External Defibrillator

 

External Defibrillator

Why a External Defibrillator is needed?

 

  • Sudden cardiac arrest usually happens without a warning , and the majority of people have not previously recognized symptoms of the heart disease. Patient survival depends on a rapid sequence of rescue events that may include the successful delivery of a high-energy shock from an external defibrillator. 
  • This depolarizes a critical mass of the heart muscle, terminates the arrhythmia and allows normal sinus rhythm to be re-established by the sinoatrial node.
  • Ventricular Fibrillation is the most common electrical mechanism in cardiac arrest.

Why a External Defibrillator is needed

External Defibrillator

  • Defibrillators are devices that are designed to deliver electrical energy to the heart for the purposes of stopping a life-threating cardiac arrhythmia and restoring the heart’s normal rhythm. 
  • The term “external defibrillator” is generally used to refer to any device that operates outside the body and delivers energy through paddles or electrode pads for the purposes of restoring normal rhythm.
  • A capacitor is charged with an appropriate level of voltage for the device (either by an operator (press of a button) or automatically) and upon initiation of the shock, current is delivered directly to the heart to interrupt the arrhythmia and restore the normal conduction.

 

Type of External defibrillator

  • Manual External Defibrillator (These units are primarily found in hospitals)
  • Wearable Defibrillator
  • Automated External Defibrillator
  • Semi-automated External Defibrillator

Type of External defibrillator

Understanding Defibrillation Waveforms

Let’s define a few terms:

Energy: A joule (J) is the unit of work associated with one amp of current passed through one ohm of resistance for one second.

When we express it in a formula, it is generally stated as follows:

Joules (Energy) = Voltage  X  Current  X  Time

Joules have become a surrogate for current in modern defibrillator language.

Current: Current is what actually defibrillates the heart. It is also expressed as Voltage/Impedance (resistance).

Impedance: Resistance to Flow; there is resistance in the electrical circuit itself as well as in the patient. The amount of impedance in a patient is difficult to determine as it relates to body mass, temperature, diaphoresis quality of the contact with paddles or pads. Impedance is expressed in ohms.

 

Monophasic Waveforms: 

  • A shock is delivered to the heart from one vector.
  • In this waveform, there is no ability to adjust for patient impedance, and it is generally recommended that all monophasic defibrillators deliver 360J of energy in adult patients to ensure maximum current is delivered in the face of an inability to detect patient impedance.

Monophasic Waveforms

Biphasic Waveforms:

  • A shock is delivered to the heart via two vectors. 
  • Biphasic waveforms were initially developed for use in implantable defibrillators and have since become the standard in external defibrillators.
  • All biphasic waveforms have been shown to allow termination of VF at lower current than monophasic defibrillators. 
  • There are two types of waveforms used in external defibrillators:

Biphasic Waveforms

Manual external defibrillator

  • Require the expertise of a healthcare professional. 
  • They are used in conjunction with an electrocardiogram, which can be separate or built-in. 
  • A healthcare provider first diagnoses the cardiac rhythm and then manually determines the voltage and timing for the electrical shock. 
  • These units are predominantly in hospitals and on some ambulances. 

Manual external defibrillator

Manual external defibrillator…

  • Sync mode uses a defibrillator discharge to correct certain arrhythmias, such as VT; a shock is delivered only when the control circuits sense the next R wave. 
  • The delivery of energy is synchronized with and shortly follows the peak of the R wave, preventing discharge during the vulnerable period of ventricular repolarization, which is represented by the T wave. 
  • An audible/visible indicator informs when the capacitor is charged and the device is ready. 
  • ECG monitoring can be performed before, during, and after a discharge, usually through ECG electrodes, although most external paddles and disposable electrodes have ECG monitoring capability. 
  • Many defibrillators are equipped with optional monitoring capabilities (SpO2, ETCO2, Temperature, NIBP).

Manual external defibrillator

Physics behind Defibrillator

3 major components of a defibrillator:   

  1. Power supply
  2. Capacitor
  3. Inductor

Physics behind Defibrillator

Block Diagram of defibrillator 

Block Diagram of defibrillator