1.4 AED Operation & Special Resuscitation Circumstances

Biophysics of Defibrillation & Universal Operational Steps

Automated External Defibrillators (AEDs) are computerized devices capable of analyzing cardiac rhythms and delivering precise electrical shocks to terminate lethal ventricular arrhythmias. Early defibrillation is the single most critical determinant of survival in adult cardiac arrest presenting with Ventricular Fibrillation (VF) or pulseless Ventricular Tachycardia (pVT). For every minute that passes between collapse and defibrillation without CPR, survival declines by 7% to 10%. Even with high-quality CPR, survival decreases by 3% to 4% per minute of shock delay.

Defibrillation does not 'start' an arrested heart. Instead, it delivers a high-voltage electrical current through the myocardium that simultaneously depolarizes a critical mass of chaotic cardiac muscle cells. This brief period of total electrical cutaneous standstill ('asystole') allows the heart's natural pacemaker—the Sinoatrial (SA) Node—to resume organized electrical pacing.

Modern AEDs utilize biphasic waveforms, which pass electrical energy through the heart in one direction for a specified duration before reversing flow back through the heart. Biphasic defibrillators adjust current based on individual patient transthoracic impedance, delivering equal or superior efficacy at lower energy levels (typically 120 J to 200 J) compared to older monophasic devices (360 J), thereby minimizing post-shock myocardial stunning.

Regardless of AED make or model, rescuers must follow the Universal Four-Step Operational Sequence:

1. Step 1: Power ON the AED. This is the most crucial initial step because turning on the device activates automated voice prompts and visual instructions that guide rescuers through all remaining steps.
2. Step 2: Attach Electrode Pads. Expose the patient's chest and apply self-adhesive pads directly to dry skin in the proper anatomical vector.
3. Step 3: Clear the Patient and Analyze Rhythm. Instruct all rescuers to stop touching the patient ('Clear!'). Motion artifact from chest compressions or rescue breathing will distort rhythm interpretation.
4. Step 4: Clear the Patient and Deliver Shock (if indicated). If a shockable rhythm is detected, the device charges. The rescuer performs a 360-degree visual sweep, declares loudly: 'Clear the patient, shock on three—one, two, three, shocking!', presses the shock button, and immediately resumes chest compressions.

Pad Vector Geometry & Transthoracic Impedance Dynamics

For defibrillation to successfully terminate VF/pVT, an adequate density of electrical current must pass directly through the ventricular myocardium. Current density is governed by pad placement vector and Transthoracic Impedance (TTI)—the anatomical resistance of the chest wall to electrical current flow.

Rescuers must utilize one of two validated pad placement configurations:

1. Anterolateral Pad Placement (Standard):

   • Right (Anterior) Pad: Placed directly below the right collarbone (clavicle), to the right of the sternum.
   • Left (Lateral) Pad: Placed on the lower left chest, lateral to the left nipple, along the mid-axillary line (mid-axillary 5th/6th intercostal space).
   • Vector: Electrical current travels diagonally through the anterior wall, septum, and apex of the left ventricle.

2. Anteroposterior (AP) Pad Placement (Alternative):

   • Anterior Pad: Placed over the left precordium (center-left chest).
   • Posterior Pad: Placed on the back, directly between or below the left scapula.
   • Vector: Current travels directly from front to back through the heart. AP placement is highly effective in thin adults, pediatric patients, or when anterior chest skin trauma limits pad placement.

Factors Influencing Transthoracic Impedance: High TTI resists electrical current flow, reducing the delivered dose to the heart. Key factors increasing TTI include poor pad skin contact, presence of air trapped under pads, thick chest hair, wet skin, small pad surface area, and improper pad pressure. Rescuers must press adhesive pads down firmly from center to edge to eliminate air bubbles.

Managing Special Resuscitation Circumstances

Rescuers frequently encounter environmental or patient-specific barriers that require rapid troubleshooting to ensure safe and effective defibrillation:

• Excessive Chest Hair: Dense hair traps air beneath electrode pads, increasing TTI and creating a severe risk of electrical arcing (sparks) and thermal skin burns. Action: Press pads firmly down. If the AED displays a 'check pads' or 'check electrodes' error, rapidly rip the first set of pads off the chest—this acts as an immediate depilatory stripping away the hair. Then apply a fresh set of pads to the cleared skin. If a razor is instantly accessible in the AED case, quickly shave the pad contact sites.
• Water, Moisture, and Standing Water: Water is an exceptionally strong electrical conductor. Shocking a patient lying in standing water (puddles, pool decks, boat bilges) causes electrical current to travel across the surface water, potentially shocking rescuers and bypassing the patient's heart. Action: Quickly move the patient out of standing water. Briskly wipe the patient's torso dry with a towel or clothing before applying pads. (Chest moisture from sweat or rain must be wiped dry; however, dry snow or minor ambient moisture on surrounding ground is safe once the chest skin is dried).
• Implanted Pacemakers and ICDs: Patients with cardiac devices have a hard, visible or palpable sub-Q bulge (about the size of a pocket watch) under the skin, usually below the right or left clavicle. Action: NEVER place an AED pad directly over an implanted device. The metallic casing of the pacemaker/ICD will absorb the electrical shock, block current from reaching the heart, and damage the internal circuitry. Place the pad at least 1 inch (2.5 cm) away from the bulge, or use Anteroposterior (AP) placement.
• Transdermal Medication Patches: Patches (e.g., nitroglycerin, nicotine, fentanyl, clonidine) contain active medication and frequently feature aluminum foil backings. Placing an AED pad over a patch blocks electrical current and causes arcing, explosive sparks, and severe skin burns. Action: Wearing gloves (to avoid absorbing concentrated medication through your own skin), peel the patch off, wipe the skin clean of remaining ointment, and apply the AED pad.

Clinical Scenario: Rescuers respond to an adult male in cardiac arrest on a wet boat dock. The patient has excessive chest hair, a visible pacemaker bulge under his right collarbone, and a nitroglycerin patch on his upper chest. The team acts systematically: 1) Move the patient off the wet dock onto dry land, 2) Wipe his chest completely dry, 3) Put on gloves, remove the nitroglycerin patch, and wipe the skin clean, 4) Apply the AED pads, positioning the right pad 1.5 inches below the pacemaker bulge, and 5) Firmly press the pads down to displace hair. The AED analyzes VF, charges, and a shock is delivered safely with immediate resumption of CPR.