Ilyas K. Colombowala, MD, FACC, FHRS
Cardiac Electrophysiology · Houston, TX · colombowala.com
EP Staff Education

Emergency

Defibrillation in the EP Lab

Defibrillating a patient with intracardiac catheters in place is the EP lab's bread-and-butter emergency. Pads in standard position, full energy, brief catheter management, sedation considerations — the steps that separate routine VF termination from a complicated event.

Indications

  • ▸VF or pulseless VT during a case — immediate defibrillation per ACLS
  • ▸Hemodynamically unstable VT that is not responding to medication
  • ▸Induced VF during EP testing that doesn't terminate spontaneously within seconds
  • ▸Failed cardioversion in a patient with hemodynamic deterioration

Equipment / drugs

  • ▸External defibrillator with pads in place (standard at EP-lab case start)
  • ▸Synchronized cardioversion capability for stable wide-complex rhythms
  • ▸Sedation drawn up: propofol 30–50 mg or etomidate 10–20 mg if patient is awake and needs cardioversion
  • ▸Ablation generator paused/disconnected before the shock
  • ▸Backup catheters and sheaths in case manipulation is needed afterward

Pitfalls

  • ▸Defibrillating an intracardiac sheath or catheter without pausing ablation energy — risk of damage to the catheter, sheath, or the heart
  • ▸Synchronizing in true VF — sync won't fire because there's no R wave to sync to; switch to unsynchronized defibrillation
  • ▸Pad position over an implanted device generator — move to antero-posterior to avoid damaging the device
  • ▸Continuing CPR over the chest with catheters in place without considering catheter displacement — pause briefly to retract or stabilize sheaths if hemodynamically possible
  • ▸Failing to verify defibrillator is charged and ready before announcing the shock — wastes seconds in arrest

Setting the stage

Every patient in the EP lab has external defibrillator pads on at case start. The pads are part of the pre-procedure checklist for a reason: VF and unstable VT are not exceptional events in the lab. They are part of normal practice, especially during VT mapping and ablation, induction testing in ICD recipients, and certain rapid-pacing protocols.

The team should know exactly how to deliver a shock without thinking about it — which means drilling the steps when nothing is happening.

Standard setup

  • Pads in anterolateral position (right of sternum below clavicle + left chest just below the apex) at case start. Move to antero-posterior if there’s a generator in the way or if the procedure requires the anterior chest in the sterile field
  • Defibrillator powered on at case start, set to monitor mode with the lab’s standard lead displayed
  • Charged status verified — pads recognized, leads recognized, battery OK
  • Cables routed clear of the table edge and not under personnel

VF or pulseless VT (the most common scenario)

  1. Recognize. Sudden loss of pulsatile arterial trace + corresponding rhythm on the monitor. Audible alarm.
  2. Pause ablation. If RF or PFA energy is being delivered, stop. Most ablation generators have a foot pedal — release it. Some have a dedicated stop button.
  3. Unscrub or retract. The operator at the table should momentarily back off the sheath. The catheter doesn’t need to come out, but the operator should not be touching it during the shock.
  4. Charge to maximum biphasic energy (typically 200 J). Lower energies have no role in arrest.
  5. Announce, clear, shock. “Charging 200, all clear” — verify nobody is touching the patient or table — deliver the shock.
  6. Resume CPR if no pulse returns. Compressions per ACLS standards. The catheters do not need to come out for compressions; just pause RF or PFA if it was running.
  7. Reassess in 2 minutes per ACLS. Recheck rhythm and pulse.
  8. Epinephrine 1 mg IV every 3–5 minutes, amiodarone 300 mg IV after second or third unsuccessful shock if VF/VT persists.

If the patient is intubated, continuous chest compressions are reasonable. If not intubated, 30:2 compression-to-ventilation ratio.

Cardioversion of unstable wide-complex tachycardia

If the patient has a pulse but is hemodynamically unstable in VT, the goal is synchronized cardioversion. Distinct from defibrillation:

  1. Sedation first if patient is awake — propofol 30–50 mg IV or etomidate 10–20 mg IV. Wait 30–60 seconds for effect.
  2. Sync mode engaged. Verify the defibrillator is marking R waves with a sync indicator.
  3. Start at 100–200 J biphasic for unstable VT (institutional variation). Higher for refractory rhythms.
  4. Announce, clear, shock.
  5. Reassess. If unsuccessful, increase energy and repeat. If patient now in VF, switch to unsynchronized defibrillation.

A common pitfall: trying to deliver synchronized cardioversion in true VF. The sync indicator won’t find R waves and the shock won’t fire. Recognize this within 2–3 seconds and switch to unsynchronized.

Pad position considerations

Anterolateral is default and works for the vast majority of patients. Right anterior pad just right of sternum below the clavicle; left lateral pad in the left mid-axillary line at approximately the 5th intercostal space.

Antero-posterior is preferred when:

  • An implanted device generator is at the standard anterior pad location
  • The anterior chest is in the sterile field for the procedure
  • The patient has had a prior failed anterolateral cardioversion (try AP before escalating energy further)
  • Body habitus argues for AP (very large breasts, prior chest-wall surgery)

In AP position: anterior pad over precordium (V3–V4 area), posterior pad between scapulae at the same level. Both pads facing each other through the heart maximizes current flow through the myocardium.

Catheters and sheaths during the shock

The current concern is not “is it safe to shock with a catheter in?” — it is. The concerns are practical:

  • Sheath kinking if the operator is leaning on it during the shock (a brief muscle contraction)
  • Ablation catheter contact damage if RF or PFA energy was being delivered at the moment of the shock — pause energy first
  • Operator handle position — the operator should not be the conductor between two anatomic structures during the shock; brief unscrub or hand-off the catheter is good practice if the timing allows

In a true emergency where seconds matter, do not delay the shock to manage the catheter. Shock first, manage the catheter afterward.

Sedation for awake patients needing shocks

Unsedated cardioversion or defibrillation is genuinely traumatic. Always sedate when there is time:

  • Propofol 30–50 mg IV bolus, additional 10–20 mg as needed. Fast onset, fast offset. Watch for hypotension on top of the underlying compromise.
  • Etomidate 10–20 mg IV bolus. Faster hemodynamic profile than propofol, ideal for already-compromised patients. Some risk of myoclonus.
  • Midazolam 2–5 mg IV — slower onset, longer offset, less hemodynamic neutrality. Used when propofol/etomidate aren’t available.

The sedation does not need to be deep — the patient just needs to be unaware of and amnestic for the shock.

After the shock — checklist

  1. Rhythm check. Sinus? Recurrent VF? Asystole? VT? Each requires a different next move.
  2. Pulse check. Mechanical capture matters more than the monitor rhythm.
  3. Hemodynamic check. Arterial line trace returning? Pulse oximetry recovering?
  4. Catheter status. Where is everything? Anything need repositioning?
  5. Tamponade check. Especially if the patient was hemodynamically unstable around the shock — quick ICE look.
  6. Anticoagulation status. If you stopped heparin in the moment, you’ll need to restart or reverse depending on the next step.
  7. Decide on next steps. Continue the case? Abort? Transfer to ICU? This conversation happens in the moments after stability.

Failed shocks

If the first 200 J shock doesn’t terminate VF:

  • Continue CPR for 2 minutes
  • Increase energy to 360 J (some defibrillators allow higher)
  • Switch pad position from anterolateral to antero-posterior — meaningfully changes the current vector
  • Verify pad-to-skin contact — check for pad lift, gel migration
  • Consider double-sequential defibrillation in refractory cases — two defibrillators, two pad sets, shocks delivered simultaneously
  • Amiodarone 300 mg IV bolus, then 150 mg IV in 3–5 minutes if needed
  • Look for reversible causes — particularly hyperkalemia, profound acidosis, ongoing ischemia
  • Mechanical CPR device if extended resuscitation is anticipated

Refractory VF is among the most challenging EP-lab events. Early escalation to ECMO consult is reasonable in select centers.

Last reviewed by Dr. Colombowala on May 24, 2026.

Clinical-reference content, not medical advice. This page is written for EP staff and does not create a doctor-patient relationship. It does not replace institutional policy, current device manuals, or attending direction during a case. See the full disclaimer.

© 2026 Ilyas K. Colombowala, MD. All rights reserved. Reproduction, redistribution, or republication of this content in any form without written permission is prohibited.

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