Abstract:
Apparatus for monitoring one or more vital signs of a subject has a number of sensors. There is at least one redundant sensor. Each sensor originates a signal. A selection system determines performance criteria for a number of groups of signals. Each group includes one or more signals. The apparatus computes a predicted value for a vital sign by either computing a value from each group of signals and taking a weighted average with weights based upon the performance criterion or by selecting one of the groups of signals for which the performance criterion is best and computing the output value from that group of signals. The output values are relatively insensitive to artifacts and to errors caused by the disconnection or malfunctioning of one sensor.

Description:
TECHNICAL FIELD 
     This invention relates to devices for monitoring vital signs such as blood pressure, pulse rate, and oxygen saturation. The invention has particular application in devices for determining blood pressure by measuring a pulse wave velocity or pulse transit time. 
     BACKGROUND 
     Equipment for monitoring the vital signs of subjects is widely used in clinical settings. Such devices may monitor various physiological signs including blood pressure, oxygen saturation, pulse rate and the like. Such devices typically include one or more sensors placed at suitable locations on the subject&#39;s body. Various different types of sensors may be used. The sensors may be of invasive types or of non-invasive types. Signals from the sensors are carried to the vital signs monitoring equipment where they are amplified, conditioned, and processed to determine values for the physiological parameters being measured. 
     In general, it is desirable to provide non-invasive monitoring of vital signs. While invasive systems are sometimes used, surgery is required to introduce sensors of invasive types into the subject&#39;s body. The sensors typically have leads which emerge from the subject&#39;s body through a fistula. The fistula can provide a pathway for infection. 
     As an example of non-invasive monitoring of a vital sign, blood oxygen saturation may be measured by providing a small clip-on sensor which includes one or more light sources and one or more light detectors. Variations in the oxygen saturation of the subject&#39;s blood cause resulting variations in the intensity of light reaching the detector. These variations are superimposed upon a variation in the intensity of light reaching the detector which results from the subject&#39;s heartbeat pulses. A device equipped with this type of sensor can also be used to measure pulse rate. Various such devices are known. 
     One type of system for measuring a subject&#39;s blood pressure relies upon the fact that the speed at which pulse waves propagate through a blood vessel is dependent upon blood pressure. Consequently, if one detects the arrival of a pulse at two different points of a subject&#39;s circulatory system there will be, in general, a difference in the time at which the pulse wave arrives at the two points. This time difference varies according to the blood pressure. One system for measuring blood pressure as a function of such a time difference is described in PCT application No. PCT/CA00/010552, and in the commonly owned and co-pending application entitled CONTINUOUS NON-INVASIVE BLOOD PRESSURE MONITORING METHOD AND APPARATUS which is being filed simultaneously herewith, both of which are fully incorporated herein by reference. Such systems require at least two sensors, one for detecting the arrival of the pulse wave at each of the two points. This type of device may use sensors of the same type as are used to detect oxygen saturation although other types of sensor could also be used. 
     One problem with such vital signs monitoring equipment is that the accuracy of measurements obtained can depend upon the stability of the signals received from the sensors. Artifacts may be caused by movement of the subject. In the worst case, a sensor may become disconnected from the subject and monitoring may be interrupted until the sensor is replaced. 
     There is a need for cost effective methods and apparatus for monitoring one or more vital signs of a subject which provide improved accuracy and are affected less by artifacts than current vital signs monitoring equipment. 
     SUMMARY OF INVENTION 
     This invention provides an apparatus for monitoring one or more vital signs of a subject by using a number of sensors. Each sensor originates a signal, typically a pulse signal. A selection system determines performance criteria for a number of groups of signals. Each group includes one or more signals. The apparatus computes a estimated value for a vital sign by either computing a value from each group of signals and taking a weighted average with weights based upon the performance criterion or by selecting one of the groups of signals for which the performance criterion is best and computing the output value from that group of signals. 
     The output values are relatively insensitive to artifacts and to errors caused by the disconnection or malfunctioning of one sensor. Further advantages and features of the invention are described below. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     In drawings which illustrate non-limiting embodiments of the invention: 
     FIG. 1 is a block diagram of a vital signs monitoring system according to the invention; 
     FIG. 2 is a view illustrating possible sensor locations for a vital signs monitoring system according to the invention; and, 
     FIG. 3 is a block diagram of a vital signs monitoring system according to a specific embodiment of the invention. 
    
    
     DESCRIPTION 
     Throughout the following description, specific details are set forth in order to provide a more thorough understanding of the invention. However, the invention may be practiced without these particulars. In other instances, well known elements have not been shown or described in detail to avoid unnecessarily obscuring the invention. Accordingly, the specification and drawings are to be regarded in an illustrative, rather than a restrictive, sense. 
     FIG. 1 shows a system  10  according to the invention. FIG. 1 comprises a plurality of sensors  12 . The illustrated embodiment comprises three sensors,  12 A,  12 B and  12 C. As shown in FIG. 2, sensors  12 A,  12 B and  12 C may be located on a subject&#39;s earlobe, finger and toe. Sensors  12  include at least one redundant sensor. That is, there is at least one more sensor  12  than is required for the type of measurement being made by system  10 . The signal from each sensor  12  is conditioned and digitized in a signal conditioner  13 . In the illustrated embodiment there are three separate signal conditioners  13 A,  13 B and  13 C which correspond respectively with sensors  12 A,  12 B and  12 C. 
     The resulting digitized signals are passed to a controller  14 . In the illustrated embodiment of the invention, controller  14  comprises a processor  14 A. Processor  14 A runs software  15  stored in a program memory  15 A. Software  15  receives the signals from all of sensors  12 A through  12 C by way of a suitable interface  18 . 
     In the illustrated example there are three sensors. This means that there are three pairs of sensors. Software  15  computes a blood pressure for the subject from the signals of each of the three pairs of sensors. This yields three computed values for the subject&#39;s blood pressure. Computation of the subject&#39;s estimated blood pressure may be done, for example, according to the methods described in PCT patent application No. PCT/CA00/010552 which is incorporated herein by reference. The methods described in that application involve measuring a time difference between a pair of signals to obtain a differential pulse transit time (DPTT). Separate DPTT values are derived for systolic and diastolic portions of the signals. A known relationship between DPTT and blood pressure is used to compute an estimated blood pressure from a DPTT value. The known relationship is obtained during a calibration process which involves measuring the subject&#39;s blood pressure by a separate accurate mechanism, and substantially simultaneously measuring the DPTT. These methods may be separately applied to each pair of signals from sensors  12 . 
     Controller  14  includes an automated selection system  16 . Selection system  16  takes the signals from sensors  12  in distinct groups. In this case, “distinct” means that each group has a combination of signals from a different set of one or more sensors  12 . Each group includes sufficient signals for the determination of a vital sign of interest. Where the groups include more than one signal, a signal from one sensor  12  may be included in more than one group. Selection system  16  identifies the best group of sensors  12  to be used for monitoring the vital sign in question. For example, where the vital sign is a blood pressure determined from a DPTT, each group of sensors comprises a pair of two sensors. For vital signs, such as pulse or blood oxygen saturation, which can be measured on the basis of the signal from one sensor the groups of sensors include one sensor each. 
     The determination of the best group of sensors to use is preferably made based upon the stability of the signals originating at the sensors. For example, the following equation may be used to provide a performance criterion for the pair of signals (p i ,p j ) to be used in a DPTT blood pressure estimation: 
     
       
           C   ij =[Corr( p   i   ,p   j )/σ i σ j ] max   (1) 
       
     
     Where, C ij  is the maximum value of the correlation coefficient of the paired signals (p i ,p j ) and σ i  and σ j  are deviation measures for the two signals which are given as follows:                σ   i     =         1     N   -   1              ∑     k   =   0       N   -   1              [         p   i          (   k   )       -     μ   i       ]     2                   (   2   )                 σ   j     =           1     N   -   1              ∑     k   =   0       N   -   1              [         p   j          (   k   )       -     μ   j       ]     2           .             (   3   )                                
     The correlation function (Corr) between the two signals can thus be calculated as:                Corr        (       p   i     ,     p   j       )       =       1   N            ∑     k   =   0       N   -   1              [         p   i          (   k   )       -     μ   i       ]          [         p   j          (   k   )       -     μ   j       ]                   (   4   )                                
     where N is the number of samples used to calculate DPTT and determine the performance of the considered pair in a certain period of time. μ i  and μ j  are respectively average signal values for the paired signals from the sensors under consideration. 
     μ i  may be given by the following equation:                μ   i     =       1   N            ∑     k   =   0       N   -   1              p   i          (   k   )                   (   5   )                                
     μ j  may be given by the following equation:                μ   j     =       1   N            ∑     k   =   0       N   -   1              p   j          (   k   )                   (   6   )                                
     The performance criterion C ij  given by equation (1) has a number of features which makes it suitable for use as a performance criterion for the pair of sensors under consideration. In particular, C ij  is independent of the amplitudes of pulse signals p i  and P j  and −1≦C ij ≦1. 
     To reduce the complexity of these computations, a histogram-based calculation may be used in practice. Histogram-based calculation techniques which may be applied in this invention are described in Smith, “ The Scientist and Engineer&#39;s Guide to Digital Signal Processing  ( Second Edition )” California Technical Publishing, 1999. Histogram-based techniques have the advantage that computational complexity is not dependent upon the number of samples collected. 
     Selection system  16  preferably comprises a function which tests for unacceptable signal values (as might result, for example, from the disconnection of a sensor) and, when such conditions are detected, forces the affected performance criterion C ij  to be zero (or some other value that will cause the selection system  16  to not select the pairs of sensors affected by the unacceptable signal). 
     In the preferred embodiment of the invention, selection system  16  computes a blood pressure as a weighted average of the blood pressures computed from the signals of each of three pairs of sensors. This weighted average may be expressed as follows:              P   =       ∑   ij            a   ij          P   ij                 (   7   )                                
     Where there are three sensors, this reduces to: 
     
       
           P=a   12   P   12   +a   23   P   23   +a   31   P   31   (8) 
       
     
     where a 12 , a 23  and a 31  are weighting factors with a 12 +a 23 +a 31 =1 and P 12 , P 23  and P 31  are blood pressures calculated from the signals produced by each of the three pairs of sensors respectively. 
     The weighting factors a ij  (where i and j are indices which represent the sensors in the pair of sensors under consideration) may be given by the following equation, where C ij  is the performance criterion for the pair of sensors under consideration:                a   ij     =       C   ij         ∑   ij          C   ij                 (   9   )                                
     In the alternative, selection system  16  may select one of the pairs of sensors which provides the best value for the subject&#39;s blood pressure (i.e. for which the performance criterion is the highest). In this case, apparatus  10  presents the blood pressure derived from the signals of that pair of sensors  12  as the subject&#39;s blood pressure. This alternative embodiment of the invention is a special case of the weighted average according to equation (8) in which the values of a ij  which do not correspond to the group of signals having the best performance criterion are all zero and the a ij  corresponding to the group of signals having the best performance criterion is 1. 
     A display  20  displays the computed blood pressure. A user input device,  24  such as a button panel, a graphical user interface, a touch screen, or the like is provided to permit users to control the operation of apparatus  10 . Preferably, user input device  24  permits a user to control whether selection system  16  selects signals from a specified pair of sensors  12 , selects signals from the pair of sensors  12  which has the best performance criterion, or uses signals from all of sensors  12  in a blended average such as that of equation (7). 
     A digital input/output (I/O) connection  26  permits results to be delivered to other devices, for example, a printer, or a data collector/concentrator, or a computer being used for data analysis. A non-invasive blood pressure (NIBP) measurement module  28  provides reference blood pressures for calibration and re-calibration purposes. Display  20 , user input  24 , I/O connection  26  and measurement module  28  communicate with processor  14 A by way of one or more suitable interfaces  18 A. 
     Apparatus  10  according to the invention preferably uses a similar strategy to that described above for obtaining and displaying a value representing the oxygen saturation of a subject&#39;s blood and the subject&#39;s pulse rate. Each of the three sensors produces a signal which can be used to derive an oxygen saturation value and a pulse rate value. As the performance criteria are calculated in real time for CNIBP estimation, one signal can be selected from the best pair of sensors for the purpose of obtaining an oxygen saturation value and a pulse rate value. 
     Once again, the system may be set to display a best one of the oxygen saturation or pulse rate values or, in the alternative, may present an oxygen saturation value which is a blended average of the oxygen saturation or pulse rate values derived from the signals originating from each sensor. 
     Those skilled in the art will notice that system  10  has at least one redundant sensor. If any one sensor becomes disconnected or malfunctions then the signal performance of all parameter values which are calculated based upon a signal from that sensor will be poor or useless. Such values will be given either a very small weighting or no weighting at all in the computation of the parameter to be displayed. In the preferred embodiment of the invention, processor  14  causes apparatus  10  to generate a visual or audible warning if the performance criterion for one group of sensors is lower than a threshold value. Most preferably the visual or audible warning is generated if the performance criterion remains below the threshold value for longer than a predetermined time period. 
     It is preferable to re-calibrate system  10  relatively frequently. For example, it is preferable to compare the blood pressure values produced by system  10  to a calibration value obtained by another measurement technique approximately every 30 minutes. Re-calibration is especially important if a sensor is relocated. Re-calibration is also desirable if the extremities of the subject where sensors are located are moved in relation to the subject&#39;s heart. For example, if a sensor is on a subject&#39;s finger and the arm to which the finger is attached is elevated then it would be preferable to re-calibrate the system after the subject has assumed a comfortable position with the arm elevated. Similarly, if a vasoactive medication is administered to the subject, or the dose of a continuously delivered vasoactive medication is altered, re-calibration is desirable. 
     FIG. 3 illustrates apparatus  10 A according to an alternative embodiment of the invention. Apparatus  10 A comprises a separate processor  40  for computing the performance criterion and the blood pressure determined by each pair of sensors. For example, processor  40 A computes the performance criterion and an estimated blood pressure from the signals measured by sensors  12 A and  12 B. Processor  40 B computes the performance criterion and an estimated blood pressure from the signals produced by sensors  12 B and  12 C. Sensor  40 C computes the performance criterion and an estimated blood pressure from the signals produced by sensors  12 A and  12 C. Each of processors  40  also computes values for oxygen saturation for one of the sensors. The results of the computations by the processors  40  are delivered to a host processor  42 . Host processor  42  coordinates the operation of apparatus  10 A and also computes an appropriate value for the systolic and diastolic blood pressures, pulse rate, blood volume and oxygen saturation from the received signals. In doing so, processor  42  may implement the functions of selection system  16  which is described above. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. For example: 
     the number of sensors may be varied. If four sensors are used then there are potentially six pairs of sensors from which pulse transit time information for blood pressure estimation may be derived. There are four sensors from which pulse and oxygen saturation information may be derived. 
     various functions which are described above as being performed by software running on a computer processor may be performed using appropriate hardware. 
     Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.