calculated from data obtained by multichannel impedance plethysmography using purpose-built electrode arrays. Several signal processing techniques were ...
Computation of the pulse wave velocity in limbs from multichannel impedance plethysmography. F. Risacher', J. Jossinet', E. McAdams", C. Eynard', J. McLaughlin", M. Schmitt"'. O
INSERM Unit6 28 1,151 Cours Albert Thomas, 69424 Lyon cedex 03, France
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The Northern Ireland BioEngineeringCentre, University of Ulster at Jordanstown
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Laboratoire dExplorations Thermiques et Vasculaires, E. Herriot Hospital, Lyon
A b s t r a c t - Pulse Wave Velocity in limbs was calculated from data obtained by multichannel impedance plethysmography using purpose-built electrode arrays. Several signal processing techniques were applied to data recordled in normal subjects. The first harmonic and the derivation techniques exhibit a greater dispersion than the other used methods. The smallest dispersion was obtained when the PWV was calculated from the time interval between peaks of dZ/dt signals.
INTRODUCTION Blood circulation in arteries can be studied by measuring either the movement of blood inside t'le vessel (e.g. Doppler techniques) or the deformation of the arterial wall (e.g. strain gauge plethysmography). The periodic changes observed in the total impedance, observed in limb impedance plethysmography, reflect the heart related changes in the volume of blood contained in the arteries. These changes are due to the differencebetween the resistivity of blood and that of surrounding tissues (muscle, bone.. .). The amplitude of the first time derivative of this signal has been used for many years for the estimation of limb blood flow or for the estimation of cardiac output. Impedance measurement is achieved in this case by means of a single channel, four electrode device. Two electrodes are used to inject a longitudinal current into the limb and the other two, encompassing the region of interest, are used to measure the resulting voltage difference. It is also feasible to make several simultaneous measurements. In this instance, a series of voltage differences are measured between several pairs of sensing electrodes located along a limb. If the distance between these sites is known, the measurement of the time lag between the recorded signals enables the callculation of the propagation speed of the arterial wall deformation, termed "Pulse Wave Velocity" (PWV). PWV is dependent of, among other things, the deformability of the artery. The measurement of PWV may, therefore, be expected to be a non-invasive, indirect method for arterial wall quality estimation [ 11. The present study analyses results obtained using several methods of PWV calculation from the above impedance plethysmographicsignals.
EQUIPMENT AND METHOD A. Electrodes
The constant amplitude current was injected into the legs by means of two standard ECG disposable electrodes located on the upper portion of each foot. Voltage differences were measured using purpose-built electrode arrays. These ;mays consisted of parallel conductive strips screenprinted on to a supple substrate sheet using silver-silver chloride: ink, according to a technology developed at the NIBEC. The electrodes were covered with a layer of conductive, adhesive hydrogel, thus ensuring good electrical contact and the firm attachment of the array to the skin [2].In the present study, each array consisted of strips 0.9 cm wide and 6 cm long. Three pairs of electrodes were used. The gap betweem the electrodes of a given channel was 1.7 cm and the distance between consecutive pairs, i.e. the distance between measurement sites, was 6.8 cm (Fig 1). B . Instrumentation
The instrumentationused in this study consisted of a constant amplitude current generator (1mA rms, 64 H z ) and three differentialvoltmeters. After amplitude detection, filtering and differentiation, the outputs of these voltmeters consisted of periodic voltages proportional to the first time derivative of
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This work was partly supported by a INSERMMRC Exchange Fellowship.
0-7803-0785-2/92$03.OO OEEE
Fig. 1. General arrangement for PWV measurement in limbs.
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the limb impedance. Particular attention was given to the construction of the voltmeters, in order that the output phase error between any two channels was less than 0.5 degrees in the range 0.5 - 16 Hz. The signals were recorded during 8 second periods using a Tektronix Datalab 2510 data acquisition system operated at a sampling frequency of 4 kHz. This frequency was the optimal compromise between time accuracy and the amount of data to be stored and processed. The general arrangementof the electrodes and instrumentation is shown on figure 1. C . Calculation of PWV
The calculation of PWV was carried out using a variety of techniques. In the time domain, the time lag between two dZldt signals was estimated from the time difference between their respective peaks. The group velocity was then calculated by dividing the distance between sites by the above time interval. This technique was applied to each of the cardiac cycles in the 8 second signal samples. This type of calculation was applied to the raw data, to bandpass filtered (0.5 - 16 Hz) data and to the time derivative of the bandpass filtered data (d2Z/dt2). A global calculation, involving the whole signal as a single, complete sample, was achieved by calculating the self-correlation function and the crosscorrelations functions between the signals from different channels. In the frequency domain, the phase velocity was calculated from the phase difference of the fundamental frequency components of two signals. D . Protocol
Multichannel recordings were carried out on 9 volunteers, 20-22 years of age, (5 males and 4 females). For each subject, 8 second signal samples were recorded at three different occasions during the same half-day, using in each case the same electrode array, which was left in place between recording sessions. During measurements the subjects were in a supine position. Recordings started following a resting period of 10 minutes for the stabilization of heart rate. RESULTS 27 sets o f data were obtained. The mean values and the standard deviations of calculated values of PWV for all methods used are given on Fig. 2. Each individual value corresponds to the mean of the results obtained from the computation of all the data for a given method. The smallest dispersion is observed when the time interval is calculated from the distance of peak values using the raw data. Filtering the data increased the standard deviation. The largest standard deviation was observed for the differentiated signals.The first harmonic also produced a large standard deviation.
DISCUSSION
Raw
filtered
diff.
Con.
Harm.1
Fig. 2. Means and 95% error bars obtained from the 27 sets of data from normal subjects. The various processing techniques are plotted on the horizontal axis. The PWV values are plotted in the vertical axis.
nature of each processing technique used. The most homogeneous values were obtained by calculating the time delay between the peaks of the dzldt signal, raw and filtered, and by the correlation technique. However, the mean values differ. This can be explained by the fact that, in the first two techniques, each cardiac cycle is processed independently of the others and for the correlation technique, the whole signal is used. The large variability differentiation technique can be explained by the decrease in the signal-to-noiseratio induced by the resultant high pass filtering. In the first harmonic method, a large portion of the frequency content of the signal is discarded; furthermore, the phase velocity is dependent on the frequency, so that the harmonics of the signal do not propagate at the same speed. This might explain why the first term of the Fourier transform is found to be insufficient in representing the whole signal. CONCLUSION The most consistent of the tested computation methods are that which involve the calculation of the time lag between peaks of the d z l d t waves and the correlation technique. However, the mean values they produce differ from each other. The sensitivity to noise of each o f these methods should therefore be investigated. Comparison with other existing techniques would also aid in assessing the validity of these computation methods. ACKNOWLEDGMENT R. Jarry, A. Matias and G. Tourtel for their respective valuable technical collaborations.
REFERENCES [ I ] F. Risacher, J. Jossinet, E. McAdams, Y. Mann and M. Schmitt,
The interpretation of the results must take into account (i) - [2] The inter-subject differences (ii) - The absence of any reference value of PWV and (iii) - The intrinsically different 1745
"Evaluation of the quality of the arterial wall using multichannel impedance plethysmography," 5th BME Int.. Symp. Peniscola, 1991.
J. Jossinet and E.T. McAdams, "The use of hydrogel for biosignals recording," 12th Ann. Int. Conf IEEE/EMBS, pp 1490 - 1491, Philadelphia, USA, 1991.
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