WO1993020748A1 - Method of determining optimum arterial applanation - Google Patents
Method of determining optimum arterial applanation Download PDFInfo
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- WO1993020748A1 WO1993020748A1 PCT/US1993/002798 US9302798W WO9320748A1 WO 1993020748 A1 WO1993020748 A1 WO 1993020748A1 US 9302798 W US9302798 W US 9302798W WO 9320748 A1 WO9320748 A1 WO 9320748A1
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- stress
- sensitive element
- applanation
- artery
- interest
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B5/00—Measuring for diagnostic purposes; Identification of persons
- A61B5/02—Detecting, measuring or recording pulse, heart rate, blood pressure or blood flow; Combined pulse/heart-rate/blood pressure determination; Evaluating a cardiovascular condition not otherwise provided for, e.g. using combinations of techniques provided for in this group with electrocardiography or electroauscultation; Heart catheters for measuring blood pressure
- A61B5/021—Measuring pressure in heart or blood vessels
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- A—HUMAN NECESSITIES
- A61—MEDICAL OR VETERINARY SCIENCE; HYGIENE
- A61B—DIAGNOSIS; SURGERY; IDENTIFICATION
- A61B2562/00—Details of sensors; Constructional details of sensor housings or probes; Accessories for sensors
- A61B2562/02—Details of sensors specially adapted for in-vivo measurements
- A61B2562/0247—Pressure sensors
Definitions
- the present invention generally relates to pressure measurement systems, and more particularly relates to a method for non-invasively determining the intra-arterial blood pressure of a wearer.
- Systems for measuring the intra-arterial blood pressure of a patient can be subdivided into two main groups ⁇ those which invade the arterial wall to access blood pressure and those which use non-invasive techniques.
- Traditionally the most accurate blood pressure measurements were achievable only by using invasive methods.
- One common invasive method involves inserting a fluid filled catheter into the patient's artery.
- invasive methods provide accurate blood pressure measurements, the associated risk of infection and potential for complications, in many cases, outweigh the advantages in using invasive methods. Because of these risks associated with invasive methods, a non-invasive method, known as the Korotkoff method is widely used.
- the Korotkoff method is known as an auscultatory method because it uses the characteristic sound made as the blood flows through the artery to mark the points of highest (systolic) and lowest (diastolic) blood pressure. Although the Korotkoff method is non-invasive, it only provides a measurement of the highest pressure point and the lowest pressure point along the continuous pressure wave. While systolic and diastolic pressure are sufficient for accurate diagnosis in many instances, there are many applications in which it is desirable to monitor and utilize the entire characteristic curve of the blood pressure wave. In these applications, the Korotkoff method is simply incapable of providing ample information.
- U.S. Patent No. 4,423,738 issued to Newgard on January 3, 1984 discloses an electromechanical force sensor which is made up of an array of individual force sensing elements, each of which has at least one dimensions smaller than the lumen of the underlying artery wherein blood pressure is to be measured.
- U.S. Patent No. 4,802,488 issued to Eckerle on February 7, 1989 discloses an electromechanical transducer that includes an array of transducer elements. The transducer elements extend across an artery with transducer elements at the ends of the array extending beyond opposite edges of the artery.
- the technician's task is simplified, it introduces certain complexities into the methodology used for determining intra-arterial blood pressure.
- the sensor face is made relatively long as compared to the lumen of the underlying artery, only a small fraction of the sensing portion of the tissue stress sensor is overlying the artery, and it is only this portion which is sensing useful forces (i.e. forces which are related to intra-arterial blood pressure).
- the remaining portion of the sensing portion is in contact with tissue which does not overlie the artery of interest, and accordingly, does not transmit forces to the sensing portion which can be used for determining intra-arterial pressure.
- the method disclosed in the '193 patent includes selecting the transducer element which has a maximum pulse amplitude output and then looking to its neighbors and choosing the neighbor having a spatially local minimum of at least one of the diastolic and systolic pressures.
- Other methods are disclosed in U.S. Patent No. 4,802,488 issued to Eckerle on
- the sensor In addition to the sensors function to measure tissue stress, the sensor also functions to applanate (or flatten) the artery of interest. Applanating the artery of interest is critical in correctly determining intra-arterial blood pressure. In fact, it has been found, that when the artery of interest is applanated to an optimum state, extremely accurate determinations of intra-arterial blood pressure can be made.
- U.S. Patent No. 4,799,491 issued to Eckerle on January 24, 1989 discloses a method for determining a "correct" hold down pressure.
- U.S. Patent No. 4,836,213 issued to Wenzel on June 6, 1989 discloses a method for computing optimum hold down pressure for a transducer indicative of blood pressure in an artery.
- the Applicants of the present invention believe that a method which is superior to those heretofore disclosed methods employs the use of stress energy. For example, it is believed, that the best area of the sensor for collecting stress data for determining optimum applanation is that portion which receives the greatest contact stress energy from the tissue overlying the artery of interest.
- Some of the disclosed methodologies include collecting tissue stress information from the area of the stress sensor which receives the greatest contact stress energy from the tissue overlying the artery of interest. This information as used, in conjunction with other information, to determine the optimum applanation state of the artery.
- the present invention provides a method of estimating optimum arterial compression by measuring the stress of tissue overlying an artery of interest.
- the disclosed method is for use in a non-invasive blood pressure monitoring systems of the type including a tissue stress sensor having a stress sensitive element, the stress sensitive element having a length that exceeds the lumen of the artery of interest.
- the method includes the steps of placing the stress sensitive element of the tissue stress sensor in communication with the tissue overlying the artery of interest; orienting the stress sensitive element such that the length spans beyond the lumen of the artery of interest; using the stress sensitive element to varyingly compress the artery of interest thereby applanating the artery of interest through a plurality of stages, and at each said applanation stage; obtaining from the tissue stress sensor at least one electrical signal representing stress data across the length of the stress sensitive element, the stress data including a plurality of stress datum, each stress datum representing stress communicated to a predetermined portion of the stress sensitive element from the tissue overlying the artery of interest, each predetermined portion of the stress sensitive element lying along the length of the stress sensitive element, and for each applanation stage; selecting and computing an applanation optimization parameter, wherein the applanation optimization parameter is selected from the group comprising pulse parameter, mean distribution breadth parameter, pulse spread parameter, spatially averaged stress parameter, stress spatial curvature parameter, and stress variation parameter and an applanation state parameter
- one or more applanation optimization parameters can be used and the results thereof can be averaged together to form an overall composite indicator.
- Individual weighting functions may be applied to the individual applanation optimization parameters so as to weigh the significance of individual factors.
- the present invention discloses 12 separate methods for determining when the optimum applanation state is achieved. Each of these methods employ one or more applanation optimization parameters as a function of an applanation state parameter and combine the two parameters in a unique way to provide a method for determining an optimum applanation state.
- the present invention discloses a method for use in a non-invasive blood pressure monitoring system of the type including a tissue stress sensor having a stress sensitive element, the stress sensitive element having a length that exceeds the lumen of the artery of interest. Specifically, a method is provided of determining which portion of the stress sensitive element is best suited for estimating intra-arterial blood pressure.
- the method includes the steps of: placing the stress sensitive element of the tissue stress sensor in communication with the tissue overlying the artery of interest; orienting the stress sensitive element such that the length spans beyond the lumen of the artery of interest; using the stress sensitive element to compress the artery of interest thereby applanating the artery of interest: obtaining from the tissue stress sensor at least one electrical signal representing stress data across the length of the stress sensitive element, the stress data including a plurality of stress datum, each stress datum representing stress communicated to a predetermined portion of the stress sensitive element from the tissue overlying the artery of interest, each predetermined portion of the stress sensitive element lying along the length of the stress sensitive element; using the stress datum to define a pulsatily energetic region along the stress sensitive element; and estimating the value of the intra-arterial blood pressure to be the value of the contact stress data found in the pulsatily energetic region of the stress sensitive element.
- Figure 1 is a perspective view of a tissue stress sensor attached to the wrist of a wearer.
- Figure 2 is a cross-sectional view taken substantially along lines 2-2 of Figure 1.
- Figure 3 is an enlarged view of encircled portion 3 of Figure 2.
- Figures 4a and 4b are diagrammatic views of the emitter and detector portions of the semiconductor assembly taken substantially along lines 4-4 of Figure 3.
- FIG. 5 is an electronic block diagram of the tissue contact stress sensor and associated supporting electronics of the present invention.
- Figure 6 is a detailed schematic of blocks 40 and 42 of Figure 5.
- Figure 7 is a graphic representation of a typical blood pressure waveform.
- Figure 8 is a graphical representation of contact stress versus distance along the length of the stress sensitive element.
- Figures 9a- 9e are diagrammatic representations of the distortion which an artery undergoes when it is compressed.
- Figure 10 is a block diagram showing the logic flow which is common to Methods 1-12 disclosed herein.
- Figure 11 is a graphical representation of the calculation of the PPAR parameter.
- Figure 12 is a graphical representation of an equivalent manner of calculating the PPAR parameter.
- Figure 13 is a combined graphical and diagrammatical representation of the method steps utilized in generating the PPAR parameter as a function of ASP.
- Figure 14 is a graph showing contact stress energy as a function of distance along the stress sensitive element.
- Figure 15 is a graphical representation showing estimating technique for estimating intra-arterial blood pressure from contact stress data.
- Figure 16 is a graphical representation of Estimating Technique B for estimating intra-arterial blood pressure from contact stress data.
- Figure 17 is a graphical representation showing Interpolation Technique A for interpolating intra-arterial blood pressure values from contact stress data.
- Figure 18 is a flow diagram of selecting and interpolating arterial blood pressure values from contact stress data.
- Figure 19 is a graphical representation of interpolation technique B depicting interpolating arterial blood pressure values from contact stress data.
- Figure 20 is a diagrammatic and graphic representation of the method steps of Method 2 utilized in generating the MDBP parameter as a function of ASP.
- Figure 21 is a graphical representation showing the calculation of the MDBP parameter as a function of a given applanation state.
- Figure 22 is a graphical representation showing the calculation of the DDBP parameter for a given applanation state.
- Figure 23 is a diagrammatic and graphic representation of the method steps of Method 3 utilized in generating the DDBP parameter as a function of ASP.
- Figure 24 is a diagrammatic and graphical representation of the method steps of Method 4 utilized in generating the PDBP parameter as a function of ASP.
- Figures 25 and 26 are identical graphical representations showing the calculation of the PDBP parameter for a given applanation state.
- Figure 27 is a diagrammatic and graphical representation of the method steps of Method 5 utilized in generating the ⁇ PDBP parameter as a function of ASP.
- Figure 28 is a diagrammatic and graphical representation of the method steps of Method 6 utilized in generating the PSP parameter as a function of ASP.
- Figure 29 is a graphical representation of the calculation of the PSP parameter for a given applanation state.
- Figure 30 is a graphical representation of the calculation of the PDBP parameter for a given applanation state.
- Figure 31 is a diagrammatic and graphical representation of the method steps of Method 7 utilized in generating the PDBP parameter as a function of ASP.
- Figure 32 is a diagrammatic and graphical representation of the method steps of Method 8 utilized in generating the DDBP parameter as a function of ASP.
- Figure 33 is a graphical representation of the calculation of the DDBP parameter for a given applanation state.
- Figure 34 is a graphical representation of the calculation of the SASP parameters for a given applanation state.
- Figure 35 is a diagrammatic and graphical representation of the method steps of Method 9 utilized in generating the SASP parameters as a function of ASP.
- Figure 36 is a graphical representation of the spatially averaged stress parameters showing the characteristic knee regions about the optimum applanation state.
- Figure 37 is a graphical representation of the second derivative of the functions of Figure 36.
- Figure 38 is a graphical representation of the calculation of the SSCP parameters for a given applanation state.
- Figure 39 is a diagrammatic and graphical representation of the method steps of Method 10 utilized in generating the SSCP parameters as a function of ASP.
- Figure 40 is a graphical representation of the SSC parameters as a function of applanation state number.
- Figure 41 is a graphical representation of the first derivative of the functions of Figure 40.
- Figure 42 is a diagrammatic and graphical representation of the method steps of Method 11 utilized in generating the SSPAR and the SDPAR parameters as a function of ASP.
- Figure 43 is a graphical representation of the calculation of the SSPAR parameters for a given applanation state.
- Figure 44 is a graphical representation of the stress spread parameters as a function of applanation state number.
- Figure 45 is a graphical representation of the first derivative of the functions of Figure 44 as a function of applanation state number.
- wrist mount apparatus 21 includes base 23 and flexible strap 25.
- Flexible strap 25 is adapted to engage base 23 to the wrist of a user.
- Tissue stress sensor housing 27 is fastened to base 23 and houses a tissue stress sensitive element 34 (tissue stress sensitive element not shown) and a means 29 for moving the tissue stress sensitive element 20 (see Figure 2) into operative engagement with the tissue overlying an artery of interest.
- tissue stress sensitive element 34 tissue stress sensitive element not shown
- means 29 for moving the tissue stress sensitive element 20 (see Figure 2) into operative engagement with the tissue overlying an artery of interest.
- Various electrical signals are derived from the tissue stress sensor located within sensor housing 27 and are made available therefrom via conductors within cable 31. These electrical signals carry data which will be used to derive the intra-arterial blood pressure of the wearer of apparatus 21.
- sensor housing 27 is mounted to base 23. Within sensor housing 27 is mounted a fluid operated slave bellows 29. Bellows 29 is attached to, at one of its end, tissue stress sensor 20. As bellows 29 receives a displacement fluid from a source of fluid via tubing 33, it expands downwardly 43 thereby causing tissue stress transducer 20 to engage tissue 24 overlying artery of interest 26.
- tissue stress sensor 20 includes wafer 30 which has a nonresponsive portion 32 and a responsive portion (also denoted as a stress sensitive element or also a diaphragm portion) 34.
- Nonresponsive portion 32 serves mainly to support responsive portion 34.
- radial artery 26' has a generally rounded opening (or lumen) as depicted at 26'.
- radial artery 26' begins to flatten (or applanate) along its top surface 36, thereby causing responsive portion 34 of wafer 30 to deflect slightly inward 38.
- the blood pressure within radial artery 26 changes (i.e.
- tissue stress transducer 20 is capable of sensing the intra-arterial pressure of radial artery 26.
- diode array 82 is arranged such that each diode 46 in the array of diodes 82 is generally arranged in a straight row substantially parallel to a long side 92 of electronic substrate 50.
- each receiver 48 in the array of receivers 84 is generally arranged in a straight row which is substantially parallel to a long side 92 of electronic substrate 50.
- Row of diodes 46 is spaced apart from the row of receivers 48 and each diode 46 is juxtaposed with two receivers 48 such that it lies generally equidistant from its two closest receivers 48.
- sensor head 40 is electronically coupled via multiple communication lines 98 to sensor base portion 42.
- Sensor base portion 42 provides conversion circuitry 100 to convert the current output signals from the array of detectors 84 to voltage output signals. These voltage signals are sent through multiplexer 102 where they are selectively digitized by A/D converter 104 and passed along to microprocessor 106.
- Microprocessor 106 performs the error correction spoken of earlier in the application and can also perform various other data compilation/analysis tasks .
- the blood pressure data can then be sent to any number of outputs such as a digital to analog converter 108 in cases where an analog representation of blood pressure is desirable. Blood pressure data may also be sent to display device 110 where it can provide the user with a continuously updated digital readout of blood pressure.
- Microprocessor 106 can be programmed to control decoding logic circuitry 112 which in turn activates select power circuits within multiplexing and power circuits 102.
- Power control circuit 118 can be used to interface microprocessor 106 to any number of mechanical actuators 120 which may be used to respond to various commands from microprocessor 106 in the utilization of sensor 40. For example, a routine may be used by microprocessor 106 which periodically queries whether sensor head 40 is properly applanating the artery of interest. If it is determined that the artery of interest is not properly applanated by wafer 30, microprocessor 106 may activate power control circuit 118 to command actuator 120 to move sensor 20 such that it properly applanates the artery of interest. Other applications may be devised where it is desirable to move, or otherwise control sensor head 20.
- sensor head 40 is comprised of a continuous responsive diaphragm portion 34 which reflects light from diodes 46(a-n) and onto receivers 48(a-n).
- Each diode 46 is fed by current source typified at 122 which can be selectively switched on and off via a respective switch 124(a-n).
- These switches 124a through 124n are all individually controlled via decoding logic circuit 112. This is the fundamental mechanism whereby each diode 46a through 46n can be selectively activated to determine what portion of diaphragm 34 is best suited to be used to transduce the tissue stress signal.
- Each receiver 48a through 48n receives a portion of the light reflected from diaphragm 34 and converts this reflected light into an electrical current signal which is converted to a voltage by each receiver's respective converter 126a through 126n.
- Converters 126a through 126n are configured as current to voltage converters which effect a linear current-to-voltage conversion of the current signal derived from the respective receiver.
- Current-to-voltage converter circuits are well known to those skilled in the art and, accordingly, will not be discussed in detail here.
- the output of each converter is made available to its respective switch 128a through 128n. Switches 128a through 128n are controlled via decoding logic 112 which enables microprocessor 106 to select any output from converter 126a through 126n and place it on cable 31 where it is digitized by A/D converter 104.
- One detector 48' is adapted to receive light 130 which is reflected from nonresponsive portion 32 of wafer 30. Detector 48' is used to generate a reference signal which will be used by microprocessor 106 to compensate for offset and gain errors due to temperature, aging and other environmental factors.
- the characteristic contour of the contact stress waveform generated by any one of the receivers 48a-48e will exhibit the following characteristics; a point of maximum (or systolic stress) 150 which corresponds to a peak or systolic blood pressure within artery 26, and a point of minimum (diastolic) stress 152 which corresponds to the diastolic blood pressure within artery 26.
- Mean stress 154 and pulse amplitude stress 156 are mathematically computed based on the following formula:
- contact stress can be plotted as a function of time (as depicted in Figure 7), it can also be plotted as a function of distance along the length of the stress sensitive element 34 (as shown in Figure 8).
- the characteristic contact stress curve of Figure 7 represented the stress sensed at location 3 (referenced at 158 in Figure 8)
- the characteristic points of systolic stress 150, diastolic stress 152, mean stress 154, and pulse amplitude stress 156 of Figure 7 would correspond to the similarly marked points in Figure 8.
- a contact stress curve resembling that of Figure 8 would result.
- the stress information present in Figure 8 is used in conjunction with the methodologies set forth hereinafter to determine optimum arterial applanation.
- a typical tonometric technique for monitoring blood pressure involves positioning a transducer over artery of interest 26 wherein the transducer is pressed against tissue overlying the artery so as to flatten (or applanate) the top surface 36 of artery 26.
- the transducer may comprise a stress sensitive element 34 which, in turn, may be comprised of a plurality of individual stress sensitive elements or a single, continuous stress sensitive element which is capable of sensing stress along overlapping portions of its length.
- the stress sensitive element is designed such that it is able to detect (and distinguish between) stresses created along its length.
- the portion of the stress sensitive element which is typically selected for monitoring blood pressure is that portion which is centered over the artery inasmuch as this portion provides an accurate measure of intra-arterial blood pressure.
- the portions of the stress sensitive elements which do not directly overlie the artery of interest do not provide as accurate a measure of intra-arterial blood pressure as the output from the centered portion.
- hold down pressure is defined as the pressure applied against the pressure transducer as the transducer is forced against the tissue overlying the artery of interest. It is Applicant's theory that the techniques taught in the prior art are improperly focused and accordingly may not produce results as accurate as the methodologies disclosed herein. Specifically, while hold down pressure is a parameter which may loosely correlate to artery applanation state, it is believed that there are a number of parameters which may perform this function much better.
- the methodologies disclosed herein set forth a number of applanation state parameters (ASP) which are believed to provide a superior measure (or indication) of applanation state.
- ASP applanation state parameters
- FIGS 3 and 9A - 9E when stress sensitive element 34 is not in contact with top surface 28 of tissue 24, opening (or lumen) 37 of artery 26 maintains a generally circular cross-section (see Figure 9A).
- Figure 9B - 9E depict various stages of deformation of artery 26 as downward displacement 45 increases.
- Figure 9B when downward displacement 45 is small lumen 37 of artery 26 is generally elliptical.
- the top surface 36 of artery 26 assumes a generally planar orientation.
- applanation means 29 can be accomplished in a step-wise fashion or in a continuously varying ashion.
- systolic, diastolic, pulsatile, and waveform mean contact stresses are derived as functions of position along the stress sensitive element (see Figure 8) and also as functions of applanation state.
- one or more optimum applanation methodologies are utilized for determining the optimum applanation compression level for intra-arterial blood pressure estimation 204. Once the optimum arterial compression level is determined, certain portions of the data which was collected during the optimum applanation level are selected for blood pressure estimation 206.
- Those selected sample points of contact stress data are then used for estimating intra-arterial blood pressure 208.
- This disclosure focuses upon methods used to determine optimum arterial compression 204 and methods used for selecting optimum sampling points along the stress sensitive element once the optimum arterial compression level has been found 206.
- Applanation Optimization Parameters are parameters which provide guidance in selecting the optimum amount of artery applanation.
- the Applanation State Parameters (ASP) are parameters which indicate the degree to which the artery has been flattened or distorted as it is acted upon by tissue stress sensor 20.
- the Applanation Optimization Parameter AOP is a function of the Applanation State Parameter AOP(ASP).
- one or more AOP(ASP) are used for determining the "best" or optimum artery applanation state.
- Each method disclosed herein generally operates by adjusting a selected ASP until a preferred or optimum AOP(ASP) is found. For example, in one method which is disclosed herein in detail, when AOP (ASP) equals 1.00, preferred conditions exist for estimating intra-arterial blood pressure based upon collected contact stress data.
- An example of an Applanation State Parameter would be simply monitoring the displacement which is applied against the stress sensor as it is displaced against the tissue overlying the artery of interest. For example, a displacement of 10 mills (one mill is equal to one-one thousandth of an inch) may receive an Applanation State Parameter value of 1, 20 mills equals an Applanation State Parameter value of 2, etc.
- Another method of deriving Applanation State Parameters is simply to measure the force against tissue stress sensor 20 (see Figure 2) as it is displaced into tissue 24 by moving means (or bellows) 29.
- Still another applanation state parameter may be derived by calculating the average contact stress across the entire length of the stress sensitive element. This method may include applanating an artery to a first state and then, while held in that state, calculating the average contact stress across the entire length of the stress sensitive element. Mathematically, this method is express as follows:
- AAS 1 First Artery Applanation State
- AASI 1 First Artery Applanation State Index
- applanation state parameters which are believed to be unique measures or indicators of the degrees or state of artery applanation.
- applanation state parameters either individually or combined
- a composite indicator representing state of artery applanation is a key in forming functional relationships which are used in the methodologies to determine optimum arterial applanation.
- tissue diastolic contact stress (across full length of stress sensitive element) divided by tissue diastolic contact stress at maximum tissue pulsatile stress location, or (2) Average tissue diastolic contact stress in passive tissue regions (remote from vessel) divided by average tissue diastolic contact stress in active tissue regions (over vessel), or
- AVERAGE DIASTOLIC CONTACT STRESS (STRESS COLLECTED OVER ENTIRE LENGTH OF STRESS SENSITIVE ELEMENT).
- D. NORMALIZED OR DIMENSIONLESS AVERAGE DIASTOLIC CONTACT STRESS FACTOR (STRESS COLLECTED IN PASSIVE REGIONS REMOTE FROM VESSEL).
- Ratio of the index computer by method H above at applanation state of interest to that index method computed at applanation state for maximum pulsatile contact stress (2) Ratio of index computed by method H above at applanation level or interest to index H above computed at any particular characteristic applanation state selected for the normalization process.
- POSITION OF DRIVING OR POSITION ADVANCEMENT MECHANISM EITHER HAND OR MOTOR ACTUATED, TURNED, OR DRIVEN
- PULSATILE STRESS PARAMETER At any given state of applanation, it is a measure of the spatial average (or weighted a spatial average) change in stress between systole and diastole [pulse stress] in the region of the sensor receiving maximum pulse energy.
- a measure of the spatial uniformity of the waveform mean stress distribution profile over the length of the stress sensitive element (normalized to the most pulsatily energetic region (s) of the stress sensitive element).
- DSP ⁇ DCSMAX - ⁇ DCSMIN within the pulsatily energetic region.
- MSP ⁇ MCSMAX - ⁇ MCSMIN within the pulsatily energetic region.
- MCPAR MEAN CURVATURE PARAMETER
- Method 1 The pulsatile stress parameter reaches an optimum fraction of its maximum value.
- Method 2 The mean distribution breadth parameter reaches an optimum value.
- Method 3 The diastolic distribution breadth parameter reaches an optimum value.
- Method 4 The pulse distribution breadth parameter reaches an optimum value.
- Method 5 The incremental change in pulse distribution breadth parameter reaches an optimum value.
- Method 6 The derivative of the pulse spread parameter reaches an optimum value.
- Method 7 The derivative of the pulse distribution breadth parameter reaches an optimum value.
- Method 8 The derivative of the diastolic distribution breadth parameter reaches an optimum value.
- Method 9 Second derivative of spatially averaged stress parameters reaches an optimum value.
- Method 10 Second derivative of stress spatial curvature parameters reaches an optimum value.
- Method 11 The derivative of the stress spread parameters or the derivative of the stress deviation parameter reaches an optimum value.
- Method 12 Two or more methods are selected from Methods 1 through 11 and combined to form additional methodologies for estimating optimum arterial applanation.
- Pulsatile Stress Parameter is defined as the average difference between the systolic contact stress ⁇ SCS (x) and diastolic contact stress ⁇ DCS (x) in the region or regions of the stress sensitive element having the greatest pulse energy content.
- PPAR is defined as follows:
- FIG. 11 A graphical representation of the method of calculating PPAR is shown in Figure 11. It is important to note that the PPAR parameter is calculated between bounds b and c. Bounds b and c represent the region having the greatest pulse energy content. Method of determining the bounds for the region of greatest pulse energy content are found later in this disclosure under the subheading Methods of Determining Limits of Integration When Calculating PPAR.
- Method 1 includes the following steps:
- the artery applanation control mechanism 29 (see Figure 2) to adjust the state of artery applanation through a broad range of applanation states (the applanation states are represented by the applanation state numbers shown in Figure 13) while acquiring contact stress data (as depicted in Figure 11) at various applanation states. 2) At each applanation state, computing PPAR and computing ASP.
- the preferred ASP for Method 1 is either displacement (as set forth in paragraphs C and D in the previous discussion dealing with applanation state parameters) or the average diastolic contact stress computed as follows:
- the optimum applanation state is found by first increasing the arterial applanation until the PPAR reaches a first maximum PPARmax 1 and then diminishes by a specified fraction of the maximum value. Next, the applanation is reduced, and typically, PPAR will increase temporarily to a second maximum PPARmax 2 . Upon further reduction of applanation, when PPAR reaches approximately 95% of the second maximum, conditions are met for estimation of true arterial blood pressure. This process is shown in the graph of PPAR(ASN) found in Figure 13. Alternatively, the estimation can be made at other points including the interval prior to PPARmax 1 in which applanation is increasing.
- stress data is collected during the interval of increasing applanation as well as during the interval of decreasing applanation.
- stress data collected during either or both of the intervals may be used for computing the applanation optimization parameter as well as the applanation state parameter
- the preferred method is to use the stress data which is collected during decreasing applanation. This is the preferred method because it is believed that stress data collected during the decreasing applanation interval more closely predicts the actual intra-arterial blood pressure than that collected during the increasing applanation interval.
- the decreasing applanation interval will be the preferred interval for all methods (Methods 1-11) disclosed herein.
- a preferred method of determining the exact optimum fraction is to use empirically collected data in which tonometric versus automatic cuff or invasive blood pressure values are statistically correlated.
- the preferred fraction may also vary depending on certain factors such as whether applanation is increasing/decreasing, sex (and age) of person being examined, etc. Initial studies indicate that results are more uniform when applanation is decreasing and therefore it is the preferred mode when collecting contact stress data.
- a preferred method for determining the limits of integration employs the concept of energy transfer. This concept is based on the theory that the energy coupling between the artery of interest and the contact stress element is greatest in the immediate vicinity of the artery of interest. The boundaries of this high energy region are used to define the integration limits (b, c). Thus, one can determine the limits of integration (b, c) by determining which portion (or portions) of the stress sensitive element is in receipt of the maximum contact stress energy. This method uses the square of the contact stress values to obtain a measure of contact stress energy and thereby construct a relationship between contact stress energy and position along the length of the stress sensitive element.
- the first method for determining the limits over which the centroid of a selected function will be computed includes using only those regions of the select function which exceed an arbitrarily selected threshold fraction of the maximum value of the function. For example, applying this method to the contact stress energy function as set out in Figure 14, first, maximum 164 is determined and then a predetermined portion of the maximum is taken. Suppose, for example, that 50 percent of maximum 164 will serve as the threshold fraction. This fraction intersects the contact stress energy function at points 166 and
- the second method of determining limits (b, c) includes using selected portions of greatest magnitude of the contact stress energy function that have a cumulative total length equal to a predetermined percentage of the total length of the stress sensitive element. This method can be easily explained in conjunction with Figures 6 and 14. As seen in Figure 6, sensing diode 48b is capable of sensing deflections along stress sensitive element 34 along regions or portions 167,
- each output value 170 through 192 corresponds to one or more portions along stress sensitive element 34.
- n the number of contact stress energy values selected when the cumulative predetermined segment lengths (as totaled in step 3) exceed a predetermined percentage of the length of the stress sensitive element.
- this method may produce selected regions which are noncontiguous. Nonetheless, the disclosed method is applied identically regardless of whether the regions are contiguous or non-contiguous.
- the above two methods for determining limits of integration have been discussed in the context of calculating the PPAR parameter, they are equally applicable to methods 2 through 12 discussed hereinafter. Because the above two methods of determining the limits of integration are executed the same, regardless of which method is used, they will not be discussed in conjunction with methods 2 through 12.
- the systolic and diastolic contact stress contours which correspond to the optimized PPAR parameter are analyzed to determine the best physical location (or locations) along the length of the stress sensitive element from which intra-arterial blood pressure may be estimated.
- a yet smaller fraction 212 (typically one-half of the width of subregion 210) is determined by finding the subregion within region 210 having the smallest diastolic contact stress ⁇ DCS (x).
- the diastolic contact stress point 214 and the systolic contact stress point 216 corresponding to subregion 212 is then used as the estimate of intra-arterial blood pressure systole and diastole points respectively.
- Technique B estimates the intra-arterial systolic and diastolic blood pressure points to be the average diastolic contact stress ⁇ DCSAVG and the average systolic contact stress ⁇ SCSAVG respectively over the interval bounded by b, c.
- the mathematical formula for computing ⁇ DCSAVG is as follows:
- the first interpolation technique ⁇ Interpolation Technique A ⁇ is the preferred interpolation technique used in conjunction with Estimating Technique A.
- the second disclosed interpolating technique ⁇ Interpolation Technique B ⁇ is the preferred interpolation technique used in conjunction with Estimating Technique B.
- contact stress data must be acquired for various compression levels over the entire length of the stress sensitive element 224, 226.
- systolic and diastolic stresses are collected over a pre-determined number of heart beats 226.
- the PPAR is computed 228 as a function of a selected ASP.
- a preferred ASP to be used with the PPAR parameter is the diastolic stress averaged over the entire length of the stress sensitive element 230 (see Section C under the earlier disclosed Section entitled, Preferred Applanation State Parameters).
- the PPAR is computed and optimized 232, and if PPAR OPT falls between two applanation states 220, 222 (as shown in Figure 17), an interpolation factor 236 is computed as follows:
- interpolation factor 236 is computed, the selected optimum region 208, 208' for each of the known bounding adjacent compression levels is selected and the corresponding systolic 216, 216' and diastolic 214, 214' values are computed as has already been disclosed in conjunction with Estimating Technique A. Finally, the optimum systolic 246 and diastolic 248 pressures are estimated at the estimated optimum compression level 242 by applying the following formula:
- SYS OPT SYS 8 + f ⁇ (SYS 9 - SYS 8 )
- DIA OPT DIA 8 + f ⁇ (DIA 9 - SDI 8 )
- Interpolation Technique B is completely analogous to Interpolation Technique A, the only point of difference being that methodology 238, 240 shown in the flow diagram of Figure 18 is altered to reflect the application of Estimating Technique B which has already been discussed in detail under the Section of this disclosure entitled Technique B. Specifically, instead of selecting the highest pulse energy subregion having the greatest energy and lowest diastolic stress 240, Interpolation Technique B calls for determining points 216, 216', 214, 214' by computing the average systolic and diastolic stress over the interval bounded by b, c. With the exception of replacing Estimating Technique A with Estimating Technique B, Interpolation Technique B is used and applied identically to the teaching of Estimating Technique A.
- Interpolation Techniques A and B have been presented in the context of Estimating intra-arterial blood pressure after applying the optimization methodology of Method 1, they are equally applicable to optimization methodologies 2-12 and their application to those methodologies is directly analogous to that which has been shown in connection with Method 1 (the optimization of PPAR parameter). For that reason, the application of Interpolation Techniques A and B .will not be detailed in conjunction with the disclosure of methodologies 2-12.
- the PPAR is defined as the average pulse stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- the Theory Behind the Method At a slightly reduced applanation state than that which causes PPAR MAX , localized arterial wall collapses occurs due to an imbalance between internal arterial pressures and external arterial pressures. The optimum fraction is statistically determined from test data.
- the Mean Distribution Breadth Parameter is a measure of the waveform mean stress distribution profile over the length of the stress sensitive element (normalized to the most pulsatily energetic regions) of the stress sensitive element. It is defined as the ratio of the spatial average waveform mean stress over the entire length of the stress sensitive element to the spatial average waveform mean stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- the optimum value occurs when MDBP is equal to approximately 1.
- the exact optimum value to use is statistically determined from actual test data in which tonometric versus automatic cuff or invasive blood pressure values are correlated.
- Figures 20 and 21 will now be used to explain the implementation of Method 2.
- MDBP mean distribution breadth parameter
- ⁇ MCS (x) is the mean contact stress averaged over n heartbeats and is calculated according to the following formula:
- ⁇ time period of one heartbeat
- n the number of heartbeats selected for time averaging
- a preferred ASP for this method is the diastolic contact stress averaged over the entire length of the stress sensitive element. Mathematically, this ASP is computed as follows:
- ⁇ MCS (x) is the mean contact stress averaged over n heartbeats and is calculated according to the following formula:
- ⁇ time period of one heartbeat
- n the number of heartbeats selected for time averaging
- MDBP is the ratio of the spatial average waveform mean stress over the entire length of the stress sensitive element to the spatial average waveform mean stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- This method utilizes the Diastolic Distribution Breadth Parameter (DDBP) to determine the optimum applanation state of the artery of interest.
- DDBP Diastolic Distribution Breadth Parameter
- the Diastolic Distribution Breadth Parameter DDBP is defined as the ratio of the average diastolic stress over the entire length of the stress sensitive element to the average diastolic stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- the DDBP parameter can be generally thought of as describing the ratio between representative diastolic stresses in the pulsatily inactive region of the stress sensitive element versus the diastolic stresses in the pulsatily active regions of the stress sensitive element.
- Mathematically DDBP is expressed as follows:
- the optimum value of DDBP occurs when DDBP equals 1.05.
- the value to be used in indicating the optimum applanation state is preferably statistically determined by comparing tonometry data and actual intra-arterial blood pressure data.
- the preferred ASP to use in Method 3 is the average diastolic contact stress calculated as follows:
- ⁇ DCSAVG average diastolic stress across the length of the stress sensitive element.
- the DDBP is defined as the ratio of the average diastolic stress over the entire length of the stress sensitive element to the average diastolic stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- This method utilizes the Pulse Distribution Breadth Parameter (PDBP) to determine the optimum applanation state of the artery of interest.
- the PDBP is defined as the number of sampling locations along the stress sensitive element (or cumulative amount of stress sensitive element length) sensing normalized pulse stress values greater than some selected threshold value.
- the PDBP is a measure of the spatial uniformity of the pulse stress distribution profile over the length of the stress sensitive element. It is an indication of the broadening out or widening of the pulsatily active regions of the stress sensitive element with increasing applanation.
- a graphical representation of the method of calculating the PDBP parameter is graphically illustrated in Figure 25.
- PDBP is defined as follows:
- Method 4 estimates the optimum applanation state of the artery of interest by assuming that the optimum value of the applanation state parameter occurs at the mid-point in the range of the maximum value of the pulsatile distribution breadth parameter PDBP (considering only vessel compression levels at or below the level producing the maximum value in the pulsatile parameter (PPAR MAX )).
- PDBP pulsatile distribution breadth parameter
- a variation of this method considers that several versions of PDBP can be computed and used in which the optimum value of the applanation state parameter is a composite of the optimum values of the ASP for each of the several versions of PDBP.
- the composite optimum value of ASP is computed by a centroidal location method using location of individual optimum ASP values in the computation.
- Method 4 when implementing Method 4, first the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state. Then, for each applanation state, the PDBP and ASP are calculated.
- the preferred ASP for ⁇ se in Method 4 i s mean diastolic stress, computed as follows:
- ⁇ DCSAVG average diastolic stress across the length of the stress sensitive element.
- a special function is created PDBP (ASP) between PDBP and ASP.
- a range of applanation state parameter values is established 2402 encompassing the region or plateau of maximum PDBP (occurring at applanation states with less applanation than that for PPAR MAX ).
- the optimum applanation state parameter occurs at the mid-point ASP value established in the previous step.
- the above referenced methodology can be used using several versions of PDBP (each having a different threshold value for computation).
- the overall optimum applanation state parameter is the combined result of the individually computed optimum ASP values.
- W TH cumulative width at ⁇ PCSTHR
- ⁇ DCSAVG average diastolic stress across the length of the stress sensitive element.
- the PDBP parameter is defined as the cumulative width W TH in a region (or multiple non-contiguous regions) above a predetermined pulsatile contact stress threshold value.
- Applanation state parameter is optimum at the mid-point in the range of the maximum value of the PDBP (considering only the range of applanation levels below that for which PPAR MAX occurs). Mid-point in PDBP maximum plateau range occurs with localized vessel wall collapse due to localized internal versus external pressure balance.
- PDBP pulse distribution breadth parameter
- ASP applanation state parameter
- This method compares incremental changes in the Pulse Distribution Breadth Parameter (for approximately equal changes or steps in ASP).
- the Pulse Distribution Breadth Parameter is a measure of the spatial uniformity of the pulse stress distribution profile over the length of the stress sensitive element. It is defined as a number of sampling locations along the stress sensitive element (or cumulative amount of length along the stress sensitive element) having normalized pulse stress values greater than some selected threshold value. It is an indication of the broadening out or widening of the pulsatively active regions of the stress sensitive element with increasing applanation.
- the graphic illustration of the calculation of PDBP (for a given applanation state) is shown in Figure 26. Mathematically, PDBP is defined as follows: where:
- W TH cumulative width at ⁇ PCSTHR
- ⁇ PCSTHR predetermined threshold value of pulsatile contact stress
- ⁇ PDBP is calculated as follows:
- ⁇ PDBP(i) W TH (i) - W TH (i+1)
- ⁇ PDBP(i) change in pulse distribution breadth parameter for the ith applanation state
- W TH (i) cumulative width at ⁇ PCSTHR for the ith applanation state
- W TH (i+1) cumulative width at ⁇ PCSTHR for the i+1 applanation state
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 to a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state.
- the PDBP is calculated along with a preferred ASP.
- a special function is created wherein PDBP is a function of the preferred ASP.
- a special function ⁇ PDBP(ASP) is created.
- the optimum applanation point occurs when ⁇ PDBP is maximum. From the function ⁇ PDBP(ASP), find the optimum value of the ASP corresponding to:
- a preferred mode of calculating the ⁇ PDBP parameter involves implementing a criteria for ignoring ⁇ PDBP calculations which are conducted at high applanation states and low applanation states. By ignoring ⁇ PDBP values at these extreme states, it has been found that more reliable predictions of optimum applanation state are possible. Implementing this type of high/low criteria is not only preferable in Method 5, but is also preferable in any of the following methods (Method 6-11) which employ the use of a difference (or delta ⁇ ) function, a first derivative function, or a second derivative function.
- W TH cumulative width at ⁇ PCSTHR
- ⁇ DCSAVG average diastolic stress across the length of the stress sensitive element.
- ⁇ PDBP is defined as a measurement of incremental changes in the PDBP for approximate equal changes or steps in the ASP.
- PDBP is a measure of the spatial uniformity of the pulse distribution profile over the length of the stress sensitive element. It is defined as the number of sampling locations along the stress sensitive element (or cumulative amount of stress sensitive element length) having normalized pulse stress values greater than some selected threshold value.
- the optimum value of the applanation parameter occurs when the increment change ⁇ PDBP is a maximum assuming the range in applanation state has been covered with approximately equal applanation increments.
- the largest value of the change in ⁇ PDBP occurs with localized vessel wall collapse due to localized internal versus external pressure balance.
- PSP Pulse Spread Parameter
- PSP ⁇ PCSMAX - ⁇ PCSENG
- ⁇ PCSENG is set equal to either ⁇ PCSb or ⁇ PCSc , which ever is the lesser.
- Method 6 estimates the optimum applanation state of the artery of interest by computing the first derivative of PSP with respect to a selected ASP.
- PSP'(ASF) is a maximum
- the optimum arterial applanation state occurs.
- the first derivative operation will be represented hereinafter with an apostrophe after the function it relates to.
- the first derivative of the PCP(ASP) function is represented as PSP'(ASP).
- the second derivative will be represented with a double apostrophe PSP''(ASP).
- Method 6 when implementing Method 6, first the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state. Then, for each applanation state, the PSP and ASP are calculated.
- the preferred ASP for use in Method 6 is mean diastolic stress, computed as follows:
- PSP(ASP) a function is created between PSP and ASP and a new function is computed PSP'(ASP).
- the optimum applanation state is defined to be that state of artery applanation which occurs when PSP'(ASP) is a maximum. From the function PSP'(ASP), the optimum value of the ASP is found according to the following formula:
- PSP ⁇ PCSMAX - ⁇ PCSENG where ⁇ PCSENG equals either ⁇ PCSB or ⁇ PCSC which ever is the lesser.
- Preferred ASP
- the pulse spread parameter is defined as the maximum spread (or deviation) in pulse stress occurring in a region (or multiple non-contiguous regions) of the stress sensitive element selected as having the greatest pulse energy content.
- the applanation state of artery 26 is changed over a broad range of arterial applanation states while acquiring contact stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state.
- This method utilizes the Pulse Distribution Breadth Parameter (PDBP) and compares the first derivative of PDBP with a selected Applanation State Parameter (ASP).
- PDBP Pulse Distribution Breadth Parameter
- ASP Applanation State Parameter
- PDBP is a measure of the spatial uniformity of the pulse stress distribution profile over the entire length of the stress sensitive element. It indicates the broadening out (or widening) of the pulsatily active regions of the diaphragm with change in applanation state. For a given applanation state, it is calculated according to the following formula:
- W TH cumulative width at threshold ⁇ PCSTHR along normalized plot of pulsatile contact stress ⁇ PCSNOR (x)
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state.
- the PDBP and ASP are calculated.
- the preferred ASP for use in Method 7 is mean diastolic stress computed as follows:
- W TH cumulative width at threshold ⁇ PCSTHR along normalized plot of pulsatile contact stress ⁇ PCSNOR (x)
- the PDBP is defined as the number of sampling locations along the stress sensitive element (or cumulative amount of segment lengths across the stress sensitive element) having normalized pulse stress values greater than a preselected threshold.
- the applanation state of artery 26 is changed to over a broad range of arterial applanation states while acquiring stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state.
- This method utilizes the Diastolic Distribution Breadth Parameter (DDBP) to determine the optimum applanation state of the artery of interest.
- the DDBP is defined as the ratio of the average diastolic stress over the entire length of the stress sensitive element to the average diastolic stress in a localized region of the stress sensitive element containing the maximum pulse energy.
- the DDBP is a measure of the spatial uniformity of the diastolic stress distribution profile over the diaphragm length (normalized to the pulsatily energetic region(s) of the stress sensitive element).
- DDBP can be thought of as the relationship between representative diastolic stresses in the pulsatily inactive versus active regions of the stress sensitive element.
- a graphical representation of the method of calculating DDBP for a given applanation state is disclosed in Figure 33. Mathematically, DDBP is defined (at any applanation state) as follows:
- Method 8 estimates the optimum applanation state of the artery of interest by computing the first derivative of DDBP(ASP). Thus, when DDBP '(ASP) is a maximum, the optimum arterial applanation state occurs. The implementation of Method 8 will now be discussed in conjunction with Figures 32 and 33.
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32 at each applanation state).
- the DDBP and ASP are calculated.
- the preferred ASP for use in Method 8 is mean diastolic stress computed as follows:
- DDBP(ASP) a special function
- DDBP'(ASP) a new function
- the optimum applanation state is defined to be that state of artery applanation which occurs when DDBP '(ASP) is a maximum. From the function DDBP'(ASP), the optimum value of the ASP is found according to the following formula:
- DDBP' a maximum.
- the applanation state of artery 26 is changed over a broad range of arterial applanation states while acquiring contact stress data (spatially distributed along the length of the stress sensitive element 32) at each applanation state.)
- This method utilizes the Spatially Averaged Stress Parameters SASPs to determine the optimum applanation state of the artery of interest.
- SASPs are a group of four parameters each of which are defined as follows: 1).
- Method 9 estimates the optimum applanation state of the artery of interest by computing the second derivative of at least one of the SASPs with respect to the selected ASP. After the second derivative is found (SASP''), the minimum of the second derivative is calculated and the optimum arterial applanation state is defined as being equal to SASP MIN .
- the approach set forth in Method 9 takes advantage of a feature found in the second derivative of the SASP functions. Namely, when the second derivative of the SASP functions is a minimum, this empirically corresponds to the "best applanation point" as it locates the abrupt "knee" in each tonometric parameter function.
- each one of the SASPs demonstrates a change in slope going from a greater slope prior to applanation state number to a lesser slope after applanation state number 5.
- This sharp demarkation in slope across applanation state from greater to lesser slope 5 is a characteristic "knee" region of negative radius.
- Figure 36 amplifies the area of negative radius as seen in Figure 35 to better demonstrate the decreasing slope which occurs in all of the SASPs as they cross the optimum applanation state.
- the abrupt "knee” in these functions is recognized as the region having a localized prominent tight negative radius generally occurring in the neighborhood of (or somewhat below) the applanation state associated with the maximum value of the pulsatile parameter.
- This knee region is the region of change in behavior of contact stress versus applanation state associated with the collapse or buckling of a portion of the arterial wall. This region marks the applanation state where the contact stress over a portion of the arterial wall becomes equilibrated with the arterial internal pressure *
- Figure 37 depicts the second derivative of the SASP functions versus the applanation state. Note the existence of the prominent minima in the second derivative functions and their association with the characteristic knee regions in the SASP parameters shown in Figure 36.
- any one of the four SASPs can be used separately to achieve the determination of optimum applanation state.
- a composite indicator can be formed from two or more of the SASPs and the resulting composite can be used to estimate the optimum applanation state.
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state.
- contact stress data spatialally distributed across the length of the stress sensitive element 32
- each of the four SASPs are calculated along with a preferred ASP.
- the preferred ASP for use in Method 9 is the mean diastolic stress computed as follows:
- the optimum applanation state is defined to be that state of artery applanation which occurs when SASP' '(ASP) is a minimum. From the function SASP''(ASP), the optimum value of the ASP is found according to the following formula:
- the applanation state of artery 26 is changed over a broad range of arterial applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state.
- the four SASP functions are computed along with a preferred ASP.
- This method utilizes the Stress Spatial Curvature Parameters SSCPs to determine the optimum applanation state of the artery of interest.
- the SSCPs are comprised of four parameters defined as follows:
- MCPAR MEAN CURVATURE PARAMETER
- the SSCPs focus on the importance of spatial contours of constituent components of the tissue contact stress distribution along the length of the stress sensitive element and also highlight the changing nature of the spatial contours with respect to applanation state.
- the focus of each of the four SSCPs is the spatial curvature of the tissue contact stress distribution function in the pulsatily active region of the stress sensitive element.
- a graphical representation of the method of calculating the SSCPs for a given applanation state is disclosed in Figure 38. Mathematically, the SSCPs are defined as follows:
- x could be established using any of the centroidal methods disclosed in co-pending U.S. patent application entitled “Method of Determining Which Portion of a Stress Sensor is Best Positioned For Use In Determining Intra-Arterial Blood Pressure”; Serial No. 07/835,635 filed February 3, 1992, which is hereby incorporated by reference.
- the behavior of the SSCPs with changing applanation state is an important ingredient of Methodology 10.
- the SSCPs are defined as a function of a selected ASP.
- Figures 39 and 40 show the SSCFs plotted as a function of an applanation state number.
- Any one of the four listed SSCP parameters can be used to find the optimum applanation state. Additionally, a composite of two or more of the SSCPs can be used in conjunction with one another to provide a composite resulting estimated optimum applanation state. The implementation of Method 10 will now be discussed in conjunction with Figures 38-41.
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state.
- contact stress data spatialally distributed across the length of stress sensitive element 32
- one or more of the SSCPs are calculated along with a corresponding preferred ASP.
- the preferred ASP for use in Method 10 is mean diastolic stress computed as follows:
- SSCP(ASP) SSCP(ASP)
- first derivative of each of the SSCP functions are also computed as a function of ASP.
- the optimum applanation state is defined to be that state of artery applanation which occurs when the first derivative of one or more of the SSCP(ASP)s is a maximum. From the function SSCP '(ASP) the optimum value of the ASP is found according to the following formula:
- the SSCP are defined as the spatial curvature of the tissue contact stress distribution function in the pulsatily active regions of the stress sensitive element.
- the region of "maxima" in the first derivative functions of each of the SSCPs corresponds to an applanation state associated with collapse or buckling of a portion of the artery wall. This occurs at an applanation state where contact stress external to the artery wall becomes equilibrated with the arterial internal pressure.
- the applanation state of artery 26 is changed over a broad range of arterial applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state.
- This method utilizes parameters defined as Stress Variation Parameters SVPARs to determine the optimum applanation state of the artery of interest.
- Stress Variation Parameters SVPARs fall into one of two possible sub-classes ⁇ Stress Spread Parameters (SSPAR) and Stress Deviation Parameters (SDPAR).
- SSPAR Stress Spread Parameters
- SDPAR Stress Deviation Parameters
- This Method is based upon the importance of local deviations existing in constituent components of the tissue contact stress occurring over the pulsatily energetic region of the stress sensitive element. More particularly, Method 11 focuses on the behavior of the contact stress deviations with changing state of applanation as the applanation and control system displaces the sensor into the tissue to create a variety of artery applanation states.
- the SSPAR applanation optimization parameters used to indicate local deviation in the contact stresses are:
- DSP ⁇ DCSMAx - ⁇ DCSMIN within the pulsatily energetic region of the stress sensitive element as defined by bounding limits b,c.
- SSP ⁇ SCSMAx - ⁇ SCSMIN within the pulsatily energetic region of the stress sensitive element as defined by bounding limits b,c.
- MSP ⁇ MCSMAX - ⁇ MCSMIN within the pulsatily energetic region of the stress sensitive element as defined by bounding limits b,c.
- the standard deviation is computed for all the sample points located in the local pulsatily energetic region.
- the SDPAR applanation optimization parameters used to indicate local deviation in the contact stresses are: (1) Pulsatile Deviation Parameter (PDP)
- the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed along the length of stress sensitive element 32 at each applanation state.
- the SSPARs are calculated along with a corresponding preferred ASP.
- the preferred ASP for use in Method 11 is mean diastolic stress computed as follows: ⁇ DCSAVG - J 7 DCS (x) dx
- SSPAR'(ASP) a new function (first derivative) is computed SSPAR'(ASP).
- the optimum applanation state is defined to be that state of artery applanation which occurs when SSPAR'(ASP) is a minimum. From the function SSPAR'(ASP) the optimum value of the ASP is found according to the following formula:
- ⁇ PCSMAX and ⁇ PCSMIN are selected from the pulsatily energetic region.
- ⁇ DCSMAX and ⁇ DCSMIN are selected from the pulsatily energetic region.
- ⁇ SCSMAX and ⁇ SCSMIN are selected from the pulsatily energetic region.
- MSP ⁇ MCSMAX - ⁇ MCSMIN
- ⁇ MCSMAX and ⁇ MCSMIN are selected from the pulsatily energetic region.
- the SSPAR and the SDPAR are measures of the spread or deviation within the contact stress profile over the pulsatily energetic region of the stress sensitive element.
- the applanation state of artery 26 is changed over a broad range of arterial applanation states while acquiring contact stress data (spatially distributed across the length of the stress sensitive element 32) at each applanation state.
- pulsatile spread or deviation parameter PSP or PDP b. diastolic spread or deviation parameter, DSP or DDP c. systolic spread or deviation parameter, SSP or SDP d. mean spread or deviation parameter.
- MSF or MDF mean spread or deviation parameter.
- MDP'(ASP OPT ) MDP' MIN
- SSPARs and the SDPARs closed form mathematical expressions of these functions can be generated using polynomial functions (e.g., fourth or fifth order expressions) derived using best fit (e.g. least squares fit) of the data. These functions can also be expressed in tabular or numerical form. Also, the derivatives can be numerically approximated utilizing difference methods.
- Method 11 should be considered more general than simply the detailed mathematical definitions given above. Method 11 should be considered as encompassing the general concept of using any mathematical description, function, or formula that examines the property of spread or deviation in contact stress profiles occurring over a defined pulsatily energetic region of the stress sensitive element.
- the mathematical definitions defined herein are merely important examples of the general concept.
- the procedure herein described in conjunction with Method 11 can be used independently with any one of the eight listed spread/deviation functions to achieve a resulting optimum applanation point. Additionally, a composite of two or more of the spread/deviation functions may be used to generate an optimum applanation point.
- This method of estimating optimum applanation is based on the concept that a better result is obtained from a consideration and use of some or all of the "best" applanation estimates from the 11 individual methods that have been previously described.
- the best overall estimate of the optimum applanation is a weighted average utilizing results of several of the optimum applanation methods.
- AOPCOM OPT composite value of optimum applanation estimate
- AOP OPT (i) the value of the applanation optimization parameter associated with the ith Method of estimating optimum arterial compression
- Method 12 is preferably as follows. First, the artery applanation control mechanism is used to adjust the applanation state of artery 26 through a broad range of applanation states while acquiring contact stress data (spatially distributed across the length of stress sensitive element 32) at each applanation state. Next, the "best" applanation estimatation methods are selected. For each selected method, using the step by step procedure for estimating the best applanation (as previously described in Method 1-11). For each selected method, choose the appropriate weighting factor to use in computing the combined applanation estimate. Finally, computing the combined estimated optimum applanation point using the combined equation:
- AOPCOM OPT composite value of optimum applanation estimate
- AOP OPT (i) the value of the applanation optimization parameter associated with the ith Method of estimating optimum arterial compression
- Methods 1-12 may be utilized by way of closed form mathematical functions, thereby providing an alternative to the use of the interpolation schemes.
- many of the methodologies have been defined in terms of finding the maximum or the minimum of a particular function and thereby locating the optimum applanation state. It will be understood by those skilled in the art that by simply altering the sign conventions used in constructing the various functions, that a minima on a graph can be mathematically transformed into a maxima and vise-a-versa. Accordingly, it is to be understood that the subject matter sought to be afforded protection hereby should be deemed to extend to the subject matter defined in the appended claims, including all fair equivalents thereof.
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EP93908577A EP0639062A4 (en) | 1992-04-15 | 1993-03-25 | Method of determining optimum arterial applanation. |
JP51835193A JP3457311B2 (en) | 1992-04-15 | 1993-03-25 | Optimal arterial applanation determination device |
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US07/869,553 US5273046A (en) | 1992-04-15 | 1992-04-15 | Method of determining optimum artery applanation |
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EP0625025B1 (en) * | 1992-12-05 | 1997-04-16 | AVL Medical Instruments AG | Sensor and device for measuring blood pressure |
US5908027A (en) * | 1994-08-22 | 1999-06-01 | Alaris Medical Systems, Inc. | Tonometry system for monitoring blood pressure |
KR0165522B1 (en) * | 1996-05-23 | 1999-03-20 | 김광호 | Optimal point detector for non-invasive diagnosis of blood constituents and non-invasive diagnostic device using the same |
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- 1993-03-25 WO PCT/US1993/002798 patent/WO1993020748A1/en not_active Application Discontinuation
- 1993-03-25 JP JP51835193A patent/JP3457311B2/en not_active Expired - Lifetime
- 1993-03-25 EP EP93908577A patent/EP0639062A4/en not_active Withdrawn
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Title |
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Also Published As
Publication number | Publication date |
---|---|
JPH07508431A (en) | 1995-09-21 |
EP0639062A1 (en) | 1995-02-22 |
JP3457311B2 (en) | 2003-10-14 |
US5273046A (en) | 1993-12-28 |
EP0639062A4 (en) | 1998-09-30 |
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