Abstract:
A device for sensing blood pressure of an underlying artery of a patient includes a housing having a sensing region and a pivot region. The sensing region is pivotable about the pivot region in response to a hold down pressure applied at the sensing region by a user. The device includes a sensor interface assembly that is supported by the sensing region. The sensor interface assembly includes a sensing surface suited for engaging tissue adjacent the artery for sensing pressure from the artery. A wrist connection holds the housing adjacent the patient&#39;s wrist.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to systems for measuring arterial blood pressure. In particular, the invention relates to a method and apparatus for conveniently positioning a non-invasive blood pressure measurement device over an underlying artery for accurate measurement. 
     Blood pressure has been typically measured by one of four basic methods: invasive, oscillometric, auscultatory and tonometric. The invasive method, otherwise known as an arterial line (A-Line), involves insertion of a needle into the artery. A transducer connected by a fluid column is used to determine exact arterial pressure. With proper instrumentation, systolic, mean and diastolic pressure may be determined. This method is difficult to set up, is expensive and involves medical risks. Set up of the invasive or A-line method poses problems. Resonance often occurs and causes significant errors. Also, if a blood clot forms on the end of the catheter, or the end of the catheter is located against the arterial wall, a large error may result. To eliminate or reduce these errors, the set up must be adjusted frequently. A skilled medical practitioner is required to insert the needle into the artery. This contributes to the expense of this method. Medical complications are also possible, such as infection or nerve damage. 
     The other methods of measuring blood pressure are non-invasive. The oscillometric method measures the amplitude of pressure oscillations in an inflated cuff. The cuff is placed against a cooperating artery of the patient and thereafter pressurized or inflated to a predetermined amount. The cuff is then deflated slowly and the pressure within the cuff is continually monitored. As the cuff is deflated, the pressure within the cuff exhibits a pressure versus time waveform. The waveform can be separated into two components, a decaying component and an oscillating component. The decaying component represents the mean of the cuff pressure while the oscillating component represents the cardiac cycle. The oscillating component is in the form of an envelope starting at zero when the cuff is inflated to a level beyond the patient&#39;s systolic blood pressure and then increasing to a peak value where the mean pressure of the cuff is equal to the patient&#39;s mean blood pressure. Once the envelope increases to a peak value, the envelope then decays as the cuff pressure continues to decrease. 
     Systolic blood pressure, mean blood pressure and diastolic blood pressure values can be obtained from the data obtained by monitoring the pressure within the cuff while the cuff is slowly deflated. The mean blood pressure value is the pressure on the decaying mean of the cuff pressure that corresponds in time to the peak of the envelope. Systolic blood pressure is generally estimated as the pressure on the decaying mean of the cuff prior to the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, systolic blood pressure is the pressure on the decaying mean of the cuff prior to the peak of the envelope where the amplitude of the envelope is 0.57 to 0.45 of the peak amplitude. Similarly, diastolic blood pressure is the pressure on the decaying mean of the cuff after the peak of the envelope that corresponds in time to where the amplitude of the envelope is equal to a ratio of the peak amplitude. Generally, diastolic blood pressure is conventionally estimated as the pressure on the decaying mean of the cuff after the peak where the amplitude of the envelope is equal to 0.82 to 0.74 of the peak amplitude. 
     The auscultatory method also involves inflation of a cuff placed around a cooperating artery of the patient. Upon inflation of the cuff, the cuff is permitted to deflate. Systolic pressure is indicated when Korotkoff sounds begin to occur as the cuff is deflated. Diastolic pressure is indicated when the Korotkoff sounds become muffled or disappear. The auscultatory method can only be used to determine systolic and diastolic pressures. 
     Because both the oscillometric and the auscultatory methods require inflation of a cuff, performing frequent measurements is difficult. The frequency of measurement is limited by the time required to comfortably inflate the cuff and the time required to deflate the cuff as measurements are made. Because the cuff is inflated around a relatively large area surrounding the artery, inflation and deflation of the cuff is uncomfortable to the patient. As a result, the oscillometric and auscultatory methods are not suitable for long periods of repetitive use. 
     Both the oscillometric and auscultatory methods lack accuracy and consistency for determining systolic and diastolic pressure values. The oscillometric method applies an arbitrary ratio to determine systolic and diastolic pressure values. As a result, the oscillometric method does not produce blood pressure values that agree with the more direct and generally more accurate blood pressure values obtained from the A-line method. Furthermore, because the signal from the cuff is very low compared to the mean pressure of the cuff, a small amount of noise can cause a large change in results and result in inaccurate measured blood pressure values. Similarly, the auscultatory method requires a judgment to be made as to when the Korotkoff sounds start and when they stop. This detection is made when the Korotkoff sound is at its very lowest. As a result, the auscultatory method is subject to inaccuracies due to low signal-to-noise ratio. 
     The fourth method used to determine arterial blood pressure has been tonometry. The tonometric method typically involves a transducer including an array of pressure sensitive elements positioned over a superficial artery. Hold down forces are applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array typically have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured. The transducer is positioned such that at least one of the individual pressure sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. The pressure measured by the selected pressure sensitive element is dependent upon the hold down pressure used to press the transducer against the skin of the patient. These tonometric systems measure a reference pressure directly from the wrist and correlate this with arterial pressure. However, because the ratio of pressure outside the artery to the pressure inside the artery, known as gain, must be known and constant, tonometric systems are not reliable. Furthermore, if a patient moves, recalibration of the tonometric system is required because the system may experience a change in gains. Because the accuracy of these tonometric systems depends upon the accurate positioning of the individual pressure sensitive element over the underlying artery, placement of the transducer is critical. Consequently, placement of the transducer with these tonometric systems is time-consuming and prone to error. 
     The oscillometric, auscultatory and tonometric methods measure and detect blood pressure by sensing force or displacement caused by blood pressure pulses as the underlying artery is compressed or flattened. The blood pressure is sensed by measuring forces exerted by blood pressure pulses in a direction perpendicular to the underlying artery. However, with these methods, the blood pressure pulse also exerts forces parallel to the underlying artery as the blood pressure pulses cross the edges of the sensor which is pressed against the skin overlying the underlying artery of the patient. In particular, with the oscillometric and the auscultatory methods, parallel forces are exerted on the edges or sides of the cuff. With the tonometric method, parallel forces are exerted on the edges of the transducer. These parallel forces exerted upon the sensor by the blood pressure pulses create a pressure gradient across the pressure sensitive elements. This uneven pressure gradient creates at least two different pressures, one pressure at the edge of the pressure sensitive element and a second pressure directly beneath the pressure sensitive element. As a result, the oscillometric, auscultatory and tonometric methods produce inaccurate and inconsistent blood pressure measurements. 
     There has been a continuing need for devices which will measure blood pressure non-invasively, with accuracy comparable to invasive methods. Medwave, Inc. the assignee of the present invention, has developed non-invasive blood pressure measurement methods and devices which are described in the following United States patents, hereby incorporated by reference: U.S. Pat. No. 5,649,542 entitled CONTINUOUS NON-INVASIVE BLOOD PRESSURE MONITORING SYSTEM; U.S. Pat. No. 5,450,852 entitled CONTINUOUS NON-INVASIVE PRESSURE MONITORING SYSTEM; U.S. Pat. No. 5,640,964 entitled WRIST MOUNTED BLOOD PRESSURE SENSOR; U.S. Pat. No. 5,720,292 entitled BEAT ONSET DETECTOR; U.S. Pat. No. 5,738,103 entitled SEGMENTED ESTIMATION METHOD; U.S. Pat. No. 5,722,414 entitled CONTINUOUS NON-INVASIVE BLOOD PRESSURE MONITORING SYSTEM; U.S. Pat. No. 5,642,733 entitled BLOOD PRESSURE SENSOR LOCATOR; U.S. Pat. No. 5,797,850 entitled METHOD AND APPARATUS FOR CALCULATING BLOOD PRESSURE OF AN ARTERY; and U.S. Pat. No. 5,941,828 entitled HAND-HELD NON-INVASIVE BLOOD PRESSURE MEASUREMENT DEVICE. 
     As described in these patents, blood pressure is determined by sensing pressure waveform data derived from an artery. A pressure sensing device includes a sensing chamber with a diaphragm which is positioned over the artery. A transducer coupled to the sensing chamber senses pressure within the chamber. A flexible body conformable wall is located adjacent to (and preferably surrounding) the sensing chamber. The wall is isolated from the sensing chamber and applies force to the artery while preventing pressure in a direction generally parallel to the artery from being applied to the sensing chamber. As varying pressure is applied to the artery by the sensing chamber, pressure waveforms are sensed by the transducer to produce sensed pressure waveform data. The varying pressure may be applied automatically in a predetermined pattern, or may be applied manually. 
     The sensed pressure waveform data is analyzed to determine waveform parameters which relate to the shape of the sensed pressure waveforms. One or more blood pressure values are derived based upon the waveform parameters. The Medwave blood pressure measurement devices include both automated devices for continually monitoring blood pressure (such as in a hospital setting) and hand-held devices which can be used by a physician or nurse, or by a patient when desired. These devices represent an important improvement in the field of non-invasive blood pressure measurement. Still further improvements, specifically with respect to convenient and accurate placement of the measurement device over an underlying artery, are highly desirable. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a wrist-mounted blood pressure measurement device that consistently positions a sensor assembly over an underlying artery, and through a pivoting or cantilever type action, provides an axial force to the underlying artery. 
     In a preferred embodiment, the device for sensing blood pressure of an underlying artery of a patient according to the present invention includes a housing having a sensing region and a pivot region. The sensing region is pivotable about the pivot region in response to a hold down pressure applied at the sensing region by a user. The device includes a sensor interface assembly that is supported by the sensing region. The sensor interface assembly includes a sensing surface suited for engaging tissue adjacent the artery for sensing pressure from the artery. A wrist connection holds the housing adjacent the patient&#39;s wrist. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a perspective view of a preferred embodiment of a blood pressure measurement device of the present invention positioned over the wrist of a patient. 
     FIG. 2A is a side view of the blood pressure measurement device of FIG.  1 . 
     FIG. 2B is a bottom view of the blood pressure measurement device of FIG.  1 . 
     FIG. 3 is an electrical block diagram of the blood pressure measurement device of the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. I illustrates a blood pressure measurement device being used to measure and display blood pressure within an underlying artery within wrist  12  of a patient. Blood pressure measurement device  10  includes placement guide  13 , main housing  14 , display panel  16 , patient identification toggle  18 , power switch  20 , and sensor interface assembly  22  (best shown in FIGS.  2 A and  2 B). 
     Using placement guide  13  of measurement device  10 , measurement device  10  is placed at the projection of the styloid process bone perpendicular to wrist  12 . With device  10 , a small amount of force is manually applied to the radial artery, which runs along the styloid process bone. As the force is manually applied, blood pressure waveforms are recorded and the corresponding hold down pressure which is being manually applied is also recorded. Using the shape of the blood pressure waveforms, waveform parameters are generated. These parameters, along with universal coefficients, are used to calculate pressure values which then can be displayed. 
     Placement guide  13  is connected to housing  14  at the base of housing  14 . Placement guide  13  straddles the styloid process bone, automatically placing sensor interface assembly  22  over the underlying artery. Housing  14  contains all of the electrical components of measurement device  10 . The shape and configuration of housing  14  allows it to hang on the patient&#39;s wrist, using placement guide  13  as a type of hook. Housing  14  includes pressure platform  15 , which is a flattened depression directly above sensor interface assembly  22 . In operation, the user (medical personnel) applies pressure on pressure platform  15  with a thumb or finger. The hold-down force from the user&#39;s thumb applies a force in an axial direction to wrist  12  of the patient. The axial force is transmitted from pressure platform  15  of housing  14  to sensor interface assembly  22 . 
     In a preferred embodiment, display panel  16  simultaneously displays the following values based upon blood pressure measurements: systolic pressure, diastolic pressure, pulse rate, and mean blood pressure. Display panel  16  also preferably provides visual prompting for manually applying a varying hold down pressure. 
     Power switch  20  is actuated to turn on power to the circuitry within housing  14 . Timing circuitry within housing  14  automatically turns power off after a predetermined period of inactivity. Actuation of switch  20 , after the unit is turned on, causes display panel  16  to indicate previous readings of blood pressure and pulse rate. 
     Patient identification toggle  18  is used to organize the recorded blood pressure information with respect to a particular patient. After actuating power switch  20 , the user selects the specific patient for which blood pressure will be measured by pressing patient identification toggle  18 . In one embodiment, display panel  16  displays a patient identification number for the currently selected patient. The patient identification number changes as patient identification toggle  18  is pressed. In one embodiment the user can scroll through a list of 16 patient identification memory locations. 
     FIG. 2A is a side view of blood pressure measurement device  10 , and FIG. 2B is a bottom view of blood pressure measurement device  10 . As can be seen from FIGS. 2A and 2B, placement guide  13  is generally U shaped. Placement guide  13  includes hook  23 , pad  25 , and opening  27 . Opening  27  is a generally circular aperture that has a notch  29  near hook  23 . Guide ribs  17  and  19  encircle opening  27  and notch  29 , and meet at the base of hook  23 . 
     When device  10  is placed on the patient, pad  25  contacts the palm side of the wrist of the patient, while hook  23  wraps around the backside of the wrist. Placement guide  13  is made of a flexible plastic so as to fit all patients, with the styloid process bone fitting into notch  29  of opening  27 . Opening  27  also allows sensor interface assembly  22  to come in contact with the patient&#39;s wrist. Pad  25  becomes a pivot point about which force is applied. 
     Relying on a cantilever type action, device  10  allows the user to apply a force at pressure platform  15  of housing  14 . Housing  14  pivots about pad  25 , and sensor interface assembly  22  applies an axial force to the underlying artery. Sensor interface assembly  22  is pivotally mounted to housing  14 . As pressure is manually applied by moving housing  14  toward the artery, that force is transferred from housing  14  to sensor interface assembly  22 . 
     Device  10 , with placement guide  13  and the cantilever type action, allows sensor interface assembly  22  to be consistently placed in the proper position, and the hold-down force to be consistently applied in the axial direction with respect to wrist  12 . This improvement greatly simplifies the procedure of applying pressure by the user, because the user no longer controls the direction and angle at which pressure is applied with respect to the patient&#39;s wrist. 
     Instead of having to palpate wrist  12  to identify the location of the radial artery, a user simply places device  10  adjacent wrist  12  so that placement guide  13  hooks onto the patient&#39;s wrist with guide ribs  17  and  19  straddling the projection of the styloid process bone. The measurement process is significantly simplified with the present invention. 
     The force applied to the artery is swept in an increasing fashion so the pressure waveform data from a series of pulses are obtained with different amounts of force being applied. To achieve the desired pattern of variable force, user feedback is preferably provided with device  10 . 
     In a preferred embodiment, feedback is in the form of a visual counter on display panel  16 . As the user begins to apply pressure, a number is displayed corresponding to the amount of pressure applied by the user. As the user increases the applied pressure, the displayed number proportionally increases. The user (medical personnel) is previously instructed to increase pressure smoothly so that the displayed counter increases one integer at a time, approximately one per second. If the user increases the hold-down pressure too quickly, the displayed counter will also jump quickly through the corresponding numbers to indicate the choppy applied pressure. The user applies greater pressure until device  10  shows the resulting blood pressure measurements on display panel  16 . Preferably, the user applies enough pressure to get the counter up to the number 15, but it could be as low as 4 or 5, or as high as 27 or 28, depending on the patient. If a patient has higher blood pressure, greater applied force will be necessary, and the corresponding ending counter number will be a higher integer. 
     After the measurement, the user can then view the blood pressure reading. In a preferred embodiment, display panel  16  provides a digital readout of systolic, diastolic, and mean blood pressure, as well as pulse rate. An indication of memory location (by number) corresponding to the patient is also displayed. 
     As soon as the reading is complete, device  10  is ready to take another reading. There is no need to clear display  16 . Device  10  stores a predetermined number of previous readings (such as the last 10 readings). To review prior readings, patient identification toggle  18  or power switch  20  is pressed. This causes a different reading from memory to be displayed on display  16 . 
     Alternatively, the feedback to the user can be audible tones and/or visual movable bars. The process of applying force in response to audible tones and/or visual movable bars on display  16  is fully described in U.S. Pat. No. 5,941,828, entitled “Non-Invasive Blood Pressure Sensor With Motion Artifact Reduction”, which is incorporated herein. 
     As can be seen in FIG. 2B, device  10  includes external connector  30 . External connector  30  is a five pin connector that is used to transmit and receive data, recharge battery  36  (see FIG. 3) contained within housing  14  and provide an alternative power source to device  10 . External connector  30  allows device  10  to be connected to a docking station (not shown) so that its internal battery can be recharged, and the collected blood pressure information can be downloaded to a central system. Device  10  can be used by a nurse or other employee in a hospital setting to collect blood pressure and heart rate information from a series of patients. 
     After blood pressure and heart rate data are obtained, the nurse places device  10  into a docking station and a central computer (not pictured), which can transmit a command via external connector  30  to device  10 . In response, device  10  outputs blood pressure and heart rate information, already organized with respect to particular patients (with the patient identification toggle  18 ), via external connector  30 . Concurrently, the rechargeable battery  36  within device  10  is being recharged, and power is supplied to device  10  from the central computer (not pictured) via external connector  30 , while device  10  is in the docking station (not pictured). The central computer can then maintain a central database for all of the patients in the hospital, with the heart rate and blood pressure information automatically being downloaded into the database from device  10 . 
     FIG. 3 is an electrical block diagram of device  10 . Device  10  includes patient marker switch  18 , power supply circuit  42 , sensor interface assembly  22 , connectors  58  and  60 , amplifiers  62 A and  62 B, analog-to-digital (A/D) converter  64 , microprocessor  68 , display driver and memory circuit  82 , display panel  16 , non-volatile memory  78  and real-time clock  80 . Power supply circuit  42  includes external connector  30 , amplifiers  32  and  34 , rechargeable battery  36 , supply switch  38 , reverse battery protection  40 , switch  20 , integrated power switch  44 , OR circuit  46 , voltage divider  48 , analog regulator  50  and supervisor circuit  52 . 
     Device  10  can be powered through an external power source. An external power source couples to device  10  through external connector  30 . Power from external connector  30  on the VSUPPLY line causes supply switch  38  to disconnect rechargeable battery  36  from supplying power to supply circuit  42 . Instead, rechargeable battery  36  is recharged using the CHRGR line while the external power source supplies power to supply circuit  42  on the VSUPPLY line. External connector  30  also allows device  10  to receive and transmit data, such as blood pressure information and device serial number, to an external device over the RX (receive) line and TX (transmit) line. The RX and TX lines are coupled to amplifiers  32  and  34 , respectively, which amplify the signals transmitted and received by microprocessor  68 . Amplifiers  32  and  34  are enabled when power is received through the VSUPPLY line, and are disabled when no power is received through the VSUPPLY line. 
     Switch  20  is partially a monitoring pushbutton switch. Pressing switch  20  causes OR circuit  46  to turn on integrated power switch  44 . Integrated power switch  44  supplies power to all digital circuits, including microprocessor  68 , display panel  16  and associated display driver and memory circuit  82 . Integrated power switch  44  supplies power to microprocessor  68 , which in turn latches on OR circuit  46 . The turn off of the circuit is controlled by microprocessor  68  discontinuing a signal to OR circuit  46 . This occurs through a fixed time of no activity. 
     Analog regulator  50  outputs electrical power which is used to energize analog circuitry, including amplifiers  62 A and  62 B, and analog-to-digital (A/D) converter  64 . 
     Pressure transducers  56 A and  56 B and nonvolatile memory  54  within sensor interface assembly  22  are connected through connector  58  and connector  60  to circuitry within housing  14 . Transducers  56 A and  56 B sense pressure communicated within sensor interface assembly  22  and supply electrical signals to connector  58 . In a preferred embodiment, transducers  56 A and  56 B are piezoresistive pressure transducers. Nonvolatile memory  54  stores offsets of transducers  56 A and  56 B and other information such as a sensor serial number. Nonvolatile memory  54  is, in a preferred embodiment, an EEPROM. 
     The outputs of transducers  56 A and  56 B are analog electrical signals representative of sensed pressure. These signals are amplified by amplifiers  62 A and  62 B and applied to inputs of A/D converter  64 . The analog signals to A/D converter  64  are converted to digital data and supplied to the digital signal processing circuitry  66  of microprocessor  68 . 
     Microprocessor  68  includes digital signal processing circuitry  66 , read only memory (ROM) and electrically erasable programmable read only memory (EEPROM)  70 , random access memory (RAM)  72 , timer circuitry  74 , and input/output ports  76 . A/D converter  64  may be integrated with microprocessor  68 , while some of the memory may be external to microprocessor  68 . 
     Based upon the pressure data received, microprocessor  68  performs calculations to determine blood pressure values. As each pulse produces a cardiac waveform, microprocessor  68  determines a peak amplitude of the waveform. Microprocessor  68  controls display driver  82  to create the visual counter on display  16  that counts in correlation to the hold down pressure applied by the user. The visual counter guides the user in applying a variable force to the artery. 
     When a measurement cycle has been completed, microprocessor  68  reorders the cardiac waveforms in increasing order of their corresponding hold down pressure and performs calculations to determine systolic pressure, diastolic pressure, mean blood pressure, and pulse rate. The process of calculating pressure using shape, amplitude, and hold down is described in the previously mentioned Medwave patents, which are incorporated by reference. If patient identification toggle  18  is pressed, a signal is supplied to microprocessor  68 , causing it to toggle to a new pressure reading with a new memory location. In one embodiment, the memory location of that pressure reading is also displayed. 
     The blood pressure calculations, organized by patient, are preferably time-stamped at the time of calculation using real-time clock  80 , and stored in nonvolatile memory  78 , so that the calculations are not lost when power to device  10  is turned off. Non-volatile memory is preferably an EEPROM. As discussed above, the blood pressure information can then be transferred through external connector  30  to an external device. In a preferred embodiment, the sensor serial number is also output through external connector  30 , so that blood pressure information can be organized with respect to particular measurement devices. The information output through external connector  30  may be stored on a computer and accessed through a local area network, the Internet, or other means. 
     Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.