Fingertip pulse wave sensor

Abstract: A fingertip pulse wave sensor includes a fingertip cushion supporting member (10) having a fingertip cushion supporting surface (11), and a nail supporting member (20) having a nail supporting surface (21) which is disposed in opposed relationship with the fingertip cushion supporting surface (11) at an interval smaller than the standard thickness of fingertips (1). Either of the fingertip cushion supporting member or the nail supporting member (20) is fixed, and the other of the fingertip cushion supporting member (10) and the nail supporting member is hinged. The hinged fingertip cushion supporting member (10) or nail supporting member is urged in the direction in which they are moved to each other by an elastic body (30) which is capable of applying a substantially fixed amount of pressure with respect to the variations in the pivot stroke caused by the variations in the thickness of the fingertips (1) resting on the fingertip cushion supporting member (10). The fingertip pulse wave sensor also includes a light-emitting element (15) which is disposed on either of the fingertip cushion supporting surface (10) and the nail supporting surface, and a light-receiving element (26) which is disposed on the other of the fingertip cushion supporting surface and the nail supporting surface.

Description: The present invention relates to a fingertip pulse wave sensor for detecting changes in the volume of a blood vessel in a fingertip held between a light-emitting element and a light-receiving element, in which the changes are caused by pulsation and detected as electrical signals produced in response to changes in the intensity of the light transmitted through the fingertip. The United States Patent US-A 4 685 464 discloses a sensor for illuminating a patient's tissue to measure blood constituents comprising a first housing and a second housing pivotally connected to one another and biased toward one another by resilient means. A first deformable means is provided in the first housing for securely gripping and complying to the patient's tissue, the second housing also having a second deformable means for gripping and complying to the patient's tissue. A light source and a light detector are secured proximal to the surfaces of the first and second deformable means respectively. The resilient means bias the first and second housings together so that the first and second deformable means grip and conform to the patient's tissue without substantially altering the blood flow. Another conventional fingertip pulse wave sensor includes a pad with either a light-receiving element or a light-emitting element buried therein which makes contact with a fingernail, and a pad which has the other of the light-receiving and light-emitting elements buried therein and which makes contact with a fingertip cushion. These pads are disposed in opposed relationship with each other. The conventional fingertip pulse wave detector also includes a plate spring having a U-shaped form and is mounted on the rear surface of either of the pads so as to hold the fingertip between the two pads and under pressure. With this arrangement, although the fingertip can be elastically held in place in spite of the variations naturally occurring in the shape of the fingertips of different individuals or any irregularities in the top dead point of the plate spring employed, these variations or irregularities serve to cause variations in the amount of pressure applied to the finger to a considerable extent. For example, in the case of a spring designed to provide a force that will press a fingertip with a deflection of 3 mm, if a deflection of 6 mm is caused due to differences in the shape of different individuals' fingertips or irregularities in the top dead point of the spring employed, it is possible that the pressure applied to the fingertip will be completely different. Excessive pressure exerted on a portion to be measured, increases the amount of blood in the venula of the fingertip, which is being returned to the associated vein. This has an effect on the waveform of a pulse, which is produced during arterial pulse wave detection at the fingertip. This is undesirable in terms of the measurement precision in a plethysmograph which is designed to estimate the form of a waveform in its detected state. This greatly affects the results of measurements obtained using an "acceleration-type" fingertip pulse wave detector which must be able to non-invasively monitor the state of a peripheral circle by converting the waveform detected to an acceleration curve. This conversion is achieved by differentiating twice the detected waveform with respect to the time. This conversion requirement, in effect, prevents such an "acceleration-type" fingertip pulse wave detector from being employable in clinical applications. Accordingly, an object of the present invention is to provide a fingertip pulse wave sensor which is capable of detecting a fingertip pulse wave in a consistent manner and with a high degree of accuracy in spite of differences naturally occurring in the shape of fingertips among different individuals. The fingertip pulse wave sensor should be based on the confirmation that application of a certain amount of pressure is inevitable for stable pulse wave detection. A fingertip pulse wave sensor is provided by the present invention as defined in claims 1 and 2. The present invention allows a fingertip to be gripped between photoelectric elements under a substantially fixed and appropriate pressure which is sufficient to keep the fingertip in place, and yet not to compress a blood vessel to the extent of affecting the measurement accuracy. This enables detection of a stable pulse waveform signal accurately corresponding to changes in the volume of a blood vessel. In particular, application of the present invention to an acceleration-type pulse wave detector which requires that a stable pulse waveform is detected with a higher degree of accuracy, enables highly reliable monitoring of the conditions of peripheral circulation. This feature of the present invention, expectedly will open the way to use of the acceleration-type fingertip pulse wave detectors in clinical applications, including prevention and cure of degenerative diseases caused by circulatory disorders. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of the central portion of a fingertip pulse wave sensor according to a first embodiment of the present invention. Fig. 2 is a section taken along the line 2 - 2 of Fig. 1. Fig. 3 is a cross-sectional view of the central portion of a fingertip pulse wave sensor according to a second embodiment of the present invention. Fig. 4 is a section taken along the line 4 - 4 of Fig. 3. Figs. 1 and 2 show a first embodiment of a fingertip pulse wave sensor according to the present invention. In this embodiment, the rear end portion of a fingertip cushion supporting plate 10 which acts as a fingertip cushion supporting member, is hinged on shaft rods 7 protruding from the side walls of casing 6. The fingertip cushion supporting plate 10 has at its forward end, a fingertip stopper 12 which is lower than the height of the nail of a fingertip 1 so as not to catch the long nail. The fingertip cushion supporting plate 10 also has guide walls 13 at the two sides thereof which are separated from each other by a distance slightly larger than the standard width of the fingertip 1. A fingertip cushion 1a is placed on the central portion of a fingertip cushion supporting surface 11. The central portion of the fingertip cushion supporting surface is provided with a through-hole 14. A light-emitting element 15 is mounted on the fingertip cushion supporting surface 11 below the through-hole 14. A nail supporting plate 20 which acts as a nail supporting member, is provided in the upper portion of the casing 6. The nail supporting plate 20 has a nail supporting surface 21 which is inclined at substantially the same angle as that at which the fingertip cushion supporting surface 11 is inclined in a normal state. A hole 22 is formed in the nail supporting plate 20 at a position at which it faces the hole 14. A light-receiving element 26 is mounted on the nail supporting plate 20 behind the hole 22. A constant-load spiral spring 30 is accommodated in the casing 6 below the fingertip cushion supporting plate 10. The inner end of the spiral spring 30 is pressed onto a rotary shaft 32 rotatably supported by arms 31 handing from the fingertip cushion supporting plate by virtue of its elasticity. An outer end 3Oa thereof is fixed to the front surface of the casing 6. When the measurement is to be conducted, the fingertip 1 is advanced until a forward end 1b thereof abuts against the stopper 12, as shown in Fig. 1. At that time, the fingertip cushion supporting plate 10 is lowered by a distance corresponding to the thickness of the fingertip 1, and the constant-load spiral spring 30 is thereby unwound by a length corresponding to the lowering stroke of the spring 30. Further, the constant load characteristics of the spring enable the pressure generated by the winding force of the spring to be maintained at an appropriate fixed value. In this way, the fingertip 1 is applied with an appropriate pressure sufficient to keep it in place and yet not to generate distortion in the pulse waveform, which is caused by the excessive pressure applied to the fingertip due to variations in the thickness of the fingertips resting on the fingertip cushion supporting plate. The light emitted from the light-emitting element 15 passes through the hole 14, is transmitted through the fingertip 1, and is received by the light-receiving element 26 which is disposed in opposed relationship with the light-emitting element 15. In the light-receiving element 26, the received light is converted into electric signals. The signals generated, which accurately represent the changes in the volume of blood in a vein, are supplied to the body (not shown) of a fingertip pulse wave analyser. Figs. 3 and 4 show a second embodiment of the present invention. In this embodiment, a fingertip cushion supporting plate 40 is provided in a casing 9. The fingertip cushion supporting plate 40 has a guide surface 44, guide walls 43 and a fingertip stopper 42. The central portion of the fingertip cushion supporting surface 41 is provided with a hole 47. A light-emitting element 46 is disposed on the fingertip cushion supporting surface 41 behind the hole 47. A nail supporting plate 50 is disposed in the casing 9 substantially parallel to the fingertip cushion supporting plate 40. Bearing portions 51 provided on the two sides of the nail supporting plate 50, are hinged on a pin 52 extending between the side walls of the casing 9. A coil spring 53 is provided on the pin 52. The one end of the coil spring 53 is fixed to the upper surface of the casing 9, and the other end thereof is fixed to the nail supporting plate 50. The coil spring 53 urges the nail supporting plate 50 in the direction in which a curved rear end portion 50a thereof abuts against the upper surface of the casing 9. The nail supporting plate 50 has a nail supporting surface 54. A hole 55 is formed in the nail supporting plate 50 at a position at which it faces the hole 47. A light-receiving element 56 is mounted on the nail supporting plate 50 behind the hole 55. When the fingertip 1 is inserted along the guide surface 44 and is then advanced until it abuts against the fingertip stopper 42, the nail supporting plate 50 is raised against the pressure of the coil spring 53 by a distance corresponding to the thickness of the fingertip. At this time, since the rear portion of the nail supporting plate 50 is hinged, the amount of pivot of the coil spring 53 is small with respect to the stroke of the nail supporting plate 54 by virtue of the lever principle. Further, constant load characteristics are ensured by using a sufficient number of turns (e.g. 10 turns in the coil spring), and the pressure applied by the spring can be maintained at a constant value with respect to the variations in the thickness of the fingertips. In either of the above-described embodiments, the fingertip cushion supporting member and the nail supporting member, have a plate-like form. However, the fixed member may also be shaped in a block-like form. Air springs and other types of mechanical springs may be employed as the elastic body which is adapted to maintain the pressure at a constant value with respect to the variations in the pivot stroke which are caused by the differences in individuals. Furthermore, the first embodiment may also employ a coil spring, and the second embodiment may also use a constant-load spiral spring.

Claims: A fingertip pulse wave sensor, comprising: a fingertip cushion supporting member (40) having a fingertip cushion supporting surface (41); a hinged nail supporting member (50) having a nail supporting surface (54) which is disposed in opposed relationship with said fingertip cushion supporting surface (41) at a distance smaller than the standard thickness of fingertips (1); an elastic body (53) for urging said hinged nail supporting member (50) toward said fingertip cushion supporting surface (41) in such a manner that the pressure applied by said elastic body (53) becomes substantially constant with respect to the variations in the pivot stroke caused by the variations in the thickness of said fingertips (1) resting on said fingertip cushion supporting member (40); a light-emitting element (46) disposed on one of said fingertip cushion supporting surface and said nail supporting surface; and a light-receiving element (56) disposed on the other of said fingertip cushion supporting surface and said nail supporting surface, characterized in that said fingertip wave pulse sensor comprises a casing (9); said fingertip cushion supporting member (40) is fixed relative to said casing (9); said hinged nail supporting member (50) has a rearward portion located distant from a forward portion at which said light-emitting element (46) or said light-receiving element (56) is disposed; and said hinged nail supporting member (50) is hinged relative to said casing (9) at said rearward portion. A fingertip pulse wave sensor, comprising: a hinged fingertip cushion supporting member (10) having a fingertip cushion supporting surface (11); a nail supporting member (20) having a nail supporting surface (21) which is disposed in opposed relationship with said fingertip cushion supporting surface (11) at a distance smaller than the standard thickness of fingertips (1); an elastic body (30) for urging said hinged fingertip cushion supporting member (10) toward said nail supporting surface (21) in such a manner that the pressure applied by said elastic body (30) becomes substantially constant with respect to the variations in the pivot stroke caused by the variations in the thickness of said fingertips (1) resting on said fingertip cushion supporting member (10); a light-emitting element (15) disposed on either of said fingertip cushion supporting surface and said nail supporting surface; and a light-receiving element (26) disposed on the other of said fingertip cushion supporting surface and said nail supporting surface; characterized in that said fingertip wave pulse sensor comprises a casing (6); said hinged fingertip cushion supporting member (10) has a rearward portion located distant from a forward portion at which said light-emitting element (15) or said light-receiving element (26) is disposed; said hinged fingertip cushion supporting member (10) is hinged relative to said casing (6) at said rearward portion; and said nail supporting member (20) is fixed relative to said casing (6). A fingertip pulse wave sensor according to claim 1, wherein said elastic body (53) comprises a constant-load spiral spring which is fixed at its inner end to a rotary shaft (52) and at its outer end to said hinged nail supporting member (50) or a fixed portion (9), said rotary shaft (52) being rotatably supported on said fixed portion (9) or said hinged nail supporting member (50). A fingertip pulse wave sensor according to claim 2, wherein said elastic body (30) comprises a constant-load spiral spring which is fixed at its inner end to a rotary shaft (32) and at its outer end to said hinged fingertip cushion supporting member (10) or a fixed portion (6), said rotary shaft (32) being rotatably supported on said fixed portion (6) or said hinged fingertip cushion supporting member (10). A fingertip pulse wave sensor according to claim 1, wherein said elastic body (53) comprises a coil spring which is mounted on said hinged rear portion of said hinged nail supporting member (50). A fingertip pulse wave sensor according to claim 2, wherein said elastic body (30) comprises a coil spring which is mounted on said hinged rear portion of said hinged fingertip cushion supporting member (10).

Inventors: HASEBE NOBORU, ITO SHOJI, HASEBE, NOBORU, ITO, SHOJI
Applicants: MISAWA HOMES RES INST, MISAWA HOMES INSTITUTE OF RESEARCH AND DEVELOPMENT
Application year: 1989
Main classification: A61B5
IPC codes: A61B5