Optical multi-channel monitoring of skin blood pulsations
for cardiovascular assessment
Janis Spigulis*, Renars Erts and Maris Ozols
Time resolved detection and analysis of the skin back-scattered optical signals (reflection photoplethysmography or PPG) provide rich information on skin blood volume pulsations and can serve for cardiovascular assessment. The multi-channel PPG concept has been developed and clinically verified in this work. Simultaneous data flow from several body locations allows to study the heartbeat pulse wave propagation in real time and to evaluate the vascular resistance. Portable two- and four-channel PPG monitoring devices and special software have been designed for real-time data acquisition and processing. The multi-channel devices were successfully applied for cardiovascular fitness tests and for early detection of arterial occlusions.
Keywords: Photoplethysmography, optical bio-sensing, cardio-vascular assessment. fitness control.
Photoplethysmography (PPG) is a non-invasive method for studies of the blood volume pulsations by detection and temporal analysis of the tissue back-scattered or transmitted 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. The AC-component of back-scattered PPG signals reliably reflects skin blood volume pulsations, therefore PPG technique has good potential to become a routine tool for express diagnostics and early screening of cardio-vascular pathologies, for self-monitoring at home and in public facilities, as well as and for the tele-diagnostics via Internet or LAN.
In general, each recorded PPG pulse contains useful information for cardio-vascular assessment. More detailed information can be obtained by analysis of the PPG signal sequences recorded over some period of time, e.g. one to few minutes. The PPG signal amplitude, baseline and period are changing with time in result of respiration, neural activities and body movements 1. One can assume that the real bio-signals are fluctuating around some stable mean single-period (SP-PPG) signal that can be determined by averaging a sequence of PPG pulses using specific algorithms and PC-processing programs. Our first robust SP-PPG fingertip sensor devices 2, 3 had undergone several series of clinical tests in laboratory, classroom and hospital environments. Analysis of fingertip signals taken from a number of 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. Later a portable single-channel PPG sensor model based on a laptop computer was developed, and new data on cardio-vascular conditions and skin micro-circulation have been reported 4, 5.
Recently portable versions of two-channel and four-channel PPG sensor devices have been developed and tested. These sensors comprise a set of universal optical contact probes, electronic converters and a laptop computer with specially designed software to provide real-time display, processing and storage of the PPG signals that are recorded simultaneously at each channel. The proposed multi-channel PPG approach offers two basic advantages:
– increased data flow - more comparative cardio-vascular data can be recorded and analyzed in real time,
– additional physiological parameter - vascular resistance - can be estimated and compared by measurements of time shifts between the corresponding PPG pulses detected at different anatomical sites.
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2. DUAL-CHANNEL PPG SENSOR DEVICE
The dual-channel device comprises two optical contact probes (applied simultaneously during the measurements), the bio-signal acquisition/conversion circuit and a laptop computer with specially developed software. All equipment is placed in a hand-held case of size 44x32x9 cm and weight 4.1 kg; it is battery-powered and can operate up to 3 hours without recharging.
Fig. 1. The dual-channel PPG sensor device (a) and application of the optical contact probes (b).
Each optoelectronic contact probe emits cw-radiation into the skin tissues and detects the back-scattered radiation; the separated AC-component of the signals precisely reflects the skin blood pulsations at the probe-covered volume. Both contact probes comprise a pair of 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 ~2-3 mm) and Si photodiode (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 or a finger-clip. Advantage of the band-probe design is the possibility of its flexible extension by means of spare sticky bands, so allowing to take the PPG measurements from practically any location of the body, e.g. forehead, forearm, neck, belly, calf, etc.
Fig. 2. The dual-channel PPG sensor device: functional scheme.
Special software was developed for the PPG bio-signal acquisition, processing and data storage from 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;
· Real-time display of both PPG signals;
· 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-shift between two corresponding PPG signals at both channels, etc.;
· Display of the selected PPG parameter set with corresponding cardio-vascular assessment results;
· Storage of the obtained data.
The data sampling rate 100 s -1 was usually chosen; it could be increased up to 950 s -1 for special cases when higher time resolution was needed. Screen-shot of the two-channel PPG measurement process is presented at Fig. 3.Time-shift between the signals of both channels (detected at the neck artery and fingertip) is clearly observable.
Fig. 3. The monitor screen-shot taken during the two-channel PPG measurements.
3. APPLICATION EXAMPLE: FITNESS TESTS WITH DUAL-CHANNEL PPG SENSING
We performed series of dual-channel PPG measurements before, during and after intensive physical exercises. The goal was to look for specific exercise-induced features of bio-signals that might give evidence of possible cardio-vascular disorders of the monitored person, as well as to assess his/her adaptability to physical loads. The monitored volunteers – about 200 in total - were persons of different ages, genders and training background, including about 30 professional sportsmen. The PPG signals were detected simultaneously from the left middle fingertip and from the Carotid artery area on the left side of neck.
The test protocol included four stages – 1minute horizontal relax, 1minute steady standing position, 3 minutes metronome-controlled stepping up and down (so-called Harvard step-test 6), and 5 minutes relax in sitting position. The dual-channel PPG signals were recorded continuously over the whole test, 10 minutes for each volunteer 7, 8.
As result, a Microsoft Access database with Visual Basic 6.0 processing program was created for further analysis of the functional parameters at different test phases. The pulse rate variations (Fig. 4) were calculated from the varying time intervals between the neighboring PPG peaks, the dominant pulse rate oscillation frequencies - using Fourier analysis, the mean recreation time after the exercise - by finding the time constant of exponential pulse rate decay, the pulse wave propagation velocity – from the time shift between two corresponding PPG pulses detected at both channels.
Fig. 4. Data extraction example: pulse rate variations during the rest phases (horizontal, vertical and sitting relax).
Fig. 5. The variations of pulse wave transit time during the fitness test (4 volunteers).
Additional advantage of the two-channel approach is the possibility to verify suspicious features of signals appearing at one channel by comparing with those detected at the other channel. For instance, the after-exercise heart arrhythmias (if appeared) had been always convincingly detected simultaneously at both channels. Following all phases of the test, we often observed notable variations not only in the pulse rate, but also in the time-shift between the PPG pulses in both channels, so indicating to changes of pulse wave transit time (proportional to the vascular resistance) during the test. Such individual dependencies (Fig. 5) eventually might serve as vascular health markers, as well.
4. THE FOUR-CHANNEL PPG MONITORING: FIRST CLINICAL DATA
Following similar principles as described for the dual-channel sensor, a more advanced four-channel PPG device has been designed and clinically tested. Fig. 6 illustrates its basic concept – to provide simultaneous PPG data flow from four body locations, e.g. the same fingertips of both arms and the same toes of both legs. Such measurement scheme looks promising for fast detection and/or monitoring of vascular occlusions that are often located in the arm or leg arteries. If there is a notable vascular narrowing or occlusion, the vascular resistance increases and pulse wave propagates slower. Consequently, some time shift between corresponding PPG signals in the “right finger – left finger” or the “right toe – left toe” channels should appear; differing PPG signal shapes are also expected.
Fig. 6. The four-channel PPG monitoring scheme.
Fig. 7. The monitor screen-shot taken during the four-channel PPG measurements.
The 4-channel PPG sensing results are represented in real time (Fig. 7); all data are also stored digitally for their further analysis. Direct on-line display of the time shift between the PPG peaks of any two selected channels is also available.
The four-channel system had undergone its first clinical tests at the Cardiology Department. As example - a patient, male of age 65, had signs of arterial occlusion in his left hand (diagnosed by MD after the blood pressure examinations: 135 + 5 mm Hg for the right hand, 105 + 5 mm Hg for the left hadnd); his right leg artery had been occluded and was surgically treated several weeks ago. A fragment of his 4-channel peripheral PPG signals set is presented at Fig. 8, A. There is clear difference in shapes of both finger-signals (c, d), and also (less notable) of both toe-signals (a, b). Besides, the signal from the left-hand finger (c) is delayed relatively to the signal from the right-hand finger (d). Obviously, the vascular resistance in the left hand is higher - this confirms the occlusion diagnosis. To compare, in the case of healthy volunteer (Fig. 8, B) the PPG signal shapes in all four channels were similar and no time shifts between the finger-finger (c-d) or toe-toe (a-b) signals have been detected.
Fig. 8. Comparison of the 4-channel PPG recordings taken from a patient with signs of the left arm occlusion (A) and from a healthy volunteer (B). The four contact probes were mounted accordingly to the scheme on Fig. 6.
The time shift between the right-left fingertip PPG signals in the case of patient was quite unstable – its changes over half-a-minute are illustrated on Fig. 9, a. This might be additional feature to confirm increased vascular resistance causing arterial blood flow irregularities. In the case of healthy volunteer (Fig. 9, b), no time shift within the measurement errors has been detected (note the error bar).
Fig. 7. Variations of the time shift between PPG pulses detected from the fingertips of both arms:
a – 65-year patient with diagnosis of one-side arterial occlusion, b – 26-year healthy volunteer.
· The multi-channel PPG concept has been developed by design and clinical tests of portable two- and four-channel sensor devices
· The proposed two-channel PPG methodology and sensor device (44x32x9 cm, 4.1 kg, battery-powered) are well adapted for cardio-vascular monitoring at steady state and during complex fitness tests
· The second PPG channel is important and useful for reference and for obtaining additional physiological data, in particular - time-shift between the PPG pulses simultaneously detected at two different body sites directly reflects vascular blood flow resistance and its changes.
· The newly developed four-channel PPG methodology and sensor device confirmed good potential for early complex diagnosis of arterial occlusions in arms and legs, using as criteria the inter-channel time shifts and signal shape variations
· The discussed technique appears to be well suited for early mass screening and non-invasive monitoring of cardio-vascular disorders at specific conditions where stationary equipment is unavailable.
The authors are sincerely thankful to Dr. Indulis Kukulis for his clinical support and to Prof. Juris Aivars for his valuable physiological 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.
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