A portable two-channel PPG cardiovascular sensor device

 

Janis Spigulis*, Renars Erts and Maris Ozols

 

University of Latvia, Physics Department and IAPS, Raina Blvd. 19, Riga, LV-1586, Latvia

 

 

ABSTRACT

 

A portable sensor device for simultaneous detection and processing of skin-remitted optical signals from any two sites of the body has been developed and tested. The photoplethysmography (PPG) principle was applied to follow the dilatation and contraction of skin blood vessels during the cardiac cycle. The newly developed two-channel approach allows to estimate the vascular blood flow resistance by analysis of time shifts between the PPG pulses detected at different body sites. Potential of the sensor device for express-assessment of human cardio-vascular condition and for body fitness tests has been demonstrated.

Keywords: Photoplethysmography, optical bio-sensing, cardio-vascular assessment. fitness control.

 

 

1.      INTRODUCTION

 

The human’s cardio-vascular condition can be assessed both invasively and non-invasively. Photoplethysmography (PPG) is a non-invasive method for studies of the blood volume pulsations by detection and analysis of the tissue back-scattered or absorbed optical radiation. Blood pumping and transport dynamics can be monitored at different body locations - fingertip, earlobe, forehead, forearm, etc. – with relatively simple and convenient PPG contact probes. PPG technique potentially may become a routine everyday tool for express diagnostics and early screening of cardio-vascular pathologies, as well as for self-monitoring of the vascular condition.  Tele-diagnostics by means of PC-connections via Internet or LAN is another area where advanced PPG-technology becomes very important.

 

In general, each recorded PPG pulse contains information, useful for cardio-vascular assessment. However, all detected heartbeat pulses are not equal – the PPG signal amplitude, baseline and period are changing with time 1. The real bio-signals are fluctuating around their mean value that we call single-period photoplethysmography (SP-PPG) signal. It can be determined by averaging a sequence of about 50…80 PPG pulses by means of specific algorithms and PC-processing programmes 2 - 4.

 

Our first wheel-table SP-PPG fingertip sensor devices 2, 3 had undergone several series of clinical tests in laboratory, classroom and hospital environments. Analysis of signals taken from numerous volunteers had lead to conclusion that each person has his/her specific shape of the mean SP-PPG signal, and this "PPG-fingerprint" reflects the individual's cardio-vascular condition. This observation confirmed the diagnostic potential of the developed devices; however, their hospital bed-site and field applications were limited due to considerable weight/size of the table-top computer placed on the wheel-table and dependency on the wall power plugs, therefore a smaller sensor model based on a lap-top computer was created as the next step 4.

 

Recently a portable version of two-channel PPG sensor device was developed and tested. The sensor comprises a set of universal optical contact probes, electronic converters and a laptop computer with specially designed software providing real-time display, processing and storage of the PPG signals recorded at both channels. The newly developed two-channel approach is more informative – on-line monitoring of two body sites not only doubles the data flow but also opens possibility to estimate the vascular blood flow resistance. It is proportional to the heartbeat wave propagation time - the measurable time shift between the two corresponding PPG pulses detected at two different body sites. Our studies confirmed the promising potential of this methodology and sensor design for easy and fast cardio-vascular assessment and for body fitness tests.

 

*) E-mail: janispi@latnet.lv, tel/fax: +371 7228249

2. DESIGN OF THE TWO-CHANNEL PPG SENSOR DEVICE

 

The PPG sensor device consists of a set of optical contact probes (two of them are used simultaneously during the measurements), the bio-signal acquisition circuit, and the lap-top computer with specially developed software. All equipment is placed in a hand-held case 4  of size 44x32x9 cm and weight 4.1 kg; it is battery-powered and can operate up to 3 hours without recharging.

 

Each optoelectronic contact probe emits cw-radiation into the skin tissues and detects the back-scattered radiation; its separated AC-component precisely reflects the skin blood volume pulsations. Each contact probe comprises a GaAs emitting diode (diameter of the emitting area ~2 mm, radiant power ~10 mW, peak wavelength ~940 nm, the estimated mean penetration depth under the skin surface ~2 mm), and a Si photodiode with square detection area ~5x5 mm. Both diodes are closely mounted on a soft plastic pillow and fixed onto the measurement site by means of a sticky band – see Fig. 2,a. The band length is adjusted for the fingertip measurements (Fig. 2, b); also a standard pulse oximetry finger-clip was adapted for our measurements, maintaining the same emitter-detector geometry as in the band-mounted probe.

 

               

                               

 

 

 

 

 

 

 

 

 

 

 

                                a                                                                                                              b

 

Fig. 1. The PPG contact probe (a) and its application for monitoring of blood pulsations in fingertip (b).

 

                                                                               

       

 

a                                                                                                              b

 

Fig. 2. Two-channel PPG measurements: placing the contact probes at fingertip and belly (a), and at calf and toe (b).

 

Advantage of the band-based probe design is the possibility of its flexible extension by means of spare sticky bands, so allowing to take the PPG measurements from various locations of the body, e.g. fingertip, belly, calf, toe (Fig. 2).

 

Special software was developed for the PPG bio-signal acquisition, processing and data storage related to both input channels, offering the following options:

·         Filling the first window for patient data - name, age, gender, complains, doctor’s comments, etc.;

·         Pre-setting the measurement time schedule;

·         The PPG signal real-time display on the monitor;

·         Signal clean-up (special filtering algorithm) and calculation of the mean single-period PPG (SP-PPG) signal shape;

·         Calculation of specific cardio-vascular parameters for the registered signals - heartbeat rate, anacrota rise-time, time delay and relative amplitude of the secondary peak (dycrotic notch), time-delay between two corresponding PPG signals at both channels, etc.;

·         Display of the corresponding PPG parameter set with subsequent cardio-vascular assessment results;

·         Storage of the measurement/assessment data.

 

The data sampling rate 100 s -1 was usually chosen; it can be increased up to 950 s -1 for special cases when higher time resolution is needed. The device monitor screenshot during two-channel PPG measurements is presented at Fig. 3.

 

 

Fig. 3. The monitor screenshot during two-channel PPG measurements.

 

 

3. APPLICATION 1: CARDIO-VASCULAR TELE-MONITORING VIA INTERNET

 

In order to check suitability of the new PPG sensor device for telemedicine needs, an Internet transmission experiment was carried out, using the WinSock interface between the TCP/IP protocol and our Windows measurement program. The measured PPG signal values of both channels were first packed together with the corresponding time reading value, and then the packages were transmitted in real time with rate ~ 6 Kb/s via the Realtek 56 K modem. The signals were further registered by another computer (with 225 MHz processor and appropriate software) that was connected to Internet via the radio-link. Practically unchanged signal data were received in few seconds (Fig. 4). Full analysis of the transmitted and received data gave the signal loss probability less than 0.7 % in this case.

 

Consequently, the developed sensor device and software can be used in telemedicine systems, e. g. for patient home monitoring under doctor’s tele-control.

 

Fig. 4. The telemedicine experiment: comparison of the two-channel PPG signals transmitted via Internet.

 

 

4.      APPLICATION 2: CARDIO-VASCULAR MONITORING DURING THE FITNESS TESTS

 

Advantage of the newly developed two-channel approach is availability of additional diagnostic information on the vascular blood flow resistance – it can be obtained by measuring the heartbeat wave propagation time between two body contact sites as the time shift between the two corresponding PPG pulses. For example, Fig. 5 illustrates distinct time delay between the PPG signals recorded simultaneously at the left fingertip and left toe (a), and at the left toe and belly (b). Time resolution of the device is high enough to provide reliable blood pulse wave propagation velocity estimations for diagnostic needs.

b
 

a
 
                                                                                                          

               

Fig. 5. Time-shifted PPG signals detected at different body sites: a – finger and toe, b – toe and belly.

 

We performed series of PPG measurements before and after intensive physical exercises aiming to follow the cardio-vascular relaxation process that reflects adaptability of the body to physical loads. The monitored volunteers – about 200 in total - were persons of different ages, genders and training background. We studied the PPG signals detected at fingertip only and at fingertip simultaneously with signals detected over the carotid artery on the neck. The goal of this study was to find out specific exercise-induced features of bio-signals, giving evidence on possible cardio-vascular disorders of the monitored person.

 

In single-channel experiments, such evidence might be sharp spasmatic peaks in the fingertip PPG signals appearing for some persons immediately after intensive running (Fig. 6, a - upper curve) or after a short relaxation time (Fig. 6, b - the next curve from the top). Probably they can also serve as markers of the cardio-vascular adaptation to physical loads.

 

                                                a                                                                                                              b

 

Fig. 6. The spasmatic peaks observed in fingertip PPG signals after intensive running.

 

The two-channel PPG methodology can ensure more informative fitness tests – in addition to the traditional physiological parameters (e. g. pulse rate, blood pressure), monitoring of vascular resistance changes reflected as the changes in time shifts of corresponding PPG signal pulses is also available.  We performed a number of measurements during a complex fitness tests comprising 1 minute horizontal relax, 1 minute standing position, 3 minutes metronome-controlled stepping up and down (the Harvard step-test 5), and 5 minutes relax in sitting position. The dual-channel PPG signals were recorded continuously all 10 minutes at two body locations – the Carotid artery on the neck and the left middle fingertip. As result, several functional parameters were calculated 6 - the heartbeat pulse rate changes (from time intervals between the PPG peaks in each channel), the dominant pulse rate oscillation frequencies (by Fourier analysis), the recreation time after the exercise (by calculating the time constant of exponential pulse rate decay), as well as the  pulse wave propagation time between the two contact sites (from time shifts between the corresponding PPG pulses detected in both channels).

 

As example, the pulse rate changes and the 2-channel PPG time shifts for the same person (male, 23 years) are presented on Fig. 7; the mean SP-PPG fingertip signal shapes for this person are presented on Fig. 8. Following all the test phases, there are notable changes not only in pulse rate, but also in time-shifts between the PPG pulses in both channels, so indicating that the vascular resistance was changing.  Such sensitivity of time shifts we observed only for few persons; most of the monitored volunteers had roughly the same time shifts at all phases of this test.

 

One more advantage of the two-channel approach is possibility to verify the suspicious features of signals at one channel by comparing with those detected at the other channel. For instance, Fig. 9 shows the after-exercise heart arrhythmia case convincingly detected simultaneously at both channels.

 

a
 

2.
 

1.
 
 

b
 

            Fig 7. Recorded pulse rate changes during the fitness step-test (a), and the corresponding changes in time

          delay between the PPG signals detected at two body sites - Carotid artery and left fingertip (b).

 

                Fig. 8. The averaged single-period PPG signals                       Fig. 9. Heart arrhythmia, detected at both channels.

                           at different phases of the fitness test.

 

5. SUMMARY

 

·         A small-size portable PPG sensor device (44x32x9 cm, 4.1 kg, battery-powered) with two simultaneous detection channels is constructed and tested.

·         The newly developed sensor is adapted for use in tele-medicine, e. g. for distant cardio-vascular monitoring via Internet.

·         The proposed two-channel PPG methodology is well suited for cardio-vascular monitoring at steady state and during complex fitness tests; the second channel can serve for reference and for obtaining additional physiological data.

·         In particular, time-shift between the PPG pulses that are detected at two different body sites provides additional information on the vascular blood flow resistance and its changes.

·         The portable design and presented results confirm good potential of the device for fast non-invasive cardio-vascular monitoring and mass screening at different conditions, including bed-site and field environments.

 

ACKNOWLEDGMENTS

 

The authors would express their sincerest thanks to Prof. Juris Aivars and Dr. Indulis Kukulis for their valuable clinical comments. Financial support from Latvian Council of Science (grant # 01.0067) and Latvian Ministry of Education and Science (grant # TOP 02-13) is highly appreciated.

 

REFERENCES

 

1.        M. Nitzan, H. de Boer, S. Turivnenko et al., “Power spectrum analysis of spontaneous fluctuations in the photoplethysmographic signal”, J. Bas. Clin. Physiol. Pharmacol., 5, No. 3-4, pp. 269-276, 1994.

2.        J. Spigulis, G. Venckus, M. Ozols, “Optical sensing for early cardiovascular diagnostics”, Proc. SPIE 3911, pp. 27-31, 2000.

3.        J. Spigulis, I. Kukulis, E. Fridenberga, G. Venckus, “Potential of advanced photoplethysmography sensing for non-invasive vascular diagnostics and early screening”, Proc. SPIE 4625, pp. 38-43, 2002.

4.        J. Spigulis, M. Ozols, R. Erts, K. Prieditis, “A portable device for optical assessment of the cardiovascular condition”, Proc. SPIE 5123, pp. 313-319, 2003.

5.        L. Brouha, A. Graybiel, C. W. Heath, “The step test. A simple method of measuring physical fitness for hard muscular work in adult men”, Rev. Canadian Biol., 2, pp. 86-92, 1943.

6.        J. Spigulis, R. Erts, V. Bernhards, “Optics for fitness assessment: potential of two-channel photoplethysmography techniques”, Abstr. of “Northern Optics 2003”, Espoo, p. 67, 2003.