Sensor for non-invasive and continuous determination of the duration of arterial pulse waves

A sensor for non-invasive and continuous determination of the duration of arterial pulse waves is provided in which at least two spaced apart piezoelectric pressure sensors are disposed in succession in the flow direction of the artery, with the sensor being provided with pressure sensitive surfaces and being integrated in a casing. The piezoelectric pressure sensors are provided with a pressure-sensitive, strip-shaped surface, with the strips each being disposed in their longitudinal extension perpendicular to the flow direction of the artery. The casing is provided with at least two recesses adapted to the contours of the strip-shaped surfaces into which the pressure-sensitive, strip-shaped surfaces of the pressure sensors are disposed flush to the surface of the casing.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a sensor for non-invasive and continuous
 determination of the duration of arterial pulse waves.
 2. Description of the Related Art
 A desirable goal for many manufacturers of blood-pressure measuring devices
 is continuous non-invasive measurement of human blood pressure without the
 aid of uncomfortable compression cuffs.
 It has been known for quite some time that human blood pressure, differing
 from individual to individual, correlates to the velocity of pulse waves.
 Hitherto this fact could not be exploited for continuous measurement of
 blood pressure, because there are no reliable, inexpensive sensors
 available for determination of the pulse-wave velocity.
 Hitherto, attempts have been made to determine the pulse-wave velocity,
 which is about 10 m/s at the wrist, via the change in the color, the form
 or in the electric resistance of the skin. Attempts have also been made to
 measure the pulse-wave velocity with the aid of the ultrasonic Doppler
 method, a reliable but not exactly inexpensive method. One such attempt is
 described in DE-OS-1 905 620. Two spaced apart piezoelectric oscillator
 systems, the conical-shaped sound of which irradiates a to-be-examined
 vessel and a Doppler reception device permit determination of the
 vessel-wall velocity with which the vessel wall is extended by the
 blood-pressure waves flowing through the vessel. Ultimately information on
 the pulse-wave velocity is obtained via a special evaluation algorithm.
 Another example for determining the pulse-wave velocity is indicated in
 U.S. Pat. No. 4,245,648 describing a process and a device for measuring
 blood pressure and for determining the pulse rate. Two pressure-sensitive
 sensors housed in an arm cuff are placed along a blood-conducting vessel.
 The increase-in-pressure values determined in intervals can be utilized
 for calculating the pressure-wave velocity. However, disadvantageous is
 the large size of the device making it impossible to use at sites that are
 difficult to reach. Furthermore, application of the device involves
 considerable motoric impediment.
 DESCRIPTION OF THE INVENTION
 The object of the present invention is to further improve the sensor known
 from U.S. Pat. No. 4,245,648 in such a manner that it is designed so small
 and compact that it can be combined, by way of illustration, with a
 wristwatch.
 The solution to the object of the present invention is set forth in claim
 1. Advantageous embodiments are the subject matter of the subclaims.
 An element of the present invention is that a sensor for non-invasive and
 continuous determination of the duration of arterial pulse waves, in which
 at least two piezoelectric pressure sensors are placed at intervals in
 succession in the running direction of the arteries, with the sensor being
 provided with pressure sensitive surfaces and integrated in a casing, is
 designed in such a manner that the piezoelectric pressure sensors have a
 pressure-sensitive, strip-shaped surface which are each placed in their
 longitudinal extension perpendicular to the running direction of the
 arteries, and that the casing is provided with at least two recesses
 adapted to the contours of the strip-shaped surfaces into which the
 pressure-sensitive, strip-shaped surfaces of the pressure sensors are
 disposed flush to the casing surface.
 The invented sensor determines non-invasively the pressure pulsations of
 the arteria radialis preferably at the wrist level at two closely adjacent
 positions of which the one is located more proximally and the other more
 distally at the measuring point of the arteria radialis in the wrist
 region. The pulse-wave velocity and therefore, using previously patient
 related calibration, the average blood pressure can be determined from the
 time lag of the pulse maximum of the two measuring positions. The level of
 the systolic and diastolic pressure can be immediately determined from the
 measured difference between the pulse-pressure maximum and pulse-pressure
 minimum at one of the positions. The device is so small that it can be
 constantly worn on the wrist like a wristwatch, thereby largely avoiding
 any discomfort to the patient.
 The sensor immediately determines the difference in the duration of the
 pulse wave propagating in the artery thereby setting stricter measurement
 criteria so that the invented sensor works more accurately than hitherto
 known measurement methods in which the indirect determination of the pulse
 wave occurs via the color, resistance, form of the skin. Inexpensive mass
 production of the sensor is also feasible.
 For determination of the pressure, the invented sensor is provided with at
 least two separately operating pressure sensors which each are provided
 with a pressure-sensitive, strip-shaped surface and which are disposed in
 their longitudinal extension perpendicular to the running direction of the
 artery. The individual pressure sensors composed of piezoelectric material
 are integrated in a semi-cylindrical casing in such a manner that they are
 inserted in the angular running recesses in the casing wall at the convex,
 hemispherical nappe of the semi-cylinder.
 Therefore, the individual pressure-sensitive surfaces essentially follow
 the convex-shaped hemispherical surface contour of the semi-cylinder which
 is pressed against the surface of the skin in such a manner that the
 curved pressure-sensitive surfaces intersect perpendicularly the running
 direction of the artery with radial polarization.
 The purpose of the convex curvature of the semi-cylinder and the pressure
 sensitive surfaces connected thereto is to ensure, upon lightly pressing
 the sensor casing against the natural form of the surface of the skin,
 improved adaptation to the measuring site and therewith improved
 mechanical contacting of the measuring object. Furthermore, the purpose of
 the semi-cylindrical surface contour of the sensor casing is largely
 insensitive to overturning at the longitudinal axis.
 Moreover, the pressure-sensitive surfaces of the pressure sensor has to be
 designed narrow in the direction of the running direction of the artery so
 that a small as possible duration of the pulse wave at each individual
 sensor area can be attained, thereby permitting obtaining high temporal
 resolution. The distance between the two pressure sensitive surface areas
 has to be selected so small that both pressure sensors still lie close to
 the surface in the region of the course of the arteria radialis. Only in
 this manner, can both pressure sensors detect the same temporal pulse
 duration. On the other hand, the distance has to be large enough in order
 to be able to still pick up the time lag of the pulse maximum between the
 two pressure sensors. Tests have shown that these conditions can be
 realized if the individual pressure-sensitive surfaces have a width of 1
 mm and are spaced 1 cm apart.
 The individual piezoelectric pressure sensors are composed of piezoelectric
 material and their surface facing the artery projects through the
 aforementioned recesses worked into the convex shaped casing wall. In this
 way. the individual piezo electric pressure sensors assume a hemispherical
 shape, in which upon external pressure polarization charges, which lead to
 an electric voltage between the pressure sensor surfaces, are released
 between their external and their internal surface proportional to the
 application of mechanical pressure or tension.
 In view of the fact that when pressure is applied, a high voltage is
 generated between the surfaces of the piezoelectric pressure sensors by
 relatively small charges, low-frequency pressure fluctuations, such as
 arterial pressure pulsations, can no longer be detected if the receiver is
 directly connected to a low-ohmic signal processing system. Therefore a
 preamplifier with a high as possible input resistor and a low as possible
 output resistor is required which is placed as close as possible to the
 piezoelectric element.
 Preferably, a simple, as small as possible to-be-realized impedance
 converter circuit, which by way of illustration is composed of a field
 effect transistor and two resistors, which are immediately integrated
 inside the sensor casing, should be provided for each piezoelectric
 pressure sensor. Due to its high-ohmic input, the entire sensor
 electronics should be completely screened off against electric
 interferences.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 FIG. 1 shows an invented preferred embodiment of a sensor for non-invasive
 and continuous determination of the duration of arterial pulse waves,
 which is provided with a semi-cylindrical casing 1. Two spaced apart
 piezoelectric pressure sensors 2 are worked into the hemispherical, convex
 outer contour of the casing. The invented sensor is pressed with its
 convex, hemispherical semi-cylindrical surface at the area of an artery 3
 through which pulse waves 4 travel. The sensor casing 1 is preferably
 provided with a radius of curvature of 2.5 mm in the convex, hemispherical
 nappe area and of about 14 mm on the overall length of the casing.
 Preferably two angularly running recesses, through which piezoelectric
 pressure sensor materials 2 project through from the inside, are provided
 in the convex, hemispherical nappe area of casing 1.
 FIG. 2 shows a longitudinal representation through an invented sensor,
 whose convex hemispherical nappe surface faces artery 3. As the
 representation shows. The top side of the semi-cylindrical casing 1 is
 interrupted by two recesses through which a piezoelectric sheeting 5
 projects from the inside. The piezopolymer sheeting 5 slightly protrudes
 beyond the surface of casing 1. The outward facing pressure-sensitive
 surface of the sheeting is, in addition, metallized and thus in electrical
 contact with the metal sensor housing. Preferably the contacting occurs
 via a press contact or adhesion contacting.
 In order to tap polarization charges generated on the bottom side of the
 sheeting due to the deformation of the piezopolymer sheeting, hemispheric
 disks of a conductive elastomer 6 are provided, which connect the
 piezopolymer sheeting to an impedance converter circuit which is placed on
 a substrate 7 for each individual pressure sensor. The voltage tapping at
 the internal side of each pressure sensor occurs in the area of the
 recesses via the respective conducting elastomer hemispheric disk 6 of
 approximately 1 mm thickness. The elastomer hemispheric disks 6 are
 contacted by means of press contacting to the non-metallized internal side
 of the piezo sheeting 5 with the respective signal input of both
 integrated impedance converter circuits on the substrate 7.
 Simultaneously, due to the elastic lining of the rear side of the sheeting
 by using elastic elastomer hemispheric disks ensures that the sheeting
 surface is pressed slightly outward at the recesses and therefore is
 essentially flush on the front to the surface of the casing or slightly
 raised.
 Evaluation of the time lag of the pulse maxima occurs by means of
 differentiating the impedance converter/output signals. The time-lagged
 zero passes of the two differentiated pulse pressure courses can be
 utilized to start or stop an electronic stopwatch. The small size of the
 sensor could permit wearing the sensor on a wristband together with a
 miniaturized evaluation electronics including a display for showing the
 pulse frequency and blood pressure.