The following information is an excerpt from a comprehensive educational resource regarding the science of defibrillation provided by Resuscitation Central.  The complete discussion can be accessed at the Resuscitation Central Website. Resuscitation Central is sponsored by an educational grant from ZOLL Medical Corporation.


Defibrillation is based upon the understanding that contraction of the heart, and the resulting circulation, is under the control of the electrical conduction system of the heart.

The sinoatrial node, (SAN) located within the wall of the right atrium, normally generates electrical impulses that are carried by special conducting tissue to the atrioventricular node

Upon reaching the AVN, located between the atria and ventricles, the electrical impulse is relayed down conducting tissue (bundle of HIS) that branches into pathways that supply the right and left ventricles. These paths are called the right bundle branch (RBBB) and left bundle branch (LBBB), respectively. The left bundle branch further divides into two sub branches
(called fascicles).

Electrical impulses generated in the SAN cause the right and left atria to contract first. Depolarization (heart muscle contraction caused by electrical stimulation) occurs nearly simultaneously in the right and left ventricles 1­2 tenths of a second after atrial depolarization. The entire sequence of depolarization, from beginning to end (for one heart beat), takes 2­3 tenths of a second.

The SAN is known as the “heart’s pacemaker” because electrical impulses are normally generated here. At rest, the SAN usually produces 60­70 signals a minute. It is the SAN that increases its rate due to stimuli such as exercise, stimulant drugs, or fever.

Should the SAN fail to produce impulses the AVN can take over. The resting rate of the AVN is slower, generating 40­60 beats a minute. The AVN and remaining parts of the conducting system are less capable of increasing heart rate, due to stimuli previously mentioned, than the SAN.

Problems with signal conduction, due to disease or abnormalities of the conducting system, can occur any place along the heart’s conduction pathway. Abnormally conducted signals, or arrhythmias, result in alterations of the heart’s normal beating. This is visualized on the electrocardiogram (EKG).

Ventricular Fibrillation

Ventricular fibrillation is the most common electrical mechanism in cardiac arrest. Fibrillation is the manifestation of chaotic electrical excitation of the chambers of the heart. The consequence is the loss of coordinated contraction of the myocytes around the chambers so that the heart no longer pumps blood adequately or at all.
VF begins as a quasiperiodic reentrant pattern of excitation in the ventricles, with resulting poorly synchronized and inadequate myocardial contractions. Multiple foci within the ventricles are firing rapidly and independently. As the initial reentrant pattern of excitation breaks up into multiple smaller wavelets, the level of disorganization increases. There is no coordinated mechanical activity of the ventricles and, thus, no effective ventricular contraction.

The sudden loss of cardiac output with the subsequent tissue hypo­ perfusion creates global tissue ischemia. The brain and the myocardium itself are most susceptible to the loss of oxygenation, and tissue death begins within minutes.

The Etiology of Ventricular Fibrillation (VF)

The etiology of VF is not completely understood. It often occurs in the setting of acute cardiac ischemia or acute myocardial infarction (MI). It is diagnosed in up to half of sudden­death survivors.

Abnormal rapid stimulation of the ventricles (ventricular tachycardia, or V­tach) can lead to fibrillation. Severe left ventricular dysfunction, a variety of cardiomyopathies and acquired or idiopathic long QT syndrome also increase the risk of fibrillation.

Pathophysiology of VF

Sudden cardiac death can be viewed as a continuum of electromechanical states of the heart:

• Ventricular Tachycardia (VT)
• Ventricular Fibrillation (VF)
• Asystole or pulseless electrical activity (PEA)

VF/VT is the most common initial state, and due to insufficient perfusion of vital cardiac tissues, it degenerates to asystole if left untreated.

VF begins as a coarse, irregular deflection on the ECG, then degenerates to a fine, irregular pattern, and eventually becomes asystole. See Figure 1 below. At the onset of VF, the QRS complexes are regular, widened and of tall amplitude, suggesting a more organized ventricular tachyarrhythmia. Over a brief period of time, though, the rhythm becomes more disorganized, with high amplitude fibrillatory waves – this is coarse VF. After a longer period of time, the fibrillatory waves become finer, culminating in asystole.

Figure 1. Degeneration of VF

Course VF Fine VF Asystole

Principle of Defibrillation

Defibrillation is the definitive treatment for the life-threatening cardiac arrhythmias ventricular fibrillation and pulseless ventricular tachycardia. Defibrillation consists of delivering a therapeutic dose of electrical energy to the affected heart with a device called a defibrillator. This depolarizes a critical mass of the heart muscle, terminates the arrhythmia and allows normal sinus rhythm to be re­established by the sinoatrial node.
Defibrillators can be external, wearable or implanted, depending on the type of device used, but all operate on the same principle. A ventricular arrhythmia is detected by the monitoring circuit of the device. A capacitor is charged with an appropriate level of voltage for the device (either by an operator or automatically) and upon initiation of the shock (automatically or upon the press of a button) current is delivered directly to the heart to interrupt the arrhythmia and restore normal conduction. When the current is delivered via an internal defibrillator far less is required as there is not a lot of resistance in the circuit. External defibrillators, however, have to be able to deliver sufficient current to reach the heart through the mass of skin, hair and tissue.

Until recently, most external defibrillation shocks were delivered via paddles placed upon the patient’s chest and the waveform was monophasic: Current traveled in one direction through the heart (monophasic waveform). Two major changes have occurred in the past decade. Today in the US, the majority of defibrillation shocks are delivered through defibrillation electrodes, pads that are placed directly on the patient’s skin. These defibrillation pads are safer for rescuers, and because they conform to the chest are generally able to deliver the current more effectively.

The second major change in defibrillation is the use of biphasic waveform to deliver the current to the heart. With a biphasic waveform, current is delivered to the heart in two vectors. Because of the two­vector approach, the peak current required to convert the arrhythmia is reduced, and the efficacy of the shock is greatly enhanced. It is generally accepted that biphasic defibrillation results in less myocardial damage from the shock itself.

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 Waveforms: A type of defibrillation waveform where 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.

While 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.  These are shown below.

Defibrillator manufacturers have approached biphasic defibrillation differently.

Both Physio Control and Philips use the biphasic truncated exponential (BTE) waveform originally developed for internal defibrillators, though they use different energy settings with the waveform. Physio Control uses what they term a “high energy” biphasic waveform, which they term ADAPTIV™ Biphasic. Physio Control energy settings go up to 360 joules of energy and they essentially distribute the voltage and current available over a wider range of energy settings. Additionally they vary the voltage and extend the duration of the shock in higher impedance patients.

Therefore, with a Physio Control BTE Waveform, you might see the following differences in the waveform when patient impedance differs:

Philips Medical also uses the biphasic truncated exponential waveform in their SMART Biphasic device, but in this case, they distribute the voltage and current available over a more narrow range of energy with the maximum current delivered at 200J, roughly equivalent to that delivered by the Physio Control device at 360J.

The Rectilinear Biphasic Waveform (RBW) is used by ZOLL Medical, and it differs from both of the BTE waveform devices. ZOLL fixes voltage at the maximum and varies resistance in order to deliver constant current across the broad range of patients. Like Philips, 200 Joules is the maximum setting on the defibrillator, however this maximum represents more voltage on the capacitor than either Physio Control or Philips has available. Additionally, the duration of the ZOLL RBE waveform is fixed at 10 msec based upon work by Gliner et al.1 which indicates that the defibrillation threshold decreases with increasing time up to a point around 10­12 msec, after which is begins to increase. As there is concern in the literature about the effects of current on myocardial stunning, ZOLL chooses not to go beyond that threshold.2
The ZOLL RBW defibrillator actually divides impedance into two components: equipment­based impedance and patient­based impedance. Rather than adjusting the secondary variables, such as voltage and time, the ZOLL RBW adjusts the equipment­based impedance, and adds or subtracts resistors in the equipment as required to control for an essentially “constant” current during the course of the first phase.

For example, for a 200J energy setting, the ZOLL RBW charges the capacitor to the maximum voltage regardless of patient impedance. In the case of a patient with 50 ohms of impedance, the defibrillator controller adds ohms of resistance to effectively “dampen” the amount of current being delivered to the patient. For a patient with 150 ohms of impedance, no equipment­based resistors are added, and the full amount of current is delivered to the patient. In laboratory bench tests, at 200J, ZOLL delivered 27.8A peak current and 24.0A average current to a 50 ohm resistor, and 14.8A peak current and 12.5A average current to a 150 ohm resistor. At energy settings less than 200J, the difference between peak and average current is even less, typically a maximum of 1A.


It is really not a good idea to try to compare manufacturers’ biphasic waveforms as each is appropriate for the device in which it is found and none has been shown to be superior to others despite a number of clinical trials.

Types of Defibrillators

There are four major categories of defibrillators:

Advanced Life Support (ALS) Units
ALS defibrillators, used by healthcare professionals in hospitals and ambulances, allow professionals to monitor the patient rhythm and manually intervene if it is determined that a shock is required. In addition, most of these units offer an Advisory or AED function, in which waveform analysis and shock recommendations are made based upon sophisticated algorithms contained within the device.

ALS units can be used with either paddles or electrodes, though the trend today is to use the defibrillation electrode as it is much safer for the rescuer and delivers the shock more uniformly and effectively.


Beyond the ability to deliver a shock, ALS defibrillators are often outfitted with a number of parameters to aid rescuers.

Most in­hospital ALS units will have an external pacing capability to allow external pacing of bradycardias.  Many will also offer SPO2, a means to monitor the oxygenation level of the patient via an external sensor, and non­invasive blood pressure (NIBP) units to automatically measure the patient’s blood pressure via a cuff.

Invasive Blood Pressure (IBP) used mainly with advanced transport units where patients with invasive lines can be managed during transport, either within the hospital or via ambulance or aircraft.
Temperature to monitor patient temperature.

Widely used by paramedics in the field is 12­lead EKG, which allows for rapid identification and classification of myocardial infarction. The EKG reading can be transmitted to receiving hospitals and alert cardiology teams that a patient requiring intervention is on the way.
CPR support:

A growing number of ALS defibrillators now also provide support for cardiac compressions. It has become exceedingly clear that good CPR is vital to improving resuscitation outcomes; it has also been determined that delivering good consistent CPR is difficult – even for highly trained professionals. Therefore, there is growing acceptance of the need for defibrillation products to not only be capable of delivering a shock, but also capable of assisting with delivery of optimal circulatory support

Automatic External Defibrillators (AEDs)
These units are designed for use by laypersons and basic life support­ trained personnel. They are widely available in airports, schools, casinos and other public areas. They guide users through the application of the electrodes and automatically analyze the patient’s rhythm and either tell the rescuer to deliver a shock, or actually deliver the shock automatically. Many will also tell bystanders to start CPR, but only one AED, the ZOLL AED Plus currently coaches rescuers to deliver the correct rate and depth of compressions via the use of an accelerometer
built into the electrode pad. As the importance of CPR delivery is increasingly realized to be a critical part of a successful rescue, this capability will most likely expand to other manufacturers.

Implantable Cardioverter Defibrillators (ICDs)
These units are implanted directly into the patient’s chest and designed to protect those patients at high risk of sudden death. Generally, these are patients who have either a known medical condition that puts them at risk, or have actually experienced an episode of VF/VT. These products are beyond the scope of this website, and an in­depth discussion of these products can be found at the manufacturers’ websites highlighted in the links to the right.

Wearable Defibrillators
These are an intermediate care option for patients with a short­term known risk of sudden death or who are not candidates for an implantable device.

Section Citations

1. Gliner et al. Circulation 1995;92:1634­45

2. Tang et al. Journal of American College of Cardiology 1999;34:815­822.