective anticoagulation must be completed before cardioversion.
• Check electrolytes: Hypokalemia and hypomagnesemia lower the threshold for malignant arrhythmias during cardioversion.
• Fasting status: For elective cardioversion, a fasting period of at least 6 hours should be observed to minimize the risk of aspiration during sedation.
• Thyroid function: In new-onset atrial fibrillation, consider hyperthyroidism – cardioversion without treating the underlying cause has a high recurrence rate.

Post-Procedure Care and Documentation

After successful cardioversion, the work is not finished:

• Rhythm monitoring for at least 2–4 hours (for elective cardioversion) up to continuous monitoring (for emergency cardioversion in the intensive care unit)
• 12-lead ECG to document the converted rhythm and to rule out signs of ischemia
• Anticoagulation after cardioversion of atrial fibrillation for at least 4 weeks (even after successful conversion to sinus rhythm), as the risk of thromboembolic events is elevated in the post-cardioversion period
• Skin inspection at the electrode sites (burns, erythema)
• Documentation: indication, rhythm, selected energy, number of shocks, sedation agent used with dose, outcome, complications

Practical Training

Synchronized cardioversion is a procedure that should be practiced regularly – precisely because it is rarely performed in everyday clinical practice. The correct activation of SYNC mode, the timing of sedation, and the handling of the defibrillator under stress are best learned in a realistic simulation environment. In the ACLS courses offered by Simulation Tirol, you train exactly these scenarios on simulated patients, with real defibrillators, and under expert guidance. This way, you gain the confidence you need when it matters most. You can find all information about upcoming ACLS course offerings on the Simulation Tirol website.