DOUBLE HEADED PROBE FOR LOCAL PULSE WAVE

VELOCITY ESTIMATION

A New Device for Hemodynamic Parameters Assessment

H. C. Pereira, J. B. Simoes, J. L. Malaquias

ISA – Intelligent Sensing Anywhere, Rua D. Manuel I, Coimbra, Portugal

Instrumentation Centre, Physics Department, University of Coimbra, R. Larga, Coimbra, Portugal

T. Pereira, V. Almeida, E. Borges, E. Figueiras, J. Cardoso, C. Correia

Instrumentation Centre, Physics Department, University of Coimbra, R. Larga, Coimbra, Portugal

Keywords: Local Pulse Wave Velocity, Double Headed Probe, Piezoelectric Sensor, Test Bench, Impulse Response.

Abstract: The present work proposes a new device for local pulse wave velocity (PWV), by using an innovative

configuration of a double piezoelectric (PZ) sensor probe. PWV is assessed in one single location and

involves the determination of time delay, between the signals acquired simultaneously by two PZs, 23 mm

apart. The double probe (DP) is characterized in a dedicated test bench system, where two main studies

were carried out. In the first one, the impulse response (IR) for each PZ sensor is determined and evaluated

through the deconvolution method. In the second one, DP time resolution is estimated from a set of time

delay algorithms and compared with the reference values, obtained through the signals of two pressure

sensors. Results demonstrate the effectiveness of the inferred IRs in deconvolution purposes and the

possibility of measure higher PWV values (≈ 19m/s), through the DP, with an error less than 10%.

1 INTRODUCTION

The velocity of propagation of the pulse pressure

wave is recognized as the simplest and most

reproducible process of assessing non-invasively

arterial stiffness, pointed out as an important key

factor of cardiovascular risk (Laurent et al, 2006).

The use of electromechanical technology remains

the golden standard for pulse wave velocity (PWV)

measurement, as a result of its signal robustness,

large bandwidth, low-price and ease of use. The

methodology used in PWV assessment, requires the

acquisition of pressure waves at two positions,

separated from a distance, d, and the determination

of time delay between the waves, defined as the

pulse transit time, ∆t. The main problems of this

technique are related with the determination of time

delay that strongly depends on the method that is

used, and with the complexity in accurately

estimating the distance between the recording sites

(Segers et al, 2009).

An alternative approach that is presented in this

work is to decrease the distance between the two

measurement sites, as a solution for more accurate d

estimations, thus providing information on local

hemodynamics. Local PWV is an accurate marker of

the degree of atherosclerosis and it is generally

measured over carotid artery. The existing devices

are essentially based on ultrasound methods and

echotracking techniques but they have not been yet

generalized to the clinical practice, due to poor PWV

precision, in phantom studies or in-vivo experiments

(Meinders et al, 2001). The most recent studies on

this matter, investigated the precision of different

signal processing methods for local PWV

assessment, over short segments, by means of a M-

line ultrasound system. The discrepancy between the

results in phantom and in-vivo was considerably

high, resulting in an unsuitable technique for clinical

practice (Hermeling et al, 2008).

In the present work, an alternative device based

on a double headed configuration of piezoelectric

(PZ) sensors for local PWV assessment, is proposed.

444

Pereira H., Simoes J., Malaquias J., Pereira T., Almeida V., Borges E., Cardoso J. and Correia C..

DOUBLE HEADED PROBE FOR LOCAL PULSE WAVE VELOCITY ESTIMATION - A New Device for Hemodynamic Parameters Assessment.

DOI: 10.5220/0003293704440447

In Proceedings of the International Conference on Biomedical Electronics and Devices (BIODEVICES-2011), pages 444-447

ISBN: 978-989-8425-37-9

 2011 SCITEPRESS (Science and Technology Publications, Lda.)

The probe is widely characterized on a test bench

system, capable of reproducing important features of

the cardiovascular system (Pereira et al, 2010).

2 MATERIALS

2.1 The Double Headed Probe

The configuration of the developed double headed

probe (DP) is shown, in figure 1. The DP consists of

two circular-shaped PZ sensors (MURATA® 7BB-

12-9 Sounder, 12mm diameter), placed 23 mm apart

and mounted on a triple double layer printed circuit

board (PCB). The first and second PCB layers

support the PZ discs, assuring the PZs oscillations

due to their perforation and the third one

incorporates the local signal conditioning electronics

which is based on a voltage follower amplifier, set to

a gain of ≈ 2, for each PZ. The probe’s mechanical

interface consists of two mushroom-shaped PVC

pieces (15 mm diameter in top), located in the centre

of the PZ discs. These elements are responsible for

transmitting the distension imparted to tissues by the

pressure waveform, to each PZ sensor.

Figure 1: Cross section scheme of the double headed PZ

probe. A, B - support layers; C- signal conditioning layer;

D- ‘mushroom’ PVC interface E - PZ metal disk; F- PZ

material G – PZ signal conductor H – ground conductor I

– coaxial cable 1- PZ

;

2- PZ

2.2 The Test Bench System

For testing the probe, it was developed a dedicated

test bench, diagrammatically shown in figure 2.

A pressure wave is generated by a piston

mechanism coupled to a 0.7 mm stroke actuator,

ACT, driven by a high voltage linear amplifier, HV,

(Physik Instrumente GmbH P-287 and E-508,

respectively) and launched into an 8mm internal

diameter, 0.5 mm wall thickness silicone rubber

tube, filled with water. The wave is then captured by

the DP placed along the tube and by two pressure

sensors PS1 and PS2 (Honeywell, 40PC015G1A),

placed at the tube’s extremities. The acquired signals

are sampled at 12.5 ksps, through NI DAQ USB-

6210, and stored for offline analysis using Matlab®.

The input waveforms are programmed into an

Agilent 33220A arbitrary waveform generator, WG

and the DC level is controlled by a piston–mass

combination, P-m, placed at the extremity of the

tube, on the opposite side of the ACT ((Pereira et al,

2009).

∆x

NI USB6210HV

PS2

PS1

ACT

Workstation

200 cm4.5 cm

1.5 cm

3.6 cm

3.7 cm

DP1 DP2

PZa

PZb

Figure 2: Schematic drawing of the test bench system.

3 METHODS

3.1 Double Probe Characterization

3.1.1 Impulse Response Determination

The electrical equivalent of a PZ sensor is more

complex than a simple RC circuit, mainly if the

sensor is attached to another mechanical structure.

The first experiment carried out for DP

characterization, consisted in determining the

impulse response (IR) for each one of its sensing

elements (i.e., mushroom probe plus PZ sensor

electronics). To achieve this purpose, a technique

based on a chirp signal that sweeps linearly a wide

frequencies range (from 500 mHz to 1 kHz) was

used. This sweep was generated by the WG and fed

to the ACT, with direct actuation on the probe’s

interface. The spectra of the PZ output and of the

sweep input signal were computed, and the

correspondent transfer function was inferred.

Through the inverse fast Fourier transform (IFFT), it

was possible to determine the referred IR.

Test bench acquisitions were accomplished in

order to apply the deconvolution principle to the DP

output, and thus determine the effectiveness of the

determined IRs.

3.1.2 Time Resolution Assessment

The main potential of the DP is focused on the

ability to locally measure the PWV, with enough

accuracy to be considered a valid device for clinical

use. In order to assess DP time resolution

performance, two main studies were carried out. The

DOUBLE HEADED PROBE FOR LOCAL PULSE WAVE VELOCITY ESTIMATION - A New Device for

Hemodynamic Parameters Assessment

445

first one aimed at studying the PWV progress of two

uncoupled PZs, regarding their separating distance,

Δx. Two DPs, initially separated by 50 cm, were

used: one of the probes (DP1) was static, while the

other one (DP2) approached successively at intervals

of 2 cm (figure 2). For each position, a Gaussian

pressure wave (150 ms width) was delivered to the

system. The second experiment intended to

determine the precision of PWV measurements

obtained with the DP, in 25 different tube’s positions

as well as the accuracy concerning the reference

PWV values, estimated with pressure sensors. For

each position, a burst of 10 Gaussian pressure waves

(400 ms width) was reproduced by the ACT.

3.1.3 Algorithms for Time Delay Estimation

Time delay was estimated for two different settings:

between the signals of both pressure sensors

(considered as a reference time for PWV estimation)

and between the signals of the DP PZ sensors. Three

different algorithms were considered:

(a) Cross-Correlation - The ACT driving signal

is used as a reference and time delay is calculated as

the difference between the two maxima values,

obtained from the cross-correlation between the

ACT signal and the pressure/PZ signals amplitude;

(b) Zero-crossing - Due to the differentiator

nature of the PZ sensors, zero-crossing point is used

as a time reference for time delay estimation and it is

determined through a linear fit on the zero crossing

vicinity; and,

6th degree polynomial fit in the maximum region to

guarantee an accurate identification of the peaks.

4 RESULTS

4.1 Impulse Response Determination

The IRs obtained for each one of the PZ sensors are

presented, in figure 3. The profiles obtained for each

sensing element are equivalent, both in shape and in

amplitude, and are very close to those expected for a

simple differentiator circuit.

In figure 4, the effectiveness of the referred IRs

is evaluated, through deconvolution method.

From the DP output and its IRs, it was possible

to recover a programmed triangular pressure

waveform of 500 ms width, sensed at the end of the

tube. It is visible the striking similarity between the

deconvolved pressure waves, determined for each

PZ, as also the high correlation with the input

Figure 3: DP impulse responses.

Figure 4: Deconvolved signals obtained from DP output

and DP IRs.

waveform. The presence of inflection points

correspond to the influence of reflected waves,

generated by the two reflection sites at the

extremities of the tube. Although deconvolution was

used in the present study, as an auxiliary technique

in DP characterization, the encouraging results

obtained above, suggest that it can be used as an

alternative method that allows the precise recovery

of the original pressure waveform. Future work will

be pursued up in order to determine the application

of this IRs to other systems (e.g.: human carotid).

4.2 Time Resolution Assessment

In figure 5, the PWV obtained for two uncoupled PZ

sensors, in successively smaller separation distances

is illustrated.

In general, the relative error in PWV increases as

the distance between the two PZ decreases. For Δx

longer than 12 cm, the PZs PWV values are close to

the reference values. For Δx smaller than 12 cm, the

PZs PWV values diverge from the pressure sensors

values and an increase of the error is visible, mainly

in the maximum amplitude algorithm.

Figure 6 compares the PWV values obtained

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446

Figure 5: PWV values of unclouped PZs and pressure

sensors, yielded by the three algorithms.

through DP, along 25 different consecutive positions

with the reference PWV values. Only the cross-

correlation algorithm was implemented for time

delay estimation, due to the previous best results.

Figure 6: Dispersion of PWV values obtained through DP

and pressure sensors, along 25 consecutive locations.

Both PWV distributions are correlated however

the dispersion of them is still quite different.

Actually, the referred PWV distributions present a

mean value and a standard deviation of 19.26 ± 0.04

m/s (pressure sensors set) and 19.55 ± 2.02 m/s (DP

set), matching up a coefficient of variation of 0.21%

and 10.32%, respectively. In addition, the relative

error between the two sets is about 8.11%.

Since this higher DP dispersion may have its

basis in lower statistics or mechanical drawbacks

(e.g. geometry of mushroom interfaces, effective

distance between the PZ sensors, etc.), future work

will be pursued up at this stage.

5 CONCLUSIONS

A novel device for PWV assessment, based on a

double configuration of PZ sensors is presented and

successfully characterized on a dedicated test bench

system.

Local PWV was measured with enough accuracy

(relative error < 10%) in test bench experiments,

through DP. Although studies to validate the clinical

use of DP are still required, this device seems to be a

valid alternative to local PWV stand alone devices.

ACKNOWLEDGEMENTS

We acknowledge support of Fundação para a

Ciência e Tecnologia, as well as of company ISA-

Intelligent Sensing Anywhere.

REFERENCES

Hermeling E, Reesink K, Reneman R and Hoeks A 2008

Confluence of incident and reflected waves interferes

with systolic foot detection of the carotid artery

distension waveform J Hypertens. 26 2374-80

Laurent S., Cockcroft J., Van Bortel L., Boutouyrie P.,

Giannattasio C, Hayoz D, et al. 2006 Expert consensus

document on arterial stiffness: methodological issues

and clinical applications Eur Heart J. 27 2588-2605

Meinders J., Kornet L., Brands P. and Hoeks A. 2001

Assessment of local pulse wave velocity in arteries

using 2D distension waveforms Ultrason. Imaging 23

199-215

Pereira H. C., Cardoso J. M., Almeida V. G., Pereira T.,

Borges E., Figueiras E., Ferreira L.R., Simões J.,

Correia C. 2009 Programmable testbench for

hemodynamic studies IFMBE Proceedings 25/IV

1460ff

Pereira, H. C., Lopes, T., Almeida, V., Borges, E.,

Figueiras, E., Basílio J. B., Cardoso, J., and Correia

C., 2010 Characterization of a double probe for local

pulse wave velocity assessment Physiol Meas. 31

697-714

Segers P., Kips J., Trachet B., Swillens A., Vermeersch S.,

Mahieu D., Rietzchel E., Buyzere M. and Bortel L.

2009 Limitations and pitfalls of non-invasive

measurement of arterial pressure wave reflections and

pulse wave velocity Artery Research 3 79-88

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Hemodynamic Parameters Assessment

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