**2. Cardiac external defibrillation – Basic science**

#### **2.1. History**

In Switzerland, 1899, Prevost and Batelli discovered that small electric shocks could induce ventricular fibrillation in dogs and that larger charges would reverse the condition. Howev‐

© 2013 Delgado et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

er it was not until 1956 when alternating current was first used for transthoracic defibrilla‐ tion to treat ventricular fibrillation in humans [11]. Following this breakthrough, direct current defibrillators were introduced into clinical practice around 1962 [12] when it was demonstrated that electrical countershock or cardioversion across the closed chest could abolish other cardiac arrhythmias in addition to ventricular fibrillation [13]. Later on, Diack et al. [14] described the first clinical experience with an AED. Subsequently, further studies provided solid evidence on the potential role of these devices in the early defibrillation and survival.

direction for the remaining duration of the electrical discharge *(figure 2A)*. With biphasic waveforms there is a lower defibrillation threshold (DFT) that allows reductions of the en‐ ergy levels administrated and may cause less myocardial damage [21-24]. The use of bi‐

Principles of External Defibrillators http://dx.doi.org/10.5772/52512 5

**• Triphasic waveform.** There are no human studies to support the use of multiphasic waveforms over biphasic. Investigation in animals suggests that the benefits of biphasic waveform could be harnessed through the use of a triphasicwaveform in which the sec‐ ond phase has the larger strength to lower the DFT and the third phase the lower

phasic waveforms permits a reduction in the size and weight of AEDs.

**Figure 1.** Monophasic waveforms. A. Damped sinusoidal wave (A) and truncated exponential (B).

Cardioversion is one of the possible treatments for arrhythmias that imply a re-entrant cir‐ cuit. By delivering a synchronized electric shock all excitable tissue of the circuit is simulta‐ neously depolarised making the tissue refractory and the circuit no longer able to sustain reentry. As a result, cardioversion terminates arrhythmias resulting from a single reentrant circuit, such as atrial flutter, atrioventricular nodal reentrant tachycardia or monomorphic ventricular tachycardia. This term is also applied when using an electrical shock to termi‐

strength, to minimize damage [25] *(figure 2B)*.

**Figure 2.** A. Biphasic waveform. B. Triphasic waveform.

**2.4. Cardioversion and defibrillation**

### **2.2. Types of defibrillators**

**•** Most defibrillators are energy-based, meaning that the device charges a capacitor to a se‐ lected voltage and then delivers a prespecified amount of energy in joules. The amount of energy which arrives at the myocardium is dependent on the selected voltage and the transthoracic impedance (which varies by patient).

Most current AEDs are energy-based but there are two other types of defibrillators less fre‐ quently used in clinical practice.


#### **2.3. Waveforms and its importance**

Energy-based defibrillators can deliver energy in a variety of waveforms, broadly character‐ ized as monophasic, biphasic or triphasic.


direction for the remaining duration of the electrical discharge *(figure 2A)*. With biphasic waveforms there is a lower defibrillation threshold (DFT) that allows reductions of the en‐ ergy levels administrated and may cause less myocardial damage [21-24]. The use of bi‐ phasic waveforms permits a reduction in the size and weight of AEDs.

**• Triphasic waveform.** There are no human studies to support the use of multiphasic waveforms over biphasic. Investigation in animals suggests that the benefits of biphasic waveform could be harnessed through the use of a triphasicwaveform in which the sec‐ ond phase has the larger strength to lower the DFT and the third phase the lower strength, to minimize damage [25] *(figure 2B)*.

**Figure 1.** Monophasic waveforms. A. Damped sinusoidal wave (A) and truncated exponential (B).

**Figure 2.** A. Biphasic waveform. B. Triphasic waveform.

#### **2.4. Cardioversion and defibrillation**

er it was not until 1956 when alternating current was first used for transthoracic defibrilla‐ tion to treat ventricular fibrillation in humans [11]. Following this breakthrough, direct current defibrillators were introduced into clinical practice around 1962 [12] when it was demonstrated that electrical countershock or cardioversion across the closed chest could abolish other cardiac arrhythmias in addition to ventricular fibrillation [13]. Later on, Diack et al. [14] described the first clinical experience with an AED. Subsequently, further studies provided solid evidence on the potential role of these devices in the early defibrillation and

**•** Most defibrillators are energy-based, meaning that the device charges a capacitor to a se‐ lected voltage and then delivers a prespecified amount of energy in joules. The amount of energy which arrives at the myocardium is dependent on the selected voltage and the

Most current AEDs are energy-based but there are two other types of defibrillators less fre‐

**• Impedance-based defibrillators** allow selection of the current applied based upon the transthoracic impedance (TTI). TTI is assessed initially with a test pulse and subsequently the capacitor charges to the appropriate voltage. In patients with high TTI there was a sig‐ nificant improvement in shock success rate using this approach when compared to the en‐

**• Current-based defibrillators** deliver a fixed dose of current which results in defibrillation thresholds that are independent of TTI [16]. The optimal current for ventricular defibrilla‐ tion appears to be 30 to 40 amperes independently of both TTI and body weight thus ach‐ ieving defibrillation with considerably less energy than the conventional energy-based method [17-19]. Current-based defibrillation was proved superior to energy-based defib‐ rillation with monophasic waveforms in one clinical study [20] but this concept merits

Energy-based defibrillators can deliver energy in a variety of waveforms, broadly character‐

**• Monophasic waveform.** Defibrillators with this type of waveform deliver current in one polarity and were the first to be introduced. They can be further categorized by the rate at which the current pulse decreases to zero.If the monophasic waveform falls to zero grad‐ ually, the term damped sinusoidal is used. If the waveform falls instantaneously, the term truncated exponential is used *(figure 1).*The damped sinusoidal monophasic waveforms

**• Biphasic waveform.** This type of waveform was developed later. The delivered current flows in a positive direction for a specified time and then reverses and flows in a negative

further exploration in the light of biphasic waveforms now available.

have been the mainstay of external defibrillation for over three decades

survival.

4 Cardiac Defibrillation

**2.2. Types of defibrillators**

quently used in clinical practice.

ergy-adjusting defibrillators [15].

**2.3. Waveforms and its importance**

ized as monophasic, biphasic or triphasic.

transthoracic impedance (which varies by patient).

Cardioversion is one of the possible treatments for arrhythmias that imply a re-entrant cir‐ cuit. By delivering a synchronized electric shock all excitable tissue of the circuit is simulta‐ neously depolarised making the tissue refractory and the circuit no longer able to sustain reentry. As a result, cardioversion terminates arrhythmias resulting from a single reentrant circuit, such as atrial flutter, atrioventricular nodal reentrant tachycardia or monomorphic ventricular tachycardia. This term is also applied when using an electrical shock to termi‐ nate atrial fibrillation although this arrhythmia involves multiple, micro-reentrant circuits. The term cardioversion implies to syncronize the delivery of the shock with the QRS com‐ plex of the patient.

AED which is capable of administering a shock without the need for outside interventions.

**Semi-automatic AEDs Fully automatic AED**

rescuers

Capable of administering a shock without the

Principles of External Defibrillators http://dx.doi.org/10.5772/52512 7

• Easier to use and more appropriate for lay-

• Better compliance with resuscitation protocols

need for outside interventions

• Longer times until shock delivery • Risk of electrocution for the rescuer if

• No possibility to override the device

• Not recommended by current guidelines except

inappropriately used

for special situations

See Table 1.

**Definition** Indicates the need for defibrillation but

pushing a button

**Advantages** • Recommended by current resuscitation guidelines • Widely used

the rescuer

corrosive and highly toxic substances.

**Disadvantages** • More complex to use for the untrained responders

independently of prompts.

maneuvers for lay rescuers

requires an operator to deliver the shock by

• Allows healthcare professionals to override the device and deliver a shock manually,

• Safer, no risk of inappropriate shocks to

• More difficult to synchronize with CPR

**Table 1.** Definition, main advantages and disadvantages for the different types of AED available.

designed to analyze the rhythm and send an electric shock if is needed.

gy delivered by this system can be anywhere from 30 to 400 joules.

they increase the speed of shock and improve defibrillation technique.

Basically these devices consist of a battery, a capacitor, electrodes and an electrical circuit

**• Batteries.** Essentially they are containers of chemical reactions and one of the most impor‐ tant parts of the AED system. Initially lead batteries and nickel-cadmium were used but lately non-rechargeable lithium batteries, smaller in size and with longer duration with‐ out maintenance (up to 5 years), are rapidly replacing them. Since extreme temperatures negatively affect the batteries, defibrillators must be stored in controlled environments. Also it is important to dispose of the batteries using designated containers as they contain

**• Capacitor**. The electrical shock delivered to the patient is generated by high voltage cir‐ cuits from energy stored in a capacitor which can hold up to 7 kV of electricity. The ener‐

**• Electrodes** are the components through which the defibrillator collects information for rhythm analysis and delivers energy to the patient's heart. Many types of electrodes are available including hand-held paddles, internal paddles, and self-adhesive disposable electrodes. In general, disposable electrodes are preferred in emergency settings because

Defibrillation is used to describe the utilization of an electric shock to terminate ventricular fibrillation (VF). VF is known to be a very persistent arrhythmia, and total elimination of the fibrillatory activity is obtained only with a relatively high energy shock that uniformly de‐ polarizes the entire myocardium.

Current European Society of Cardiology and AHA guidelines suggest the following initial energy selection for specific arrhythmias [26-28]:


Cardioversion is most commonly used for the treatment of atrial fibrillation and the devel‐ opment of biphasic defibrillators proved to be very useful. At least 2 randomized trials illus‐ trated the benefit of the biphasic waveform when compared to escalating monophasic shocks [29, 30]. First shock efficacy was greater with a biphasic waveform (68 versus 21 per‐ cent), delivered energy was 50 percent less, and the overall cardioversion rate was higher (94 versus 79 percent) [29]. There were fewer total shocks (1.7 versus 2.8), less energy deliv‐ ered (217 versus 548 joules), and a lower frequency of dermal injury (17 versus 41 percent) [30].

Similar findings were reported for patients with atrial flutter, in whom cardioversion was successful more frequently and at lower energy levels when using biphasic waveforms [31].
