WO2011048556A2

WO2011048556A2 - Photoplethysmography at multiple depths

Info

Publication number: WO2011048556A2
Application number: PCT/IB2010/054746
Authority: WO
Inventors: Reuven Gladshtein; Benjamin Maytal
Original assignee: Reuven Gladshtein; Benjamin Maytal
Priority date: 2009-10-20
Filing date: 2010-10-20
Publication date: 2011-04-28
Also published as: WO2011048556A3

Abstract

A photoplethysmograph (PPG) includes a unit to measure blood flow through capillaries, a unit to measure blood flow through small blood vessels and a unit to measure blood flow through large blood vessels. Another PPG includes a multiplicity of light transmitters, each transmitting at a different wavelength and at least one photodetector to detect light from at least one of the multiplicity of transmitters after the light has reflected off elements within a body. Each the wavelength has its own penetration depth and each transmitter is geometrically placed with respect to its photodetector to create a measurement depth for its measurement arc which is close to the penetration depth for its wavelength.

Description

PHOTOPLETHYSMOGRAPHY AT MULTIPLE DEPTHS

CROSS-REFERENCE TO RELATED APPLICATIONS

[001] This application claims benefit from U.S. Provisional Patent Application No. 61/279,305, filed October 20, 2009, which is hereby incorporated in its entirety by reference.

FIELD OF THE INVENTION

[002] The present invention relates to photoplethysmography generally.

BACKGROUND OF THE INVENTION

[003] Photoplethysmography is a technology for measuring physiological parameters by shining light at a specific wavelength into the body and measuring the return signal (either through an extremity such as a finger or from reflections off the body). PPG technology for measuring oxygen level and of heart and breath rates has existed for more then 30 years.

[004] Many PPG units have at least two different wavelengths of light, because different compounds in the body react differently to different wavelengths. For instance, maximal absorption of light by oxyhemoglobin is observed at different wavelengths of light than for deoxyhemoglobin (i.e. non-oxygenated hemoglobin). The difference of absorption indicates the amount of each compound in the blood and can indicate how oxygenated the blood is.

[005] The original pulse oximeters of Nellcor Company of the United States are described in US 4,653,498. More modern devices, for example from OrSense of the Israel, are described in US 6,191,860 and US 7,386,336. All of these patents describe using light radiation at multiple wavelengtlis for measurement of the saturation of oxygen in blood, of the concentration of different hemoglobin groups, of glucose concentration, etc.

[006] Unfortunately, PPGs are very sensitive to noise and cannot distinguish between the different phenomena, including heart rate, breath rate and body motion, affecting the blood flow. SUMMARY OF THE PRESENT INVENTION

[007] There is therefore provided, in accordance with a preferred embodiment of the present invention, a photopiethysmograph including a multiplicity of light transmitters, each transmitting at a different wavelength and at least one photodetector to detect light from at least one of the multiplicity of transmitters after the light has reflected off elements within a body. Each wavelength has its own penetration depth and each transmitter is geometrically placed with respect to its the photodetector to create a measurement depth for its measurement arc which is close to the penetration depth for its wavelength.

[008] Moreover, in accordance with a preferred embodiment of the present invention, there is a green transmitter and an infra-red transmitter (IR) and one photodetector. Alternatively, there is a green transmitter, a red transmitter and an infra-red transmitter (IR) and one photodetector.

[009] Further, in accordance with a preferred embodiment of the present invention, there is one photodetector and the photopiethysmograph also includes an overlap processor to reduce the effects of overlapping measurement arcs.

[0010] Still further, in accordance with a preferred embodiment of the present invention, the photopiethysmograph also includes a processor to determine effects which modulate the flow of blood in the body.

[0011] Moreover, in accordance with a preferred embodiment of the present invention, the processor includes a distinguishing unit to distinguish between physiological and non- physiological blood flow phenomena. For example, at least one non-physiological blood flow phenomena is motion of the body.

[0012] Additionally, in accordance with a preferred embodiment of the present invention, the photopiethysmograph also includes a depth differentiator to differentiate signals associated with the transmitters to differentiated signals of their particular measurement regions.

[0013] Further, in accordance with a preferred embodiment of the present invention, the differentiated signals generally do not overlap.

[0014] Still further, in accordance with a preferred embodiment of the present invention, the photopiethysmograph also includes a distinguishing unit to distinguish between physiological and non-physiological blood flow phenomena using the differentiated signals.

[0015] Additionally, in accordance with a preferred embodiment of the present invention, the distinguishing unit includes a spectrum analyzer to analyze spectral components of the differentiated signals. [0016] There is also provided, in accordance with a preferred embodiment of the present invention, a photopiethysmograph including a unit to measure blood flow tlirough capillaries, a unit to measure blood flow through small blood vessels and a unit to measure blood flow through large blood vessels.

[0017] Additionally, in accordance with a preferred embodiment of the present invention, each the unit has a light transmitter of a different wavelength, wherein each wavelength has its own penetration depth and each the transmitter is geometrically placed with respect to its photodetector to create a measurement depth for its measurement arc which is close to the penetration depth for its wavelength.

[0018] Further, in accordance with a preferred embodiment of the present invention, the photopiethysmograph also includes an overlap processor to reduce the effects of overlapping measurement arcs.

[0019] There is also provided, in accordance with a preferred embodiment of the present invention, a method for photoplethysmography which includes for a multiplicity of light transmitters each having its particular wavelength, geometrically placing each transmitter with respect to its photodetector to create a measurement depth for its measurement arc which is close to a penetration depth for its wavelength and detecting light from at least one of a multiplicity of transmitters after the light has reflected off elements within a body.

[0020] Further, in accordance with a preferred embodiment of the present invention, the method also includes reducing the effects of overlapping measurement arcs.

[0021] Moreover, in accordance with a preferred embodiment of the present invention, the method also includes determining effects which modulate the flow of blood in the body.

[0022] Additionally, in accordance with a preferred embodiment of the present invention, the determining includes distinguishing between physiological and non-physiological blood flow phenomena. For example, one non-physiological blood flow phenomena is motion of the body.

[0023] Still further, in accordance with a preferred embodiment of the present invention, the method also includes differentiating signals associated with the transmitters to differentiated signals of their particular measurement regions.

[0024] Moreover, in accordance with a preferred embodiment of the present invention, the differentiated signals generally do not overlap. [0025] Additionally, in accordance with a preferred embodiment of the present invention, the method also distinguishes between physiological and non-physiological blood flow phenomena using the differentiated signals.

[0026] Finally, in accordance with a preferred embodiment of the present invention, the distinguishing includes analyzing spectral components of the differentiated signals.

BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The subject matter regarded as the invention is particularly pointed out and distinctly claimed in the concluding portion of the specification. The invention, however, both as to organization and method of operation, together with objects, features, and advantages thereof, may best be understood by reference to the following detailed description when read with the accompanying drawings in which:

[0028] Fig. 1 is a schematic illustration of the characteristic relationships of blood vessels of different sizes to different depths of biologic tissue, in the general area of an arm;

[0029] Fig. 2 is a schematic illustration of the basic operation of a single transmitter-receiver pair placed on a surface;

[0030] Figs. 3A and 3B are schematic illustrations of photoplethysmographs, constructed and operative in accordance with a preferred embodiment of the present invention, having three and two light transmitters, respectively;

[0031] Fig. 4 is a block diagram illustration of processing elements of a photopiethysmograph, constructed and operative in accordance with a preferred embodiment of the present invention;

[0032] Figs. 5A and 5B are exemplary graphs of frequency spectra generated by some of the processing elements of Fig. 4; and

[0033] Fig. 6 is a flow chart illustration of the operation of distinguishing heart rate and body motion, operative in accordance with a preferred embodiment of the present invention.

[0034] it will be appreciated that for simplicity and clarity of illustration, elements shown in the figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements may be exaggerated relative to other elements for clarity. Further, where considered appropriate, reference numerals may be repeated among the figures to indicate corresponding or analogous elements. DETAILED DESCRIPTION OF THE PRESENT INVENTION

[0035] In the following detailed description, numerous specific details are set forth in order to provide a thorough understanding of the invention. However, it will be understood by those skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known methods, procedures, and components have not been described in detail so as not to obscure the present invention.

[0036] Applicants have realized that more accurate measurements may be obtained by measuring different phenomena at different depths of tissue since different physical phenomena effecting blood flow behave differently at different depths and in different types of blood vessels. Thus, as Applicants have realized, motion may be determined, since it affects the flow of blood in the veins, but does not affect the flow in the capillaries and minimally affects the flow in the arteries. Heart rate, on the other hand, may be measured in the arteries. However, performing this kind of calculation requires measurements at known depths; otherwise, one signal may contain phenomena from multiple depths. Applicants have realized that depth measurements are affected both by the wavelength of the light shined into the body and the shape of the diffusive curve of the light from the transmitter to the receiver.

[0037] The present invention may be a photoplethysmograph (PPG) which, in accordance with a preferred embodiment of the present invention, may comprise transmitters transmitting at multiple wavelengths of light which reach to selected depths and receivers placed so as to shape diffusive curves which may read optimally at or near those depths. This may reduce the likelihood of receiving measurements from other depths and may enable separation of the signals, as described in more detail hereinbelow.

[0038] Reference is now made to Fig. 1, which illustrates the characteristic relationships of blood vessels of different sizes to different depths of biologic tissue, in the general area of an arm 8. Capillaries 10 generally lie close to the surface, labeled 11, of the skin and generally are very narrow. Veins 16 and arteries 18 are generally relatively wide. Arteries 18 generally lie deep within the body, in a region 17, while veins 16 generally lie below surface 11, in a region 15. Venules 12 and arterioles 14 connect between capillaries 10 and veins 16 and arteries 18, respectively.

[0039] As is known, blood flows in one direction, from the heart, through arteries 18, through arterioles 14, through capillaries 10, through venules 12, through veins 16, back to the heart. Accordingly, arteries 18 and arterioles 14 are pulsating vessels, which assist the heart in supplying blood volumes to all extremities of the body. Capillaries 10 may transfer oxygen and nutrients to tissues 19 and may receive waste products therefrom.

[0040] When we 011 our lungs with air, the lungs increase in volume and, in turn, push against the liquids of the body, resulting in increased static pressure on all liquids. The blood vessels become squeezed and push the blood towards the exfremities. Thus, in the extremities, the blood volume increases. Applicants have realized that changes in the blood flow due to the breathing process may be strongly measurable in veins 16 and venules 12, while they may be somewhat measurable in capillaries 10.

[0041 ] Furthermore, as Applicants have realized, changes in blood flow due to motion of arm 8 may be more measurable in veins 16. The changes in. blood flow may cause changes in the volume of blood at the measurement point, as veins 16 expand and shrink to accommodate the wave caused by the motion.

[0042] As mentioned hereinabove, Applicants have realized that the pulsating heart beats may be measurable in arteries 18 and arterioles 14. Thus, since the arteries 18 may be within region 17 and arterioles 18 may be within region 15, the heart rate may be measured therein while breath rate may be measured in veins 16 and venules 12, within region 15.

[0043] As mentioned hereinabove, the present invention may utilize transmitters which may transmit at wavelengths of light which may reflect off items in regions 15 and 17. For example, green light (~530nm) may have a characteristic penetration depth of ~2-3mm and, accordingly, may reflect off capillaries 10 and arterioles 12 while near infra red (NIR) light (~940nm) may enable measurement from deeper layers of tissue, such as layers 15 and/or 17, due to its longer penetration depth.

[0044] Fig. 2 illustrates the basic operation of a single transmitter-receiver pair (labeled "source" and "detector") placed on surface 11 of the skin. The Beer-Lambert law provides the physical and mathematical basis for near-infrared spectroscopy (NIRS) and states that transmission of light passing through a solution of a colour compound (chromophore) is absorbed by the compound resulting in a reduction in the intensity of the emerging light.

[0045] In reflective PPG, photons generally do not travel directly from the source to a receiver, in fact, they may be scattered in all directions, but measurable amounts of photons (typically of about 60 - 80% of the optical energy) may be picked up by a detector from a shallow arc (described as banana-shaped) extending between source and detector. The effective pathlength L of the arc naturally is longer than the inter-optode distance d and its measurement depth D into the tissue may be of approximately one half of the separation distance d. This is particularly true when measuiing the optical properties of biologic tissue. Thus, an equation for measurement depth D may be:

[0046] D~0.5*d (Equation 1)

[0047] In order to achieve good signal sensitivity and a high signal-to-noise ratio, the distance d between transmitter and detector may be between 1.6 and 2.4 times the desired measurement depth D, where, in accordance with a preferred embodiment of the present invention, measurement depth D may be defined to match the penetration depth of the selected wavelength of light. For example, for near infra red light, the penetration depth may be 20 - 30 mm.

[0048] Moreover, in order to provide good results (high sensitivity and high signal-to-noise ratio), the characteristic size (such as the side of a square array or the diagonal of a circular an'ay) of an active area of a detector array may be not less then 0.2D.

[0049] As an example, in order to measure a PPG signal from a depth of 1.5 - 2mm, the transmitter may emit light in the spectral range from blue to green and may be placed 3 - 4mm from the detector. In order to measure a PPG signal from a depth of 4mm, the transmitter may emit light in the spectral range from yellow to near infra red and may be placed 6.4 - 9.6mm from the detector. For such a device, the active area of the detector may be 3 - 4mm of its characteristic size.

[0050] Table 1 provides suggested measurement depths of the measurement arc, distances between "optode" pair (i.e. the transmitter and receiver) and the size of the detector array for three common wavelengths.

[0051] TABLE 1 :

[0052] distances between geometrical centers of transmitter and receiver [0053] It will be appreciated that measurement depth D may also be affected by the geometry of placement. For example, the angles of the transmitter and/or the detector to the measurement surface, which may be the skin or any other measurement surface, also affect measurement depth D. Thus, the shape and measurement depth of the measurement arc are a function of the distance from the transmitter and detector, of their angles related to surface plane of the measured tissue, the characteristic size of the detector and the wavelength of the irradiated light.

[0054] Reference is now made to Fig. 3A, which illustrates an exemplary photopiethysmograph unit 20 having multiple transmitters. Unit 20 may comprise a green light emitting diode (LED) 22, a red LED 24 and an IR LED 26. Unit 20 may comprise a single photodetector (PD) 28.

[0055] As per Table 1 above, the active area of photodetector 28 may be large enough to receive NIR signals and thus, is between 4 - 8mm. Such an array may be large enough to detect signals of all three wavelengths.

[0056] Fig. 3A, to which reference is now briefly made, shows the shallow arcs for all three transmitters, where arc 30 measures at a depth Dg, which is the measurement depth for green LED 22, arc 32 measures at a depth Dr, which is the measurement depth for red LED 24 and arc 34 measures at a depth Dir, which is the measurement depth for IR LED 26.

[0057] Fig. 3B, to which reference is now briefly made, shows a simpler implementation with two transmitters, a green LED 42 and an IR LED 46, and a photodetector 48, overlaid on the expanded view of Fig. 1. Fig. 3B shows the two arcs, labeled 52 and 56, respectively, and indicates that arc 52, the shallower arc, spans the region close to surface 1 1 (thereby to measure blood flow in the capillaries), while arc 56 spans the regions 15 and 17 (thereby to measure blood flow in the veins and, possibly, in the arteries) .

[0058] It will be appreciated from the discussion herein that the photopiethysmograph of the present invention measures blood flow through the different types of blood vessels. Thus, the data from arc 52, once processed, measures blood flow in the capillaries the data from arc 56 measures blood flow through the other vessels. If there are three transmitters, the data can be separated into flow through capillaries, flow through small blood vessels and flow through large blood vessels.

[0059] Reference is now made to Fig. 4, which illustrates the elements of a photopiethysmograph (PPG) 60, constructed and operative in accordance with a preferred embodiment of the present invention. PPG 60 may comprise a detector, such as detectors 28 or 48, an overlap remover 62, a spectrum analyzer 64, a peak detector 66 and a ratio determiner 68.

[0060] Detectors 28 or 48 may provide the signals from the multiple transmitters to overlap remover 62 to remove portions of the signals common to all measurement arcs, as described hereinbelow. Spectrum analyzer 64 may generate a frequency spectrum of the signal from each transmitter once its overlap has been removed. Peak detector 66 may find the peaks in each frequency spectrum and may provide the peak frequencies and amplitudes to ratio determiner 68. Ratio determiner 68 may determine the ratios of peak amplitudes of the signals from different transmitters and, as described hereinbelow, may distinguish tlie peaks for at least two of heart rate, breath rate and body motion.

[0061] Returning to Figs. 3 A and 3b, it is noted that, in Fig. 3 A, arcs 30, 32 and 34 overlap and in Fig. 3B, arcs 52 and 56 overlap, particularly in regions close to the respective transmitters and detectors. Applicants have realized that all of the arcs pass through the same initial tissue layers, at both ends of the arc). In accordance with a preferred embodiment of the present invention, overlap remover 62 may remove the overlap in order to receive signals which relate mostly to their particular measurement depth. Since the detector - transmitter geometry is fixed and the PPG unit generally does not move, the overlap amount may be fixed and may be measured, as follows:

[0062] A standard reflective PPG signal may be considered to have the following time- dependent components:

[0063] Background (Tissue) component - Bt

[0064] Capillary component - Ct

[0065] Vein component - Vt

[0066] Pulsative (Arterial) component - Pt

[0067] Thus, a received PPG signal St, from each transmitter i may be written as:

[0068] StrBti+Ctj+Vt.+Pti (Equation 2)

[0069] Applicants have realized that an LED in tlie blue-green range of light has a short wavelength and therefore, may detect generally the upper tissue layer, which may include mostly skin and capillaries 10 (Fig. 1). The background signal Bt may be determined by looking at the signal from such a blue-green LED while moving an arm up and down. When an arm is raised, the fluid in tlie body flows downwards, which minimizes capillary pressure at the extremity of the arm while, when an arm is lowered, the fluid pressure is maximized. [0070] As Applicants have realized, since blue-green light generally reflects from the upper tissue layer, the detected signal may be mostly affected by two components, the background component Btj and the capillary component Cti. The capillary component Ct, changes as the arm is moved up and down. However, the constant bias in the two signals may generally stay the same. This is the background component Btj.

[0071] Overlap remover 62 may remove capillary and background components Cti and Btj from all signals not detecting from the upper tissue layer. In the unit of Fig. 3 A, overlap remover 62 may remove the two components from signals detected from red LED 24 and IR LED 26. In the unit of Fig. 3 A, overlap remover 62 may remove the two components from signals detected from red LED 46.

[0072] For a unit like that of Fig. 3 A with more than two light transmitters, it is important to also remove the common vein and pulsative components Vt, and Ptj. These may be detected from the flow of blood in the smaller blood vessels (venules 14 and arterioles 12), which may be measured using a green-red wavelength, such as may be provided by red LED 24.

[0073] Overlap remover 62 may first remove background and capillary components Btj and Ctj, determined hereinabove, leaving the common vein and pulsative components Vt, and Ptj.

[0074] This tissue layer has capillaries, venules and arterioles. Blood flow in capillaries is laminar and the capillaries themselves have a strong ability to filter high frequency flow Applicants have realized that, accordingly, the non-pulsative blood volume (venules) in this tissue layer is nearly balanced with the pulsative (arterioles) blood volume. Thus, their two components may have similar mean amplitudes and thus, the mean RMS (root mean square) of the pulsative component, which may be of the form Asin(HRt), where A is the amplitude and HR is the heart rate, may be generally the same as the mean RMS of the vein signal, which may be of the form At. Overlap remover 62 may determine the mean RMS of the received signal, may determine an initial heart rate value from an at rest signal and may generate a common pulsative component Asin(HRt) using A = mean RMS. Overlap remover 62 may also generate a common vein component defined as an RMS bias BI of the level Bi - A/sqrt(2).

[0075] Overlap remover 62 may then remove the resultant common vein and pulsative components Vtj and Ptj of arterioles and venules, close to the skin laye_r may be removed from any signals detected from deeper within the body.

[0076] With the tight control on measurement depth D and the removal of the overlapping signals, the signals produced by overlap remover 62 may comprise measurements at known measurement depths. In accordance with a preferred embodiment of the present invention, the measurements may be taken over time, in order to identify at least the heart rate, breathing and motion which modulate the signals.

[0077] Spectrum analyzer 64 may generate frequency spectra for each signal received from overlap remover 62. The results of two exemplary experiments are shown in the frequency spectra of Figs. 5 A and 5B, to which reference is now made.

[0078] For the curves of Fig. 5 A, the patient was shaking his wrist at a fast pace, resulting in a frequency of motion of about 85 beats/min. As can be seen, both the green signal and the IR signal have peaks, labeled 74 and 76, respectively, at that frequency. There is a peak 70 in the green signal at about 12 beats/min, while there is hardly any peak (marked 72) in the IR signal at that frequency. Peak 70 corresponds to the rate of breathing. Finally, the heart rate may be seen at about 105 beats/min in the green signal, marked by a peak 78.

[0079] For the curves of Fig. 5B, the patient moved his arm much more slowly, at a rate of about 45 beats/min. This corresponds to peaks 84 and 86 in the green and IR signals, respectively, where peak 86 of the IR signal is much larger. Since the motion was so slow, there is a harmonic peak at about 90 beats/min, but the amplitudes have been reversed. Peak 88 of the IR signal is smaller than peak 90 of the green signal. There is a small peak at 110 beats/min in the green signal which corresponds to the heart rate.

[0080] In comparing the two graphs, it can be seen that the amplitude of the motion signal is significantly higher than either of the two physiological signals of heart rate and rate of breathing. Moreover, the frequency of motion is not fixed - in Fig. 5A it was about 85 beats/min while in Fig. 5B it is at 45 beats/min.

[0081] Thus, as mentioned hereinabove, the motion of the body interferes with the measurements, since it is not easily distinguishable from the physical parameter of breath rate (a person generally breathes at about 10 - 20 breaths/min and moves his body in a range of 30 - 90 motions/min). However, the longer light wavelengths provide measurements of the relatively wide blood vessels displaced in the inner regions of biologic tissue and the blood flow therein has some freedom of movement. Therefore, the blood flow in the wide blood vessels is strongly modulated by the motion while the motion minimally affects the flow of blood in the smaller blood vessels. This can be seen in Figs. 5A and 5B where the IR signal is of a higher amplitude at the frequency of motion. [0082] In accordance with a preferred embodiment of the present invention, ratio determiner 68 may compare the ratios of the peak amplitudes of the short and long wavelength measurements to distinguish the motion component from the physiologic components.

[0083] As mentioned hereinabove, the blood volume in the arterioles and venules in the upper tissue layer, measured by green LED 22 or 42, is roughly equivalent and thus, Vag and Vvg are of the same order and thus, their ratio is on the order of 1. Accordingly,

[0084] Rbg = Vag/Vvg ~ 1 (Equation 3)

[0085] where Rb is a ratio and Vag and Vvg are the arterial and venous blood volumes, respectively, as measured by green LED 22 or 42s.

[0086] However, in vein region 15, the venous blood volume is much larger than pulsating blood volume of the arterioles. Vein region 15 may be measured by red LED 24 or, if there is only IR LED 46, then care should be taken that the expected measurement area does not include any arteries. With this proviso, the ratio Rbr for the vein region, preferably measured by the red signal, is:

[0087] Rbr = Var/Vvr < 1 (Equation 4)

[0088] Accordingly, the ratio from the vein region, preferably measured by red signal is less than the ratio from the venule/arteriole region, preferably measured by green signal or:

[0089] Rbr < Rbg (Equation s)

[0090] Due to the pulse from the heart, the heart rate may be measured best in the arteries and thus, the arterial volume is most affected by the heart rate. In contrast, while body motions may cause changes in blood volumes in any part of body, these changes are strongest in the veins (due to the lack of the pulse from the heart). Thus, the venous blood volume is mostly affected by body motion.

[0091] Accordingly, the ratio Rb may be rewritten as :

[0092] Rb ~ Ahr/Abm (Equation 6)

[0093] where Ahr is the spectral amplitude due to heart rate and Abm is the spectral amplitude due to body motion. Equation 6 is not exact since correction coefficients, due to the different absorption in venous blood of (deoxi-)hemoglobin and versus the absorption in arterial blood of (oxi-)hemoglobin for light of different wavelengths, have to be taken into account hereinabove.

[0094] We can put equation 6 into equation 5:

[0095] Ahrr/Abmr < Ahrg/Abmg (Equation 7) [0096] Or, rewriting equation 7:

[0097] Ahrr/Ahrg < Abmr/Abmg (Equation 8)

[0098] Thus, ratio determiner 68 may take two peaks suspected of being body motion and heart rate and which need to be distinguished, either because the frequencies are too close to each other or because their peak amplitudes are too similar, and may take their red/green ratios. The peak whose ratio is less is the heart rate.

[0099] Reference is now briefly made to Fig. 6 which is a flow chart illustration of the operation of distinguishing heart rate and body motion. In step 200, the signals from the detector are received and in step 102, the spectral frequencies are determined (with or without the overlap removal). In step 104. the two amplitudes which need to be distinguished are chosen and the ratios are generated. In steps 106 and 110, the ratios are checked. If Avl/Acl > Av2/Ac2 (step 108), the Al component indicates the heart rate. Otherwise, (step 1. 12), the A2 component indicates the heart rate.

[00100] Unless specifically stated otherwise, as apparent from the preceding discussions, it is appreciated that, throughout the specification, discussions utilizing terms such as "processing," "computing," "calculating," "determining," or the like, refer to the action and/or processes of a computer, computing system, or similar electronic computing device that manipulates and/or transforms data represented as physical, such as electronic, quantities within the computing system's registers and/or memories into other data similarly represented as physical quantities within the computing system's memories, registers or other such information storage, transmission or display devices.

[00101 ] The processes and displays presented herein are not inherently related to any particular computer or other apparatus. Various general-purpose systems may be used with programs in accordance with the teachings herein, or it may prove convenient to construct a more specialized apparatus to perform the desired method. The desired structure for a variety of these systems will appear from, the description below. In addition, embodiments of the present invention are not described with reference to any particular programming language. It will be appreciated that a variety of programming languages may be used to implement the teachings of the invention as described herein.

[00102] While certain features of the invention have been illustrated and described herein, many modifications, substitutions, changes, and equivalents will now occur to those of ordinary skill in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.

Claims

CLAIMS [00103] What is claimed is:

1. A photoplethysmograph comprising:

a multiplicity of light transmitters, each transmitting at a different wavelength; and at least one photodetector to detect light from at least one of said multiplicity of transmitters after said light has reflected off elements within a body,

wherein each said wavelength has its own penetration depth and each said transmitter is geometrically placed with respect to its said photodetector to create a measurement depth for its measurement arc which is close to said penetration depth for its wavelength.

2. The photoplethysmograph according to claim 1 and wherein said multiplicity of light transmitters is a green transmitter and an infra-red transmitter (IR) and said at least one photodetector is one photodetector.

3. The photoplethysmograph according to claim 1 and wherein said multiplicity of light transmitters is a green transmitter, a red transmitter and an infra-red transmitter (IR) and said at least one photodetector is one photodetector.

4. The photoplethysmograph according to claim 1 and wherein said at least one photodetector is one photodetector and also comprising an overlap processor to reduce the effects of overlapping measurement arcs.

5. The photoplethysmograph according to claim 1 and also comprising a processor to determine effects which modulate the flow of blood in the body.

6. The photoplethysmograph according to claim 5 and wherein said processor comprises distinguishing means to distinguish between physiological and non-physiological blood flow phenomena.

7. The photoplethysmograph according to claim 6 and wherein at least one said non- physiological blood flow phenomena is motion of the body.

8. The photoplethysmograph according to claim 1 and also comprising a depth differentiator to differentiate signals associated with said transmitters to differentiated signals of their particular measurement regions.

9. The photoplethysmograph according to claim 8 and wherein said differentiated signals generally do not overlap.

10. The photoplethysmograph according to claim 8 and also comprising distinguishing means to distinguish between physiological and non-physiological blood flow phenomena using said differentiated signals.

11. The photoplethysmograph according to claim 10 and wherein said distinguishing means comprises a spectrum analyzer to analyze spectral components of said differentiated signals.

12. A photoplethysmograph comprising:

a unit to measure blood flow through capillaries;

a unit to measure blood flow through small blood vessels; and

a unit to measure blood flow through large blood vessels.

13. The photoplethysmograph according to claim 12 and wherein each said unit has a light transmitter of a different wavelength, wherein each said wavelength has its own penetration depth and each said transmitter is geometrically placed with respect to its photodetector to create a measurement depth for its measurement arc which is close to said penetration depth for its wavelength.

14. The photoplethysmograph according to claim 13 and also comprising an overlap processor to reduce the effects of overlapping measurement arcs.

15. A method for photoplethysmography, the method comprising:

for a multiplicity of light transmitters each having its particular wavelength, geometrically placing each transmitter with respect to its photodetector to create a measurement depth for its measurement arc which is close to a penetration depth for its wavelength; and

detecting light from at least one of a multiplicity of transmitters after said light has reflected off elements within a body.

16. The method according to claim 15 and also reducing the effects of overlapping measurement arcs.

17. The method according to claim 15 and also comprising determining effects which modulate the flow of blood in the body.

18. The method according to claim 17 and wherein said determining comprises distinguishing between physiological and non-physiological blood flow phenomena.

19. The method according to claim 18 and wherein at least one said non-physiological blood flow phenomena is motio of the body.

20. The method according to claim 15 and also comprising differentiating signals associated with said transmitters to differentiated signals of their particular measurement regions.

21. The method according to claim 20 and wherein said differentiated signals generally do not overlap.

22. The method according to claim 20 and also distinguishing between physiological and non- physiological blood flow phenomena using said differentiated signals.

23. The method according to claim 22 and wherein said distinguishing comprises analyzing spectral components of said differentiated signals.