Abstract:
A non-invasive blood pressure system is disclosed herein. The non-invasive blood pressure system includes a pressure transducer configured to obtain pressure data comprising a transient baseline effects component. The non-invasive blood pressure system also includes a processor adapted to receive the pressure data from the pressure transducer. The processor is configured to generate a transient baseline effects model, and to implement the transient baseline effects model to at least partially remove the transient baseline effects component of the pressure data. The removal of the transient baseline effects component from the pressure data eliminates a potential source of error and thereby enables a more accurate blood pressure estimate.

Description:
BACKGROUND OF THE INVENTION 
     The subject matter disclosed herein relates to a system and method for a non-invasive blood pressure monitor. More specifically, the subject matter disclosed herein relates to a system and method for a non-invasive blood pressure monitor that is configured to more accurately estimate one or more blood pressure parameters. 
     Non-invasive blood pressure (NIBP) monitors typically inflate a pressure cuff above the patient&#39;s systolic pressure and measure very small amplitude pressure oscillations within the cuff as the cuff is deflated either in steps or continuously. The pressure oscillations in the cuff are due to volume oscillations resulting from the heart beating and pumping blood through the arterial system. The size of the cuff pressure oscillations changes as the cuff pressure itself changes. The data set which describes the cuff oscillation size as a function of the cuff pressure is commonly known as the oscillometric envelope. The resulting oscillometric envelope obtained from the cuff pressure data is used to determine the patient&#39;s 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 by finding the cuff pressure levels at which a fixed ratio of the maximum oscillation amplitude occurs. Some NIBP monitors also use details in the shape of the oscillometric envelope to compute the Systolic and Diastolic pressures. 
     The cuff pressure data can, in some cases, contain various types of artifacts that may hinder the ability of the NIBP device to estimate blood pressure values accurately. Two primary classes of artifacts are patient motion and transient baseline effects. Conventional NIBP techniques are not capable of handling these artifact problems effectively and this can often introduce imprecision into the blood pressure estimates, and may also result in longer determination times which can be uncomfortable to the patient. Transient baseline effects are well known to those skilled in the art, and may include such phenomena as the heating and cooling of the air within the cuff, the visco-elastic effects of the cuff material which influence the time needed to reach pressure-volume equilibrium, and physiological changes in fluid and tissue volume under the cuff. 
     BRIEF DESCRIPTION OF THE INVENTION 
     The above-mentioned shortcomings, disadvantages and problems are addressed herein which will be understood by reading and understanding the following specification. 
     In an embodiment, a non-invasive blood pressure system includes a pressure transducer configured to obtain pressure data comprising a transient baseline effects component. The non-invasive blood pressure system also includes a processor adapted to receive the pressure data from the pressure transducer. The processor is configured to generate a transient baseline effects model, and to implement the transient baseline effects model to at least partially remove the transient baseline effects component of the pressure data. The removal of the transient baseline effects component from the pressure data eliminates a potential source of error and thereby enables a more accurate blood pressure estimate. 
     In another embodiment, a non-invasive blood pressure system includes an inflatable cuff, and a pressure transducer pneumatically coupled with the inflatable cuff. The pressure transducer is configured to obtain pressure data comprising an oscillatory component and a transient baseline effects component. The non-invasive blood pressure system also includes a processor adapted to receive the pressure data from the pressure transducer. The processor is configured to generate a non-linear transient baseline effects model, and to implement the non-linear transient baseline effects model to at least partially remove the transient baseline effects component of the pressure data such that substantially only the oscillatory component remains. The processor is further configured to estimate a blood pressure parameter based on the oscillatory component of the pressure data. The non-invasive blood pressure system also includes a display configured to visually convey the estimated blood pressure parameter. The removal of the transient baseline effects component from the pressure data eliminates a potential source of error and thereby improves the accuracy of the estimated blood pressure parameter. 
     In another embodiment, a method includes implementing a pressure transducer and an inflatable cuff to obtain pressure data comprising an oscillatory component and a transient baseline effects component, and implementing a processor to generate a non-linear transient baseline effects model based on a plurality of data points acquired from a plot of the pressure data. The method also includes implementing the transient baseline effects model to remove at least a portion of the transient baseline effects component from the pressure data such that substantially only the oscillatory component remains, and implementing the oscillatory component of the pressure data to estimate a blood pressure parameter. The removal of the transient baseline effects component from the pressure data eliminates a potential source of error and thereby enables a more accurate estimate of the blood pressure parameter. 
     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 
         FIG. 1  is a schematic representation of a non-invasive blood pressure system in accordance with an embodiment; 
         FIG. 2  is a graph of cuff pressure versus time illustrating a method for estimating a blood pressure parameter; 
         FIG. 3  is an exemplary plot of pressure data comprising an oscillatory component and a transient baseline effects component; and 
         FIG. 4  is the plot of pressure data from  FIG. 3  and a transient baseline effects model in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     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. 
     Referring to  FIG. 1 , a non-invasive blood pressure (NIBP) system  10  is shown in accordance with an embodiment. The NIBP system  10  includes an inflatable cuff  12 , a processor  14 , a pressure transducer  16 , an inflation valve  18 , a source of pressurized air  20 , a deflation valve  22  and a display  24 . In the embodiment depicted, the inflatable cuff  12  is wrapped around the patient&#39;s upper arm  26 , however other locations (e.g., forearm, wrist, finger) and other limbs could also be used. The inflatable cuff  12  is pneumatically coupled with the pressure transducer  16 , the inflation valve  18  and the deflation valve  22  via a flexible tube  28 . The processor  14  is electronically coupled with the pressure transducer  16 , the inflation valve  18 , the deflation valve  22  and the display  24 . 
     The processor  14  is configured to coordinate the operation of valves  18 ,  22  in a manner adapted to regulate cuff  12  inflation and deflation. More precisely, the processor  14  can selectively open the inflation valve  18  in order to allow the source of pressurized air  20  to inflate the cuff  12 , and selectively open the deflation valve  22  to release the pressurized air and thereby deflate the cuff  12 . The pressure transducer  16  is configured to sense or identify pressure pulses referred to hereinafter as NIBP pulses at the portion of the patient&#39;s arm  26  to which the cuff  12  is attached. Thereafter, the pressure transducer  16  can transmit pressure data comprising data pertaining to the NIBP pulses to the processor  14 . 
     The processor  14  is configured to estimate a blood pressure parameter such as mean arterial pressure (MAP), systolic blood pressure (SBP), and/or diastolic blood pressure (DBP) based on the pressure data from the pressure transducer  16 . The processor  14  is also configured to transmit the estimated MAP, SBP and/or DBP values to the display  24 . The display  24  is configured to visually convey the estimated MAP, SBP and/or DBP values. With reference to  FIGS. 1 and 2 , an exemplary process of estimating MAP, SBP and DBP will be described in accordance with one embodiment. 
     The exemplary process of estimating MAP, SBP and/or DBP is performed by increasing and decreasing the pressure of the cuff  12  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 inflating the cuff  12  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  12 , systolic pressure is the cuff pressure level at which blood just begins flowing through the artery under the cuff  12 , and diastolic pressure is the cuff pressure level at which blood flow through the artery under the cuff  12  is unobstructed. After cuff  12  pressure is increased to a supra-systolic pressure level, the cuff  12  is deflated (via valve  22 ) in a controlled manner adapted to produce a series of decreasing pressure level steps  42 . 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. Additional possible embodiments include other cuff inflation and deflation patterns. 
     After the cuff  12  reaches systolic pressure, the pressure level measured by the pressure transducer  16  oscillates due to the force generated by the entry of blood into the artery under the cuff  12 . 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 to guarantee consistency in the measurement of the pulse properties for that step and thereby reject artifact. 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 )). The actual fractions of OA max  used to find systolic and diastolic estimates depends upon the signal processing implemented and can be further influenced by the experience of those skilled in the art of oscillometry. 
     It should be appreciated that transient baseline effects can introduce imprecision into the blood pressure measurement. More precisely, transient baseline effects can vary the pressure level reading acquired by the pressure transducer  16  at each step  42 , and can also complicate the process of detecting and measuring pulse property details. This pressure level variation correspondingly varies the oscillation amplitude measurements and the resultant MAP, SBP and DBP estimates. In a non-limiting manner, transient baseline effects may include any effect caused by the heating and cooling of the air within the cuff, the visco-elastic properties of the cuff material, and any physiological changes in fluid volume or tissue response under the cuff. Transient baseline effects are well known to those skilled in the art and therefore will not be described in further detail. 
     Referring to  FIG. 3 , a plot  50  represents filtered pressure data acquired from the pressure transducer  16  (shown in  FIG. 1 ) during one of the steps  42  (shown in  FIG. 2 ) of the pressure curve  36  (shown in  FIG. 2 ). Filtering is implemented to more clearly identify the constituent parts of the pressure data, and it should be appreciated that there are a variety of filtering methods that may be implemented for this purpose. According to one embodiment, the pressure data may be filtered with an amplified band-pass filter. The depicted portion of the plot  50  comprises an oscillatory component  52  attributable to the patient&#39;s cardiac activity, and a transient baseline effects component  54 . While shown and described hereinafter as producing a steadily increasing pressure level, the transient baseline effects component  54  may alternatively cause the pressure level to decrease during some of the steps  42  of a blood pressure determination. 
     It should be appreciated that the transient baseline effects component  54  of the plot  50  is responsible for introducing imprecision into the MAP, SBP and DBP estimates since it may cause uncertainty in the estimation of the pulse amplitude and corrupt the oscillometric envelope data. Accordingly, as will be described in detail hereinafter, the transient baseline effects component  54  may be modeled or otherwise approximated and thereafter subtracted from the plot  50  such that substantially only the oscillatory component  52  remains. Thereafter, the oscillatory component  52  may be implemented to estimate MAP, SBP and DBP in a manner that minimizes the imprecision associated with the transient baseline effects. 
     The transient baseline effects component  54  of the plot  50  can be modeled or estimated in a variety of different ways. The following will describe an embodiment wherein the processor  14  (shown in  FIG. 1 ) generates an exponential transient baseline effects model  56  (shown in  FIG. 4 ) as an estimate of the transient baseline effects component  54  of the plot  50 . It should, however, be appreciated that the transient baseline effects component  54  may alternatively be modeled based on other types of equations such as, for example, a polynomial based model. 
     Referring to  FIG. 4 , the transient baseline effects model  56  is initially established by implementing the processor  14  (shown in  FIG. 1 ) to identify points (t 1 , p 1 ) and (t 2 , p 2 ) that are common to both the plot  50  and the transient baseline effects model  56 . According to one embodiment, the points (t 1 , p 1 ) and (t 2 , p 2 ) of the transient baseline effects model  56  are defined as consecutive relative minimum values of the plot  50 . The selection of relative minimum values is convenient in that they are generally easy to identify, however, it should be appreciated that any other consistent phase point (e.g., relative maximum values) of the plot  50  may alternatively be implemented. 
     After identifying the points (t 1 , p 1 ) and (t 2 , p 2 ), the remainder of the transient baseline effects model  56  can be solved by the processor  14  (shown in  FIG. 1 ) using the following exponential equation: P t =Ae −(t−t     1     )/τ +B. The variable P t  represents the pressure of the transient baseline effects model  56  at time t, the variable A represents the magnitude of the change of the transient baseline effects model  56 , the variable B represents the equilibrium or steady state pressure of the transient baseline effects model  56 , and the variable τ is a time constant. Experimentation based on a variety of different transient baseline effects models has revealed that the time constant τ is generally approximately equal to one second. Alternate embodiments may generate a transient baseline effects model based on any line or curve incorporating the points (t 1 , p 1 ) and (t 2 , p 2 ). 
     The exponential equation P t =Ae −(t−t     1     )/τ +B can be solved by deriving two equations in the following manner. The first derived equation is obtained by evaluating the exponential equation at time t 1  such that t=t 1  and P t =p 1 , and solving for B. The first derived equation becomes: B =p 1 −A. The second derived equation is obtained by evaluating the exponential equation at time t 2  such that t=t 2  and P t =p 2 , replacing the variable B with the quantity (p 1 −A) per the first derived equation, and solving for A. The second derived equation becomes A=(p 1 −p 2 )/(1−e −(t     2     −t     1     )/τ ). The two derived equations can be algebraically solved by setting the time constant τ equal to one second and inputting relative minimum values from the plot  50  for points (t 1 , p 1 ) and (t 2 , p 2 ). This yields two equations and two unknowns such that the variables A and B can be solved. Alternatively, the variables A, B and τ can all be solved for by obtaining a third relative minimum point (t 3 , p 3 ) from the plot  50 , and evaluating the exponential equation at time t 3  to derive a third equation, such that there are three equations and three unknowns. 
     After solving for or otherwise obtaining values for A, B and τ, the exponential equation P t =Ae −(t−t     1     )/τ +B is solvable to completely define the transient baseline effects model  56 . Thereafter, the complete transient baseline effects model  56  or any portion thereof can be removed (e.g., subtracted) from the pressure data of plot  50  in order to eliminate or at least minimize the imprecision introduced by the transient baseline effects. It should be appreciated that the elimination or minimization of the transient baseline effects imprecision in the manner described provides a more accurate estimate of MAP, SBP, and DBP. 
     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.