4.8 Synchronized Cardioversion and Defibrillation
Key Takeaways
- Synchronized cardioversion times the shock to the R wave to avoid the vulnerable relative refractory period (R-on-T) and requires an organized QRS; pulseless or unstable polymorphic VT is DEFIBRILLATED because its beat-to-beat variation makes synchronization impossible.
- Initial biphasic energies: narrow regular 50-100 J, atrial fibrillation 120-200 J, monomorphic VT with a pulse 100 J, and defibrillation at the manufacturer dose (generally 120-200 J biphasic, 360 J monophasic), escalating stepwise.
- Atrial fibrillation lasting 48 hours or longer or of unknown duration requires 3 weeks of therapeutic anticoagulation before cardioversion or a TEE-guided approach, and at least 4 weeks of anticoagulation afterward in every patient because atrial stunning persists for days to weeks after electrical conversion.
- Most defibrillators revert to unsynchronized mode after each shock, so SYNC must be re-pressed and the markers re-verified on the R waves before every subsequent cardioversion.
- Remove transdermal patches, keep pads at least 8 cm from an implanted generator, and have atropine and transcutaneous pacing ready because bradyasystole can follow cardioversion in sinus node dysfunction.
The Distinction the Exam Is Built On
Test-plan items III.D.7 and III.D.8 separate synchronized cardioversion from defibrillation, and nearly every CMC question on this material turns on that separation.
Synchronized cardioversion delivers the shock on the R wave. The defibrillator senses each QRS complex, marks it, and times the discharge to coincide with depolarization, deliberately avoiding the vulnerable period — the relative refractory phase represented by the upstroke and peak of the T wave, when part of the myocardium has repolarized and part has not. A shock landing there (R-on-T) creates the dispersion of refractoriness that initiates ventricular fibrillation. Synchronization therefore requires an organized, reliably sensed QRS complex.
Defibrillation is unsynchronized. The energy is delivered the instant the button is pressed, because in ventricular fibrillation there is no organized QRS to synchronize to, and any delay costs perfusion.
Which Rhythm Gets Which
| Rhythm | Therapy | Why |
|---|---|---|
| Unstable narrow, regular tachycardia (SVT, atrial flutter) | Synchronized cardioversion | Organized QRS present |
| Unstable atrial fibrillation (narrow, irregular) | Synchronized cardioversion | Organized QRS present despite irregular rhythm |
| Unstable monomorphic VT with a pulse | Synchronized cardioversion | Uniform QRS can be sensed |
| Polymorphic VT / torsades, pulseless or unstable | DEFIBRILLATION (unsynchronized) | QRS amplitude and morphology vary beat to beat, so the device cannot reliably find an R wave; attempting to sync produces a dangerous delay or no discharge at all |
| Ventricular fibrillation | DEFIBRILLATION | No organized activity |
| Pulseless VT of any morphology | DEFIBRILLATION | Cardiac arrest algorithm |
| Asystole, PEA | Neither — shocking these rhythms is never indicated | No shockable substrate |
The single most heavily tested trap in this content area: pulseless or unstable polymorphic VT is defibrillated, not cardioverted. A candidate who reflexively answers "synchronize any VT" gets it wrong. In practice, if you press sync and the machine will not discharge, or the sync markers are absent or erratic, turn sync off and defibrillate.
Energy Selection
Biphasic waveforms are the standard of care; the values below reflect the AHA adult tachycardia and cardiac arrest algorithms. Escalate stepwise if the first shock fails.
| Rhythm | Initial biphasic energy | Notes |
|---|---|---|
| Narrow, regular (SVT, atrial flutter) | 50–100 J | Atrial flutter often converts at 50 J |
| Narrow, irregular (atrial fibrillation) | 120–200 J | Atrial fibrillation is the most energy-demanding of the supraventricular rhythms; escalate to maximum |
| Wide, regular (monomorphic VT with a pulse) | 100 J | Synchronized; escalate stepwise |
| Wide, irregular (polymorphic VT) | Defibrillation dose, unsynchronized | Treat as ventricular fibrillation |
| Ventricular fibrillation / pulseless VT | Manufacturer-recommended dose, generally 120–200 J; if unknown, use the maximum available | Subsequent shocks equal or higher; monophasic devices use 360 J |
| Pediatric defibrillation | 2 J/kg first, 4 J/kg subsequent, up to 10 J/kg or the adult dose | Included for completeness |
Energy is only half the equation. Transthoracic impedance — chest wall size, lung volume, pad contact, hair, moisture, and pad-to-pad distance — determines how much current actually reaches myocardium, which is why pad technique matters as much as the joule setting.
Elective Cardioversion of Atrial Fibrillation: The Checklist
Elective cardioversion is the most common planned use of this therapy on a cardiac unit, and the stroke-prevention rules are examinable in detail.
Anticoagulation and the 48-Hour Rule
- Atrial fibrillation of less than 48 hours' duration may generally be cardioverted without a preceding 3-week anticoagulation course — but the 2023 ACC/AHA/ACCP/HRS guideline now advises that for episodes lasting 12–48 hours in a patient with a CHA2DS2-VASc of 2 or more in men or 3 or more in women, either 3 weeks of anticoagulation or a TEE-guided approach is preferred. Treating "under 48 hours" as automatically safe is an outdated simplification.
- Duration 48 hours or longer, or unknown, requires either:
- At least 3 weeks of documented therapeutic anticoagulation before cardioversion (for warfarin, INR 2.0–3.0 verified weekly; a single subtherapeutic INR restarts the clock), or
- A transesophageal echocardiogram excluding left atrial and left atrial appendage thrombus, with anticoagulation started before the procedure and continued afterward.
- Anticoagulation is required for at least 4 weeks after cardioversion in every patient, regardless of the pre-procedure route and regardless of the fact that the patient is now in sinus rhythm. Long-term continuation is then decided by CHA2DS2-VASc.
- Why: atrial stunning. Electrical restoration of sinus rhythm does not immediately restore mechanical atrial contraction. Atrial and appendage mechanical function remains depressed for days to several weeks, so a thrombus can form after a successful cardioversion in an atrium that looks like it is in sinus rhythm on the monitor. Most cardioversion-related strokes occur in the first 10 days. This is the reasoning behind the 4-week rule and a frequent exam item.
- Hemodynamically unstable patients are cardioverted immediately regardless of anticoagulation status, with anticoagulation started as soon as feasible.
Other Pre-Procedure Requirements
| Check | Detail |
|---|---|
| Digoxin toxicity | A relative contraindication. Cardioversion in a digitalis-toxic patient can precipitate refractory ventricular arrhythmias or asystole. Look for the clinical picture (nausea, visual halos, confusion) plus atrial tachycardia with block or junctional tachycardia, and check the level. If shock is unavoidable, start at very low energy (10–50 J) with lidocaine and pacing readiness |
| Potassium and magnesium | Correct hypokalemia (target 4.0 or above) and hypomagnesemia (target above 2.0 mg/dL) before an elective shock: both reduce conversion success and increase post-shock arrhythmia |
| NPO and consent | Typically nothing by mouth 6–8 hours; verified informed consent |
| Baseline data | 12-lead ECG documenting the rhythm, vital signs, weight, IV access, current INR or last DOAC dose with adherence confirmed, thyroid and renal function as ordered |
| Airway readiness | Suction, bag-mask, oral airway, oxygen, capnography, reversal agents (naloxone, flumazenil), and a provider credentialed in airway management present |
| Sedation | Short-acting agents such as etomidate, propofol, or midazolam with fentanyl, titrated to brief deep sedation; monitor blood pressure, oxygen saturation and end-tidal CO2 continuously |
| Standby equipment | Transcutaneous pacing pads, atropine, and emergency drugs, because post-cardioversion bradyasystole is a recognized event in sinus node disease |
Pad Placement and Procedural Technique
- Anterolateral (right infraclavicular parasternal plus left mid-axillary at the V6 level) and anteroposterior (left parasternal/apical anteriorly, between the spine and left scapula posteriorly) are both acceptable to the AHA. Anteroposterior is preferred when a large-breasted or obese habitus prevents good lateral contact, when an implanted device generator sits in the anterolateral path, and in many institutions for atrial arrhythmias, because it directs current through the atrial mass.
- Remove all transdermal patches (nitroglycerin, clonidine, fentanyl, nicotine) and wipe the skin: the metallic backing can arc, burn the skin and impede current delivery.
- Keep pads at least 8 cm from any implanted generator, and interrogate the device after external cardioversion.
- Clip excess hair, ensure the skin is dry, and press pads flat without air pockets.
- Press SYNC and verify. A marker must fall on each R wave, not on T waves. If markers land on T waves or are missed, change the monitoring lead or increase the ECG gain until the R wave is the tallest signal. In atrial fibrillation the marker spacing will be irregular — that is expected and correct as long as every marker sits on a QRS.
- Most defibrillators default back to unsynchronized mode after each shock. If a second cardioversion is needed, the operator must press SYNC again and re-verify markers. Failing to do so delivers an unsynchronized shock to an organized rhythm and can induce ventricular fibrillation — a classic, examinable error.
- Clear the patient: state "I'm clear, you're clear, everybody clear," visually confirm no one is touching the patient or bed, remove or disconnect oxygen from the immediate field, and then discharge while holding the buttons until the shock is delivered (synchronized discharge is delayed until the next R wave).
Complications and Post-Procedure Nursing Care
| Complication | Mechanism and recognition | Nursing response |
|---|---|---|
| Skin burns and erythema | High impedance from poor pad contact, hair, moisture, repeated high-energy shocks | Inspect skin under both pads after the procedure, document, apply topical care per order; prevent with proper technique |
| Transient ST-segment elevation | Direct myocardial current effect; usually resolves within minutes | Repeat the 12-lead; persistent elevation with chest pain requires ischemia evaluation and troponin |
| Bradyasystole, sinus arrest, junctional escape | Underlying sinus node dysfunction unmasked when the fibrillating atrium is silenced (tachy-brady syndrome) | Have atropine and transcutaneous pacing immediately available, keep pads on, monitor continuously; this may reveal a permanent pacemaker indication |
| Thromboembolism / stroke | Dislodged or newly formed atrial thrombus, most often within the first 10 days | Baseline and serial neurologic assessment, verify anticoagulation continuity, activate the stroke pathway for any deficit |
| Ventricular fibrillation | Unsynchronized shock delivered to an organized rhythm, digitalis toxicity, or severe electrolyte derangement | Immediate defibrillation and ACLS; prevention is verification of sync markers and pre-procedure electrolyte correction |
| Pulmonary edema | Rare; post-cardioversion left atrial mechanical dysfunction | Assess work of breathing and oxygenation, upright positioning, notify provider |
| Sedation-related hypotension, hypoventilation, aspiration | Short-acting agent effect | Continuous pulse oximetry and capnography, airway support, position for airway protection, monitor blood pressure every 3–5 minutes until baseline |
| Recurrence of atrial fibrillation | Common, often within hours to days | Reassure, verify rate-control and antiarrhythmic orders, ensure anticoagulation continues |
After the Shock
- Immediately confirm rhythm on the monitor and pulse/blood pressure — electrical conversion without mechanical output is possible. Obtain a post-procedure 12-lead ECG.
- Maintain the airway and supplemental oxygen until the patient is awake and protecting the airway; monitor level of consciousness, respiratory rate and end-tidal CO2 until back to baseline.
- Vital signs and rhythm every 5 minutes for the first 15–30 minutes, then per protocol, with continuous telemetry for a minimum monitoring period defined by institution (commonly 1–4 hours for outpatient elective cardioversion).
- Assess the skin under the pads, the neurologic examination, and the presence of chest pain or dyspnea.
- Documentation is examinable in itself: rhythm before and after each attempt, whether the device was in synchronized mode, energy delivered for each shock, number of shocks, pad position, sedation agent and dose, airway support, vital signs throughout, complications, skin condition, and post-procedure 12-lead findings.
- Discharge teaching after elective cardioversion: no driving or operating machinery for 24 hours after sedation; a responsible adult accompanies the patient home; continue anticoagulation for at least 4 weeks and often indefinitely, exactly as prescribed, even though the heart is now in sinus rhythm; continue antiarrhythmic and rate-control medications; report palpitations, dyspnea, chest pain, syncope, or any neurologic symptom immediately; expect mild chest wall soreness and pad-site redness for a few days; keep the follow-up appointment for rhythm reassessment.
The nursing bottom line for this entire topic: verify the mode, verify the marker, verify the anticoagulation. Those three verifications prevent the three events that turn an elective procedure into a code — an unsynchronized shock inducing ventricular fibrillation, a shock delivered to an unanticoagulated atrium causing a stroke, and a shock delivered without pacing and airway backup in a patient whose sinus node cannot resume.
A patient with a prolonged QT after starting sotalol develops polymorphic ventricular tachycardia at 240 bpm and becomes unresponsive and pulseless. The nurse at the defibrillator selects 120 J, presses SYNC, and the device will not discharge. What should the nurse do?
A 68-year-old woman underwent successful elective cardioversion of atrial fibrillation of 3 months' duration after 4 weeks of therapeutic warfarin. She is in sinus rhythm and asks whether she can stop the warfarin now that her rhythm is normal. Which explanation is correct?
During synchronized cardioversion of atrial fibrillation, the first shock at 150 J is unsuccessful and the provider asks for a second shock at 200 J. Before the second attempt, which action is most important?