A portable device for optical assessment of the cardiovascular condition

 

 

Janis Spigulis*, Maris Ozols, Renars Erts and Karlis Prieditis

 

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

 

 

ABSTRACT

 

A hand-held prototype device for detection and processing of the tissue-remitted optical signals 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. Cardiovascular condition of the monitored person was assessed by temporal analysis of the recorded PPG signals as well as by shape analysis of the mean single-period PPG signals.

Keywords: Photoplethysmography, optical bio-sensing, cardio-vascular equipment.

 

 

1. INTRODUCTION

 

The human’s cardio-vascular condition can be assessed by various techniques, 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 (remitted) or absorbed optical radiation. Blood pumping and transport can be monitored at different body locations - fingertip, earlobe, forehead, forearm, etc. – with relatively simple and convenient PPG contact probes.

 

Progress in microelectronics and computer technologies has opened new possibilities to improve the PPG sensing technology since its origins in 1937 1.  PPG is becoming a powerful, safe and easy-to-use 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 can appear useful for cardio-vascular assessment. However, all detected heartbeat pulses are not equal – the PPG signal amplitude, baseline and period are changing with time 2. The real bio-signals are fluctuating around the mean single-period photoplethysmography (SPPPG) signal that can be determined by averaging a sequence 50…80 PPG pulses with subsequent mean shape determination. Special hardware, algorithms and PC-processing programmes were developed to obtain the mean SPPPG signals, suitable for further clinical analysis 3-7.

 

Our first table-top SPPPG sensor prototype devices 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 SPPPG signal and this "SPPPG-fingerprint" reflects the individual's cardio-vascular condition. This feature confirmed the diagnostic potential of the developed devices; however, their hospital bed-site and field applications were limited due to considerable weight and size.

 

As the next step, we have recently developed a portable version of the reflection-type PPG/SPPPG sensor device consisting of a universal contact probe, bio-signal processing electronics and a lap-top computer. Design of the new device will be described here, and some obtained clinical results will be presented and discussed.

 

 

 

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

2. DESIGN OF THE DEVICE

 

The basic design of the device is relatively simple. It consists of optical contact probe, bio-signal amplifying/filtering circuit (both powered by a rechargeable battery) and a lap-top computer with specially developed software for AD-conversion, storage, processing and display of the PPG or SPPPG signals. All equipment is placed in a hand-held case – see Fig. 1.

 

                                                a                                                                                                              b

 

Fig. 1. The hand-held equipment case: a – closed, b – open (dimensions: 44x32x9 cm, weight: 4.1 kg).              

 

 

2.1. The PPG contact probe

 

The optoelectronic contact probe continuously emits radiation into the under-skin tissues with blood vessels and detects the AC-component of the back-scattered radiation that reflects the blood volume pulsations. The probe comprises a GaAs emitting diode

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

                                                a                                                                                                              b

 

Fig. 2. The PPG contact probe (a) and its application for the fingertip monitoring (b).

(diameter of the emitting area ~2 mm, power ~10 mW, peak wavelength ~940 nm), 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 to the fingertip measurements (Fig. 2, b); however, the band easily can be extended by spare bands, if necessary, so providing possibility to take PPG measurements from different locations of the body, e.g., forehead, neck, forearm, knee (Fig. 3).

 

 

 

 

 

 

 

 

 

 

Fig. 3. Application of the PPG contact probe at forehaed, neck, forearm and knee.

 

 

2.2. Acquisition and processingof the bio-signals

 

The AC-component of the photodiode output signals is selected, pre-amplified and converted into digital format, then accumulated and processed by the computer. The signal amplitude-to-digital conversion is provided in somewhat original manner, by means of the built-in computer sound card 8,  9. Frequency of the sinusoidal output signal of the sound card determines the time resolution of measurements; in our studies the upper limit for the sound card was 44 100 Hz, so theoretically time interval around 23 microseconds between the neighbouring points could be achieved. In order to save the resources, we selected 200 times lower working frequency - 220.5 Hz; the corresponding time gap between the measured points was less than 5 milliseconds, quite satisfactory for recording well-resolved heartbeat signals.

 

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

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

·         Pre-setting of the measurement time schedule;

·         The PPG signal registration and display in real time;

·         Signal clean-up (special filtering algorithm) and calculation of the mean single-period PPG (SPPPG) 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), etc.;

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

·         Storage of the measurement/assessment data.

 

 

3. THE MEASUREMENT RESULTS: SOME EXAMPLES

 

The newly developed bio-sensor device had undergone several tests, and some interesting clinical results were obtained; they will be presented and discussed below. Most of these measurements were taken from the middle fingertip of the left hand.

 

3.1. The heartbeat irregularities

 

We observed and recorded several abnormalities of heart function, including partial or total lack of one heartbeat in the cardiac sequence – see fig. 4. Typically, the next heartbeat after the missing one is more intensive than others in the sequence, so obviously the heart is auto-compensating the short-term lack of blood pumping.  The monitored persons did not feel any discomfort during the missing heartbeat. This phenomenon was recorded several times, so there was little doubt that both persons had trouble with heart functioning, and they were recommended to visit cardiologist for further investigations.

 

Consequently, the new PPG sensor device appeared helpful for early warning of cardio-vascular dysfunction, so it seems to have good potential for primary cardio-vascular assessment and early screening of the risk patient groups in future.

 

       

                                                                a                                                                                              b

 

Fig. 4. The observed heartbeat irregularities for two monitored persons.

 

3.2. Adaptation of the cardio-vascular system to physical exercises

 

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 were young sportsmen (16-22 years) dealing with field athletics; the measurements were taken in stadium right during their training.

 

Fig. 5, a shows a typical relaxation process of the body as reflected by the PPG signals. Right after the exercises the heartbeat rate is increased, but regular, and the secondary (dycrotic) notch is nearly disappeared; it gradually recovers with time – see the

                                                a                                                                                                              b

 

Fig. 5. A normal (a) and arrythmic (b) response of the cardio-vascular system to intensive physical exercises.

 

                                                a                                                                                                              b

 

Fig. 6. The spasmatic peaks observed in PPG signals recorded after intensive physical exercises.

signals 1 min. and 2 min. after the load. Fig. 5, b illustrates the over-load situation – the heart function right after exercises is irregular, reflecting difficulties of the cardiovascular system to adapt the increased physical load. One can see that 1 minute of rest is enough to restore the regular cardiac cycle for this particular person. The secondary notch is well pronounced as evidence of good elasticity of the blood vessels.

 

We observed also sharp spasmatic peaks in the skin blood flow PPG signals immediately after the exercises (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 body adaptation to physical loads; however, this effect has to be studied in more details before drawing any conclusions.

 

3.3. The mean SPPPG signals from different body sites

Fig. 7. Comparison of the mean SPPPG signals taken at different body locations - middle fingertips (both hands), forehead and carotid artery - for two persons.

               

 

Some of the mean SPPPG signals taken at various locations of the body are presented on Fig. 7. Differences in shapes of the signals detected from the middle fingertips of both hands, forehead and the carotid artery (on the neck) for the same person were clearly observed. One can also note also person-to-person differences in the mean SPPPG signals taken at the same locations of the body. Each of the mean SPPPG signals obviously contains specific features related to the person’s cardio-vascular condition to be assessed; the assessment criteria are still elaborated in discussions with medical doctors.

 

 

4. SUMMARY

 

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

·         The new contact probe design provides reliable pulsating blood flow measurements at different sites of the body.

·         Several interesting clinical conditions have been recorded with the new device by temporal analysis of the PPG signals – heartbeat irregularities, arrythmic and spasmatic responses to intensive physical exercises, right-left hand fingertip blood flow differences, etc.

·         Shapes of mean SPPPG signals recorded at various locations of the body contain valuable cardiovascular information.

·         The proposed approach and sensor design proved to be suitable for fast primary cardiovascular assessment and early screening.

ACKNOWLEDGMENTS

 

The authors are very grateful to Dr. Indulis Kukulis for his valuable clinical comments. Financial support from Latvian Council of Science (grant # 01.0067) and Ministry of Education and Science (grant # TOP 02-13) is highly appreciated.

 

 

REFERENCES

 

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2.        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.

3.        J. Spigulis, U. Rubins, “Photoplethysmographic sensor with smoothed output signals”, Proc. SPIE. 3570, pp. 195-199, 1998.

4.        G. Venckus, J. Spigulis, “Frequency filtering effects on the single-period photoplethysmography signals”, Med. Biol. Eng. Comput., 37, Suppl. 1, pp. 218-219, 1999.

5.        J. Spigulis, G. Venckus, “Single-period photoplethysmography: a potential tool for noninvasive cardiovascular diagnostics”, Springer Series “Optics for Life Sciences” OFLS-VI, Berlin (in press).

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

7.        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.

8.        M. Ozols, J. Spigulis, “Acquisition of biosignals using the PC sound card”, Proc. Int. Conf. “Biomedical Engineering” (KTU, Kaunas, LT), pp. 24-27, 2001.

9.        M. Ozols, J. Spigulis, “Analog-to-digital conversion of bio-signals by means of the PC sound card“, Proc. Baltic Electronics Conference BEC’2002 (TTU, Tallinn, EE), 2002 (in press).