Description

Defibrillator

Defibrillator is a device used to perform defibrillation for the purpose of turning abnormal ECG rhythm into normal rhythm. When heart enters into ventricular fibrillation, the process of returning it to its normal sinus rhythm by giving proper amount of external electric current is called defibrillation, and device used in giving such electric current is called defibrillator.  Should electrical energy is given in the form of shocks, it creates stimulation namely, myocardium is depolarized and systole occurs. To give shock to heart by means of defibrillator, heart should be in ventricular fibrillation or ventricular tachycardia rhythm without pulse.

A defibrillator is an electrical device that provides a shock to the heart when there is a life-threatening arrhythmia present. The arrhythmia that we worry about is called ventricular fibrillation. This is a very rapid erratic beating of the heart. Multiple parts of pacemakers in the heart starts sort of beating erratically and the heart can’t rhythmically contract. And what the defibrillator does, it provides shock that basically shocks the heart to stop so that it can start rhythmically contracting again.

Defibrillation is a procedure used to treat life threatening conditions that affect the rhythm of the heart such as cardiac arrhythmia, ventricular fibrillation and pulseless ventricular tachycardia.

The procedure involves the delivery of an electric shock to the heart which causes depolarisation of the heart muscles and re-establishes normal conduction of the heart’s electrical impulse. The machine used to deliver this therapeutic shock to the heart is called a defibrillator.

Understanding Defibrillation Waveforms

Before we start, let’s define a few terms:
Energy: Energy in a defibrillator is expressed in joules. A joule 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 type of defibrillation waveform where a shock is delivered to the heart from one vector as shown below. It is shown graphically as current vs. time.

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 insure maximum current is delivered in the face of an inability to detect patient impedance.

Biphasic Defibrillation 

Unlike conventional monophasic defibrillators, biphasic defibrillators deliver current in two directions. In the first phase, the current moves from one paddle to the other as with monophasic defibrillators. During the second phase, the current flow reverses direction. The underlying physiologic mechanisms aren’t fully understood yet, but it is clear that biphasic waveforms lower the electrical threshold for successful defibrillation. The shape of the current’s delivery is determined by the current, direction of energy and duration of the delivered energy. Animal research has demonstrated that the most effective waveforms maintain their shape and duration regardless of patient impedance. Biphasic waveforms adjust for impedance by varying the characteristics of the waveforms. This is intended to ensure that high-impedance persons will have the same chance for survival as those who are of low impedance.
Biphasic defibrillation offers equal or better efficacy at lower energies than traditional monophasic waveform defibrillators, with less risk of post-shock complications such as myocardial dysfunction and skin burns. This is why almost all manufacturers of external defibrillators are now using biphasic waveforms in their devices

What is Biphasic Waveform?

Biphasic defibrillator gives bilateral current as to be positive and negative after its electrical energy is discharged. Namely, it conducts electrical current bilaterally. On the first phase, current flows from one electron to the other as it is in monophasic defibrillator. On the second phase, current starts to flow on reverse direction. According to the studies done, it is observed that biphasic waves produce more successful defibrillation with lower energy than monophasic waves and so they are less harmful. In addition, since same results can be obtained with low energy use, some adverse effects such as burns are also reduced.

What is the difference between a monophasic and biphasic defibrillator?

Defibrillator is a device used to shock the heart back into action when it stops contracting due to a disorder of the rhythm known as ventricular fibrillation (VF). The electrodes used to deliver the shock could be either defibrillator paddles or patches, directly applied to the chest below the left collar bone and at the apex of the heart.

The direct current shock given can have a monophasic or biphasic wave form. In monophasic shock, the shock is given in only one direction from one electrode to the other. In a biphasic shock, initially direction of shock is reversed by changing the polarity of the electrodes in the latter part of the shock being delivered. Usually the initial voltage applied is higher than the reversed polarity shock. Biphasic wave forms were initially developed for use in implantable cardioverter defibrillators (ICD) and later adapted to external defibrillators. Biphasic truncated exponential wave form and rectilinear biphasic waveform are two types of biphasic waveforms used by different manufacturers. Defibrillators can sense the thoracic impedance and increase or decrease their internal resistance so that the selected level of energy is delivered to the subject.

Biphasic shocks are more effective than monophasic shocks and need lesser energy. Typically when 360 Joules are delivered for defibrillation in a monophasic defibrillator, 200 Joules are given in a biphasic defibrillator. This could theoretically reduce the potential damage to the heart muscle by the high voltage shock.