5.3 Synchronized Cardioversion and Transcutaneous Pacing

Synchronized Cardioversion vs. Defibrillation

The single most tested distinction in this section is synchronized cardioversion versus defibrillation. Synchronized cardioversion times the shock to land on the R wave, deliberately avoiding the relative refractory period on the T wave — the so-called vulnerable period where a shock can induce ventricular fibrillation (the R-on-T phenomenon). You press SYNC and the defibrillator waits for the next R wave before discharging, so there is a brief, expected delay after you push the shock button. Cardioversion is for unstable tachycardia WITH a pulse: unstable SVT, atrial fibrillation/flutter, and monomorphic VT.

Defibrillation is unsynchronized — it fires the instant the button is pressed at full energy. It is used for pulseless VF/pVT and for polymorphic VT, because the chaotic, ever-changing QRS gives the machine no consistent R wave to sync to; trying to sync a polymorphic rhythm causes the device to hang, refusing to fire while the patient deteriorates. The rule for the exam: a pulse + an organized rhythm = SYNC; no pulse OR a polymorphic/chaotic rhythm = no sync (defibrillate).

Modality decision table

The vulnerable period explained

During the ventricular cycle, the T wave represents repolarization, and the upstroke of the T wave is the relative refractory (vulnerable) period. Cells are partially repolarized and at uneven readiness, so a stimulus landing here can fragment conduction and trigger ventricular fibrillation — the R-on-T phenomenon. An unsynchronized shock dropped randomly onto a perfusing rhythm can land on the T wave and convert a survivable tachycardia into VF. That is precisely why, whenever the patient has a pulse and an organized rhythm, you press SYNC so the device deliberately delivers on the R wave, the safest part of the cycle.

Cardioversion Energy Selection

The AHA's current guidance is to use the energy your specific device recommends to maximize first-shock success, and it has stepped away from rigidly prescriptive numbers. That said, ACLS courses still teach widely accepted initial biphasic starting energies, and exam items expect you to know them. If the first shock fails, escalate in a stepwise fashion.

Initial synchronized cardioversion energies (biphasic)

Narrow regular rhythms (SVT, flutter) convert at the lowest energies because the atrial mass involved is small and the rhythm is organized. Atrial fibrillation needs higher energy (120-200 J biphasic) because of the disorganized atrial activity. Monomorphic VT with a pulse starts at 100 J. Polymorphic VT is the trap: even if the patient still has a pulse, you do not synchronize — you deliver an unsynchronized high-energy shock because the rhythm is too irregular to sync.

Practical cardioversion sequence

1. Confirm a pulse and an organized rhythm — cardioversion is only for perfusing tachyarrhythmias.
2. Sedate the conscious patient if time and perfusion allow.
3. Press SYNC and confirm sync markers appear on each R wave (the monitor flags the R waves).
4. Select energy per rhythm (above), charge, and clear the patient ("I'm clear, you're clear, everyone clear").
5. Press and HOLD the shock button — the device waits for the next R wave before discharging, so expect a short delay.
6. Reassess rhythm and pulse; if the patient deteriorates to pulseless VF, switch the machine to defibrillation (unsynchronized) and shock immediately.

A frequently tested nuance: many defibrillators revert to unsynchronized mode after each synchronized shock, so if you need a second cardioversion you must press SYNC again before the next shock. Forgetting this re-arming step is a classic error.

Transcutaneous Pacing and Procedure Safety

Transcutaneous pacing (TCP) is the electrical answer for unstable bradycardia when atropine is ineffective, contraindicated, or the block is high-grade. Apply pacing pads (anterior-posterior or sternal-apex), select pacer mode, set a rate of about 60-80/min, and increase the output (mA) from zero until you see electrical capture — a pacer spike immediately followed by a broad QRS and T wave. Then confirm mechanical capture by palpating a femoral pulse; checking the carotid is unreliable because the pacing current causes muscle twitching that feels like a pulse.

Once capturing, set the mA slightly above the capture threshold for a safety margin.

Sedation and comfort

Both cardioversion and TCP are painful in conscious patients. When time and perfusion allow, provide sedation and/or analgesia (e.g., a benzodiazepine plus an opioid per protocol) before the procedure. The overriding rule: do not delay lifesaving electricity for a profoundly unstable or arresting patient just to arrange sedation.

Common traps

• Forgetting to press SYNC before cardioverting a patient with a pulse (R-on-T risk).
• Synchronizing a polymorphic VT — the device will not fire; defibrillate instead.
• Pacing a patient who is actually in pulseless arrest rather than starting high-quality CPR.
• Confirming TCP capture on the carotid (twitch artifact) instead of the femoral pulse.

Scenario anchor: A conscious patient in monomorphic VT at 170/min is hypotensive and clammy. The correct electrical therapy is synchronized cardioversion (start ~100 J) with sedation if feasible — not default defibrillation, and not adenosine.