Abstract:
A method for non-invasively estimating blood pressure is disclosed herein. The method includes inflating a cuff and collecting first oscillation amplitude data at a first plurality of cuff pressure levels while inflating the cuff. The method also includes deflating the cuff and collecting second oscillation amplitude data at a second plurality of cuff pressure levels while deflating the cuff. The method also includes fitting a curve to the first oscillation amplitude data and to the second oscillation amplitude data and estimating a blood pressure parameter based on the curve. A non-invasive blood pressure system is also disclosed.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The subject matter disclosed herein relates to a method and system for non-invasive blood pressure determination. 
         [0002]    Human heart muscles periodically contract, forcing blood through the arteries. As a result of this pumping action, pressure pulses exist in these arteries and cause them to cyclically change volume. The minimum pressure for these pulses during a cardiac cycle is known as the diastolic pressure and the peak pressure is known as the systolic pressure. A further pressure parameter, known as the “mean arterial pressure” (MAP), represents a time-weighted average of the blood pressure. Blood pressure parameters such as systolic pressure, MAP and diastolic pressure for a patient are useful in monitoring the cardiovascular state of the patient, and in treating disease. 
         [0003]    A conventional method of measuring blood pressure is referred to as oscillometry. Typically, the measurement of blood pressure by oscillometry requires the inflation of a cuff to a cuff pressure level above the patient&#39;s systolic pressure to fully occlude the artery. Blood pressure is then determined by measuring an oscillation amplitude value at multiple cuff pressure levels during the deflation of the cuff. The patient&#39;s systolic pressure is not known during the initial inflation. One problem with the conventional method is that the cuff may be inflated to an unnecessarily high cuff pressure level since the patient&#39;s systolic pressure is not known during the initial inflation of the cuff. This may lead to patient discomfort. Another problem with the conventional method is that if the initial cuff pressure level is too low, it may be necessary to inflate the cuff to a higher pressure level as part of one or more subsequent steps. If subsequent steps are required to reach a correct initial cuff pressure level, this causes the blood pressure measurement to take longer than necessary. Additionally, the extra time needed to inflate the cuff to the right cuff pressure level may also lead to patient discomfort. 
       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 non-invasively estimating blood pressure includes inflating a cuff and collecting first oscillation amplitude data at a first plurality of cuff pressure levels while inflating the cuff. The method also includes deflating the cuff and collecting second oscillation amplitude data at a second plurality of cuff pressure levels while deflating the cuff. The method also includes fitting a curve to the first oscillation amplitude data and to the second oscillation amplitude data and estimating a blood pressure parameter based on the curve. 
         [0006]    In another embodiment, a method for non-invasively estimating blood pressure includes inflating a cuff and collecting first oscillation amplitude data at a first plurality of cuff pressure levels while inflating the cuff. The method also includes deflating the cuff and collecting second oscillation amplitude data at a second plurality of cuff pressure levels while deflating the cuff. At least one of the second plurality of cuff pressure levels differs from each of the first plurality of cuff pressure levels. The method also includes fitting a curve to the first oscillation amplitude data and to the second oscillation amplitude data and estimating a blood pressure parameter based on the curve. 
         [0007]    In another embodiment, a system for non-invasively estimating a blood pressure parameter includes a cuff, a source of pressurized gas attached to the cuff, a deflation valve attached to the cuff, and a transducer attached to the cuff. The transducer is configured to acquire a first plurality of oscillation amplitude values while the cuff is inflated by the source of pressurized gas and the transducer is further configured to acquire a second plurality of oscillation amplitude values while the cuff is deflated via the deflation valve. The system also includes a controller attached to the cuff, the transducer, the source of pressurized gas, and the deflation valve. The controller is configured to fit a curve to the first plurality of oscillation amplitude values and to the second plurality of oscillation amplitude values and estimate a blood pressure parameter based on the curve. 
         [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 illustrating a non-invasive blood pressure system in accordance with an embodiment; 
           [0010]      FIG. 2  is a flow chart illustrating a method in accordance with an embodiment; 
           [0011]      FIG. 3  is a cuff pressure level versus time plot in accordance with an embodiment; and 
           [0012]      FIG. 4  is graphical representation of oscillation amplitude versus cuff pressure level in accordance with an embodiment. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0013]    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. 
         [0014]    Referring to  FIG. 1 , a schematic representation of a non-invasive blood pressure (NIBP) system  10  is shown in accordance with an embodiment. The NIBP system  10  includes a cuff  12  disposed about an arm  13  of a patient  14 . The cuff  12  comprises one or more inflatable bladders (not shown) that can be selectively filled with air. While the cuff  12  is depicted around the arm  13  of the patient  14  in this embodiment, it should be appreciated that the cuff  12  could also be disposed about a leg or any other limb. 
         [0015]    A transducer  16  is attached to the cuff  12  and configured to obtain a cuff pressure signal. The cuff pressure signal is used to determine measurements of a cuff pressure level and an oscillation amplitude. For the purposes of this disclosure, the “cuff pressure level” is defined to include a lower frequency portion of the cuff pressure signal, while the “oscillation amplitude” is defined to include the amplitude of a higher frequency portion of the cuff pressure signal that varies with the expansion and contraction of the patient&#39;s arteries. Both cuff pressure level and oscillation amplitude are well-known values in the oscillometric field. 
         [0016]    A source of pressurized gas  18  is connected to the cuff  12  in a manner that allows gas to travel into the cuff  12  to increase the cuff pressure level. A deflation valve  20  is also connected to the cuff  12  and the deflation valve  20  functions to selectively lower the cuff pressure level. A controller  22  is operatively connected to the transducer  16 , the source of pressurized gas  18 , the deflation valve  20 , and a display  24 . The controller  22  is configured to regulate the source of pressurized gas  18  and the deflation valve  20  in order to attain a desired cuff pressure level. The display  24  is attached to the controller  20  and is adapted to display a blood pressure parameter as will be discussed in detail hereinafter. 
         [0017]      FIG. 2  is a flow chart illustrating a method  200  in accordance with an embodiment. The individual blocks  202 - 222  of the flow chart represent steps that may be performed in accordance with the method  200 . The technical effect of method  200  is the estimation of blood pressure parameters based on oscillation amplitude data collected both during an inflation process of the cuff  12  (shown in  FIG. 1 ) and during a deflation process of the cuff  12 . Steps  202 - 222  of the method  200  need not be performed in the order shown. 
         [0018]    Referring to  FIGS. 1 and 2 , at step  202 , the cuff  12  is inflated to a predetermined cuff pressure level. The controller  22  controls the source of pressurized gas  18  and the deflation valve  20  in a manner adapted to bring the cuff pressure level close to the predetermined cuff pressure level. The predetermined cuff pressure level may be based on data from a previous estimation of the patient&#39;s blood pressure, it may be based on empirical data, or it may be manually set by an operator. 
         [0019]    At step  204 , oscillation amplitude data is collected at the predetermined cuff pressure level of step  202 . For the purposes of this disclosure, it should be understood that oscillation amplitude data may comprise an oscillation amplitude value or a plurality of oscillation amplitude values. 
         [0020]    At step  206 , the cuff  12  is inflated to a higher cuff pressure level. At step  208 , oscillation amplitude data is collected at the higher cuff pressure level of step  206 . At step  210 , after the oscillation amplitude data has been collected, the controller  22  determines if oscillation amplitude data from a higher cuff pressure level is required. This determination may be made by comparing the number of cuff pressure levels from which oscillation amplitude data has been collected to a previously determined number of cuff pressure levels needed for a specific blood pressure estimation. It should be appreciated by those skilled in the art that additional methods of determining if oscillation amplitude data from a higher cuff pressure level is required may also be employed at step  210 . 
         [0021]    If oscillation amplitude data from a higher cuff pressure level is required at step  210 , the method  200  returns back to step  206 , where the cuff  12  is inflated to a higher cuff pressure level. As the method  200  iteratively cycles through steps  206  through  210 , the cuff pressure level is increased in either a stepwise or a continuous manner. For the purposes of this disclosure, “increased in a stepwise manner” is defined to include an inflation process where the cuff pressure level is increased in steps and where the cuff pressure level is maintained at a generally constant value at times when the oscillation amplitude data is collected. For the purposes of this disclosure, “increased in a continuous manner” is defined to include an inflation process where the cuff pressure level is continuously increased while oscillation amplitude data is collected. It should be appreciated by those skilled in the art that “increased in a continuous manner” includes methods employing both a generally constant rate of inflation of the cuff  12  and a variable rate of inflation of the cuff  12 . It should also be appreciated that it may be possible to adaptively change either the rate of inflation or the size of the steps between cuff pressure levels depending upon the oscillation amplitude data collected during the steps  202 - 210 . 
         [0022]    By collecting data while increasing the cuff pressure level according to steps  206 - 210 , it is also possible to ensure that the cuff  12  is not inflated to an unnecessarily high cuff pressure level. For example, if the NIBP system  10  is no longer obtaining oscillation amplitude values because the patient&#39;s  14  artery is fully occluded, it may not be necessary to inflate the cuff  12  to a higher cuff pressure level. 
         [0023]    If oscillation amplitude data from a higher cuff pressure level is not required at step  210 , the method  200  proceeds to step  212 . At step  212 , the controller  22  controls the deflation valve  20  to deflate the cuff  12  to a lower cuff pressure level. At step  214 , oscillation amplitude data is collected from the lower cuff pressure level. The collection of oscillation amplitude data at steps  204 ,  208 , and  214  will be described in accordance with an illustrative embodiment wherein the collection of oscillation amplitude data comprises collecting a single oscillation amplitude value at each cuff pressure level. It should, however, be appreciated that in alternate embodiments, the collection of oscillation amplitude data at steps  204 ,  208 , and  214  may comprise collecting multiple oscillation amplitude values at each cuff pressure level. The collection of multiple oscillation amplitude values at each cuff pressure level may be implemented to provide a type of quality check. For example, if multiple oscillation amplitude values are collected at a single cuff pressure level, a comparison between the multiple oscillation amplitude values may be conducted. If the multiple oscillation amplitude values collected at a single cuff pressure level are found to vary by more than a predetermined amount, this may be a sign that one or more of the oscillation amplitude values contains an artifact. According to an embodiment, it may be desirable to collect additional oscillation amplitude values at a specific cuff pressure level if the multiple oscillation amplitude values collected at a single cuff pressure level are found to vary by more than the predetermined amount. According to an embodiment, the controller  22  may control the deflation of the cuff  12  during step  212  in a manner so that the cuff pressure level where oscillation amplitude data is collected at step  214  is different from the cuff pressure levels where oscillation amplitude data was collected at steps  204  and  208 . 
         [0024]    At step  216 , the controller  22  determines if oscillation amplitude data from a lower cuff pressure level is required. This determination may, for example, be made by calculating if oscillation amplitude data from enough cuff pressure levels has been collected at steps  204 ,  208 , and  214  in order to complete an oscillometric envelope for a specific blood pressure estimation as is known by those skilled in the art. If oscillation amplitude data from an additional lower cuff pressure level is needed to complete the oscillometric envelope, then the method  200  returns back to step  212 , where the cuff  12  is deflated to a lower cuff pressure level. It should be appreciated that additional methods of determining if oscillation amplitude data from a lower cuff pressure level is required may also be employed. As the method  200  iteratively cycles through steps  212  through  216 , the cuff pressure level is decreased in either a stepwise manner or a continuous manner. For the purposes of this disclosure, “decreased in a stepwise manner” is defined to include a deflation process where the cuff pressure level is decreased in steps and where the cuff pressure level is maintained at a generally constant value at times when the oscillation amplitude data is collected. For the purposes of this disclosure, “decreased in a continuous manner” is defined to include a deflation process where the cuff pressure level is continuously decreased while oscillation amplitude data is collected. It should be appreciated by those skilled in the art that “decreased in a continuous manner” includes methods employing both a generally constant rate of deflation of the cuff  12  and a variable rate of deflation of the cuff  12 . It should also be appreciated that it may be possible to adaptively change either the rate of deflation or the size of the steps between cuff pressure levels. It should be understood that it may also be possible to use additional patterns of inflation and deflation in order to most efficiently acquire oscillation amplitude data. 
         [0025]    Referring to  FIGS. 2 and 3 , a cuff pressure level versus time plot  38  of steps  202 - 216  of an embodiment of the method  200  is shown. From a time T 10  until a time T 12 , the cuff pressure level is increased to a predetermined cuff pressure level, graphically represented by cuff pressure level P 18 , according to step  202 . According to an embodiment, the cuff pressure level is generally maintained at the cuff pressure level P 18  for a period of time as indicated by the generally horizontal portion of the graph from the time T 12  to a time T 14 . Between the time T 12  and the time T 14 , oscillation amplitude data is collected according to step  204 . From the time T 14  until a time T 16 , the cuff pressure level is increased according to steps  206  through  210 . According to an embodiment, the collection of oscillation amplitude data at step  208  takes place at times when the cuff pressure level is generally constant, such as at cuff pressure levels P 20 , P 30 , P 40 , and P 50 . 
         [0026]    From the time T 16  until a time T 18 , the cuff pressure level is decreased according to the steps  212 - 216 . According to an embodiment, the collection of oscillation amplitude data at step  214  takes place at times when the cuff pressure level is generally constant. In the embodiment depicted in  FIG. 3 , the decreasing cuff pressure levels (i.e. the cuff pressure levels P 60 , P 70 , P 80 , P 90 , and P 100 ) are all distinct from the increasing cuff pressure levels (i.e. the cuff pressure levels P 18 , P 20 , P 30 , P 40 , and P 50 ). An example of this is depicted by the cuff pressure level P 60 , which is shown as being between the cuff pressure level P 50  and the cuff pressure level P 40 . It should be understood that in additional embodiments some or all of the decreasing cuff pressure levels may be the same as the increasing cuff pressure levels. 
         [0027]    After oscillation amplitude data is collected for the cuff pressure level P 100 , the controller  22  (shown in  FIG. 1 ) determines that no additional deflation steps are required in accordance with an exemplary embodiment. Since additional oscillation amplitude data is not required, the controller  22  stops collecting data for the patient  14  (shown in  FIG. 1 ). If oscillation amplitude data from a lower cuff pressure level is not required at step  216 , the method  200  proceeds to step  218 . 
         [0028]    Referring to  FIGS. 2 and 4 , at step  218  a curve  30  is fit to the oscillation amplitude data collected at steps  204 ,  208 , and  214 . Fitting a curve is a well known mathematical technique and may comprise fitting one of the following nonlimiting list of functions to the oscillation amplitude data: linear, piecewise linear, quadratic, exponential, and Gaussian. Different functions used for the curve may be chosen depending upon the preferred properties of the blood pressure estimation and the amount of oscillation amplitude data available. 
         [0029]    At step  220 , one or more blood pressure parameters of the patient  14  (shown in  FIG. 1 ) are estimated based on the curve  30 . For this exemplary embodiment, the oscillation amplitude data comprises oscillation amplitude values  31  that are depicted as bars on a graphical representation  26  of oscillation amplitude versus cuff pressure level. The curve  30  is implemented to determine an estimate of a mean arterial pressure  32  of the patient  14 . The mean arterial pressure  32  may be estimated by implementing the curve  30  to find the cuff pressure level where the curve  30  reaches a maximum oscillation amplitude  33 . Once an estimate of the mean arterial pressure  32  has been made, estimates of a diastolic pressure  34  and a systolic pressure  36  may be made based on well-known relationships of the diastolic pressure  34  compared to the mean arterial pressure  32  and the systolic pressure  36  compared to the mean arterial pressure  32 . According to an exemplary embodiment, the diastolic pressure  34  may be estimated by finding the cuff pressure level below the mean arterial pressure  32  where the ratio of the oscillation amplitude at the diastolic pressure  34  to the oscillation amplitude at the mean arterial pressure  32  equals a first established value, typically chosen to be between 0.4 and 1.0. A point  38  represents the location on the curve  30  where the ratio of the oscillation amplitude at the diastolic pressure  34  to the oscillation amplitude at the mean arterial pressure  32  equals a first established value. According to an exemplary embodiment, the systolic pressure  36  may be estimated by finding the cuff pressure level above the mean arterial pressure  32  where the ratio of the oscillation amplitude at the systolic pressure  36  to the oscillation amplitude at the mean arterial pressure  32  equals a second established value, typically chosen to be between 0.4 and 1.0. A point  40  represents the location on the curve  30  where the ratio of the oscillation amplitude at the systolic pressure  36  to the oscillation amplitude at the mean arterial pressure  32  equals a second established value. While the mean arterial pressure  32 , the systolic pressure  36 , and the diastolic pressure  34  are examples of blood pressure parameters, it should be understood that it would be possible to use the curve  30  to estimate other blood pressure parameters as well. 
         [0030]    Referring to  FIG. 2 , at step  222 , the blood pressure parameters estimated at step  220  are displayed. It is within the scope of this invention for the blood pressure parameters to be displayed as a numeral, a graph, or any other form that conveys the blood pressure parameters to an observer. 
         [0031]    This written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.