Abstract:
A method for estimating systolic and diastolic pressure is disclosed herein. The method includes obtaining a predetermined type of blood pressure data from a patient, and providing previously acquired blood pressure data obtained from a plurality of different subjects. The method also includes implementing the previously acquired blood pressure data to select systolic and diastolic amplitude ratios that most closely correlate with the predetermined type of blood pressure data obtained from the patient. The selected systolic and diastolic amplitude ratios are adapted to compensate for the effects of arterial compliance. The method also includes implementing the selected systolic and diastolic amplitude ratios to generate a systolic and diastolic blood pressure estimates.

Description:
FIELD OF THE INVENTION 
       [0001]    This disclosure relates generally to a method for non-invasively determining a patient&#39;s blood pressure. 
       BACKGROUND OF THE INVENTION 
       [0002]    An accurate and reliable technique for continuously measuring blood pressure involves inserting a saline filled catheter through the patient&#39;s vascular system to the point at which it is desired to perform the measurements. The catheter is connected to a pressure sensor, which measures the pressure in the vessel. An alternative method uses a catheter with a pressure sensor at the tip that directly senses the blood pressure. Procedures such as these are commonly referred to as “invasive procedures” because they involve making an incision through the patient&#39;s skin and inserting the catheter into a blood vessel. A problem with invasive procedures is that they can cause patient discomfort and increase the risk of complications such as infection. 
         [0003]    Non-invasive blood pressure (NIBP) algorithms typically inflate a pressure cuff above the patient&#39;s systolic pressure and measure oscillations under the cuff as the cuff is deflated either in steps or continuously. The resulting oscillometric envelope is used to determine the patients&#39; blood pressure. The cuff pressure corresponding to the maximum oscillation amplitude is typically taken as the mean arterial pressure (MAP). Systolic and Diastolic pressures are computed using a fixed ratio of the maximum oscillation amplitude. Some NIBP monitors also use the shape of the oscillometric envelope to compute the Systolic and Diastolic pressures. The problem with conventional NIBP techniques is that they do not compensate for arterial compliance changes and are therefore imprecise. 
       BRIEF DESCRIPTION OF THE INVENTION 
       [0004]    The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
         [0005]    In an embodiment, a method for estimating systolic blood pressure and diastolic blood pressure includes obtaining a predetermined type of blood pressure data from a patient, and providing previously acquired blood pressure data obtained from a plurality of different subjects. The previously acquired blood pressure data is adapted to convey the manner in which a systolic amplitude ratio and a diastolic amplitude ratio vary with respect to the predetermined type of blood pressure pulse data obtained from the patient. The method also includes implementing the previously acquired blood pressure data to select a systolic amplitude ratio and a diastolic amplitude ratio that most closely correlate with the predetermined type of blood pressure data obtained from the patient. The selected systolic amplitude ratio and diastolic amplitude ratio are adapted to compensate for the effects of arterial compliance. The method also includes implementing the selected systolic amplitude ratio and the selected diastolic amplitude ratio to generate a systolic blood pressure estimate and a diastolic blood pressure estimate. 
         [0006]    In another embodiment, a method for estimating systolic blood pressure and diastolic blood pressure includes providing a non-invasive blood pressure monitor having a cuff configured to apply a selectable pressure level to a patient. The method also includes estimating a first pulse transit time at a first cuff pressure level, and a second pulse transit time at a second cuff pressure level. The method also includes calculating a pulse transit time ratio, which is defined as the first pulse transit time divided by the second pulse transit time. The method also includes providing blood pressure data adapted to correlate a plurality of pulse transit time ratios with a corresponding plurality of systolic amplitude ratios and diastolic amplitude ratios. The method also includes selecting one of the systolic amplitude ratios and one of the diastolic amplitude ratios that most closely correlate with the calculated pulse transit time ratio. The selected systolic and diastolic amplitude ratios are adapted to compensate for the effects of arterial compliance. The method also includes implementing the selected systolic and diastolic amplitude ratios to generate a systolic blood pressure estimate and a diastolic blood pressure estimate. 
         [0007]    In yet another embodiment, a method for estimating systolic blood pressure and diastolic blood pressure includes estimating a pulse wave velocity of a blood pressure pulse being transmitted through a patient. The method also includes providing blood pressure data adapted to correlate a plurality of pulse wave velocity values with a plurality of systolic amplitude ratios and a plurality of diastolic amplitude ratios. The method also includes selecting one of the plurality of systolic amplitude ratios and one of the plurality of diastolic amplitude ratios that are most closely correlated with the estimated pulse wave velocity. The selected systolic and diastolic amplitude ratios are adapted to compensate for the effects of arterial compliance. The method also includes implementing the selected systolic and diastolic amplitude ratios to generate a systolic blood pressure estimate and a diastolic blood pressure estimate. 
         [0008]    Various other features, objects, and advantages of the invention will be made apparent to those skilled in the art from the accompanying drawings and detailed description thereof. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]      FIG. 1  is a schematic diagram of a patient monitoring system in accordance with an embodiment; 
           [0010]      FIG. 2  is a graph of cuff pressure versus time illustrating a method for estimating blood pressure using a non-invasive blood pressure monitoring system; 
           [0011]      FIG. 3  is a block diagram illustrating a method in accordance with an embodiment; 
           [0012]      FIG. 4  is a block diagram illustrating a method in accordance with an embodiment; 
           [0013]      FIG. 4   a  is a graph of oscillation amplitude versus PTT ratio ; 
           [0014]      FIG. 5  is a block diagram illustrating a method in accordance with an embodiment; 
           [0015]      FIG. 5   a  is a graph of PTT versus cuff pressure; 
           [0016]      FIG. 5   b  is a graph of oscillation amplitude versus PTT slope ; and 
           [0017]      FIG. 6  is a block diagram illustrating a method in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments that may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the embodiments. The following detailed description is, therefore, not to be taken as limiting the scope of the invention. 
         [0019]    Referring to  FIG. 1 , a patient monitoring system  10  is shown in accordance with an embodiment. The patient monitoring system  10  includes a pulse oximeter  12  and a non-invasive blood pressure (NIBP) monitor  14 . The pulse oximeter  12  is connected to a probe  16  that is attachable to a finger  18  of a patient  20 . The pulse oximeter  12  is operable to sense or identify volume pulses referred to hereinafter as SpO2 pulses at the patient&#39;s finger  18 , and to thereafter transmit data pertaining to the SpO2 pulses to a processor  22 . 
         [0020]    The NIBP monitor  14  is connected to an inflatable cuff  24  via a flexible tube  26 . The NIBP monitor  14  includes a pump  28  adapted to inflate the cuff  24 , and one or more valves  30  adapted to deflate the cuff  24 . In the embodiment depicted, the inflatable cuff  24  is wrapped around the patient&#39;s upper arm  32 , however other locations (e.g., forearm) and other limbs could also be used. The NIBP monitor  14  includes a pressure transducer  34  operable to sense or identify pressure pulses referred to hereinafter as NIBP pulses at the portion of the patient&#39;s arm  32  to which the cuff  24  is attached. Thereafter, the NIBP monitor  14  can transmit data pertaining to the NIBP pulses to the processor  22 . 
         [0021]    The NIBP monitor  14  is configured to measure mean arterial pressure (MAP), systolic blood pressure (SBP), and diastolic blood pressure (DBP) in a known manner. With reference to  FIGS. 1 and 2 , a process of measuring MAP, SBP and DBP will be described for exemplary purposes in accordance with one embodiment. 
         [0022]    The exemplary process of measuring MAP, SBP and/or DBP is performed by increasing and decreasing the pressure of the cuff  24  in the manner illustrated by the cuff pressure curve  36  of  FIG. 2 , and generally simultaneously measuring a series of NIBP pulses  38 . This process is initiated by implementing the pump  28  to inflate the cuff  24  and thereby increase cuff  24  pressure to a supra-systolic pressure level. As is known in the art, at supra-systolic cuff pressure blood is completely occluded or obstructed from flowing through the artery under the cuff  24 , systolic pressure is the cuff pressure level at which blood just begins flowing through the artery under the cuff  24 , and diastolic pressure is the cuff pressure level at which blood flow through the artery under the cuff  24  is unobstructed. After cuff  24  pressure is increased to a supra-systolic pressure level, the cuff  24  is deflated (via valve  30 ) in a controlled manner adapted to produce a series of decreasing pressure level steps. It should be appreciated that while the exemplary embodiment has been described and depicted as including a stepwise cuff pressure reduction, other embodiments may alternatively implement a generally continuous cuff pressure reduction. 
         [0023]    After the cuff  24  reaches systolic pressure, the pressure level measured by the pressure transducer  34  oscillates due to the force exerted on the cuff  24  by the entry of blood into the artery under the cuff  24 . The term “oscillation” refers to a measurable pressure level oscillation produced by this change in volume. Two consecutive oscillations are generally measured at each cuff pressure level step. As shown in  FIG. 2 , MAP is identifiable as the cuff pressure level at which oscillation amplitude is maximum (OA max ). SBP is identifiable as the cuff pressure level at which oscillation amplitude is approximately equal to (0.5*(OA max )), and DBP is identifiable as the cuff pressure level at which oscillation amplitude is approximately equal to (0.625*(OA max )). A plurality of SpO2 pulses  40  are also shown in  FIG. 2  to illustrate typical SpO2 data acquired during the previously described cuff inflation/deflation sequence. 
         [0024]    The processor  22  is operable to calculate pulse transit time (PTT) in response to data from the pulse oximeter  12  and the NIBP monitor  14 . For purposes of this disclosure, PTT is defined as the time required for a given pressure pulse to travel from one reference point (e.g., the patient&#39;s arm  32 ) to another reference point (e.g., the patient&#39;s finger  18 ). It will be understood by those skilled in the art that a pressure pulse is accompanied by a volume pulse, which is what is measured by the NIBP cuff  24  and the probe  16 . As an example, if the probe  16  and cuff  24  are attached to the same limb, PTT can be calculated by measuring the time interval between a NIBP pulse and an immediately subsequent SpO2 pulse. PTT can be measured, for example, as the “foot-to-foot delay”, the “peak-to-peak delay”, or the delay between maximum slope points. The “foot-to-foot delay” refers to the time interval measured between the foot of a NIBP pulse and the foot of an immediately subsequent SpO2 pulse. Similarly, the “peak-to-peak delay” refers to the time interval measured between the peak of a NIBP pulse and the peak of an immediately subsequent SpO2 pulse. 
         [0025]      FIG. 3  is flow chart illustrating a method  100  that is also referred to hereinafter as the algorithm  100 . The individual blocks of the flow chart represent steps that may be performed in accordance with the method  100 . Unless otherwise specified, the steps  102 - 110  need not be performed in the order shown. 
         [0026]    Referring now to  FIGS. 1 and 3 , at step  102 , cuff  24  pressure is increased to a supra-systolic pressure level. At step  104 , the cuff  24  pressure is reduced in a controlled manner which may include, for example, a stepwise pressure reduction or a generally continuous pressure reduction. Also at step  104 , while cuff  24  pressure is being reduced, the processor  22  measures PTT. As previously described, PTT can be measured by measuring the time interval between each NIBP pulse and the immediately subsequent SpO2 pulse. 
         [0027]    At step  106 , the algorithm  100  determines whether the current cuff  24  pressure value is below diastolic pressure. This determination can be made by comparing a current cuff  24  pressure value measured by the pressure transducer  34  with the calculated DBP value. The DBP value can be calculated using a baseline amplitude ratio that is not adjusted for pulse transit time such as, for example, the previously described DPB amplitude ratio of 0.625, or can alternatively be calculated in any other known manner. If, at step  106 , the current cuff  24  pressure is not below diastolic pressure, the algorithm  100  returns to step  104 . If, at step  106 , the current cuff  24  pressure is below diastolic pressure, the algorithm  100  proceeds to step  108 . 
         [0028]    At step  108 , cuff  24  pressure is reduced. If cuff  24  pressure is being reduced in a stepwise manner, the cuff  24  pressure is further reduced by one step. If cuff  24  pressure is being reduced in a generally continuous manner, the cuff  24  pressure is further reduced in a continuous manner by 10 mm Hg. At step  110 , the processor  22  measures PTT. The PTT measurement of step  110  is taken at a sub-diastolic pressure level. 
         [0029]    Referring to  FIG. 4 , a flow chart illustrates a method  200  adapted for use in combination with the method  100  (shown in  FIG. 3 ) to precisely estimate SBP and DBP. The method  200  may also be referred to hereinafter as the algorithm  200 . The individual blocks of the flow chart represent steps that may be performed in accordance with the method  200 . Unless otherwise specified, the steps  202 - 208  need not be performed in the order shown. 
         [0030]    At step  202 , PTT ratio  is calculated according to the equation PTT ratio =(PTT MAP /PTT subdias ). The variable PTT MAP  represents the pulse transit time measured at the mean arterial pressure level, and is acquired by the processor  22  (shown in  FIG. 1 ) at step  104  of the algorithm  100  (shown in  FIG. 3 ) in the manner previously described. The variable PTT subdias  represents the pulse transit time measured at a sub-diastolic pressure level, and is acquired by the processor  22  at step  110  of the algorithm  100  in the manner previously described. 
         [0031]    At step  204 , previously acquired blood pressure data is provided. The previously acquired blood pressure data generally represents multiple blood pressure measurements taken in a known manner (e.g., via intra-arterial, oscillometric and/or auscultatory procedures) from a plurality of different individuals. The previously acquired blood pressure data is preferably provided in a format adapted to correlate PTT ratio  with systolic and diastolic amplitude ratios. As an example, the previously acquired blood pressure data may be provided in the form of a graph as depicted in  FIG. 4   a , however it should be appreciated that the data may alternatively be provided in any known format including, for example, a look-up table, a spreadsheet or a database. 
         [0032]    Referring to  FIG. 4   a , a graph of oscillation amplitude versus PTT ratio  is shown to illustrate a method for compiling previously acquired blood pressure data in accordance with step  204  of the algorithm  200  (shown in  FIG. 4 ). PTT ratio  may be calculated, for example, in accordance with the previously provided equation PTT ratio =(PTT MAP /PTT subdias ). The graph of  FIG. 4   a  can be generated by calculating SBP amplitude ratio, DBP amplitude ratio and PTT ratio  values for each of the previously acquired blood pressure measurements. Thereafter, a SBP data point  210  having (X, Y) coordinate values of (PTT ratio , SBP amplitude ratio), and a DBP data point  212  having (X, Y) coordinate values of (PTT ratio , DBP amplitude ratio) are plotted for each previously acquired blood pressure measurement. An SBP best-fit line  214  is calculated for the SBP data points  210  and a DBP best-fit line  216  is calculated for the DBP data points  212 . The process of calculating a “best-fit line” is well known mathematical process and therefore will not be described in detail. While a linear fit is shown in  FIG. 4   a , the data might also be fitted to a polynomial, exponential or other curvilinear function. 
         [0033]    A non-limiting example will now be provided to better illustrate the previously described method for generating the graph of  FIG. 4   a . For purposes of this example, assume that the previously acquired blood pressure of a single test subject was intra-arterially measured, and that this test subject was determined to have a PTT of 95 milliseconds at MAP, a PTT of 70 milliseconds at a sub-diastolic pressure level, a systolic oscillation amplitude ratio of 0.475, and a diastolic oscillation amplitude ratio of 0.610. The “systolic oscillation amplitude ratio” refers to the test subject&#39;s oscillation amplitude at SBP divided by their oscillation amplitude at MAP, and the “diastolic oscillation amplitude ratio” refers to the patient&#39;s oscillation amplitude at DBP divided by their oscillation amplitude at MAP. For the exemplary embodiment, PTT ratio  is calculated as (PTT MAP /PTT subdias ) or (95/70)=1.35. Accordingly, the exemplary SBP data point  210   a  having (X, Y) coordinate values of (1.35, 0.475), and the exemplary DBP data point  212   a  having (X, Y) coordinate values of (1.35, 0.610) are plotted as shown in  FIG. 4   a . After plotting SBP data points  210  and DBP data points  212  for each of a plurality of different test subjects in the manner previously described, the SBP best-fit line  214  is calculated for the SBP data points  210  and the DBP best-fit line  216  is calculated for the DBP data points  212 . 
         [0034]    Referring to  FIG. 4 , at step  206  the PTT ratio  value calculated at step  202  is compared with previously acquired blood pressure data of step  204  in order to obtain optimal systolic and diastolic ratios. As a non-limiting example, assume that the PTT ratio  calculated at step  202  is equal to 1.50, and that the previously acquired blood pressure data provided at step  204  is represented by the graph of  FIG. 4   a . For purposes of this non-limiting example, the optimal systolic ratio is 0.480 which is the Y-axis value corresponding to the point of intersection between the X-axis PTT ratio  value (i.e., 1.50) and the SBP best-fit line  214 . Similarly, the optimal diastolic ratio is 0.620 which is the Y-axis value corresponding to the point of intersection between the X-axis PTT ratio  value (i.e., 1.50) and the DBP best-fit line  216 . It should be appreciated that, unlike conventional fixed systolic and diastolic amplitude ratios, the previously described optimal systolic and diastolic amplitude ratios are variable to compensate for the effects of arterial compliance. 
         [0035]    Referring again to  FIG. 4 , at step  208  the optimal systolic and diastolic ratios that were obtained at step  206  are used to recalculate SBP and DBP. The previously calculated optimal systolic amplitude ratio value 0.480 and optimal diastolic amplitude ratio value 0.620 will again be used for illustrative purposes. Referring to  FIG. 2  and according to the illustrative embodiment, SBP can be recalculated as the cuff pressure level at which NIBP oscillation amplitude is approximately equal to (0.480*(OA max )), and DBP can be recalculated as the cuff pressure level value at which NIBP oscillation amplitude is approximately equal to (0.620*(OA max )). The recalculated SBP and DBP values are generally more accurate than conventional SBP/DBP estimates because the recalculated values are based on optimal systolic and diastolic amplitude ratios selected to compensate for the effects of arterial compliance. 
         [0036]    Referring to  FIG. 5 , a flow chart illustrates a method  300  adapted for use in combination with the method  100  (shown in  FIG. 3 ) to precisely estimate SBP and DBP. The method  300  may also be referred to hereinafter as the algorithm  300 . The individual blocks of the flow chart represent steps that may be performed in accordance with the method  300 . Unless otherwise specified, the steps  302 - 312  need not be performed in the order shown. 
         [0037]    At step  302 , PTT versus cuff pressure data points  314  are plotted as shown in  FIG. 5   a . The data points  314  represent the pulse transit time measured during the process of reducing cuff pressure level, and are obtained from steps  104  and  110  of the algorithm  100  (shown in  FIG. 3 ). At step  304 , a best-fit line  316  (shown in  FIG. 5   a ) is calculated for the data points  314 . At step  306 , PTT slope  is calculated as the slope of the best-fit line  316 . 
         [0038]    At step  308 , previously acquired blood pressure data is provided. The previously acquired blood pressure data generally represents multiple blood pressure measurements taken in a known manner (e.g., via intra-arterial, oscillometric and/or auscultatory procedures) from a plurality of different individuals. The previously acquired blood pressure data is preferably provided in a format adapted to correlate PTT slope  with systolic and diastolic amplitude ratios. As an example, the previously acquired blood pressure data may be provided in the form of a graph as depicted in  FIG. 5   b , however it should be appreciated that the data may alternatively be provided in any known format including, for example, a look-up table, a spreadsheet or a database. 
         [0039]    Referring to  FIG. 5   b , a graph of oscillation amplitude versus PTT slope  is shown to illustrate a method for compiling previously acquired blood pressure data in accordance with step  308  of the algorithm  300  (shown in  FIG. 5 ). The graph of  FIG. 5   b  can be generated by calculating SBP amplitude ratio, DBP amplitude ratio and PTT slope  values for each of the previously acquired blood pressure measurements. Thereafter, a SBP data point  318  having (X, Y) coordinate values of (PTT slope , SBP amplitude ratio), and a DBP data point  320  having (X, Y) coordinate values of (PTT slope , DBP amplitude ratio) are plotted for each previously acquired blood pressure measurement. An SBP best-fit line  322  is calculated for the SBP data points  318  and a DBP best-fit line  324  is calculated for the DBP data points  320 . While a linear fit is shown in  FIG. 5   b , the data might also be fitted to a polynomial, exponential or other curvilinear function. 
         [0040]    Referring to  FIG. 5 , at step  310  the PTT slope  value calculated at step  306  is compared with previously acquired blood pressure data of step  308  in order to obtain optimal systolic and diastolic ratios. According to the embodiment wherein the blood pressure data is complied in the form of a graph, the optimal systolic ratio is the Y-axis value corresponding to the point of intersection between the X-axis PTT slope  value (obtained at step  306 ) and the SBP best-fit line  322  (shown in  FIG. 5   b ). Similarly, the optimal diastolic ratio is the Y-axis value corresponding to the point of intersection between the X-axis PTT slope  value (obtained at step  306 ) and the DBP best-fit line  324  (shown in  FIG. 5   b ). At step  312  the optimal systolic and diastolic ratios are used to recalculate SBP and DBP in a manner similar to that previously described with respect to step  208  of the algorithm  200  (shown in  FIG. 4 ). The recalculated SBP and DBP values are generally more accurate than conventional SBP/DBP estimates because the recalculated values are based on optimal systolic and diastolic amplitude ratios selected to compensate for the effects of arterial compliance. 
         [0041]    Referring to  FIG. 6 , a flow chart illustrates a method  400  adapted for use in combination with the method  100  (shown in  FIG. 3 ) to precisely estimate SBP and DBP. The method  400  may also be referred to hereinafter as the algorithm  400 . The individual blocks of the flow chart represent steps that may be performed in accordance with the method  400 . Unless otherwise specified, the steps  402 - 410  need not be performed in the order shown. 
         [0042]    At step  402 , the distance D (shown in  FIG. 1 ) between the cuff  24  and the probe  16  is estimated. The distance D can be estimated in any known manner such as, for example, by physically measuring this distance along the arm  32  of the patient  20  (shown in  FIG. 1 ). At step  404 , pulse wave velocity (PWV) is calculated according to, the equation PWV=D/PTT. PTT values for this calculation can be obtained at steps  104  and/or  108  of the algorithm  100  (shown in  FIG. 3 ). 
         [0043]    At step  406 , previously acquired blood pressure data is provided. The previously acquired blood pressure data generally represents multiple blood pressure measurements taken in a known manner (e.g., via intra-arterial, oscillometric and/or auscultatory procedures) from a plurality of different individuals. The previously acquired blood pressure data is preferably provided in a format adapted to correlate PWV with systolic and diastolic amplitude ratios. The form in which the blood pressure data is provided may include, for example, a look-up table, a spreadsheet, a graph or a database. 
         [0044]    At step  408 , the PWV value calculated at step  404  is compared with previously acquired blood pressure data of step  406  in order to obtain optimal systolic and diastolic ratios. According to an illustrative embodiment wherein the blood pressure data is complied in the form of a look-up table (not shown), the optimal systolic and diastolic ratios are obtainable by indexing the previously acquired systolic and diastolic ratios that most closely corresponds to the PWV value calculated at step  404 . At step  410  the optimal systolic and diastolic ratios are used to recalculate SBP and DBP in a manner similar to that previously described with respect to step  208  of the algorithm  200  (shown in  FIG. 4 ). The recalculated SBP and DBP values are generally more accurate than conventional SBP/DBP estimates because the recalculated values are based on optimal systolic and diastolic amplitude ratios selected to compensate for the effects of arterial compliance. 
         [0045]    While the invention has been described with reference to preferred embodiments, those skilled in the art will appreciate that certain substitutions, alterations and omissions may be made to the embodiments without departing from the spirit of the invention. Accordingly, the foregoing description is meant to be exemplary only, and should not limit the scope of the invention as set forth in the following claims.