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Paramedic Synchronized Cardioversion vs Defibrillation Study Guide

By Forrest Munden, MundenInteractive.com

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Same box, same pads, two very different jobs. Synchronized cardioversion and defibrillation both push current across the heart, but one is timed to the patient’s own QRS and the other is not, and mixing them up can turn a patient with a pulse into a patient without one. This guide walks through what each shock is for, why timing matters, monophasic versus biphasic, adult versus pediatric pad placement, what to do about medication patches and implanted devices, where the shock sits relative to epinephrine and amiodarone or lidocaine, and how the 2025 AHA guidelines sort shockable from non-shockable rhythms. It is written for paramedics and ACLS providers.

Guideline version verified: 2025 American Heart Association Guidelines for CPR and Emergency Cardiovascular Care (published October 22, 2025, in Circulation), Part 6 (Pediatric BLS), Part 7 (Adult BLS), Part 8 (PALS), and Part 9 (Adult ALS). Checked October 30, 2026: no 2026 AHA guideline or focused update has replaced them. AHA’s February 6, 2026 change notice for the 2025 print edition corrects only algorithm wording (for example, “1 breath every 6 seconds” with an advanced airway). Medication patch and implanted-device guidance comes from the 2010 AHA guidelines, which the 2025 parts do not restate (see Section 6).

Disclaimer: This is a study aid, not official. For PDF, copy into a word processor and export. Always follow current local protocols, medical direction, NREMT skill sheets, your defibrillator manufacturer’s instructions, and the latest AHA 2025 ACLS/PALS guidelines.

Section 1: Two Shocks, Two Purposes

Defibrillation (Unsynchronized)Synchronized Cardioversion
TimingFires the moment you press the shock button, wherever the patient is in the cardiac cycle.The monitor marks each R wave and delivers the shock on the QRS complex.
PatientPulseless: VF or pulseless VT. Also polymorphic VT, which the AHA calls always unstable.Has a pulse but is unstable from a tachyarrhythmia: wide-complex (monomorphic VT) or narrow-complex (SVT, atrial fibrillation, atrial flutter).
GoalDepolarize enough myocardium at once to stop chaotic electrical activity so an organized rhythm can return.Interrupt the tachyarrhythmia without dropping a shock on the vulnerable T wave.
EnergyAdult: per manufacturer; if unknown, the maximum may be considered. Pediatric: 2–4 J/kg first, then 4 J/kg, up to 10 J/kg or the adult maximum.Adult AF: ≥200 J. Adult flutter: 200 J (new in 2025). Pediatric: 0.5–1 J/kg, then 2 J/kg.

Instability is what moves a tachycardia toward electricity. AHA examples include hypotension, acutely altered mental status, signs of shock, ischemic chest discomfort, and acute heart failure. The 2025 ALS guideline recommends synchronized cardioversion for unstable wide-complex and narrow-complex tachycardias. For a stable patient, there is time for a 12-lead, vagal maneuvers, adenosine, or antiarrhythmics per protocol.

Section 2: Why Synchronization Matters (R-on-T and the Vulnerable Period)

On the ECG, the T wave is repolarization. Part of it is a relative refractory (vulnerable) period: some cells have recovered and others haven’t. A stimulus landing there, whether a PVC (the classic “R-on-T”), a precordial thump, or an unsynchronized shock, can set up the re-entry that becomes VF. The 2025 ALS guideline says it directly: when VT is of uniform morphology, “electric cardioversion synchronized to the QRS complex minimizes the risk of provoking VF during the vulnerable period of the cardiac cycle (T wave).”

Section 3: Shockable vs Non-Shockable Rhythms (2025)

RhythmShock?Core 2025 Actions
Ventricular fibrillationYes: defibrillateSingle shock, then immediately resume CPR starting with compressions. Stacked shocks are not preferred.
Pulseless VTYes: defibrillateSame pathway as VF.
AsystoleNoHigh-quality CPR and epinephrine as soon as feasible. Pacing is not recommended in established arrest.
PEANoHigh-quality CPR, epinephrine as soon as feasible, and hunt for reversible causes.

Time is the enemy. The 2025 Adult BLS guideline reports that each minute of delay to the first shock was associated with about a 6% lower probability of survival to discharge. For witnessed or monitored VF/pVT, defibrillate immediately, with brief CPR only while the defibrillator is readied.

Section 4: Monophasic vs Biphasic

Section 5: Pad Placement: Adults vs Children, and Can You Swap Them?

Adults (Part 7, 2025):

Infants and Children (Parts 6 and 8, 2025):

Swapping Pads:

SituationAnswerWhy
Adult pads / no attenuator on a childYes, if nothing else is available. If adult pads won’t fit without touching, go AP.AHA: if neither a manual defibrillator nor an attenuated AED is available, an AED without an attenuator may be considered (2b). Unattenuated AEDs deliver 120–360 J, more than recommended for a child under 25 kg. But defibrillation is the only effective therapy for VF/pVT, and AHA cites reports of safe use. Larger pads also lower impedance.
Pediatric pads / attenuator on an adult (or a post-pubertal teen)No.The attenuator deliberately cuts the dose to a child-sized amount, and smaller pads raise impedance. An adult would be underdosed. Post-pubertal patients get adult care.
Pads touchingNever.Current arcs pad to pad across the skin instead of through the heart. Keep them separated.

Section 6: Medication Patches, Pacemakers, and ICDs

The 2025 Adult BLS and ALS parts don’t restate guidance on transdermal patches or implanted devices. The 2025 guideline documents state that existing guidelines remain relevant unless specifically updated. The most recent AHA statements are in the 2010 Electrical Therapies part:

Section 7: When the Shock Comes Relative to the Drugs

Shockable (VF/pVT) Adult Pathway:

  1. CPR, attach the defibrillator, analyze, and shock as soon as possible.
  2. Resume CPR immediately for 2 minutes. Establish IV access; use IO if IV fails.
  3. Epinephrine 1 mg every 3–5 minutes, given after initial defibrillation attempts have failed. In the AHA algorithm that falls after the second shock.
  4. Amiodarone or lidocaine may be considered for VF/pVT unresponsive to defibrillation (2b). In the algorithm that falls after the third shock. Dose per the ACLS algorithm and your protocol.
  5. Every 2 minutes: rhythm check, shock if still shockable, and minimize the pause.

Non-Shockable (Asystole/PEA):

Other 2025 Points:

Section 8: What Changed From 2020

Section 9: Putting It Together

Question: Your 64-year-old patient has a regular wide-complex tachycardia at 190, BP 72/40, and is confused. While you prepare to cardiovert, the rhythm becomes polymorphic, then degenerates into VF. What do you do with the sync button?

Answer: Start with synchronized cardioversion for the unstable monomorphic VT. When the rhythm turns polymorphic, turn sync off and deliver a high-energy unsynchronized shock. Once it is VF, defibrillate (unsynchronized), resume CPR immediately, and follow the shockable pathway: epinephrine after initial shocks fail, then consider amiodarone or lidocaine.

Reasoning: Synchronization exists to keep the shock off the vulnerable T wave in a heart that still has an organized rhythm. Once the QRS becomes polymorphic, the monitor cannot sync reliably. Once it is VF, there is no QRS to sync to, and every second of waiting costs survival.

Sources (American Heart Association only)