Detector For Physiological Quantities

Abstract

A device for detecting physiological quantities, comprising two hinged arms which are provided on their end with freely movable end members. When a part of the body, for example, an ear lobe, is clamped between the end members, the latter will be immovably retained in a mount.

Description

The invention relates to a detector for physiological quantities, comprising two arms which are connected to each other by a hinge near one end, each arm accommodating an end member near its free end, at least one of the end members being displaceable with respect to the associated arm, it being possible to hold a part of a body such as an earlobe or a finger tip between said end members, at least one of said end members comprising a measuring head.

A detector of this kind is known from U.S. Pat. No. 3,152,587 and is often used for measuring, for example, the blood pressure or the heart frequency of patients, in particular patients whose life is in danger, such as after major surgery. It is then of essential importance that all relevant signals generated by the body are detected by the detector, whilst the detector itself should not cause any interference signals.

For example, when the heart frequency is measured use is often made of a detector which comprises a lamp and a photosensitive element between which an earlobe of the patient is placed so that the light produced by the lamp passes through the earlobe before it impinges upon the photosensitive element. Any variation of the quantity of blood in the earlobe due to the heart beat will change the amount of light transmitted through the ear lobe, which results in a variation of the signal produced by the photosensitive element.

Variations in the quantity of light received by the element occur also if the detector is displaced over the earlobe or if the distance between the lamp and the element changes. These variations also lead to signal variations (so-termed movement artefacts) which the measuring apparatus connected to the element generally cannot distinguish from signal variations which are caused by the heart beat.

The invention has for its object to provide a construction by which the occurrence of such disturbing signals is substantially avoided. To this end, a detector according to the invention is characterized in that the free end of at least one of the arms is provided with a mount, the shape, the dimensions and the nature of the surface of the mount and of the surface of the associated end member which faces the mount and which is connected to the arm via a flexible connecting member being chosen such that the end member is arranged to be immovable in the mount when the end member is subjected to a force engaging the vicinity of its centre and acting in the direction of the mount.

Consequently, the end members are freely movable with respect to the arms while the detector is being arranged on the part of the body so that optimum adjustment of the end members to the contours of this body part is possible, whilst after the detector has been mounted the end members are unmovably held in their mount so that they can no longer move with respect to each other and movement artefacts are hardly possible.

A construction which was found to be very satisfactory is characterized in that the mount comprises a rotation-symmetrical recess in the arm, the shape of the surface of the end member which faces the mount having a rotation-symmetry which is adapted to the shape of the said recess. The surface of the end member which faces the mount is preferably spherical, the mount being annular and comprising three bearing points for the spherical surface which are regularly distributed over its circumference.

The flexible connecting member which is used for connecting the end member to the arm is preferably formed by an e-shaped resilient wire, the straight portion of which is located to be rotatable in the end member near the centre of the surface facing the mount, the bent portion being arranged in a groove which is recessed in the arm.

So as to press the end members sufficiently rigidly into the mounts, the pressing force must exceed a given minimum value. On the other hand, this force should not be too large as this will bother the patient, particularly in the case of prolonged use of the detector. A frequently occurring complaint associated with the use of the detectors known thus far is, for example, a pronounced irritation of the skin at the areas where the detector presses against the body part. Furthermore, it should be avoided that any accidental touching of the detector causes movement artefacts due to the variation of the angle between the arms. Consequently, a given rigidity of the detector is required, at least after it has been arranged on the relevant part of the body. Such a rigidity is achieved for the detector described in the said U.S. Patent Specification in that a screw must be turned for changing the angle between the arms. However, this is cumbersome and, in addition, the evaluation of the pressing force is difficult.

An embodiment of the detector according to the invention which eliminates these drawbacks while maintaining the advantages, is characterized in that the force to be exerted at the area of the centre of the end members in order to hinge the arms with respect to each other is composed of a combination of a force which is produced by a clamping spring and a frictional force of at least 10 grams.

The hinge of the detector can comprise a hinge pin which is inserted through eyelets on both arms in the usual manner. The frictional force which is required at the area of the end members can then be produced in the hinge in that the hinge pin is also inserted through at least one friction ring which is arranged between two eyelets, the friction pack formed by the eyelets and the friction rings being compressed in the axial direction by a compression spring.

The force exerted by the compression spring is preferably made to be adjustable so as to enable adjustment of the friction.

After the detector has been arranged on the body part, it can sometimes happen that the detector is displaced by external forces which are large compared to the force exerted by the combination of the clamping spring and the friction. Such forces occur, for example, when the detector is jolted. So as to prevent this kind of displacement, a variant of the detector according to the invention is characterized in that part of the hinge pin is formed by a screw bolt which cooperates with a nut which, by tightening, can exert a compressive force in the axial direction on the friction pack, the said force being substantially larger than the force exerted by the compression spring.

The invention will be described in detail with reference to the drawings.

FIG. 1 is a perspective view of an embodiment of a detector according to the invention,

FIG. 2 shows how an end member of the detector shown in FIG. 1 is connected,

FIG. 3 shows a mount of the same detector,

FIGS. 4, 5 and 6 illustrate how the end member of the detector can be unmovably held in the mount by a force exerted thereon,

FIGS. 7 and 8 are sectional views of two embodiments of a hinge for a detector according to the invention,

FIG. 9 is a perspective view of an end member and an associated measuring head for a detector according to the invention,

FIG. 10 shows an embodiment of such a measuring head, and

FIG. 11 illustrates how the detector according to FIG. 1 can be used.

The detector 1 for physiological quantities which is shown in FIG. 1 comprises two arms 3 which are connected to each other near one of their ends by a hinge 5. Near their free end, each of the arms 3 accommodates an end member 7 which comprises a measuring head 9. Between the two end members 7 a body part such as an ear lobe or a tip of a finger can be held, the measuring heads 9 then contacting the skin. So as to facilitate the arrangement of the detector on the body part each of the arms 3 is extended beyond the hinge 5 by an end portion 11 which is adapted to the shape of the fingers.

Each of the arms 3 is provided with an annular mount 13 which comprises a rotation-symmetrical recess 14 in the arm 3. The end members 7 are connected to the arms 3 in a floating manner so that they have a limited freedom of movement in all directions. However, if an end member 7 is subjected to a force engaging near its centre and acting in the direction of the associated mount 13, the end member will be unmovably located in the mount. These properties are due to a proper choice of the shape, the dimensions and the materials of the mount and of the surface of the end member which faces the mount; this will be described hereinafter with reference to the FIGS. 4 to 6.

FIG. 4 is a cross-sectional view of a semi-cylindrical body 15 which is arranged in a trough 17 such that it bears on the edges 19 on the trough. If a force F acting in the direction of the trough 17 is exerted on the body 15 at the area of a point P, the body will be subjected at the area of the edges 19 of the trough both to normal forces and frictional forces. The normal forces, denoted in FIG. 4 by N.sub.1 and N.sub.2, are per definition directed perpendicular to the surface of the body 15 and hence in the direction of the centre M of the section. The frictional forces W.sub.1 and W.sub.2 are parallel to the said surface and their direction opposes that in which the body 15 could start to rotate as a result of the force F. The maximum value of the frictional force is dependent of the value of the associated normal force and of the friction coefficient f which is determined by the material properties of the body 15 and the trough edges 19. This dependency is given by the formula W = fN. The resultant of the forces W and N acts along a load line which encloses an angle .mu. (called friction angle) with the direction of the normal force N. It will be obvious that on the basis of the foregoing formula tg .mu. = f. This means that .mu. is independent of the value of the force F. The load lines for the forces at the area of the two trough edges 19 are denoted in FIG. 4 by l.sub.1 and l.sub.2. They intersect each other in a point S.sub.1, the projection on the upper surface of the body 15 of which is denoted by S'.sub.1. Considering the foregoing, the location of S.sub.1 and S'.sub.1 is also independent of the value of F.

Analogous reasoning can be followed for a force F which engages on the right of M. In that case a second intersection is found which is denoted in FIG. 4 by S.sub.2, the projection thereof on the upper surface of the body 15 being denoted by S'.sub.2. The location of the point of application P with respect to S'.sub.1 and S'.sub.2 is decisive for the question whether or not the body 15 will be stable in the trough 17. If P lies between S'.sub.1 and S'.sub.2, the body 15 cannot be brought to rotation by the force F, regardless of the value of, F. In a plan view, this "stable" area has the form of a band which extends in the longitudinal direction of the body 15 on both sides of the axis thereof (see the shaded area in FIG. 5). A force which is directed towards the trough 17 and which engages outside this area will bring the body to rotation.

It will be obvious that the width of the shaded area in FIG. 5 is dependent only of the diameter of the body 15, the width of the trough 17 and the friction coefficient f. If the friction coefficients f at the area of the left-hand and the right-hand trough edge 19 are not the same, the two angles .mu. will be different and the shaded area will be asymmetrical with respect to the axis of the body 15.

The requirement that the end member must be unmovably pressed into the mount by a force which is exerted thereon and which acts in the direction of the mount can thus be satisfied by choosing an end member in the form of a semi-cylinder and a mount which comprises two trough edges. However, the position of the end members can be even better adapted to the surface of the part of the body if the mount 13 is annular (see FIG. 3) and the surface of the end member 7 which faces the mount is spherical. It is to be noted that the mount must comprise three bearing points 21 for the spherical surface which are preferably regularly distributed over its circumference. If this is not the case, it can hardly be prevented that, due to manufacturing tolerances, the end member 7 contacts the mount 13 only at two locations, so that the end member can wobble in the mount.

For the combination of an annular mount 13 with three bearing points 21 and an end member 7 having a spherical surface it is also possible to calculate an area on the end member within which a force acting in the direction of the mount 13 must engage so as to keep the end member unmovably in the mount. The two-dimensional model shown in FIG. 4, hwoever, cannot be used due to the presence of three separate bearing points 21. The very complex three-dimensional calculation which would in this case be required has been omitted for the sake of simplicity. The stable area on the surface of the end member 7 which is remote from the mount 13 which can be found by means of such a calculation or by a much simpler experimental determination, is denoted by a shaded area in FIG. 6. The locations where this area is nearest to the edge of the end member 7 correspond to the locations of the bearing points 21. The shape and the extension of the area are dependent of the diameters of the end member and the mount and of the friction coefficient at the area of the bearing point 21. Because the detector is generally used only on parts of the body which are readily deformed, the force exerted on the end member 7 by the body part is usually regularly distributed over the surface of the end member which is in contact with the body part. This means that the resultant force usually engages near the centre of this surface so that a stable area whose edge is nowhere nearer to the centre of the surface than one fourth of the radius of the circle limiting the surface is generally sufficiently large to keep the end member 7 unmovably retained in the mount 13 in all practical cases.

The floating attachment of the end member 7 to the arm 3 can be realized in various manners by means of a flexible connecting member. Such a connecting member can consist of, for example, three or more wires or bands which extend from the centre of the spherical surface to the mount. However, a very simple and effective flexible connecting member consists of an e-shaped resilient wire 23 made of, for example, spring steel, the straight portion of which protrudes through an aperture 25 in the end member 7 with some clearance so that it is located to be rotatable. The bent portion of the wire 23 lies in a groove 27 which is recessed in the arm 3 and which is coaxial with the mount 13. Owing to this connection the end member 7 can perform the following movements: a tilting movement in the plane perpendicular to the straight portion of the wire 23 in that the end member hinges about this straight portion; a tilting movement perpendicular to the former tilting movement in that the straight portion and the adjoining bent portion of the wire are deflected in a resilient manner; and a movement in the direction of the axis of the mount in which case the wire is also deflected in a resilient manner. The end member 7 can thus be optimally adapted to the skin surface of the body part on which the detector is arranged. After this adaptation has been achieved, the end member 7 is fixed in the mount in the described manner.

However, to ensure that the end member 7 remains immobile in the mount 13 also if the patient to which the detector is connected moves, the force acting on the end member in the direction of the mount must exceed a given minimum value. This minimum value was found to be 10 grammes in practice. On the other hand, the said force should not be too high as otherwise it will be annoying to the patient. It was found that in the interest of the patient a maximum value of 100 grammes must be adhered to. Furthermore, movements of the arms 3 with respect to each other cause false signals (movement artefacts), so that from this point of view a completed rigid detector would be desirable.

In the detector shown in FIG. 1 the force exerted during opening and closing at the area of the centre of the end members 7 is determined by a clamping spring 29 and by the friction in the hinge 5. For each hinging movement the friction of the hinge 5 must then be overcome. When the detector is opened, this frictional force and the force of the spring 29 cooperate; the two forces oppose each other when the detector is closed. By an appropriate choice of the resilience and the frictional force a suitable compromise can be reached between the three above-mentioned requirements. A suitable choice is, for example, the case where the force produced by the clamping spring 29 at the area of the centre of the end members 7 amounts to approximately 50 grammes, whilst at the same areas a force of at least 10 grammes, preferably approximately 30 grammes, must be exerted so as to overcome the friction in the hinge 5. During opening the force then amounts to 80 grammes whilst it amounts to 20 grammes during closing.

It will be obvious that the friction of the hinge 5 must be properly reproducible so as to satisfy the imposed requirements. In the detector shown in FIG. 1 use is made of a hinge construction by means of which properly reproducible friction can be readily obtained. A sectional view of this construction is given in FIG. 7.

Each of the arms 3 comprises two eyelets 31 through which a hinge pin 33 is inserted. Between two eyelets 31 (forming part of different arms 3) a number (three in the case shown in FIG. 7) of friction rings 35 is provided. The friction pack formed by the two eyelets 31 and the friction rings 35 is compressed in the axial direction by a helical compression spring 37 which is slid about the hinge pin. The force exerted by the compression spring 37 can be adjusted by varying the thickness of the stack of friction rings 35. To this end, the hinge pin 33 is provided with a head 39 at only one end, so that it can be readily pulled out of the eyelets 31. The two arms 3 are then separated and friction rings 35 can be removed or added as desired.

The value of the friction is not only determined by the force of the compression spring 37, but also by the material properties of the eyelets 31 and the friction rings 35. The eyelets 31 are preferably moulded, integral with the arms 3, of a suitable synthetic resin material, for example, polycarbonate. The friction rings can be made of hard-paper or of a synthetic resin material which is filled with asbestos.

A variant of the hinge shown in FIG. 7 is shown in FIG. 8. In this case a portion 41 of the hinge pin 33 is formed by a screw bolt which cooperates with a nut 43. The screw bolt 41 is centered in the friction pack 31, 35 by means of a bush 44. The nut 43 and the end of the hinge pin 33 which is opposite to the screw bolt 41 are provided with a knurled head so that the nut can be readily tightened by hand. The tightening of the nut 43 causes compression of the friction pack 31, 35 in the axial direction between a locking ring 45 and a clamping plate 47. This compressive force is substantially larger than the force exerted in the same direction by the compression spring 37, so that the friction caused by the tightening of the nut 43 also becomes very large. As a result, the arms 3 are rigidly connected to each other as if it were. Due to the tightening of the nut 43 after the detector 1 has been arranged on the body part, the distance between the arms 3 existing at that instant is fixed so that the detector is rendered substantially less sensitive to touching and movements of the patient. This is particularly useful in the case of patients who are subjected to measurements during prolonged periods of time.

The measuring head 9 is preferably mounted to be detachable in the end member 7. An example of such a construction is shown in FIG. 9. The measuring head 9 has the shape of a flat cylinder comprising a connecting piece 49 which is connected to the cylinder wall and which extends into a connecting cable 51. The cylindrical measuring head 9 fits in a cylindrical cavity 53 which is recessed in the end member 7 and which communicates with the edge of the end member 7 via a number (two in FIG. 9) of radial ducts 55 which can accommodate the connecting piece 49. The connecting piece 49 is preferably made of a somewhat elastic synthetic resin material and its dimensions are such that it has a tight fit in the radial duct 55. As a result, the measuring head 9 can be rigidly secured and still be readily detachable in the end member 7. The direction of the connecting cable 51 with respect to the axis of the opening 25 is determined by the choice of the radial duct 55 in which the connecting piece 49 is inserted. It is to be ensured that the chosen direction involves, in view of the arrangement of the detector on the body, an as small as possible risk of movement artefacts due to touching of the connecting cable 51.

In accordance with the desired measurements, the measuring head 9 can comprise, for example, mechanical, optical or electrical measuring elements. A frequently applied measurement is the measurement of the heart frequency by optical measurement of periodic variations of the quantity of blood in a finger tip of ear lobe. As already described, one of the two measuring heads 9 then usually comprises a lamp and the other element comprises a photosensitive element. If the lamp or the photosensitive element become defective, measuring becomes impossible until the defective measuring head has been replaced. If uninterrupted supervision of the patient is required such as, for example, during major surgery, this can be very dangerous. Consequently, in each detector 9 a lamp 57 and a photosensitive element 59 are preferably arranged adjacent to each other (see FIG. 10). If a defect occurs, the measurement can be simply continued by switching over. The replacement of the measuring head 9 can then be postponed until a suitable time, if necessary. If desired, switching over can even be effected fully automatically by the measuring equipment used. The lamp 57 can be formed by a light-emissive diode and the photo-sensitive element 59 by a photosensitive diode.

FIG. 11 shows how the detector can be used in practice. The detector 1 is connected to an ear lobe and the connecting cable 51 is attached to the neck of the patient with an adhesive tape 61 so that movements of the connecting cable are not transferred to the detector. If the patient lies in bed and must carry the detector 1 during a prolonged period of time, it is desirable to prevent the patient from lying on the detector so that the detector would be jammed. This would give rise to substantial movement artefacts and could, moreover, be very painful to the patient. Therefore, a thick ring 63 of a soft and rigid material, for example silicone rubber, is provided about the ear to which the detector 1 is connected. The ring 63 is attached to the head of the patient by means of adhesive tape 65.

Claims

What is claimed is:

1. A device for detecting physiological quantities comprising first and second arms connected to each other for pivotal movement of one with respect to the other, a hinge located at one end of said arms for pivotally connecting said arms together, an end member carried on the free end of each arm so as to locate therebetween a part of a body such as an ear lobe or fingertip, a mounting means carried at the free end of at least one of said arms comprising a recess in said arm for carrying its respective end member whereby said end member is immovably accomodated in said recess when a force is applied to said end member in the direction of said mounting means and in the vicinity of the center of said end member and at least one of said end members is displaceable with respect to its associated arm when no forces are applied thereto, flexible connecting means connecting said end member to said mounting means, and measuring means carried by at least one of said end members so as to detect said physiological quantities.

2. The device according to claim 1 wherein said recess is rotation-symmetrical and wherein the shape of the end member accomodated within said recess has a rotation-symmetry adapted to the shape of said recess.

3. The detector according to claim 2 wherein the surface of said end member facing its respective mounting means is spherical, and wherein said mounting means is annular having three bearing points for supporting said spherical surface, said bearing points being evenly distributed about the circumferance of said mounting means.

4. The device according to claim 1 further comprising a groove in the recess of at least one of said arms, and wherein said flexible connecting means comprises an e-shaped resilient wire, the straight portion of said e-shaped wire being located in said end member substantially at the center thereof near the surface facing said mounting means so as to be rotatable therewith, the bent portion of said e-shaped wire being arranged within said groove.

5. The device according to claim 1 further comprising a clamping spring and wherein the force required to move said arms with respect to each other is equal to the combination of the force produced by said clamping spring and a frictional force of said hinge, said frictional force being at least 10 grams.

6. The device according to claim 5 wherein said hinge comprises a hinge pin inserted through eyelets on both arms, at least one friction ring arranged between two eyelets of one of said arms forming a friction pack with said eyelets, and a compression spring carried by said pin for exerting a compression force on said friction pack.

7. The device according to claim 6 further comprising a means for adjusting said compression force exerted by said spring carried on said pin.

8. The device according to claim 6 further comprising a screw bolt extending axially through said pin and a nut for cooperative engagement therewith, whereby rotational movement of said nut produces compressive forces exerted in the axial direction on said friction pack, said compressive forces being substantially larger than the force exerted by said compression spring.

9. The device according to claim 1 wherein said measuring means is detachably connected to said end members.

10. The device according to claim 1 wherein said measuring means further comprises a photo emissive means and a photosensitive means arranged adjacent each other on each of said end members.