• Defibrillation is the application of electricity during cardiac arrest either to the chest wall or directly to the heart muscle to restore a coordinated cardiac rhythm and a spontaneous circulation.
• The first scientific work on defibrillation was published by a team of researchers at the Johns Hopkins University in 1933. Funding was provided from an electrical company as a result of an increase in the rate of sudden death among utility workers who were rewiring American homes with electricity. This work was followed by the development of closed chest cardiac massage by the same team and publication in 1960.

• Adequate current flow through the heart is required for successful defibrillation: I = V/R, where I = current, V = voltage, and R = resistance. Therefore, for the same voltage (V) applied, the current (I) delivered to the patient varies depending on the transthoracic impedance (R).
• Transthoracic impedance (70–80 Ohm) is influenced by:
  • Electrode surface area: Electrodes should be 8–12 cm in diameter for adults. Large electrode pads result in smaller transthoracic impedance.
  • Pressure on the electrodes: As per the American Heart Association guidelines, a force of 8 kg applied to paddles results in an optimal reduction of transthoracic impedance.
  • Body habitus and tissue properties of the rib cage, muscle, subcutaneous fat and skin contribute to transthoracic impedance. In order to further reduce transthoracic impedance, a conductive material such as gel pads or electrode paste should be used.
• There is no direct relationship between the energy applied (Energy = Power × Time) and current flow.
• Theories about mechanisms of defibrillation:
  • Critical mass theory (original): The delivery of current to a critical mass of myocardium can stun and make the muscle unexcitable. The uncoordinated waveforms of excitation that perpetuate ventricular fibrillation would then be extinguished and a normal cardiac excitation will resume. Tissue in close proximity to the shocking electrodes produces virtual electrodes that either hyperpolarize or depolarize the surrounding cardiac tissue.
  • Vulnerability theory (newer): Defibrillation is mediated by depolarizing fully excitable cells and also by depolarizing cells in the relative refractory period by a stimulus that exceeds the upper limit of vulnerability.
• Defibrillator waveforms:
  • Monophasic defibrillators deliver current in one direction (one polarity).
  • Biphasic defibrillators deliver current in 2 directions. The current flows in one direction during the first phase of the waveform and in the opposite direction during the second phase. Therefore, lower energy levels can be used during defibrillation for equivalent or higher success rates.
  • No specific waveform has shown to be consistently associated with a greater incidence of return of spontaneous circulation or higher survival to hospital discharge.
  • All newer defibrillators deliver biphasic shocks, but monophasic shocks are still acceptable and currently in clinical use.

• Physiological cardiac excitation is initiated in the sinus node. It travels through atrial tissue to the AV node and is then conducted through the bundle of His to the bundle branches over the Purkinje fibers and finally to the myocardium.
• Ventricular fibrillation (Vfib) is a state of disorganized electrical activity that leads to uncoordinated contraction of the myocardium, making its ejection ineffective.

Cardiac arrest is a leading cause of death in many parts of the world.

• In the US and Canada, approximately 350,000 people suffer a cardiac arrest per year and require resuscitation (half of the cases occur in-hospital).
• The incidence of in-hospital cardiac arrests is estimated at 3–6/1,000 admissions.
• for every minute that elapses between collapse and defibrillation, the chance of survival from ventricular fibrillation diminishes by 7–10% every minute if no CPR is provided and by 3–4% with CPR.
• Defibrillation is the only effective treatment of Vfib and should be performed within 10 minutes after sudden onset cardiac arrest (<3 minutes in hospital) to achieve a good neurologic outcome.

• External (transthoracic) defibrillation:
  • Initial energy level: for biphasic defibrillators, follow the recommendations by the manufacturer. If those are not readily available, it is reasonable to start with 150–200 J when using a biphasic defibrillator to treat Vfib/pulseless ventricular tachycardia (Vtach), and it is optional to increase up to 360 J for subsequent shocks or to continue with the initial amount of energy. The recommendation for monophasic defibrillators is to use 360 J for the initial and all subsequent shocks.
  • Unsynchronized shock describes the delivery of a shock that is not synchronized with the QRS wave. As there is no organized electrical activity during Vfib or pulseless Vtach, this is the treatment of choice for those entities and an integral part in the treatment of sudden cardiac arrest.
  • Pads or paddles:
    • To decrease transthoracic impedance one should use gel pads or electrode paste (American guidelines only), shave the chest if it does not delay chest compressions or shock delivery, and press the paddles firmly to the chest (8 kg of force).
    • Can be placed in the following positions: Antero-lateral (default location), antero-posterior, anterior-left infrascapular, or anterior-right infrascapular position. There are no studies in patients with Vfib or pulseless Vtach directly comparing the effects of electrode positioning to success of defibrillation. Therefore, practical considerations usually dictate electrode placement to optimize current flow through the patient's heart. In adults, antero-lateral placement is favored, while in children the antero-posterior placement is often used in order for the pads not to overlap.
    • In patients with an implantable cardioverter defibrillator (ICD), pads should be placed at least 8 cm away from the device in an anterior-posterior or anterior-lateral position.
    • Do not place pads or paddles directly over transdermal medication patches because the patch may block the delivery of energy and may cause burns to the skin.
  • Pediatric:
    • Energy levels should start at 2 J/kg and can be escalated up to 10 J/kg (not to escalate the adult maximum dose).
    • In children aged 1–8 years, pediatric electrodes should be used.
    • Pads or paddles should be applied with 5 kg of force and can be in the following positions: Antero-apical, antero-posterior, or apex-posterior position.
• Temporary transvenous defibrillation can be considered as an alternative in patients undergoing electrophysiological procedures.
• ICDs are usually implanted underneath the pectoral muscle below the left clavicle and defibrillate with 40 J of energy via an internal pacing wire embedded in the right ventricle (up to 8 times per detected Vfib/Vtach).

• I = V/R, where I = current, V = voltage, R = resistance
• Energy = power × time

• 2010 ACLS Cardiac Arrest Algorithm for Vfib or pulseless Vtach arrest:
  • Activate Emergency Response System
  • Start CPR, administer oxygen, attach monitor/defibrillator
  • Confirm Vfib or pulseless Vtach and defibrillate
  • Continue CPR for 2 minutes and establish intravenous/intraosseous access
  • Check rhythm: If shockable, defibrillate; otherwise proceed with ACLS pathway for asystole/PEA, or postcardiac arrest care
  • Continue CPR for 2 minutes, administer epinephrine every 3–5 minutes, consider advanced airway and capnography
  • Check rhythm: If shockable defibrillate; otherwise proceed with ACLS pathway for asystole/PEA, or postcardiac arrest care
  • Continue CPR for 2 minutes, give amiodarone and treat reversible causes
  • Repeat check rhythm.
• for witnessed cardiac arrest, 1 shock followed immediately by CPR is favored over 3 stacked shocks. The European guidelines allow consideration of 3 stacked shocks in the early postoperative period following cardiac surgery, in the catheterization lab, or when the patient is already connected to a defibrillator and the cardiac arrest is witnessed.

• Ventricular Fibrillation
• Ventricular Tachycardia
• Operating Room Fires

Jochen Steppan , MD

Nanhi Mitter , MD