ELECTRONIC DEVICE FOR SEISMOCARDIOGRAPHY

Noninvasive Examination and Signal Evaluation

Zdenek Trefny

, Milan Stork

and Martin Trefny

Cardiological laboratory in Prague U Pruhonu 52, 17000 Praha, Czech Republic

Univeristy of West Bohemia, Univerzitni 8, 30614 Plzen, Czech Republic

Keywords: Quantitative seismocardiography, Heart rate, Systolic force, Direct digital Synthesis, Analog/digital

converter, Savitzki-Golay filter.

Abstract: The Quantitative seismocardiography (Q-SCG) opens a new field of cardiovascular dynamics examination.

Using this absolutely non-invasive method, a new field of monitoring heart rate variability was opened up.

Systolic forces as well as heart rate variability in relation to changes in external stimuli are registered. Q-

SCG probably offers a more complex view of both isotropic and chronotropic heart functions. It will be

suitable for: examining operators exposed to stress; for assessing the effect of work, fatigue and mental

stress; for monitoring persons as part of disease prevention; for determining a person’s ability to carry out

their duties both on the ground and in the air. An electronic system for acquisition of data for noninvasive

Q-SCG and signal processing is also presented. The measuring system is based on analog filter,

analog/digital converter, microcontroller and personal computer. A special digital smoothing polynomial

filter is used for signal processing. The example of real measured and evaluated signal is also shown.

1 INTRODUCTION

1.1 Balistocardiography

In balistocardiography (BCG), the body vibrations

caused by the heart activity are registered.

Balistocardiography is a non-invasive method

enabling the examination of the cardiovascular

dynamics. This field has a longer history than is

commonly known. William Harvey (1578-1657)

who discovered blood circulation called his work,

published in 1628, „Exercitatio anatomica de motu

cordis et sanguinis in animalibus.“ As the title

suggests, this work covers two main topics:

a) Movement of the heart

b) Movement of the blood

Harvey also states that movement is one of the basic

functions that sustain circulation. This process

requires impulse and force (impetus et violentia),

which are produced by the heart (impulsor). The

heart itself may produce force and impulse, while

blood is propelled and forced to leave its source and

home, towards the peripheral parts of the body.

In 1936, Starr took part in a conference held by the

American Society of Physiology which dealt with

methods of determining cardiac output. For this

purpose, he used a bed with tight springs, whereby

by the movement against these springs increased the

instrument’s natural frequency to values higher than

the heart rate. Thus began the era of high-frequency

balistocardiography, which lasted approximately 15

years. Other types of instruments were developed

later on which measured the displacement, velocity

or acceleration of a body lying on a bed. Later

studies showed that there are difficulties when

comparing records registered on different

apparatuses. This is mainly caused by two factors:

a) The instrument’s natural frequency

b) The instrument’s damping

The instrument’s natural frequency lies within the

range of the frequencies caused by the cardiac

activity that we wish to observe. This leads to

interference and the subsequent recording is a

summation of the oscillations of the instrument and

those of the heart. The other factor that significantly

affects the shape of the registered curve is the

damping installed in these instruments (which are

basically oscillatory systems) in order to prevent the

periodic oscillations of the instruments themselves.

Records of heart activity are therefore deformed.

204

Trefny Z., Stork M. and Trefny M. (2009).

ELECTRONIC DEVICE FOR SEISMOCARDIOGRAPHY - Noninvasive Examination and Signal Evaluation.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 204-208

DOI: 10.5220/0001534602040208

 SciTePress

Figure 1: Principle of the noninvasive quantitative

seismocardiography measuring: PT - piezoelectric

transducer, ES - electronic system, PC - personal

computer.

1.2 Quantitative Balistocardiography

Following the critical evaluation of all these facts, in

1952 it was begun with our own experiments related

to the construction of an apparatus which would lack

the aforementioned shortcomings. Thus, over the

years, an apparatus was constructed whose

advantages lie not only in the simplicity of its

design, but also in its important functional qualities.

The properties of the pick-up device and bearing

structure, the subject’s sitting position in close

contact with the seat and an amplifier with a

sufficiently long time constant reduce the possibility

of shape, phase and time deformation of the records.

All this enabled to conduct a physical and

mathematical analysis of the balistocardiographic

system and to calibrate our instrument. Based on

these processes, the apparatus was designated a

quantitative balistocardiograph. This was chiefly to

distinguish it from previous instruments that

registered displacement, velocity and acceleration

and were designed to determine cardiac output on

one hand, and also because our instrument was

calibrated so that force expressed in Newton's

registers an amplitude measurable in mm, whereby

the relationship between the size of the active force

and the registered amplitude is linear, on the other

hand. The quantitative balistocardiographic method

enabled to introduce two characteristic quantities:

systolic force (F) and minute cardiac force (MF),

thus using quantitative balistocardiography in an

exact manner when studying cardiovascular

dynamics at rest and during stress. Current

applications of quantitative balistocardiography (Q-

BCG) in papers published to date the fact that the

relationship between the force acting on the pick-up

device and the amplitude of the balistocardiographic

curve is linear was proved. This enabled to study the

evolution of systolic force in relation to age and

ageing, the influence of hypoxia and hyperoxia. It

was also possible to follow the changes in Q-BCG

indices at rest and under workload in various groups

of volunteers, and to determine the linear

relationship between the skeletal muscle force and

systolic force, and determine changes in Q-BCG

indices in various pathological states. Our

parameters, determined by Q-BCG, with parameters

determined using other non-invasive methods were

compared. (

Trefny at all, 1996).

1.3 Quantitative Seismocardiography

During a visit to the Flight Psychophysiology

Laboratory at Wright-Patterson Airforce Base, a new

application field for Q-BCG emerged. This made

use of the fact that our method enables the recording

of force applied without phase or time deformation.

Thus, heart rate may be monitored and analyzed

using the method of heart rate variability. The

method of Q-BCG was designated by the laboratory

employees as absolutely non-invasive, as the persons

examined did not have any electrodes attached to the

body surface and was not connected by cables to the

registering instrument. This new field of monitoring

heart activity, whereby we determine both

amplitude-force and time-frequency relationships, is

termed Quantitative Seismocardiography (Q-SCG).

(

Trefny at all, 1998). Thus, one may determine the

force-response of the cardiovascular system to

changes in external stimuli, as well as the

autonomous nervous system regulation of the

circulation and the activity of the sympathetic and

parasympathetic systems. The basic part of the Q-

SCG is a rigid piezoelectric force transducer resting

on steel chair. The examined person sits on the seat

placed on the transducer and force caused by the

cardiovascular activity is a measured (Figure 1). The

natural frequency of the chair is higher then 1 kHz

so that there is no interference with the vibrations

caused by the heart activity. Neither damping nor

isolation from building vibrations are necessary.

These properties enabled to calibrate

seismocardiographic system and determine the

absolute value of force acting upon the pick-up-

device. (

Trefny at all, 1999).

The system described in the present study enable

better signal evaluation based on high resolution

analog/digital converter (ADC), digital filtration and

digital correction of nonlinearities and noise

suppression by means of personal computer (PC).

The heart rate (HR), systolic force (F), minute

ELECTRONIC DEVICE FOR SEISMOCARDIOGRAPHY - Noninvasive Examination and Signal Evaluation

205

cardiac force (MF) and breathing frequency (BF) is

non-invasively measured.

Figure 2: The simplified block diagram of the electronic

system for Q-SCG measuring. The main parts of the

system are: Piezoelectric force transducer, Analog Filter,

PGA - programmable gain amplifier, Sigma-Delta A/D

converter, Digital Filter, microcontroller and Personal

Computer connected to system by means of USB

(Universal serial bus).

Figure 3: The frequency and phase responses of analog,

electronically controlled filter.

2 MATERIALS AND METHODS

The electronic system used for data acquisition

consists of a piezoelectric force transducer (PT),

analog front end for low frequency measurement

applications (containing ADC), microcontroller and

PC. The block diagram of the whole system is

shown in Figure 2. It is important to note, that

amplitude of measured signal from PT is sometimes

under 1 mV (depend on subject heart activity) and

desired frequency spectrum is lower then 30 Hz.

The measured signal is corrupted by strong noise,

baseline wander, etc. therefore the analog and digital

signal processing (DSP) are used for signal

denoising. The frequency and phase responses of

electronically controlled analog bandpass filter are

shown in Figure 3. The analog front end of A/D

converter can accept either 2 low level input signals

(± 10 mV to ±1.225 V, depends on PGA setting) and

produce serial digital output. (AD7707, 2000). It

employs a sigma-delta conversion technique to

realize up to 16 bits of no missing codes

performance.

Figure 4: The frequency response of on-chip digital filter

in A/D converter.

The sigma-delta modulator output is processed by an

on-chip digital filter. The first notch of this digital

filter can be programmed via an on-chip control

Hz to 131 Hz) and output update rate (4.054 Hz to

500 Hz). The -3 dB frequency f-3dB is determined

by the programmed first notch frequency according

to the relationship (1):

-3dB

= 0.262 f

[Hz] (1)

where f

is filter first notch frequency and f

output update rate (sampling rate). The AD7707’s

digital filter is a low-pass filter with a (sinx/x)

response (also called sinc

). The transfer function for

this filter is described in z-domain by:

and in the frequency domain by:

() .

−

(2)

and in the frequency domain by:

sin( / )1

() .

sin( / )

Nf f

Nff

= [Hz]

(3)

where N is the ratio of the modulator rate to the

output rate (modulator rate is 19.2 kHz for

Xtal=2.4576 MHz).

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The frequency responses of the digital filter are

shown in Fig. 3 and Fig. 4. Phase response is given

by (4):

Phase(f) = -3

(N -2) f/f

[Rad]

(4)

The data from A/D converter are next filtered also

by Savitzky-Golay Smoothing filter (SG).

Figure. 5: Record of the Q-SCG in normal man, age 45

years, 78 kg, after lowpass and highpass filtration. Raw

signal is filtered by SG filter and Remez, finite impulse

response (FIR) filter; 250 samples = 1 sec.

Savitzki and Golay defined a family of filters which

are suitable for smoothing and/or differentiating

sampled data (commonly called Savitzki-Golay,

DISPO - Digital Smoothing Polynomial, or least-

square smoothing). (

Savitzki and Golay, 1994). The

data are assumed to be taken at equal intervals. The

smoothing strategy is derived from the least squares

fitting of a lower order polynomial to a number of

consecutive points. (

Madden, 1998). For example, a

cubic curve which is fit to 5 or more points in a least

squares sense can be viewed as a smoothing

function. (

Bromba, Ziegler, 1998), (King at all, 1999).

Figure 6: Breathing frequency derived from raw signal by

means of two SG filters.

Figure 7: The heart rate variability can be also detected

from Q-SCG signal.

An example of Q-SCG measurement is illustrated in

Figure 5. The output update rate was 250 Hz, f

-3dB

was 62.5 Hz. The tree SG filters with different

window length were used for Q-SCG and breathing

signal processing. Data on Y axis are decimal values

of A/D converter. The breathing signal detection is

shown in Figure 6.

The heart rate variability (HRV) can be also

evaluated from Q-SCG signal. The signal processing

example for beat to beat detection is shown in short

time slice of signal in Figure 7. After calibration (Y

axis in Newton), the systolic force F and minute

cardiac force MF can be computed according (5) and

(6):

F = (F

+ F

)/3 [N]

(5)

MF= F * HR [N. beats/min]

(6)

where HR is heart rate and F

, F

can be find

according Figure. 8. The systolic force represent the

force response caused by the heart activity and is

expressed in units of force [Newton]. For the total

intensity of the heart activity is introduced the

minute cardiac force which equals the systolic force

multiplied by the HR.

Figure 8: The systolic force (F) determination from Q-

SCG measured signal from points: H, I, J K.

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207

3 DISCUSSION

The anatomy and function of single organs of human

organism are in correlation. This is true for muscle

mass, the body weight and the muscle force too. The

reason of this fact is that higher body weight needs

for the defined movement greater force, which

cannot be realised but by the development of the

skeletal musculature. Consequently greater

musculature needs more energy which is transported

and distributed by the cardiovascular system. In

addition, the increased performance of the

cardiovascular must be adjusted by the heart muscle.

From these relationship it can be concluded that

there must be not only the correlation between the

skeletal muscle force and the heart mass but also

between the skeletal muscle force and the systolic

cardiac force as it was observed in the present study.

According to our opinion these results may be

extrapolated generally for healthy men without

pathological changes in cardiovascular system.

4 CONCLUSIONS

The principles of Q-SCG, measuring system for

noninvasive measuring of heart activity, breathing

and heart rate variability was presented. Also signal

processing for Q-SCG evaluation was described.

ACKNOWLEDGEMENTS

Zdenek Trefny preparing this paper has been

supported by: Grant Eureka E! 2249.

Milan Stork's participation has been supported by:

Department of Applied Electronic and Tele-

communication, University of West Bohemia and

from GACR (grant No. 102/07/0147).

REFERENCES

Trefny Z., Trefny M., David E., Machova J., Svacinka J.,

1996. Some physical aspects in cardiovascular

dynamics, J. Cardiovaasc., Diag. and Procedures, 13,

No: 2, pp. 141-45.

Trefny Z., Svacinka J., Trefny M., Trojan S., Slavicek J.,

Kittnar O., 1998. Relation between cardiac force and

maximal sceletal muscle force, Journal of Physiology,

Cambridge University Press.

Trefny Z., Svacinka J., Trefny M., David E., David V.,

Trojan S., Slavicek J., Kittnar O., 1999. Noninvasive

method - quantitative balistocardiography (Q-BCG)

and its value, XIII. Congres of the Cardiovascular

System Dynamics Society, Gent., Belgium.

AD7707, 3-Channel 16-bit, Sigma-Delta ADC, Analog

Devices, 2000, Internet site address:

http://www.analog.com.

Savitzki and Golay, 1994. Smoothing and Differentiation

of Data by Simplified Least Squares Procedures,

Analytical Chemistry, vol. 36, no. 8, pp. 1627-38.

Madden H., 1998. Comments on Smoothing and

Differentiation of Data by Simplified Least Square

Procedure, Analytical Chemistry, vol. 50, no. 9, pp.

1383-86.

Bromba U., Ziegler H., 1998. Application Hints for

Savitzki-Golay Digital Smoothing Filters, Analytical

Chemistry, vol. 53, no. 11, pp. 1583-86.

King R., Ruffin C., LaMastus F., Shaw D., 1999. The

Analysis of Hyperspectral Data Using Savitzki-Golay

Filtring - Practical Issues (Part 2), IEEE IGARSS ’99

proceedings.

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