The Effective External Defibrillators

Project - #1

Kehali B. Haileselassie & Faisal alsaadi

08/14/2013

ELC ENG 305 – Circuit Analysis II

Instructor - Ebrahim Forati

Table of Contents

Introduction…………………..…….……………………………………………………….. 3

Design Procedure….…………….…………………………………………………………... 4

Simulations……….…………….………………………………………………………….... 8

Results………….…………….……………………………………………………………… 9

Analysis………..……………………………………………………………………………. 11

Conclusion…………………………………………………………………………………... 13

References…………………………………………………………………………………… 14

Introduction:

A basic defibrillator is a device that can send a current from an external source through

the heart to restore the electric current within the heart. The shape of the current is vital to

make a heartbeat properly. It is this waveform that mimics the motion of the heart and enables

it to deliver good blood flow.

There have been two defibrillation waveforms that have had the most success in passing

an electric pulse to a heart, allowing it to receive the signal and restore normal heartbeats. The

two waveforms used in defibrillation are the Lown waveform and the Biphasic Truncated

Exponential Waveform (BTEW).

The Lown waveform is made of basic electronic components in series. An electric

charge is stored in a capacitor and then passed through an inductor and the output across a

load, namely an arrested heart. This design was the standard for defibrillation devices until the

mid-1980s. It was at this time that a biphasic waveform was thought to be more effective.

The Biphasic Truncated Exponential Waveform (BTEW) defibrillator was believed to be

an improvement over the monophasic waveform developed. Some advantages are portability,

battery life, and cost (Witt). However, the biggest advantage may be that biphasic defibrillator

has a greater success rate of working on the first pulse compared to the monophasic

defibrillator because of its fundamental signal.

Many advances in defibrillator have been made in the healthcare field. With the

application and improvement of electric waveforms, the defibrillator has also made many

advances, such as manual internal defibrillation devices and automatic external devices.

Understanding the construction of the waveform is vital to success in ventricular fibrillation.

The focus of this study will be output and application of the Lown and biphasic waveforms.

Using a limited selection of basic electrical components, the two types of waveforms

must be created while keeping cost to a minimum. Resistor, capacitors and inductors will be

the only components allowed along with a direct current power source.

Procedure

The signals of for the defibrillator have been shown in figures 1 and 2. Signal 1 is a

monophasic signal using an under damped Lown waveform and signal 2 is a Biphasic Truncated

Exponential Waveform (BTEW).

The desired output for the signal takes the form of Vo = K1*e-ζω0tsin[ωt]. Knowing the

following relationships:

ω0=1/√(LC) and ζω0 = R/2L

Fig. 1 – Original Lown waveform from design specificaitons

Fig. 2 – Original Biphasic Truncated Exponential Waveform from design specifications

ω0 can be found from the figure 3 and ζ can be calculated thereafter.

The design procedure for creating a circuit capable of mimicking the desired output is a

best done through selecting the necessary components from the list of available parts.

Inductor Values for determining ω0

[from ω0 = 1/√(LC)]L = 100 mH L = 220 mH

Capacitor Value ω0 Capacitor Value ω0

0.047 µF 14,586 rad/s 0.047 µF 9,834 rad/s0.1 µF 10,000 rad/s 0.1 µF 6,742 rad/s1.0 µF 3,162 rad/s 1.0 µF 2,132 rad/s

Fig. 3 - ω0 calculation from available parts

The choice of inductors was most limiting. The available inductors were either 100 mH or 220

mH while the choice of capacitor was one of six ranging from 22 nF to 100 µF. Through simple

evaluation of the choices presented, the value of ω0 = 9,834 radians works best when trying to

get close to a frequency:

ω = 2П/T = ω0 √(1-ζ2) = 1000П rad/s

for T = 2ms. The damping coefficient ζ calculated from ζω0 = R/2L = 4,545.45 rad/s is 0.46222.

From these values, an RLC series circuit configured as depicted in figure 4 is optimal for the

Lown waveform.

When determining the output for a BTEW, it is advisable to build a first order circuit,

which can reduce cost significantly. A capacitor in series with a resistor will give the desired

output. An RC first-order circuit has the time constant of τ = R*C. Evaluating the original

signal, the half period is observed to be 0.4 ms. The half period is used in the biphasic circuit

because the polarity is reversed halfway through the total period. By creating a circuit that has a τ

similar to the half period, a signal will emerge that resembles the wanted output. Using a 0.1 µF

and a resistor of 2.2 kΩ the time constant is 0.22 ms which approximately equals the half period

of 0.2 ms. The optimal design for the BTEW is depicted in figure 4.

Fig. 3 – Lown monophasic circuit configuration

Fig. 4 – Biphasic Truncated Exponential Waveform circuit.

Simulations

Using LTSpice as a circuit analyzer, figures 5 and 6 portray the intended output for the

derived solutions.

Fig. 5 – Simulated Lown waveform output

Fig. 6 – Simulated BTEW output

Results

The results of the output signals are depicted in figures 7 and 8. Figures 9 and 10 show

the results of the values for voltage and time for the original, simulated, and experimental

signals.

Fig. 7 – Experimental output of Lown monophasic waveform

Fig. 8 - Experimental output of BTEW

Original Peak 1 (V)

Simulated Peak 1 (V)

Experimental Peak 1 (V)

% Error Original Peak 4 (V)

Simulated Peak 4 (V)

Experimental Peak 4 (V)

% Error

13.2 12.3 12.78 3.18 -3.23 -2.30 -2.34 27.55

Original Peak 2(V)

Simulated Peak 2 (V)

Experimental Peak 2 (V)

% Error Original Peak 5(V)

Simulated Peak 5 (V)

Experimental Peak 5 (V)

% Error

9.4 9.7 10.2 -8.5 0.6 0.31 0.275 54.2

Fig. 9 – Lown waveform signal comparison of max and min voltages

Original Peak 1 (t)

Simulated Peak 1 (t)

Experimental Peak 1 (t) % Error

Original Peak 4 (t)

Simulated Peak 4 (t)

Experimental Peak 4 (t)

% Error

0.25ms 0.255ms 0.266ms 6.4 1.263 ms 1.26 ms 1.25 ms 1.02

Original Peak 2(t)

Simulated Peak 2 (t)

Experimental Peak 2 (t)

%Error Original Peak 5(t)

Simulated Peak 5 (t)

Experimental Peak 5 (t)

% Error

0.6 ms 0.653 ms 0.62 ms 3.33 1.646 ms 1.62 ms 1.63 ms 0.9

Fig. 10 – Lown waveform comparison of time differences

Analysis

When analyzing the circuit for price, the least expensive circuit will have the least

components. Because of the extremely high voltage necessary to resuscitate a heart,

component voltage, amperage, and power ratings must also be very high.

After, the most significant factor in creating the circuit is the margin of error between

the original design specifications and the physical circuit. When comparing the Lown

waveform, he first half of the signal has an average margin of error of 3.6%. The second half of

the generated signal does not reach the minimum voltage of the design specification. Instead

of reaching a minimum of 0.6 V at 5T/6, we were only able to reach 0.275 V. This left a much

higher margin of error for this part of the circuit at nearly 25%. However, the timing of the

circuit was nearly exact with an average margin of error of 1.94%.

The BTEW circuit fared even better than the Lown waveform with an average margin of

error of 9% and a timing margin of error of 0%.

The final designs are within an acceptable range compared to the original design when

taking cost and size into account.

The significant design flaw in either circuit is the lack of backup components in the case

of failure. If any single component fails, there is no way to salvage a dependable waveform.

This flaw is unavoidable and may make the product undesirable.

Conclusion

When the two types of circuits are compared against each other, it appears that the

BTEW clearly outperforms the Lown waveform Some advantages of the biphasic defibrillator

are that it is more portable because of its smaller size. The biphasic waveform can be

constructed more easily and with less parts making it less expensive. The power source

necessary for a biphasic defibrillator is usually a battery and can last longer. Perhaps the

biggest advantage of the BTEW is the ability deliver two beats for every period, increasing the

chance of successful resuscitation on the first pulse (Witt). The biphasic truncated exponential

waveform is closest to the original signal, cheaper to construct, more effective in application,

smaller, and has greater battery life. The recommendation is that the BTEW defibrillator be

used.

References

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

2. Lown, Bernard. "Biography of Dr. Lown: Co-founder of IPPNW."BernardLown.org, n.d.

Web. 29 Mar. 2013. <http://bernardlown.org/bio.html>.

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

4. Witt, Pharaba. "The Advantages of Biphasic Defibrillators." EHow. Demand Media, 20 Oct. 2010.

Web.