1 Introduction THE OPTICALtechniques most widely used to measure skin perfusion are photoplethysmography (PPG) and laser Doppler ttowmetry (LDF). Both techniques are simple to use, noninvasive and measure microvascular perfusion in a small volume, but they use two different physical principles. PPG has been used to assess skin perfusion since the 1930s (for a review see CHALLONER, 1979). In this method, intensity variations in the backscattered light from the skin are assumed to contain information about changes in blood flow. The results published so far reflect uncertainty about the origin of the photoplethysmographic signal. The orientation and packing of erythrocytes, vessel wall movements as well as scattering and absorptive processes in tissue and blood may all affect the light reaching the photodetector (CHALLONER, 1979; NIJBOER et al., 1981; ROBERTS, 1982). LDF (STERN, 1975) determines the flux of red cells through a volume of tissue by using the Doppler shift of laser light (NILSSON et al., 1980a). The output signal from the laser Doppler flowmeter is proportional to the number of scatterers (red blood cells) times their mean velocity. PPG and LDF have been used in a comparative way, though mostly not simultaneously, to study skin perfusion. TLrR et al. (1983) applied PPG to measure blood volume fluctuation and LDF to measure blood flow. Recordings were taken from several anatomical places on the human skin surface and these showed a good agreement between the two techniques. To study the erythema onset time after Correspondence should be addressed to Professor Oberg at address I. First received 8th March 1989 and in final form 5th July 1990

9 IFMBE: 1991

40

the administration of methyl nicotinate, GuY et al. (1983) used visual observation, LDF and PPG. Both LDF and PPG were found to agree with the changes observed visually. WESTER et al. (1984) used both techniques for measuring the 'blood flow' in balding scalps with a randomly assigned topical solution of Minoxidil, a directacting vasodilator. They found that the blood flow, as measured by LDF, increased when Minoxidil was applied, while the PPG method only responded weakly to this stimulus. These authors concluded that because PPG mainly measures the local blood volume, this method is unsuitable for studying blood flow. On the other hand DE TRAFFORD and LAFFERTY(1984) showed a strong correlation between the PPG signal (DC, 'blood volume') and venous occlusion plethysmography. They concluded that the PPG method seems to 'reflect changes in blood flow rather than simply blood volume'. OBERLE et al. (1988) used LDF and AC PPG to study blood flow responses to various stimuli on humans exposed to different ambient temperatures. The two methods showed similar results, although discrepancies were found occasionally. ALMOND et al. (1988) measured blood flow in the human finger with a combined LDF and AC PPG probe. Recordings were made simultaneously but not exactly from the same vascular bed and a correlation was found between the LDF and PPG signals. Existing literature evaluating the PPG and LDF methods does not discuss in detail the influence of differences in optical geometry, light intensity or wavelength of the light sources. The commercially available PPG instruments use light sources (LEDs) in the near-infra-red region 800-960nm. Most laser Doppler flowmeters in use today use helium-neon lasers with a wavelength of 632.8 nm. The

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penetration depth of optical radiation into tissue increases with the wavelength used (ANDERSON and PARRISH, 1981), which means that the two methods probe different vascular beds, even at the same incident light intensity level. In the case of radiation geometry, commercially available PPG instruments, with light-emitting diodes and photodetectors placed close to the skin, have a different optical geometry to that of laser Doppler flowmeters, which use optical fibres. There is a lack of information as to how optical geometry, light source characteristics and wavelength affect signals obtained with the LDF and PPG methods. The purpose of this paper is twofold:

2.1 Macro PPG-LED and micro PPG-LED

(i) a comparison between LDF and PPG using standardised optical and geometrical conditions (ii) a study of the PPG signal using three different optical and geometrical setups.

2 Method Blood perfusion in human skin was measured with a laser Doppler flowmeter (Periflux, PFld, Perimed KB, Sweden) and three different photoplethysmographic setups which are described in more detail below. Measurements were made at two local skin temperatures (attained by cold and hot water provocation) and at two different anatomical sites (finger and forearm).

I

macroPPG

To study the photoplethysmographic technique we used three setups with different optical and physical characteristics. Fig. la shows a block diagram of two setups. One, called macro PPG-LED, is a modified commercial photoplethysmograph (Vascular minilab III, Parks, USA). A red LED (GaAIAs, H-2000, 660 nm, Stanley Electric Co. Ltd., Japan) was mounted inside the commercial probe. The adjacent original photodetector had similar optical and electrical characteristics as the photodetector described below. The centre distance between LED and photodetector was 6 mm. In the second setup, called micro PPG-LED, light from the same type of LED (GaAIAs, H-2000) was focused into a step index optical fibre (diameter 0.75mm, numerical aperture = 0.5, Super Eska, SH3001, Mitsubishi Reyon Co., Japan) and transmitted to the tissue via the fibre. The backscattered light from the tissue was transmitted by an optical fibre of the same type to a photodetector (photodiode, silicon, HUV l108B, EG&G Inc., Electrooptics Divn., USA). The centre distance between the illuminating and receiving fibres was 0.75 mm. The signals in these two arrangements were low-pass filtered at 40 Hz to eliminate high-frequency noise (20Hz was used in some recordings which contained a lot of noise). The signals were high pass filtered at 0.5 Hz because only the pu!satile component (or the AC component) of the photoplethysmographic signal was considered.

photodetector

~'~skI'ki'~i~n

i

! ~

l~f"ter

lK

todetector

microPPG

photodetector~

J high-pass fi'ter lI ou!put

(macroPPG-LED)

high -pass ~ filter

normaliser differential amplifier

laser photodetector optical[ fibres

~

high-pass filter

~

low - pass filter

I high-pass V filter

output (microPPG-laser)

transfer function

I

output (LDF)

b

Fig. 1 (a) Block diagram of the macro and micro photoplethysmograph-LED. (b) Block diagram of the laser Doppler flowmeter and the micro photoplethysmograph-laser Medical & Biological Engineering & Computing

January 1991

41

2.2 Laser Doppler flowmetry and micro PPG-laser Fig. lb shows a block diagram of the laser Doppler flowmeter and the third PPG arrangement. The flowmeter (for details, see NILSSOr~ et al., 1980a; b) used an He/rNe laser (1-5 mW) with a wavelength of 632-8 nm. The light was transmitted to the tissue by a step index optical fibre (see above). The reflected and backscattered light from the tissue was transmitted by the same type of optical fibres to two photodetectors (HUV l l00B). The centre distance between the illuminating and receiving fibres was 0.75 mm. The signals were high-pass filtered at 40Hz, normalised and amplified in a differential amplifier. The output signal from this amplifier was fed to a transfer function network which produces a signal proportional to the flux of red cells. When used normally, the signal is further processed to give an output voltage proportional to the RMS value of the blood-flow-related signal. In our evaluation, however, a blood-flow-related parameter was produced by calculating the total power from the power spectral density of the transfer function signal. An average of 200 spectra were used throughout the study. The signal from one of the photodetectors (see Fig. lb) was used for the recording of a photoplethysmographic output signal, described as micro PPG-laser. Signal filtering was performed as described earlier. Blood perfusion could, in this way, be measured simultaneously using both methods. This permitted evaluation of the skin perfusion measurements at the same anatomical site, using identical optical geometry and the same optical wavelength. The total power of the pulsatile (AC) PPG signal was calculated. The incident radiant flux to the skin was measured with an optical multimeter (Model 22XL, Photodyne Inc., USA) and set manually to the same value (1.30 + 0.05mW) for all the light sources used. Table 1 summarises the physical and optical specifications of the methods used. All the signals were analysed by a signal analyser (HP 5451C Fourier Analyzer, Hewlett Packard, USA) by which the power spectral density and total signal power were calculated. In the case of LDF signals, the sampling rate of the A/D convertor was set to 10kHz. The lower cutoff frequency of the computer was 5 Hz. The total power was calculated in the range 5 Hz to 4 kHz according to eqn. 1.

f2

(1)

2'~4~176176 ~5

P(a~) is the power spectral density of the photodetector signal and 09 is the transfer function of a third-order Butterworth bandpass filter with upper and lower cutoff frequencies of 4kHz and 40Hz, respectively. For PPG Table 1 Physical and optical specifications of the LDF and micro~macro PPG methods

Wavelength, n m Spectral line halfwidth, nm, approximate Light source Radiant flux, m W Luminous angle

LDF

Micro P P G laser

Micro P P G LED

Macro P P G LED

632.8 0.01

632-8 0.01

660 25

660 25

laser 1-30 60 ~

laser 1.30 60 ~

LED 1-30 60 ~

LED 1-30 35 ~

Probe type

fibre

fibre

fibre

comm.

Detecting area, m m 2 Source detector separation, m m

0.43 0.75

0.43 0.75

0.43 0.75

2.25 6.0

6-0 10.95

6.0 10.95

6-0 10.95

5.2 9.78

Skin contacting area, cm 2 finger forearm Comm.: commercial

42

signals the sampling rate of the A/D convertor was set to 2kHz and the lower cutoff frequency was 0.5Hz. The average was calculated for 100 power spectra. The total power of the PPG signal was calculated in the range 0.5 Hz to 40 Hz according to eqn. 2. 2~t40

~2

(2)

,~o.5 P(co)doa

The total signal power at heat provocation was used as normalisation for each measurement and subject.

water in

" E

~ ~

PPG reads

----~ water out

~

aluminium

photodetector LED

water in

water out

oplical fibres alumJnlum b

Fig. 2 Sectional view of probes and probeholders for local temperature provocation. The top figure shows the holder for macro PPG and the bottom figure shows that for LDF/ micro PPG

Changes in blood perfusion were brought about by temperature provocation using probe holders attached to the skin as shown in Fig. 2. The material of the holders in contact with the skin was aluminium with a high thermal conductivity. Both probe holders were perfused with water by a flow-controlled pump. The water temperature was held constant at the probe holder inlet during cold (13~ and heat (42~ provocation. Skin temperature was measured under the probe holder (Exacon, Scientific Instruments Aps., Denmark) as the average temperature of the probe holder surface and the skin surface. Cold provocation was always performed before heat provocation. During finger measurements, the fingertip was held against the probe holder and secured with double-sided adhesive tape. During forearm measurements the probe holder was attached to the skin with double-sided adhesive tape. Data about skin contacting area, which varied due to methods and sites used, are shown in Table 1. Data collection was made for 5 min when the skin temperature had stabilised (after 15-30min provocation). The same site, precisely, was used for the cooling and heating experiments. Heart rate (Low Frequency Rate Meter, Dynamic Electronics Ltd., UK) and skin temperature under the probe holder were measured every minute. Recordings on subjects showing big variations in heart rate were excluded from the material.

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January 1991

2.3 Materials Experiments were performed on healthy Caucasian nonsmokers (five male and five female), with ages ranging from 22 to 30 years. The subjects were studied in a supine position and were allowed to rest for at least 10rain before the experiments began. No prescriptions with regard to food intake and the time of day for the experiments were applied. Recordings were made at the forearm and distal part of the middle finger at water temperatures of 13~ and 42~ The measuring site was approximately at heart level. The experiments were run during the Swedish winter and early spring. 3 Results

3.1 PPG recordinos Fig. 3 shows recordings from the forearm skin using all three PPG methods demonstrating the lowest quality of the signal in terms of a marked pulse shape. Signal amplitude and quality was much higher in the finger recordings.

ated by an increased skin temperature. Fig. 4b shows the macro PPG power spectra with a high peak corresponding to the heart frequency. This spectral density curve is typical of almost all PPG signals obtained at both sites. The exception is the micro PPG-laser curve measured at the forearm (Fig. 4c), which has a much broader frequency distribution. It must be pointed out that the spectral density of PPG-laser in this figure reflects the variations in intensity of the reflected light from the tissue and not laser Doppler frequencies. 3.3 Comparison of LDF and PPG The total signal power at cold and heat provocation from the two skin sites is shown in Fig. 5(a-d). For each subject and method used, the total power at heat provocation was used for normalisation. The total power of LDF (eqn. 1 and Fig. 5a) was substantially higher at 42~ at both sites for all subjects. The total power of micro PPG-laser (eqn. 2 and Fig, 5b) was higher at 42~ at the finger (except for one subject where the total power was T:42~

T= 13~

50mY

I

ls

! T= 42~

T=13~

50mY

T I

ls

I T= 42~

T=13~

25rnV

I I

I ls

Fig. 3

Typical example of the photoplethysmographic signals recorded from forearm after 40 Hz low-passfilterino : ( a) macro PPG-LED ; (b) micro PPG-LED ; (c) micro PPG-laser

The macro PPG-LED signal (Fig. 3a) was clearly synchronous with the heart frequency at both temperatures. A larger signal amplitude was found at 42~ compared with the value at 13~ In Fig. 3b the micro PPG-LED signal exhibited a less regular waveform. Heart synchronism was very evident and the amplitude changed due to increased skin temperature. The micro PPG-laser signal (Fig. 3c) did not show the same heart synchronism during cold stimulation. Increased skin temperature resulted in a more regular waveform but there were only small changes in amplitude. 3.2 Frequency content of the LDF and PPG sionals The laser Doppler power spectral density at different temperatures are shown in Fig. 4a. This figure shows an increase in signal power due to the increased flow mediM e d i c a l & Biological Engineering & Computing

lower). In the case of forearm skin, the total power of micro PPG-laser showed no noticeable change. With the exception of three subjects the total power of micro PPG-LED (Fig. 5c) increased due to heat stress at both sites, The percentage increase varied greatly among the subjects. With the exception of two subjects the total power of macro PPG-LED (Fig. 5d) measured at the finger was higher at 42~ In the case of forearm skin the total power increased substantially due to heat provocation. Table 2 summarises the total power ratio (total power at 13~ power at 42~ and related skin temperatures. Paired differences (total power at 4 2 ~ total power at 13~ were calculated and used to test the following hypothesis (Student t-test): Ho: the total power of the signal does not increase when the skin temperature is elevated.

January 1991

43

finger

%

foreorm

T=42~

E

-6 "5

"5

Q_

m

..... T:13~ o

~0

.~

I

1000

I

l

I

2000

3000

4000

~-o

1000

2000

frequency. Hz

frequency, Hz

foreorrn

foreorm

3000

4000

T:42~ D

T= 42~

L

t3

>: E OJ X~

T:13~

"6 Q.

0

O

o. 0

10

j

2O

o.

0

frequency, Hz

1'0

2'0

frequency, Hz

b

C

Fig. 4 Power spectra of(a) LDF; (b) macro PPG-LED; (c) micro PPG laser T h e test results are s h o w n in T a b l e 2 a n d imply the following: Table 2 Total power ratio = total power at 13~ power at 42~ skin temperature during cold and heat provocation and result oft-test. Values are presented as means 4-SD often subjects Site Water Total power ratio Skin temperature, temperature, ~ ~ LDF micro PPG-laser 13-0 20.6 ___2-8 Finger 0.34 • 0.18 0-74 • 0.40 42-0 37.4 + 1-3 rejected at not t-test 0.05 per cent rejected 13.0 21-3 • 1.8 Forearm 0.17 _+0.12 0.98 • 0.12 42.0 38.1 _-t-1.0 rejected at not t-test 0.05 per cent rejected micro PPG-LED 13.0 19.4 • 2.8 Finger 1.31 • 2.06 42.0 37.5 + 0-6 t-test not rejected 13.0 21.4 • 1.8 Forearm 0-55 • 0.29 42.0 38.3 • 0-6 t-test rejected at 0-05 per cent macro PPG-LED 13.0 21.4 • 2.7 Finger 0-74 • 0.81 42.0 36-0 • 2.5 t-test not rejected 13.0 22.0 4- 1.3 Forearm 0-09 • 0.09 42.0 37"5 • 0.9 /-test rejected at 0.05 per cent

44

(a) L D F shows a n increased skin perfusion due to increased skin t e m p e r a t u r e o n b o t h finger a n d forearm. (b) Micro P P G - l a s e r c a n n o t detect a n increased skin perfusion due to increased skin t e m p e r a t u r e at either the finger or the forearm. (c) Both micro P P G - L E D a n d m a c r o P P G - L E D show a n increased skin perfusion due to a n elevated skin temperature only in the case of m e a s u r e m e n t s o n the forearm. I n some P P G recordings the total power decreased at 42~ thus i n d i c a t i n g a lower m i c r o v a s c u l a r perfusion at heat provocation. This occurred particularly for measurem e n t s at the finger (Fig. 5(b, c a n d d)). These results have, of course, a very great influence o n the statistical test performed above.

4 Discussion The microvascular bed of the finger c o n t a i n s b o t h capillaries of a high density a n d a large n u m b e r of complex arteriovenous anastomoses. This is in contrast to the vascular structure of the forearm skin, which has a low capillary density a n d few or n o AV anastomoses (GREENFIELD, 1972). T o u n d e r s t a n d the P P G response following vascular reactions to cold/heat provocation, b o t h types of skin have to be studied. TUR et al. (1983) have presented an extensive study of P P G from most parts of h u m a n skin. A change in skin t e m p e r a t u r e from (on average) 21.0~ to 37"5~ gives rise to a n increased total

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January 1991

blood flow, especially in skin areas with thermoregulatory properties (GREENFIELD, 1972). Changes in skin temperature will affect both the control mechanisms of the vascular bed and the rheological properties of the blood (SVANES, 1980). It is very difficult to simulate this complex behaviour in mathematical or in physical models. Human skin gives a reproducible and controlled response to a temperature change. This is why human skin was preferred for this type of experiment rather than models of other kinds, even if such a choice makes the interpretation of the results more complicated. 4.1 Comparison of LDF and micro PPG-laser Both L D F and P P G are supposed to measure the superficial blood perfusion of the skin, but the two methods use different physical principles. Very often, the two techniques are used with widely different optical arrangements which makes a direct comparison of the signals produced difficult. In this study, these technical differences have been eliminated, thus making it easier to compare the nature of the signals. The L D F and micro PPG-laser methods are compared simultaneously. They use the same light source, incident light intensity, optical fibre probe and they measure blood flow in the same vascular bed. By using this arrangement, the two methods only differ in terms of the physical principle used, which facilitates the comparison and interpretation of responses to well defined physiological stimuli. The results in Fig. 5 (a and b) show that the L D F principle is more sensitive than micro PPG-laser in recording a change in skin perfusion that follows a skin temperature change from 21.0~ to 37.5~ When using laser light, frequency deviation (the Doppler shift) seems to be a more sensitive parameter than intensity variations for the recording of skin perfusion changes. 4.2 Comparison of micro PPG-laser and micro PPG-LED In the study of micro PPG-laser and micro PPG-LED, the same light intensity and almost the same peak wavelengths are used. The optical probes are the same for the two methods. However, the light sources used have different physical characteristics. The laser light is monochromatic (spectral width approximately 0-002 nm) in comparison with the much broader LED light source (spectral width approximately 25nm). Fig. 5 (b and c) clearly demonstrates the differences in results, which may depend on the differences in monochromatism and/or coherence of the light sources. The micro P P G - L E D recording demonstrates a perfusion change which follows a skin temperature change in the forearm. The corresponding micro PPG-laser recording does not seem to respond at all to the same perfusion change. A possible explanation to this finding is the following. Subcutaneous tissue moves very slightly and rhythmically as a response to the blood pressure pulse at each heartbeat. The speckle pattern on the skin surface generated by the laser feeding an optical fibre fluctuates because of the mechanical movements of the skin. When a laser beam is launched into a multimode optical fibre of short length, each laser longitudinal mode will generate numerous fibre modes which interfere with each other. The speckle pattern generated by the phase differencies of the modes will, when moving, create a broadband noise in the same frequency range as the P P G variations. This noise will add to the P P G signal and thus mask it. To test this hypothesis, an experiment was performed in which laser light was guided to the skin surface in two Medical & Biological Engineering & Computing

ways: through a step index optical fibre (diameter 0.75mm) and focused directly onto the skin (without passing along a fibre). In the latter case a lens was used to achieve a spreading of light of a similar angle (60 ~ as in the case usilag a fibre. The photoplethysmographic signal was picked up by a second optical fibre. The distance between the fibre ends and the skin was 0.8mm. The centre-to-centre fibre separation was 0-75mm. Measurements were made at the distal part of the middle finger on two subjects at room temperature. The spectral density of the signal was calculated for the two P P G signals. We found an increase in the high-frequency content of the signal when a fibre was used for passing the light to the skin. This experiment partly explains the broad frequency distribution in the micro PPG-laser signal (Fig. 4c) and the associated limited sensitivity of the micro PPG-laser in detecting skin perfusion changes. ALMOND et al. (1988) declared that the high frequency content of the AC P P G signal obtained at small fibre separations was associated with red cells movement being less pulsatile in the upper skin layer. 4.3 Comparison of micro PPG-LED and macro PPG-LED The comparative study between micro PPG-LED and macro PPG-LED (Fig. 5 (c and d)) shows that macro P P G - L E D is more sensitive, particularly when used on the forearm skin. The two methods use the same peak wavelength, the same type of light source and the same light intensity. Thus, any differences in results are constituted only by the optical probe used. In the micro P P G - L E D setup the illuminated skin area is 0.43 mm 2. Assuming a penetration depth of 1 mm and a semispherical light distribution at 660nm, the illuminated volume is approximately 0.11 mm 3. The adjacent optical fibre will detect a volume of the same order of magnitude. In macro PPGLED, the light-emitting diode and the photodetector are placed close to the skin. This diode illuminates a skin area of 20mm 2, and the corresponding volume is approximately 28mm 3. The volume seen by the photodetector, area 2.25 mm 2, is calculated to be 12 mm 3. These geometrical calculations tell us that macro P P G - L E D illuminates and measures a larger microvascular volume than micro PPG-LED. This means that the integrating properties of macro P P G are well suited to detect changes in skin perfusion. The differences in their respective measuring volume may be one explanation for the different results obtained in Fig. 5 (c and d) and in Fig. 3 (a and b). As indicated in Fig. 5 (a-d) the skin perfusion measured with P P G was occasionally lower when provoked by heat than when provoked by cold. This occurred mainly in the finger measurements. To study whether it was possible to reproduce this 'reverse' response to temperature provocation, one of the subjects ( 0 in Fig. 5) was exposed to repeated measurements at the finger during a period of 1.5 months. When performing L D F and micro PPG-laser simultaneously, PPG showed a decrease in total power at 42~ in four out of seven experiments, while the total power of L D F always increased significantly (t-test). Micro P P G - L E D and macro P P G - L E D were repeated four times and the results showed both increases and decreases in total power at 42~ Another subject ( ~ in Fig. 5) who showed an increase in total power at 42~ at both finger and forearm with all the methods used (except micro PPGlaser) was also exposed to a reproducibility test. Two or three recordings per method and site were performed and on each occasion this subject exhibited an increase in total power due to an elevated skin temperature. Although no conclusive explanation of these findings

January 1991

45

can be given, it is of interest to point out some aspects of the problem. The optical processes which give rise to the photoplethysmographic signal arc poorly understood. The signal consists of two components, one pulsatile (AC) in synchronism whith the heart rate and one component (DC) which fluctuates slowly, reflecting changes in total blood volume in the tissue under study. The pals9 component is generally believed to be related to (a) blood volume changes during each cardiac cycle and (b) orientation changes of red blood cells during each cardiac cycle (velocity-dependence). It is not known (CHALLONER, 1979) 2.0

which of these variables is dominant or if more processes are involved. The LDF output signal is shown to be proportional to the number of red blood cells times their mean velocity in the case of low haematocrits and velocities (NILsSON et al., 1980b). However, this relationship may not be generally valid and one of the variables may affect the signal more than the other. Cold and heat provocation changes the rheological properties of blood and may influence the skin pcrfusion signal measured. When the local blood temperature is lowered, an increase in haematocrit and red cell 2"0

a

1"5

I-5

O Q.

-6 o 13

b

0

-8

4-

1"0

4-

"13

E O c

zx 9149

+

B, O9

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io

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4 2'~ 13~C 42'~ finger forearm laser Doppler flowmetry (LDF) 13~

9

13~

i

i

J

42~

13~ 42~ finger forearm micro photoplethysmogrophy (micro PPG-laser)

c

9

d

2-o

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42J~ 42~' 13% finger forearm photoplethysmography (macro PPG- LED ) Total power of (a) LDF ; (b) micro PPG-laser ; (c) micro PPG-LED ; (d) macro PPG-LED 13~

42'~ 13'~ 42"C finger forearm micro photoplethysmography (micro PPG-LED)

46

+

V A

O

Fig. 5

-6 -~ ~-o 13

' 13~

Medical & Biological Engineering & Computing

January 1991

aggregation will occur (SVANES, 1980). When the blood temperature is high the local haematocrit may also be elevated (ScHMID-SCHtNBEIN, 1981). These variations in local haematocrit (and also haemoglobin concentration) may lead to variations in reflectance as measured by PPG. Complex optical interactions between light and blood/ tissue are underlying mechanisms resulting in intensity variations detected at the skin surface. ZDROJKOWSKI and PISHAROTY (1970) reported on nonlinear reflectance changes against haemoglobin concentration in nonhaemolysed blood at 630 nm. If we assume that P P G is mainly sensitive to changes in blood volume (and thereby red cell concentration), it m a y then be possible that the amplitude in the photoplethysmogram will be higher when provoked by cold than by heat. Let us assume that the output of L D F is mainly proportional to the velocity of red cells. An elevated skin temperature will give rise to an increase in the total power of the L D F signal. These varying rheological properties of blood may explain the different results obtained with L D F and P P G (Fig. 5 (a-d)). As indicated in Fig. 5, the sensitivity in recording perfusion changes is imposed by the anatomical site. When using L D F and micro and macro P P G - L E D , an enhancement in blood perfusion is, in general, best detected at the forearm. This may be explained by anatomical differences between the two skin sites. N u m e r o u s arterio-venous anastomoses are present in the finger. During heat provocation, a portion of the blood is transported through these open AV shunts. Some of the shunts are located at a deep vascular level, and these shunts will not be monitored by the red light of P P G - L E D and LDF. On the other hand, in the case of the forearm, which contains no or few A-V shunts, a higher percentage of the blood perfusion changes occurs within a vascular volume which can be reached by the red light. Tissue penetration data for different optical wavelengths given by ANDERSON and PARRISH (1981) indicate that infrared light in the region of 800-960 nm penetrates the tissue to a depth which is approximately twice that of laser light of 632-8nm. The use of these wavelengths means that recordings are made from two different tissue volumes and this must be taken into consideration when L D F and P P G are compared (see Part 2, LINDBERG and OBERG, 1991). Acknowledgment--The authors would like to thank H. Rohman and P. Sveider for their technical assistance. This work was supported by a grant from the National Swedish Board for Technical Development, grant 83-3907. Dr T. Tamura visited Sweden as a research scientist on sabbatical leave from the Institute for Medical & Dental Engineering, Tokyo Medical & Dental University, Tokyo, Japan.

References ALMOND, N. E., JONES, D. P. and COOKE, E. D. (1988) High quality photoplethysmograph signals from a laser Doppler flowmeter: preliminary studies of two simultaneous outputs from the finger. J. Biomed. Eng., 10, 458-462. ANDERSON,R. R. and PARRISn, J. A. (1981) The optics of human skin. J. Invest. Dermatol., 77, 13-19. CHALLONER, A. V. J. (1979) Photoelectric plethysmography for estimating cutaneous blood flow. In Non-invasive physiological measurement: ROLFE, P. Ed., Academic Press, London, 125151. DE TRAFFORD, J. and LAFFERTY, K. (1984) What does photoplethysmography measure? (Letter). Med. & Biol. Eng. & Comput., 22, 479-480. GREENFIELD,A. D. M. (1972) The circulation through the skin. In Handbook of physiology. Section 2 Circulation Vol. II. HAM-

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Authors' biographies Toshiyo Tamura received BS and MS degrees in Instrumentation Engineering from Keio University in 1971 and 1973, respectively, and a Ph.D. in Physiological Science from Tokyo Medical & Dental University in 1980. Since 1980 he has been working at the Institute for Medical & Dental Engineering, T o k y o Medical & Dental University. His research interest is focused on the develooment and application of biomedical transducers.

P. Ake Oberg received the MSEE degree from the Chalmers University of Technology, G6teborg, Sweden, in 1964, and the Ph.D. in Biomedical Engineering from Uppsala University in 1971. He is currently Professor of Biomedical Engineering at Linktping University and Director of the Department of Clinical Engineering, University Hospital: His research interests include biomedical instrumentation, transducers and clinical engineering. Dr Oberg is a Past President of the Swedish Society of Medical Physics & Medical Engineering and a board member of the IFMBE's Division of Clinical Engineering. He is a Fellow of the Swedish Academy of Engineering Sciences and the Royal Swedish Academy of Sciences.

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