Configurable External Defibrillator Devoted

to Education and Clinical Trials

Victor D. N. Santos

1,2 a

, J. Cândido Santos

and N. M. Fonseca Ferreira

1,3 c

Coimbra Polytechnic – ISEC, Rua Pedro Nunes - Quinta da Nora, 3030-199 Coimbra, Portugal

INESC Coimbra, Rua Sílvio Lima, pólo II DEEC, 3030-790 Coimbra, Portugal

GECAD, Polytechnic Institute of Porto (IPP), 4200-465 Porto, Portugal

Keywords: External Defibrillator, Defibrillation Waveforms, Rectilinear Biphasic, Biphasic Truncated Exponential.

Abstract: External defibrillators are recognized effective to revert ventricular fibrillation and pulseless ventricular

tachycardia. This paper presents a new settable defibrillator, designed to assess the effectiveness of the

following defibrillation waveforms during clinical trials: monophasic damped sinusoidal (MDS); biphasic

truncated exponential (BTE) and rectilinear biphasic waveform (RBW). The device flexibility allows the

setting of the defibrillation waveforms most relevant parameters, namely energy and pulses duration. The

device usage is also relevant in biomedical engineer and medical staff education and training programs.

1 INTRODUCTION

Cardiovascular diseases take a leading role in the

morbidity and mortality of the Western countries

populations. International experience shows that the

use of an automated external defibrillator (AED),

outside the hospitals, by non-medical personnel,

significantly increases the probability of survival of

the victims. However, only the existence of an

efficient survival chain, makes AED an effective way

to improve survival after cardiorespiratory arrest.

AED is a device capable of automatically

identifying defibrillating heart rhythms, alerting to

safety conditions, and complete the steps of the

approved algorithm on cardiopulmonary resuscitation

to produce electric discharges automatically or under

the command of an external operator, according to

predefined energy values. It also records the data

from the electrocardiographic to support later

auditing.

Patient’s defibrillation is obtained by the delivery

of a suitable electrical current through their

myocardium, able to depolarize a critical myocardial

mass and thereby re-establish a coordinated electrical

activity that leads to an organized sinus rhythm and

spontaneous circulation.

https://orcid.org/0000-0002-5441-4394

https://orcid.org/0000-0003-0494-4335

https://orcid.org/0000-0002-2204-6339

Several studies have demonstrated that biphasic

waveform shocks are superior to monophasic shocks

to revert cardiac arrest caused by ventricular

fibrillation (Keener et all., 1999; Zhang et all., 2003).

This arises from the fact that in biphasic waveforms

the current flows in a predefined direction during a

time period and thereafter reverses the current flow to

the opposite direction for the remaining pulse

duration. These type of pulses improve defibrillation

efficiency by diminishing the defibrillation threshold

value and the underling associated hazards.

Further studies (Zhang et all., 2003), performed in

animal’s, indicate that the benefits of biphasic

waveform could be further enhanced by using of a

triphasic waveform composed of three pulses with

reversed polarities i.e. the polarity of the second pulse

is opposite to that used on the first and third pulses.

Recent published studies (Mittal et all., 1999),

patents (Dascoli et all., 2017) and commercial devices

(Zoll, 2018) point to rectilinear biphasic waveform

(RBW). Those devices deliver an approximately

constant current during the first phase of a pulse,

independent of the patient’s transthoracic impedance

(TTI). The optimal current for ventricular

defibrillation appears to be 30 to 40 A (Tavakoli et

all., 2017). In the pulse second phase the current

142

Santos, V., Santos, J. and Ferreira, N.

Conﬁgurable External Deﬁbrillator Devoted to Education and Clinical Trials.

DOI: 10.5220/0008977701420148

In Proceedings of the 13th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2020) - Volume 1: BIODEVICES, pages 142-148

ISBN: 978-989-758-398-8; ISSN: 2184-4305

decays according with a first order RC circuit

transient, negative exponential pulse, being its

polarity opposite regarding the first one.

This paper presents a novel programmable

defibrillator device devoted to either clinical studies

and educational purposes. The assessment of the

therapeutic efficiency of different defibrillation

waveforms is only possible by means of the pulses

characteristics definition: waveform, amplitude,

energy and individual pulses duration time periods.

The implemented device comprises a micro

controlled electronic unit connected with a Labview

application that performs the pulse configuration and

the acquired ECG signals processing. The device

allows the implementation of different defibrillation

waveforms. The monophasic damped sinusoidal

(MDS) is obtained from the Lown circuit being the

biphasic truncated exponential (BTE) and the

triphasic waveforms implemented using a H bridge.

The current based defibrillation waveform includes a

set of low value resistances that are electronically

bypassed as the capacitor voltage drops, keeping the

current value almost constant in the initial pulse

phase. Moreover, the implemented defibrillator has

an ECG signal acquisition unit, able to identify

defibrillable heart rhythms. The acquired signal is

later processed to synchronize the location of the

defibrillation pulse with respect to the QRS complex.

The paper is organized as follows. Section 2,

describes and analyses the waveforms typically used

on defibrillators, the underlying theoretical concepts

and the employed electronic circuits. Moreover, the

results retrieved from each particular circuits are also

presented. Section 3, presents the design and

implementation issues of the defibrillator. Section 4

covers the implementation and tests on the novel

defibrillator in the biomedical engineers courses

education in the Institute Polytechnic of Coimbra

(IPC). Finally, Section 5 presents the main

conclusions and future work.

2 DEFIBRILLATION

WAVEF OR MS

The developed defibrillator is capable to generate

different defibrillation pulses: MDS, BTE or RBW by

selecting one of the implemented circuits, explained

in detail in the following sections. The circuit analysis

is added to explain the waveform generation process

and its parameterization.

2.1 Origin and Evolution of the

Defibrillators

Defibrillation was first demonstrated in 1899 by

Prevost and Batelli. They discovered that small

electric shocks could induce ventricular fibrillation in

dogs, and that larger charges would reverse that

condition. The defibrillator was invented in 1932 by

Dr. William B. Kouwenhoven and its first use on a

human was conducted in 1947 by Claude Beck

(Akselrod et all., 2009; Tavakoli et all., 2017).

Until the early 1950s, defibrillation of the heart

was possible only when the chest cavity was open

during surgery. The technique used an alternating

current (AC) delivered to the sides of the exposed

heart by paddle electrodes. During his PhD work

Gurvich discovered that direct current (DC) shocks

were significantly more efficient and less damaging

than AC in defibrillation.

In 1959 Bernard Lown began to investigate the

use of monophasic DC shocks resultant from the

discharge of capacitors over patients. The designed

method assumes that a capacitor or a bank of

capacitors is kept at a high voltage (Akselrod et all.,

2009). The energy stored in the capacitors is then

delivered to the patient’s chest through an additional

inductor, in order to produce a classical MDS

waveform of finite duration. This approach follows

previous work performed in human patients reported

by both Gurvich and Peleska (Akselrod et all., 2009).

The Lown waveform was the standard for

defibrillation until the late 1980s when research

showed that a BTE waveform with both positive and

negative pulses could provide equal or even better

results requiring lower energy levels, thereby

increasing patient safety and reducing burden on the

power supply system and batteries (Akselrod et all.,

2009, Tavakoli et all., 2017).

2.2 Lown Defibrillator

The Lown defibrillator is based on the RLC series

circuit depicted in Fig. 1.

Figure 1: Lown defibrillator circuit. (Irwin, 2015).

Conﬁgurable External Deﬁbrillator Devoted to Education and Clinical Trials

143

The analysis of the circuit assumes that the

capacitor was previously charged at a high voltage

level, being then connected in series to an inductor

and resistor at instant t = 0 s. The defibrillation pulse

applied to the patient (resistor) is the solution of the

second-order differential equation obtained by the

application of KVL around the circuit loop.

  



tdi

tid

(1)

The solution of (1) must be a function whose first

and second derivatives have the same form as the

original function in order to the left side of the

equation becoming zero (Irwin, 2015). The solution



eKti 

is then replaced in (1) resulting in

















seK

(2)

Since

eK 

is the assumed solution, the

expression in parentheses will have to be zero. That

quadratic equation is known as the characteristic

equation of the second-order differential equation (1),

since the roots of that equation dictate the behaviour



ti . Using the quadratic formula, two roots of (2)

can be found:

;



 ss

(3)

assuming that





and





(4)

The roots s

and s

are associated with the natural

response of the circuit; ω

is known as the undamped

natural frequency, expressed in radians per second

(rad/s); and α is the damping factor, expressed in

nepers per second.

If α > ω

both roots s

and s

are negative and real

(overdamped case) (Alexander, 2012). Thus, the

current decays and approaches zero as t increases.



 















(5)

with





If α < ω

the roots s

and s

are complex conjugate

being the electric current given by (underdamped

case) (Irwin, 2015; Alexander, 2012)

 













sin

(6)

with





. In (5) and (6) the constant

factor was obtained from two initial conditions: the

initial value of the current and its first derivative.

Fig. 2 presents the simulated MDS waveforms for

patients with resistance values equal to 25 Ω; 50 Ω

and 150 Ω. The results were obtained assuming the

usage of a 47 µF capacitor and a 33 mH inductor and

an initial voltage level in the capacitor, of 4.2 kV.

Figure 2: Lown waveform in function of the impedance.

Fig. 3 shows the MDS waveform acquired in the

lab using the above components and a R = 25 Ω. In

this case the system is underdamped with α = 378.78,

= 802.96 rad/s and ω

= 708 rad/s.

Figure 3: Lown waveform in function of the impedance.

In Fig. 3 the signal was acquired setting the

TIME/DIV control of the oscilloscope to 1 ms/DIV.

2.3 BTE Defibrillators

In order to generate a BTE waveform defibrillation

pulses a new circuit, represented in Fig. 4 is needed.

It uses a high-voltage capacitor and a H bridge. The

capacitor is charged with a high DC voltage value that

meets the selected energy value to be applied. After

charging the capacitor with the desired voltage value,

the energy stored in it is then delivered to the patient

as a defibrillation pulse. The H-bridge is used to

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control transfer of energy from the capacitor to the

patient. It includes four electronic switches (placed on

the H-bridge legs), that are used to define the polarity

applied to the load (patient).

Figure 4: BTE waveform defibrillator circuit.

During the first phase of the waveform, the H

bridge connects the capacitors’ two terminals with the

defibrillator paddles placed on the patient. At the end

of the pulse first phase that connection is opened and

the terminals of the capacitor are switched to connect

in the reverse polarity to the mentioned paddles. It is

advisable to have a small guard time interval between

phases in order to avoid the simultaneous connection

of the switch in the same side of the H-bridge

(Sullivan, 1997).

The didactic version of the configurable external

defibrillator device uses the TB6612FNG H bridge to

control de discharge of the capacitor over the load

(patient). This driver IC is devoted to perform the

control of up two DC motors. The mentioned device

requires three input signals, IN1, IN2 and STBY to

select the four modes of operation corresponding to

the normal operation CW, in one direction, and CCW;

in the opposite direction; the high impedance mode

and the short brake mode.

The mentioned control signals are obtained from

a ATmega328P microcontroller according with the

patient impedance value.

Table 1: BTE parameters for adults (Philips, 1997).

Load

resistance (Ω)

Phase II

duration (ms)

Phase II

duration (ms)

25 2.8 2.8

50 4.5 4.5

75 6.25 5.0

100 8.0 5.3

125 9.64 6.4

150 11.5 7.7

Fig. 5 depicts the simulated BTE waveform

delivered to patients with resistance values equal to

25 Ω; 50 Ω and 150 Ω. The presented results assume

a 100 µF capacitor and an initial voltage of 1.8 kV.

The pulse phases durations of the BTE waveform are

presented in table I.

Figure 5: BTE waveform in function of the impedance.

Figure 6: BTE waveform.

Fig. 6 shows the BTE waveform obtained in the

laboratory, from the developed device using the

following components C = 100 uF and R = 50 Ω and

the H-bridge. The signal was acquired setting the

TIME/DIV control of the oscilloscope to 1 ms/DIV.

2.4 Rectilinear Biphasic Waveform

Defibrillators

The RBW implementation is based on the circuit used

for the BTE waveforms. In order to achieve an

approximately constant current during the first pulse,

as the capacitor discharges across the patient chest, is

needed to decrease the circuit impedance accordingly.

The underlying idea of a RBW defibrillator

operation comprises the inclusion of several series-

connected resistors, in the current path, in series with

the patient impedance. Each one of those additional

resistors is connected in parallel with a shorting

switch controlled by a micro-controller providing an

extremely low resistance path to the current (Fig. 7).

The resistors should have different resistance values

to tune the current with a high accuracy to a nearly

constant value.

Conﬁgurable External Deﬁbrillator Devoted to Education and Clinical Trials

145

Figure 7: RBW defibrillator circuit.

When the H-bridge switches, of opposite legs, are

closed, in the beginning of the pulse, all the resistor-

shorting switches are open, to force the current to

passes through each one of the mentioned added

resistors.

As the voltage in the capacitor decays, with a time

constant R·C, the additional resistors in series with

the patient are successively shorted out. Each time

that one of the resistors is bypassed, the current

increases instantaneously, taking into account that the

capacitor voltage remains the same, as the event

happens. Thus, it can be observed significant ripple,

in the positive phase of the RBW (Fig. 8), matching

the repeated activation of the resistor-shorting

switches. The ripple magnitude is larger at the end of

the rectilinear phase because the time constant value

is smaller at the end of the phase than at the

beginning.

At the end of the positive phase all the H-bridge

switches legs are open to ensure a safety guard time

period before invert the direction of the current flow.

The polarity of the waveform is reversed by closing

the H-bridge switches that were open in first positive

phase and vice versa.

Figure 8: RBW waveform in function of the impedance.

Fig. 8 shows the RBW obtained by simulation. In

the conducted simulations was assumed impedances

equals to 50 Ω; 75 Ω and 100 Ω. Moreover, it was

assumed the usage of a 100 µF capacitor, three 10 Ω

additional resistor’s and an initial voltage value of

2.3 kV.

The device that generates the RBW defibrillator is

based on the TB6612FNG H bridge and on the earlier

mentioned ATMEL microcontroller. However,

additional control signals must be generated in order

to manage each one of the bypass transistors. The

signals are defined to turn on each transistor at

particular time instants. The transistors sequence of

operation is always the same and starts with the

transistor connected to the ground and ends with the

transistor connected with the H bridge.

3 DEFIBRILLATION DESIGN

ISSUES

The implemented novel defibrillator, is expected to

be comparable to the existent commercial devices

including all its features, being also compliant with

international standards. The prototype consists of a

hardware part that implements the mentioned

waveforms and performs the conditioning and

acquisition of the patient’s ECG signals and the TTI

evaluation. The device includes a power supply unit,

a micro-controller unit, and several electronic

components. The power source can come either from

a battery or from the main supply using an electronic

rectifier.

As mentioned earlier the capacitor is the vital

element of the defibrillator, it stores a large amount

of energy in its electric field, then releases it on a

patient’s chest accordingly with the system

configuration and TTI value. The capacitor charging

circuit includes a series-resistor to limit the current

value and two electronic sensors to measure, in real

time basis, the absorbed /delivered current (Hall

effect sensor) to/from the capacitor and the voltage

value across its terminals.

A signal conditioning circuit was also

implemented, depicted in Fig. 9, in order to perform

the mentioned ECG signal acquisition. That circuit

employs the low cost AD620 instrumentation

amplifier and the LM358N operational amplifier, a

band pass filter and right leg reference circuit.

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146

Figure 9: ECG signal conditioning circuit.

The amplified ECG signal is thereafter acquired

by a National Instruments low-cost, multifunction

DAQ. Fig. 10 presents the acquired ECG signal of a

healthy patient, in the test bed, using the conditioning

circuit of Fig 9.

Figure 10: Acquired ECG signal from a patient.

The described circuits are controlled by a software

component (LabView) which implements a state

machine underlying the operation of the external

defibrillator as well as all the signal processing

operations.

The procedures related with the defibrillator

capacitor are controlled by a state machine that ensure

the correct: charging state; charge maintenance,

discharge, safety discharge. The state machine also

controls other events that take place in a logical order

in order to avoid the occurrence of conditions that

compromise the device and/or the user safety.

The following features were implemented in the

software:

 Defibrillator state machine;

 Charging and discharging circuits monitoring;

 ECG signal acquisition, filtering, processing

and representation;

 QRS complex identification;

 Patient impedance value assessment;

 H bridge and control timing signals generation;

 Data storage.

To avoid the hazard of ventricular fibrillation

resultant from the application of the DC pulse, the

discharge should be synchronized with the

electrocardiogram. The defibrillation pulse must be

applied during or immediately after the downward

slope of the R wave (Zoll, 2018). The synchronization

avoids the delivery of the shock during the T wave

that corresponds to a partially refractory vulnerable

period during which ventricular fibrillation could

occur in the presence of an external electric stimulus.

4 LIFE SUPPORT SYSTEMS

EDUCATION

The Coimbra Institute of Engineering (ISEC) is a

Portuguese higher education Polytechnic school,

integrated in the Coimbra Polytechnic Institute. Its

formative offer includes, among others, the

biomedical engineering and biomedical

instrumentation, first and second cycles whose

curricula have been designed accordingly to the

Bologna process.

The presented external defibrillator was designed

and implemented on the Life Support Systems (LSS)

curricular unit of the second semester of the

biomedical instrumentation master course. This unit

is relevant taken into account that in Portugal,

cardiovascular diseases are one of the most serious

health problems of the population (Santos, 2019).

This curricular unit aims to provide skills and

knowledge regarding the existent basic and advanced

life support equipment. Furthermore, it is intended

that students acquire fundamental knowledge, in the

electrical engineering area regarding the design,

implementation and maintenance of such equipment.

Additionally, R&D activities are encouraged on

students, which result on the proposal of new

approaches and circuits in compliance with the

international standards.

Students that successfully complete the unit

should be able to understand RC and RLC transients

in order to design Lown BTE and RBW

defibrillators’; to understand, pacemakers’ and AEDs

devices’ operation and functionalities. To understand

the underling theory and sensor technology needed to

obtain the vital signals monitoring and finally to

understand the operation principles of the most

common ventilators.

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147

5 CONCLUSIONS

This paper describes a novel defibrillator capable to

implement different defibrillation pulses, namely: the

MDS, BTE and RBW waveforms, using different

electrical and electronic components and circuits.

With respect to the Lown defibrillator is only

possible to define the energy delivered to the patient.

The generated MDS waveforms in the test bed, were

validated using the results obtained from simulation

of the RLC circuit under the same assumptions. The

defibrillator settings allow BTE and RBW waveforms

provision by means of the H bridge circuit usage. An

external micro-controller is responsible for the pulses

different phases timings accordingly with the

measured patient impedance value. The retrieved

laboratory results, from the defibrillator, were

compared and validated with the ones obtained by the

circuits simulation.

Additionally, more advanced features were also

employed and implemented in order to evaluate the

effectiveness of different defibrillation waveforms

and parameters during clinical trials.

Moreover, the design, implementation and test of

the developed configurable defibrillator in the LSS

curricular unit has engaged students on R&D

activities; promoted the academic success, the

students’ motivation facing a real practice problem

and the improvement of students’ skills teamwork,

communication and leadership.

ACKNOWLEDGEMENTS

This work has been supported by the Portuguese

Foundation for Science and Technology (FCT) under

project grant UID/MULTI/00308/2019.

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