Abstract:
A system and method are provided for using an oximeter to take blood pressure readings for an extended period of time. Calibration of the oximeter for this purpose requires use of a sphygmomanometer to determine a sequence of blood pressure readings taken for a patient over a sphygmomanometer duty cycle. During the duty cycle, readings for both blood pressure (sphygmomanometer) and blood flow amplitude (oximeter) are taken simultaneously at predetermined time intervals (e.g. patient pulse rate). These readings then determine an operational ratio between the two that can be used to translate pulse magnitude readings of the oximeter for presentation as blood pressure readings. Operationally, variations from the patient&#39;s systolic pressure can then be continuously monitored in real time.

Description:
[0001]    This application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/867,005, filed Aug. 16, 2013. The entire contents of Application Ser. No. 61/867,005 are hereby incorporated by reference herein. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention pertains to systems and methods for continuously monitoring the blood pressure of a patient over an extended period of time. More particularly, the present invention pertains to systems and methods wherein a patient&#39;s blood flow, as measured by an oximeter, is evaluated in terms of blood pressure readings. The present invention is particularly, but not exclusively, useful for systems and methods wherein the incremental changes in blood pressure, that are measured by a sphygmomanometer during a duty cycle of the sphygmomanometer, are correlated with changes in pulse amplitude as measured by an oximeter during the same duty cycle, for subsequent use of the oximeter in measuring a patient&#39;s blood pressure. 
       BACKGROUND OF THE INVENTION 
       [0003]    An ability to continuously monitor the blood pressure of a patient over an extended period of time is clinically beneficial for several reasons. At present, the most commonly accepted methodology for measuring a patient&#39;s blood pressure involves the use of a sphygmomanometer. In its use, a sphygmomanometer will provide blood pressure pulse measurements during its duty cycle that include a systolic measurement and a diastolic measurement. In detail, the systolic measurement provides a blood pressure reading for the phase of the patient&#39;s heartbeat when the heart muscle contracts and pumps blood from the chambers into the arteries. On the other hand, the diastolic measurement provides a blood pressure reading for the phase of the heartbeat when the heart muscle relaxes and allows the chambers of the heart to fill with blood. Typically, these measurements are referenced together and evaluated as systolic/diastolic. Although a sphygmomanometer is both accurate and reliable, its use can be cumbersome. Consequently, the repetitive use of a sphygmomanometer to obtain continuous readings over an extended period of time may be problematic. 
         [0004]    Apart from the sphygmomanometer, an oximeter is a well-known and commonly used device for measuring blood pulse amplitudes. Specifically, an oximeter is typically used to monitor a patient&#39;s pulse rate. To do this, a sensor is merely clamped onto the finger of a patient and the oximeter is thereafter capable of continuously monitoring blood flow pulse amplitudes. This can be done for an extended period of time, without interruption. 
         [0005]    With the above in mind, several general considerations are helpful for an appreciation of the present invention. These considerations, which are all patient specific, include:
       A patient&#39;s diastolic pressure will remain substantially constant during a stabilized condition. On the other hand, the systolic pressure will vary most significantly.   Physiologically, absent an anomaly, the impedance to blood flow in a patient&#39;s cardiovascular system will generally remain substantially constant over an extended period of time.   Pulse amplitude signals taken by an oximeter are directly proportional to blood flow level.       
 
         [0009]    In light of the above, it is an object of the present invention to provide a system and a method for continuously monitoring blood flow in the vasculature of a patient. Another object of the present invention is to provide a system and method for using blood pressure pulse measurements, taken by a sphygmomanometer, to calibrate an oximeter for subsequent use in monitoring the blood pressure of a patient. Still another object of the present invention is to provide a system and method for simultaneously monitoring blood pressure and blood flow pulse amplitudes over an extended period of time which is easy to use, simple to implement and comparatively cost effective. 
       SUMMARY OF THE INVENTION 
       [0010]    In accordance with the present invention, a system and method are provided to continuously monitor blood flow in a patient for an extended period of time. In particular, this monitoring is accomplished using a conventional oximeter as the sensor, and using a sphygmomanometer to periodically calibrate the oximeter. As envisioned for the present invention, after the oximeter has been calibrated it can be employed to continuously generate blood flow pulse amplitude signals that are indicative of blood pressures generated by the patient&#39;s heart beat. 
         [0011]    For purposes of the present invention, a calibration of the oximeter begins by first connecting both the oximeter and the sphygmomanometer to the patient. In this combination, the sphygmomanometer is used for measuring a blood pressure pulse magnitude p s  for each pulse of the patient&#39;s heart. Simultaneously, the oximeter is used for measuring a blood flow pulse amplitude p o . Both measurements are taken contemporaneously during a sphygmomanometer duty cycle which extends between a systolic pressure p s(systolic)  and a diastolic pressure p s(diastolic)  of the patient. 
         [0012]    During the sphygmomanometer duty cycle that is used for calibrating the oximeter, the respective magnitude and amplitude measurements for p s0  (sphygmomanometer) and p o  (oximeter) are received as input at a computer. After completion of the duty cycle, these measurements are used by the computer to establish an operational ratio, p o /p s , that is based on contemporary measurements of p s  and p o  during the duty cycle. In detail, the operational ratio, p o /p s , is preferably established as follows. For an n number of pulses during the sphygmomanometer duty cycle, successively different blood pressure magnitude measurements p sn  are taken by the sphygmomanometer for each pulse (heart beat). Simultaneously, corresponding blood flow amplitude measurements p on  are also taken by the oximeter. An average change in blood pressure pulse magnitude Δp s  [Δp s =(Σ Δp sn )n] is then calculated, and it is compared with an average change in pulse amplitude Δp o  [Δp o =(Σ Δp on )n]. The computer then uses the ratio Δp o /Δp s  to establish the operational ratio p o /p s . As will be appreciated by the skilled artisan, conventional curve fitting techniques can be employed in this process. In any event, as implied above, the operational ratio p o /p s  is then used to determine a blood pressure value p s  that is based on pulse amplitudes p o  that are measured in real time. 
         [0013]    In an operation of the present invention, a monitor, which is connected to the computer, is used to continuously compare pulse amplitude signals p o  from the oximeter with the base amplitude p o(base) . Specifically, this comparison is done in real time, to detect variations of p o  from the base amplitude p o(base)  as an indicator of changes in blood flow and, hence, changes in blood pressure. Further, an alarm can be initiated by the computer to indicate whenever a pulse amplitude signal p o  has a maximum/minimum value that differs from the base amplitude p o(base)  by a predetermined value. For instance, these predetermined values can be based on the operational ratio p o /p s  to cause an alarm with a positive change (maximum value) of more than 60 mmHg or a negative change (minimum value) of more than 40 mmHg in blood pressure p s . 
         [0014]    Other aspects of the present invention that are noteworthy include the notion that during a calibration of the oximeter, blood pressure pulse magnitudes p s  and blood flow pulse amplitudes p o  are taken during the sphygmomanometer duty cycle at a same selected point in each pulse of the patient&#39;s heart. Also, the operational ratio Δp o /Δp s  that results from these measurements is always patient specific. Furthermore, the operational ratio Δp o /Δp s  for calibrating the oximeter is preferably recalculated at least every hour. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
           [0016]      FIG. 1  is a schematic depiction of an employment of a system in accordance with the present invention; 
           [0017]      FIG. 2  is a calibration graph showing sphygmomanometer measurements (blood pressure pulse magnitude) and corresponding oximeter measurements (blood flow pulse amplitude) taken at a same time during a sphygmomanometer duty cycle; 
           [0018]      FIG. 3  is a graph showing a relationship between blood pulse amplitude and blood pressure for use by a computer when correlating pulse amplitude signals measured by an oximeter as blood pressure readings; and 
           [0019]      FIG. 4  is a depiction of a linear scale for use by the computer when comparing the pulse amplitude signals with a reference value, in real time, to monitor blood flow. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0020]    Referring initially to  FIG. 1 , a system for continuously monitoring blood flow in the vasculature of a patient is shown, and is generally designated  10 . As shown, the system  10  includes both a sphygmomanometer  12  and an oximeter  14 . In  FIG. 1 , these components of the system  10  are shown in use together, and are connected with a patient  16  for the purpose of taking simultaneous measurements. In this combination, the sphygmomanometer  12  is used for the purpose of taking blood pressure pulse measurements, p s . Thus, it will typically include a pressure cuff  18  which is placed on an arm  20  of the patient  16 . On the other hand, the oximeter  14  is used for the purpose of taking blood flow pulse amplitude measurements, p o . Thus, it will typically include a clamp (not shown in detail) that is connected directly with a finger  22  of the patient  16 . 
         [0021]    As is well known, the sphygmomanometer  12  and the oximeter  14  are normally employed independently, for different purposes. The present invention, however, envisions their concurrent use during a set-up (i.e. calibration) of the system  10 . In particular, the set-up of system  10  is undertaken to calibrate blood flow pulse amplitudes measured by the oximeter  14 , with blood pressure measurements from the sphygmomanometer  12 . The specific purpose here is to calibrate the oximeter  14  for a subsequent, independent use of the oximeter  14 , by itself, for monitoring the blood pressure of patient  16 , without the sphygmomanometer  12 . 
         [0022]      FIG. 1  also shows that both the sphygmomanometer  12  and the oximeter  14  are connected with a computer  24 . A monitor  26  is also connected with the computer  24 . Further, it is to be appreciated that the monitor  26  will include a visual display (not shown) which provides continuous, real-time information from the oximeter  14  and from the computer  24  regarding the blood pressure of the patient  16 . An important aspect of the present invention is that this information can be provided over an extended period of time. 
         [0023]      FIG. 2  shows a calibration graph  28  which illustrates an exemplary correspondence between blood pressure pulse magnitudes p s  and simultaneous blood flow pulse amplitudes p o . For a set-up of the system  10 , measurements of both p s  and p o  are respectively taken by the sphygmomanometer  12  and the oximeter  14  during a same sphygmomanometer duty cycle  30 . 
         [0024]    As indicated by the graph  28 , exemplary blood pressure measurements (i.e. p s ) are sequentially taken for each heart beat during the duty cycle  30  (e.g. at times t 0  through t 7 ). Importantly, during the duty cycle  30 , the particular blood pressure measurement which is taken at time t o , at point  32  on graph  28 , corresponds with the systolic pressure, p s(systolic) , of the patient  16 . Similarly, the measurement at point  34  on graph  28  which is taken at time t 7 , corresponds to the diastolic pressure, p s(diastolic) , of the patient  16 . Further, for reasons more clearly established below, the systolic pressure, p s(systolic) , of the patient  16  (point  32 ) is correlated with a simultaneous measurement taken by the oximeter  14 , which is represented by the point  36  in graph  28 . The blood flow pulse amplitude measurement which is indicated at point  36 , is then subsequently used as a base amplitude measurement, p o(base) . 
         [0025]    A correlation between blood pressure pulse magnitudes, p s , and blood flow pulse amplitudes, p o , is based on changes Δp s  and Δp o  between the respective measurements taken at successive time t n  and t n+1  during the duty cycle  30 . For instance, referring to  FIG. 2  it will be seen that at the time t 3  in the duty cycle  30 , a reading p s3  is obtained for a blood pressure measurement, and a reading p o3  is obtained for a blood flow pulse amplitude measurement. Subsequently, at time t 4 , measurements p s4  and p o4  are respectively taken. Thus, during the time interval  38  between t 3  and t 4 , shown in  FIG. 2 , a change in blood pressure p s4 −p s3 =Δp s3  and a change in pulse amplitude p o4 −p o3 =Δp o3  are determined. A series of an n number of such measurements taken over a duty cycle  30  can then be represented by the line graph  40  in  FIG. 3  using well known curve fitting techniques. 
         [0026]    In detail, the line graph  40  is based on a comparison between an average change in blood pressure pulse magnitude Δp s  [Δp s =(Σ Δp sn )n] and an average change in blood flow pulse amplitude Δp o  [Δp o =(Σ Δp on )n]. For example, with n=8, the averages will be based on measurements taken sequentially at times t 0  through t 7  over the sphygmomanometer duty cycle  30 . The result here is the ability to mathematically determine an operational ratio Δp o /Δp s  (e.g. the slope of the line graph  40 ) that is patient specific, and that can be used for determining a blood pressure value p s  based on changes in pulse amplitude p o . 
         [0027]    In overview, each blood pressure pulse magnitude p s  and each blood flow pulse amplitude p o  is taken at a selected point in each heart pulse of the patient  16  (e.g. at a time t n ). These measurements are taken during the sphygmomanometer duty cycle  30 , and are provided as input to the computer  24  for calculating the operational ratio Δp o /Δp s . 
         [0028]    For an operation of the system  10 , the oximeter  14  is calibrated, and periodically recalibrated as necessary, to correlate p o  with p s . Specifically, this is done in accordance with a methodology for determining the operational ratio Δp o /Δp s  as disclosed above. Using a calibrated oximeter  14 , the monitor  26  is then continuously available for checking the blood flow/pressure condition of the patient  16 . As will be appreciated with reference to  FIG. 4 , the system  10  will monitor for when a change in blood pressure causes the pulse amplitude p o  measured by the oximeter  14  to vary from the base amplitude p o(base)  by a predetermined value. 
         [0029]    By way of example, while cross referencing  FIG. 3  with  FIG. 4 , consider a change in p s  from point  32  to point  42 . For the system  10 , this change in p s  to the point  42  is indicated by a change in p o  to the point  44  from the point  36  (i.e. p o(base) ) As shown in  FIG. 4 , this change keeps p o  within a range  46  of predetermined value (e.g. where p s  remains less than p s(systolic) +60 mmHg). Otherwise, as intended for the present invention, when p o  exceeds the value at point  48 , p s  will be greater than p s(systolic) +60 mmHg and the system  10  can be set to alarm. On the other hand, also by way of example, when p o  goes below p o(base)  and beyond a range  50  of predetermined value (e.g. where p s  is below p s(systolic) −40 mmHg), the system  10  can be set to alarm. As will be appreciated by the skilled artisan, the values given in this example can be varied as desired by the user. In any event, it is also to be appreciated that the operational ratio Δp o /Δp s  will, preferably, be recalculated to recalibrate the oximeter  14  at least every hour. 
         [0030]    While the particular Oximetry Signal, Pulse-Pressure Correlator as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.