Study Notes: Defibrillator Modes – Defibrillation, Cardioversion & Pacing

1. Overview

• Defibrillator = A key emergency tool on crash carts.

• Main modes:

  1. Defibrillation

  2. Synchronized Cardioversion

  3. Transcutaneous Pacing

• Also functions as a monitor.

• All modes use pads or paddles + electrical energy.

2. Electrical Basics

• Joules (J): Energy delivered (amps × voltage × time).

• Milliamps (mA): Current used (important in pacing).

3. Delivery Methods

• Pads (preferred):

  • Quick, easy, often pre-attached to defibrillator.

  • Placement:

    • Anterior-Posterior (preferred):

      • One pad on front (low anterior chest, over heart).

      • One pad on back (between scapulae).

    • Anterior-Apex:

      • One pad on right upper chest (below clavicle).

      • One pad on left lower chest (below pectoral muscle/breast).

• Paddles:

  • Classic, seen in movies; less common now.

  • Internal paddles used during open-heart surgery (require less energy).

4. Defibrillation

• Definition: Delivery of high-energy shock (J) → depolarizes myocardium → allows SA node to reset rhythm.

• Indications:

  • Ventricular fibrillation (VF)

  • Pulseless ventricular tachycardia (VT)

  • Not used in: Asystole or PEA.

• Steps:

  1. Attach pads.

  2. Switch to defib mode.

  3. Press charge.

  4. Ensure “all clear.”

  5. Deliver shock immediately.

  6. Resume chest compressions immediately (no pulse check yet).

• Energy settings (external):

  • Start 120 J → 150 J → 200 J.

  • Continue at 200 J for subsequent shocks.

• Energy settings (internal paddles):

  • 5 J → 10 J → 20 J → 30 J → 50 J (then maintain 50 J).

• Monophasic vs Biphasic:

  • Monophasic (older): Current flows one direction, max 360 J.

  • Biphasic (modern): Current flows both directions, max 200 J.

  • Biphasic = less energy needed, less myocardial injury, fewer burns, better first-shock success.

5. Synchronized Cardioversion

• Definition: Timed shock (J) synced to R wave → avoids shocking during T wave (risk: VF).

• Indications: Tachyarrhythmias with a pulse:

  • Atrial fibrillation (AFib)

  • Atrial flutter (Aflutter)

  • Supraventricular tachycardia (SVT)

  • Ventricular tachycardia (VT) with pulse (unstable patient).

• Steps:

  1. Attach pads.

  2. Select synchronize mode.

  3. Confirm markers (“dots”) on R waves.

  4. Press charge.

  5. Hold shock button (shock delivered on next R wave).

  6. Reassess rhythm.

• Energy: Usually 50–100 J for adults.

• Key difference vs defibrillation:

  • Defib = immediate, asynchronous shock.

  • Cardioversion = synchronized with R wave.

• Patient care: Often awake → sedation/analgesia required if possible.

6. Transcutaneous Pacing

• Definition: Delivery of electrical impulses (mA) through pads to stimulate ventricular contraction.

• Indications:

  • Symptomatic bradycardia.

  • 2nd or 3rd degree AV block.

• Temporary measure until transvenous or implanted pacemaker.

• Setup:

  1. Attach pads (prefer Anterior-Posterior).

  2. Attach 3-lead monitor (separate from pacing pads).

  3. Switch to pacer mode.

  4. Set pacing rate (desired HR).

  5. Increase output (mA) until capture.

     • Capture = pacing spike followed by wide QRS.

  6. Reduce output to find capture threshold.

  7. Increase by +10% above threshold (safety buffer).

  8. Select pacing mode:

     • Asynchronous: Delivers pulses regardless of native rhythm.

     • Demand mode: Only delivers pacing if HR < set rate.

• Output range: Usually up to 140 mA, titrated by 5 mA increments.

• Patient care: Analgesia/sedation needed (pacing is painful).

7. Key Clinical Points

• Defibrillation = shock lethal arrhythmias (VF, pulseless VT).

• Cardioversion = synchronized shock for tachyarrhythmias with pulse.

• Pacing = temporary bridge for unstable bradycardia/blocks.

• Always know your machine setup (practice in non-emergent times).

• Immediate CPR after defibrillation is critical.

• Sedation for awake patients in cardioversion/pacing whenever possible.

Takeaway: The defibrillator is more than just a “shock box”—it’s a multi-function tool (defib, cardioversion, pacing, monitoring). Mastery of pad placement, mode selection, and energy/output settings is essential for effective and safe use in emergencies.