Multi-wavelength photoplethysmography method
for skin microcirculation assessment
Lasma Gailite, PhD student
Bio-optics and Fiber Optics Laboratory, Institute of Atomic Physics and Spectroscopy
University of Latvia, Raina Blvd. 19, Riga, LV-1586, Latvia,
E-mail: [email protected], tel/fax: +371 7228249
1. INTRODUCTION
Reflection photoplethysmography (PPG) is a non-invasive method for studies of the skin blood
volume pulsations by detection and analysis of the back-scattered optical radiation. The PPG method
has gained wide clinical use due to the simple measurement procedure and the informative content of
the PPG signal. Parameters, including heart rate, respiratory rate and tissue perfusion can be monitored
via PPG, moreover: specific indicators of cardiac disorders as well as peripheral vascular diseases can
be extracted from the single-period PPG pulse shape [1,2].
The specifics of reflection PPG is that a fixed penetration volume/depth is monitored which
depends on the emitter wavelength: PPG pulsations from deeper skin layers contribute to the signal at
longer wavelengths [3, 4]. Consequently, depth-selective PPG measurements are possible, e.g., studies
of blood flow at different vascular beds described in literature [2,5]. Differences in PPG signal shapes
have been reported [4] in case of subsequent application of various emitters/wavelength bands to the
same skin spot. However, to authors knowledge, there are no data available on PPG signal shapes that
would be detected on the same skin location at the same moment and using several emitter
wavelengths.
Parallel multi-wavelength detection of reflection PPG signals related to the same heartbeats with
subsequent shape analysis may yield a sub-millimeter-scale depth characterization of skin
microcirculation in selected skin layers. The goal of this study is the development of such a new
technique, namely, the multi-wavelength photoplethysmography that enables recording of PPG signals
from the same skin location simultaneously at any selected wavelength in the range of 400 to 1100 nm.
2. MATERIALS AND METHODS
The concept of multi-wavelength PPG equipment (Fig. 1.) includes three basic components: (i)
illumination by several laser emission lines within the range of 400 to 1100 nm, (ii) fiberoptic contact
probe for measurements on skin and (iii) detection in the range of 400 up to 1100 nm via multi-channel
spectrometer.
The time-resolved diffuse reflectance spectra of skin were detected by a 2048-channel CCD array
spectrometer AvaSpec-2048-USB2 (Avantes BV, the Netherlands). Two commercial lasers (BWTek, Inc.
BWB-405-40-PIG-200-0.22-SMA 405 nm line and BWT-532-15-SMA, two lines: 532 nm and 1064 nm)
were used, as well as two laboratory-assembled diode lasers (645 nm and 807 nm). Irradiation power at
the skin contact probe output varied from 1 mW at 1064 nm to 16 mW at 405 nm, which corresponds
to power densities on skin in the range 0.35… 5.66 W/cm2. The fiberoptic probe comprised six 600micron silica core fibers arranged in a line with 1 mm distance between the neighbour fiber centers.
First fiber in the row was used for collection of reflectance light; the other five adjacent fibers were
used for delivery of laser output to the skin surface. All fiber optic components were designed and
manufactured by Z-Light, Ltd. (Latvia).
The probe was positioned directly on skin during the measurements, namely, on the inner part of
the middle fingertip of the examined person. Measurements were taken from fingertips of ten
volunteers in relaxed sitting position. The probe was loaded with calibrated weights (20, 70, 130, 230,
or 430 g) thereby changing the skin-probe contact pressure in the range of 0.5-11 kPa. Two procedures
of load application were used: (i) gradual increase of probe-skin contact pressure by adding a weight to
the probe every minute of 5 min continuous PPG recording, (ii) alternating increase and decrease of
probe-skin contact pressure by adding and removing weights every 2 minutes of a 10 min PPG
measurement session. Each measurement session was repeated three times for a single volunteer.
For measurements with the spectrometer model AvaSpec-2048-USB2, more advanced software
was created by means of the special Avantes BV tools using the CodeGear Delphi 2007 programming
language. This software made possible to choose any selected wavelength band (up to three
nanometers) for the multi-spectral PPG signal real-time recording, and to record additionally two
analogue PPG signals in real-time, along with the multi-spectral PPG recordings. In accordance with
the laser emission lines used in the setup, PPG signals were plotted in time at the laser emission
wavelengths. This principle of data processing is shown schematically in Fig. 2.
Fig. 1.
Experimental setup for the multiwavelength
photoplethysmography method.
Fig. 2.
Schematic illustration of the
time-resolved PPG signal
acquisition:
from the total detected skin
reflectance spectrum (on the
left), the intensity-time sections
(A-A) are recorded only at the
selected laser wavelengths(on the
right).
3. RESULTS AND DISCUSSION
Two types of signal comparison were performed: (i) regarding the relative amplitude changes of
signal baselines or DC levels, and (ii) regarding the relative amplitude changes of normalized singleperiod PPG signals at various wavelengths, i.e., relative AC values.
The first type of signal comparison shows the responses of multi-wavelength PPG signal baselines
to a gradual increase or decrease in probe-skin contact pressure. The increase of probe-skin contact
pressure has caused a decrease of the PPG signal baseline amplitudes at all five exploited wavelengths
for all volunteers. Typical response for a three-wavelength set is presented in Fig. 3.a. This trend
coincides with the results obtained earlier by other authors using different PPG technique modalities
[5,6]. Measurements also showed a sharp decrease of amplitudes of the PPG pulsations detected at
shorter wavelengths of 405 and 532 nm (that relate to shallow penetration) at the highest probe-skin
pressures applied, namely, the signal AC component was absent at 6.5 kPa for 405 nm signal (Fig.
3.b), and at 11.3 kPa for both, 405nm and 533 nm PPG signals for 8 of 10 volunteers (Fig. 3.a). The
loss of signal AC component indicates pressure-induced occlusions of superficial skin blood vessels; it
has been shown previously that the occlusion pressures increase with wavelength and differ among
persons [6]; our results correspond qualitatively to the above mentioned findings, however, the
quantitave disagreement can be explained by differences in the loading procedures.
The other type of results represent variations in the shapes of single-period PPG pulses at various
wavelengths. The typical feature taken into consideration was the relative amplitude of the secondary
peak of a single PPG pulse. The variations in secondary peak relative amplitude of the mean
normalized single-period PPG contours at various laser wavelengths did not exceed the calculated
StDev = 0.09 a.u. The calculated amplitude variations were ascribed to different signal depth, as well
as different signal-to-noise ratios at various wavelengths caused by skin absorption spectral
dependence. Both, statistical analysis and impovement of signal-to-noise ratios are necessary to enable
the detection of PPG shape variation with wavelength.
a
b
Fig. 3.
(a) Typical changes of PPG signals at three wavelengths with increasing probe-skin pressure from 0.5 to
11.3 kPa; the loss of the AC signal at 11.3 kPa indicates occlusions at 405 and 532 nm (Signals of one
volunteer are plotted).
(b) PPG signal response to alternating increase and decrease of probe-skin pressure; the AC component is
absent at 6.5 kPa for the 405 nm signal, while at 532 and 807 nm the AC pulsation component is still
detectable representing the differences in occlusion pressures for various wavelenghts. (Signals of one
volunteer are plotted)
4. CONCLUSIONS
Simultaneous use of several cw lasers with multi-fiber coupling to skin and further to a standard
multi-channel array spectrometer has been tested for the recording of photoplethysmography signals.
PPG baseline responses to probe-skin contact pressure have been recorded at three wavelengths at a
time combining 405; 532; 645; 807 and 1064 nm laser lines. The differences in loss of AC component
at various wavelengths, i.e., occurence of occlusions, have confirmed the depth sensitivity of the multiwavelength PPG method. The further evolution of the multi-wavelength PPG methodology involves
experimental data accumulation for the establishment of quantitative descriptive criteria, as well as
creation of physical model in order to associate measured PPG signals with particular depths in skin.
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