Abstract:
A method of detecting heart beat coincidence includes receiving first and second signals from first and second heart beat sources. The method further includes detecting first heart beat occurrences on the first signal, each first heart beat occurrence having a respective time associated therewith and detecting second heart beat occurrences on the second signal, each second heart beat occurrence having a respective time associated therewith. The method further includes comparing the times of the first and second heart beat occurrences to detect coincidence.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Not Applicable. 
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not Applicable. 
     FIELD OF THE INVENTION 
     The present invention relates generally to medical systems and methods for monitoring a heart beat. The present invention relates more specifically to medical systems and methods for monitoring first and second heart beats and detecting coincidence. 
     BACKGROUND OF THE INVENTION 
     Medical monitoring devices are becoming increasingly important in the fields of patient diagnosis and care. New technologies give patient caregivers various alternative tools for performing the tasks necessary to address the needs of their patients. 
     In one example, the heartbeat of a fetus in the womb is monitored during an examination or during birth. The health and well-being of the fetus can be monitored by studying the fetal heartbeat. Transducers are positioned on or near the fetus to monitor the fetal heartbeat and are coupled with wires to a nearby computer for display and/or charting. 
     However, existing technology relating to transducer design and placement is prone to errors, such as, noise, multipath, and other signal disturbances. One particularly troublesome error is the tendency of the fetal transducer to acquire the heartbeat of the mother instead of the fetus. When the heartbeat of the mother is recorded and displayed as that of the fetus (e.g., on a monitor or strip chart), the caregiver risks an incorrect or inaccurate diagnosis. 
     Correct positioning of the fetal transducer can avoid the mother&#39;s heartbeat while acquiring the fetal heartbeat. However, it is not easy to identify when the fetal transducer is correctly positioned. One proposed solution is a fetal monitor capable of recording the heart rate traces of a fetus and a mother. The heart rates are traced on a chart and, if the heart rates are coincident, a warning signal is generated. One drawback of this proposed solution is that heart rates from two different sources can vary widely, even crossing one another at times, particularly during childbirth. Furthermore, a threshold must be established around one heart rate signal (e.g., +/−5 beats per minute) which, under some conditions, does not precisely identify whether the incorrect heart rate is being monitored. 
     Accordingly, what is needed is a system and method for detecting heart beat coincidence that avoids the uncertainties associated with heart rate generation and monitoring. Further, what is needed is a system and method that is more precise, and provides improved coincidence detection in a shorter time period. Further still, a system and method is needed that provides a higher degree of certainty in coincidence detection. The system and method would further be more versatile than prior systems. 
     BRIEF SUMMARY OF THE INVENTION 
     According to an exemplary embodiment, a method of detecting heart beat coincidence includes receiving first and second signals from first and second heart beat sources. The method further includes detecting first heart beat occurrences on the first signal, each first heart beat occurrence having a respective time associated therewith and detecting second heart beat occurrences on the second signal, each second heart beat occurrence having a respective time associated therewith. The method further includes comparing the times of the first and second heart beat occurrences to detect coincidence. 
     According to an alternative embodiment, a system for detecting heart beat coincidence includes means for receiving first and second signals from first and second heart beat sources, respectively. The system further includes means for detecting first heart beat occurrences on the first signal, each first heart beat occurrence having a respective time associated therewith and means for detecting second heart beat occurrences on the second signal, each second heart beat occurrence having a respective time associated therewith. The system further includes means for comparing the times of the first and second heart beat occurrences to detect coincidence. 
     According to another alternative embodiment, a heart beat coincidence detection system includes a processor and an output device. The processor is configured to receive first and second cardiac signals, to detect first and second heart beats on the first and second cardiac signals, respectively, to calculate phase shifts between respective first and second heart beats, and to generate a display signal based on the phase shifts. The output device is configured to receive the display signal and to provide the display signal to an operator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will become more fully understood from the following detailed description, taken in conjunction with the accompanying drawings, wherein like reference numerals refer to like parts, in which: 
     FIG. 1 is a block diagram of a heart beat coincidence detection system according to an exemplary embodiment; 
     FIGS. 2A-2B are flowcharts illustrating a method of detecting heart beat coincidence according to an exemplary embodiment; 
     FIG. 3 is a graph illustrating several steps of the method of FIG. 2A; 
     FIG. 4 is a screen display providing coincidence status according to an exemplary embodiment; and 
     FIG. 5 is a portion of a strip chart providing coincidence status according to an exemplary embodiment. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring first to FIG. 1, a heart beat coincidence detection system  10  is shown. System  10  is implemented, for example, on a Corometrics 120 Series Maternal/Fetal Monitor manufactured by GE Marquette Medical Systems of Milwaukee, Wis. However, system  10  may be implemented on other fetal monitoring systems, or other medical devices. 
     System  10  includes an input/output device  12  coupleable via wires or wirelessly to one or more transducers  14 . Transducers  14  include a fetal heartbeat transducer  16  and a maternal heartbeat transducer  18 , and may further include additional transducers  20 . Transducers  14  may each include an electrocardiogram electrode, ultrasound transducer, blood pressure transducer, pulse oximetry transducer, or other transducer configured to monitor cardiac activity from a heart beat source and to generate a cardiac signal based on that activity. Input/output device  12  includes a port, circuit board, or other circuit configured to receive the cardiac signals from transducers  14  and provide one or more of the cardiac signals to a digital signal processor  22 . 
     Digital signal processor  22  is an integrated circuit or other circuit configured to receive analog signals from transducers  14 , digitize them, and detect heart beats on the cardiac signals. Digital signal processor  22  includes a processor and program-storage memory to perform these tasks, but may include any necessary circuit elements, such as discrete components, analog components, programmable logic, etc. Digital signal processor  22  provides a priority interrupt to a central processing unit  24  (e.g., an INTEL or MOTOROLA microprocessor, or other processing circuit) each time a heart beat is detected. Central processing unit  24  runs a heart beat coincidence detection algorithm stored in program memory  26  each time the priority interrupt is received from digital signal processor  22 . 
     The algorithm will be described below with reference to FIGS. 2A and 2B. According to an alternative structure, digital signal processor  22  and central processing unit  24  can be fabricated on one integrated circuit. Alternative methods and systems of heart beat detection for both fetal heart beats and maternal heart beats may be utilized in system  10 . 
     System  10  further includes an operator input device  28  including keypads, switches, dials, a touch-screen interface, and/or other devices configured to receive input data from a caregiver or other operator. System  10  further includes one or more output devices  30 , such as, a display  32 , a strip chart device  34 , and/or a communications link  36  coupled to the central processing unit  24  and including any necessary interface circuitry. Central processing unit  24  generates output signals, such as display signals, based on the heart beat coincidence detection algorithm stored in program memory  26  and provides these output signals to one or more of output devices  30 . 
     The following is a heart beat coincidence comparison matrix. 
     
       
         
               
               
               
               
               
               
               
             
           
               
                   
               
               
                 Mode 
                 F 1ECG   
                 F 1US   
                 F 2US   
                 M ECG   
                 M Sp02   
                 M BP   
               
               
                   
               
             
             
               
                 F 1ECG   
                 X 
                 YES 
                 YES 
                 YES 
                 YES 
                 NO 
               
               
                 F 1US   
                 YES 
                 X 
                 YES 
                 YES 
                 YES 
                 NO 
               
               
                 F 2US   
                 YES 
                 YES 
                 X 
                 YES 
                 YES 
                 NO 
               
               
                 M ECG   
                 YES 
                 YES 
                 YES 
                 X 
                 NO 
                 NO 
               
               
                 M Sp02   
                 YES 
                 YES 
                 YES 
                 NO 
                 X 
                 NO 
               
               
                 M BP   
                 NO 
                 NO 
                 NO 
                 NO 
                 NO 
                 X 
               
               
                   
               
             
          
         
       
     
     The heart beat coincidence algorithm of FIGS. 2A and 2B is configured to compare the heart beats on two or more cardiac signals and determine whether the heart beats exhibit coincidence. The above matrix illustrates the transducers which may be compared by the exemplary system and method. For example, a fetal electrocardiograph signal (F 1ECG ) is compared to a maternal electrocardiograph signal (M ECG ) as indicated by the word “YES” in the chart. However, a fetal electrocardiograph signal (F 1ECG ) is not compared to a maternal blood pressure signal (M BP ) as indicated by the word “NO” in the chart. The symbols F 1ECG , F 1US  (fetal ultrasound signal  1 ), F 2US  (fetal ultrasound signal  2 ), M ECG , M SpO2  (maternal pulse oximetry transducer), and M BP  correspond to ports of input/output device  12  configured to receive cardiac signals from corresponding transducers. Thus, F 1ECG  is not compared to F 1ECG  as indicated by the “X” since only one port on system  10  is available for this transducer. Further, it is noted that the maternal blood pressure sensor is not utilized in this exemplary embodiment for comparison to any other signal. Other alternative configurations of this matrix are contemplated, depending on the capabilities of the system. 
     Referring now to FIGS. 2A and 2B, an exemplary heart beat coincidence detection method  50  is shown. Method  50  is operable in system  10  as software, but may alternatively be operable via discrete circuit elements or other programming elements. At step  52 , cardiac signals on two channels, channel  1  and channel  2 , are monitored. When a heart beat occurrence is detected on one of channels  1  or  2 , the heart beat occurrence is registered and timestamped. HBT 1  and HBT 2  in FIG. 2A indicate the heart beat timestamps for a heart beat occurrence detected on channel  1  and a heart beat occurrence detected on channel  2 , respectively. When a heart beat occurrence is detected on the other of the two channels, the method proceeds to step  54 . A heart beat occurrence on one of channels  1  and  2  followed by a heart beat occurrence on the other of channels  1  and  2  is referred to hereinafter as a cycle. 
     At step  54 , the method identifies whether the heart beat occurrences on channels  1  and  2  bear a 1:1 correspondence. In other words, at step  54 , the method calculates whether the number of heart beat occurrences from one of channels  1  and  2  occurs twice between successive heart beat occurrences in the other channel. If a correspondence of greater or less than 1:1 is found, the method proceeds to step  55 . At step  55 , if channel  1  (representing the fetal heart beat in this exemplary embodiment) has greater than one heart beat for one heart beat of channel  2  (representing the maternal heart beat in this exemplary embodiment), the method proceeds to step  68 . If channel  1  has less than one heart beat for one heart beat of channel  2 , the method proceeds to step  72  (FIG.  2 B). Alternatively, when the heart beat occurrences indicate greater or less than 1:1 correspondence, the method may directly generate a divergence signal, as described below with respect to step  82 . 
     Once a cycle of heart beat occurrences is detected and time stamped, times associated with each heart beat occurrence are compared to detect coincidence. The following is one exemplary method for comparing heart beat occurrences to detect coincidence, though alternative methods are contemplated utilizing heart beat occurrences. At step  56 , a time offset (e.g., a phase shift) between HBT 1  and HBT 2  is calculated. Next, the running jitter between multiple cycles of occurrences is determined to indicate coincidence or divergence. 
     In this exemplary embodiment, determining jitter between cycles includes keeping a record of the minimum and maximum phase shifts occurring among a plurality of cycles. Therefore, the jitter determination assumes multiple cycles over a period of time (e.g., a time “window”). The time window is a fixed time (e.g., three seconds) in this exemplary embodiment after which the minimum and maximum phase shift variables are reset but may alternatively depend on a cycle count or all cycles during a period of 1:1 correspondence. 
     At step  58 , a minimum phase shift variable is updated with the new phase shift provided the new phase shift is smaller than the prior minimum phase shift. At step  60 , a maximum phase shift variable is updated with the new phase shift provided the new phase shift is greater than the prior maximum phase shift. At step  62 , a jitter is calculated by subtracting the minimum phase shift from the maximum phase shift. 
     Once the phase relationship has been characterized by phase shift and jitter, this data is used to determine if the heart beat occurrences are representative of coincidence or divergence. Maximum jitter and maximum phase shift criteria are applied. Thus, at step  64 , the jitter is compared to a maximum jitter threshold (J) and the maximum phase shift is compared to a maximum phase shift threshold (S). Maximum jitter threshold (J) and maximum phase shift threshold (S) are variable and may be adjusted to tune the algorithm. For example, maximum jitter threshold (J) may be set at approximately 100 ms, or as low as approximately 1 ms. In one exemplary embodiment, maximum jitter threshold (J) is less than one-half the minimum expected beat-to-beat interval. For example, if the minimum expected beat-to-beat interval is 200 ms (i.e., corresponding to 300 beats per minute), maximum jitter threshold (J) is set to one-half of 200 ms, or 100 ms. Phase shift threshold (S) may be set at approximately 200 ms, or between 1 and 2,000 ms. Alternatively, (J) and (S) may be tuned to any value, depending upon the application and such factors as transducer type/cardiac source. 
     Maximum phase shift and maximum phase jitter thresholds may be dynamically variable by the algorithm or static. The potential range of the phase shift between channels with signal peaks demonstrating a 1:1 correspondence is defined as 0-359 degrees, but would generally be expected to be within 180 degrees. In the time domain this could be from 0 to 1999 milliseconds depending upon the period between beats from channel  1  and channel  2 . Coinciding heart beats at the low end could occur offset 1999 milliseconds from one another and be 359 degrees out of phase. 
     Maximum phase jitter may be defined as a constant or as a variable to the algorithm. One method is to make J a function of the maximum phase shift. For example, J=max shift/3. In this example, the maximum allowable jitter for beats to be characterized as coinciding would be 33%. 
     An embodiment that followed these principles would, first, characterize beats from two channels to be within 359 degrees phase of each other if the beat registry is 1:1. Then, within a window of comparison, the jitter, which is evaluated as the difference between the maximum and minimum phase shift, may be qualified characteristic of coincidence if less than 33% of the maximum. Maximum shift may be further qualified in the time domain if so desired, and would be a function which takes into account system latencies, This embodiment is more forgiving of the degree of phase shift, but enforces consistency in the phase relationship by allowing only a minimum in the phase jitter. With this approach the max jitter threshold should not be implemented to exceed 49%. 
     If the jitter is less than the maximum jitter threshold and the maximum phase shift is less than the maximum phase shift threshold, a coincidence counter is incremented and a divergence counter is decremented at step  66 . Alternatively, the coincidence counter is decremented and the divergence counter is incremented at step  68 . As indicated by steps  64 ,  66 , and  68 , both an increasing jitter and an exceedingly high phase shift indicate divergence. A steady jitter and a smaller phase shift indicate coincidence. 
     At step  70  the heart beat registry is reset and prepared for registration of a new cycle of heart beat occurrences. 
     At step  72 , a coincidence index is calculated. The coincidence index represents the degree of coincidence or divergence between heart beat occurrences on channels  1  and  2  over a time window which is either fixed or variable, as described hereinabove. In this example, the time window includes all heart beat occurrences in a three second window. The time window may include between 2 and 100 heart beat cycles. At step  72 , the coincidence index may be calculated, for example, as a ratio (as in this exemplary embodiment) or as a percentage of cycles which coincide. 
     At step  74 , the coincidence index is compared to coincidence/divergence criteria (e.g., a coincidence trigger threshold (C)) which indicates when a sufficient amount of coincidence or divergence is detected to alert the operator. The coincidence/divergence criteria are variable and may be adjusted to tune the algorithm. For example, coincidence trigger threshold (C) may be set at approximately 70% of cycles being coincident, or approximately 3 coincident cycles to every 1 divergent cycle. Alternatively, (C) may range between 50% and 90%, or may be any other value, depending upon the application and such factors as transducer type/cardiac source. 
     If the coincidence index meets the coincidence/divergence criteria, a timer is started for the respective criterion. If the coincidence index continues to meet the coincidence/divergence criteria over multiple heart beat cycles for a predetermined time period, a signal will be generated to the user to notify the user of coincidence or divergence. In this exemplary embodiment, a coincidence trigger flag is used to implement the timer. At step  74 , if the coincidence index exceeds the coincidence trigger threshold (C), the coincidence trigger flag is checked at step  76  to see if it is FALSE. If the coincidence trigger flag is FALSE, at step  78  the coincidence trigger flag is set to TRUE, a coincidence timer is started, and the algorithm returns to step  52 . The coincidence timer may be set to 60 seconds, between 40 and 80 seconds, or any other time, depending upon the application. If the coincidence trigger flag is not FALSE at step  76 , at step  80  the coincidence timer is checked to see if it expired. If not, the algorithm returns to step  52 . If so, a coincidence signal is generated at step  82  and provided to one of output devices  30 . 
     Returning to step  74 , if the coincidence index does not exceed the coincidence index trigger threshold (C), the coincidence trigger flag is checked at step  84 . If the coincidence trigger flag is TRUE, at step  86 , the coincidence trigger flag is set to FALSE, a divergence timer is started, and the algorithm returns to step  52 . The divergence timer may be set to 5 seconds, between 1 and 10 seconds, or any other time, depending upon the application. If the coincidence trigger flag is not TRUE, at step  88  the divergence timer is checked to see if it expired. If not, the algorithm returns to step  52 . If so, a divergence signal is generated at step  82  and provided to one of output devices  30 . 
     In operation, when the coincidence index exceeds coincidence index trigger threshold (C) for a predetermined time (i.e., the duration of the coincidence timer), a coincidence indicia is generated on one or more of output devices  30 . When the coincidence index is below coincidence index trigger threshold (C) for a predetermined time (i.e., the duration of the divergence timer), a divergence indicia is generated on one or more of output devices  30 . According to one alternative, a divergence indicia is only provided to output devices  30  if a coincidence indicia was previously provided to output devices  30 . This alternative is particularly advantageous when the strip chart is utilized, since no indicia need be provided to the user when the heart beat occurrences are divergent unless a previous indicia indicated the heart beat occurrences were coincident. 
     Referring now to FIG. 3, a chart  89  illustrates the operation of a portion of the heart beat coincidence detection algorithm of FIG. 2A. A heart beat occurrence on channel  1  is shown at occurrence  90 . A heart beat occurrence on channel  2  is shown at occurrence  92 . The phase shift or time offset between occurrences  90  and  92  is indicated by time period  94 . The X-axis of the chart represents real time in milliseconds (ms). In this example, the phase shift between occurrence  90  and  92  is 150 ms, as shown. 
     In operation, the algorithm first updates the maximum and minimum phase shift values with the new phase shift value of 150 ms. A subsequent heart beat occurrence  96  is received on channel  1 , and a further subsequent heart beat occurrence  98  is received on channel  2 . Note that 1:1 correspondence is maintained between heart beat occurrences on channels  1  and  2  from the first cycle to the second cycle. The phase shift between occurrences  96  and  98  is calculated as 155 ms, indicating a slight difference from the previous cycle. The maximum phase shift is updated to equal 155 ms and a jitter is calculated as 5 ms. Further heart beat occurrences on chart  89  indicate phase shifts of 153 ms, 220 ms, 250 ms, and 190 ms and corresponding jitters of 5 milliseconds, 70 milliseconds, 100 milliseconds, and 100 milliseconds. 
     The maximum phase shift and jitter are compared to maximum phase shift threshold (S) and maximum jitter threshold (J) to determine whether coincidence and divergence counters should be incremented or decremented. The coincidence index is then calculated and compared to the coincidence index trigger threshold (C). This occurs over a three second time window. Processing continues in accordance with the relevant steps of FIG.  2 B. The output of the algorithm is dependent on the values of thresholds (J), (S), and (C), which may be programmed when manufactured, may be updateable, and may also be adjusted by the operator via operator input device  28  to give the operator control over the sensitivity. 
     Referring now to FIG. 4, a screen display  100  is shown. Screen display  100  is generated by display  32  (FIG. 1) in response to display signals provided by central processing unit  24 . Additional graphics cards or alternative circuitry may be implemented. Screen display  100  includes an indicia  102  (e.g., the text “HBC”) indicating that the heart beat coincidence feature is currently operational. At step  82  of the heart beat coincidence algorithm (FIG.  2 B), the algorithm generates a display signal which is one of a coincidence signal, a divergence signal, or no signal. Display  100  indicates that a coincidence signal is received by displaying heart rates for channels  1  and  2  in inverse video at indicia  104  and  106 . Other indicia may be used to indicate coincidence, such as, two side-by-side hearts, the text “COINCIDENCE DETECTED”, an audible tone, other indicia, or some combination thereof. Divergence is indicated in this example by ordinary (i.e., non-inverse) video, but may be indicated by a different indicia or by no indicia. 
     Referring now to FIG. 5, a portion  110  of a strip chart is shown. Portion  110  is generated by strip chart device  34  (FIG. 1) in response to signals provided by central processing unit  24 . Additional graphics cards or alternative circuitry may be implemented. Portion  110  includes an indicia  112  (e.g., the text “HBC”) indicating that the heart beat coincidence feature is currently operational. Indicia  112  is printed periodically (e.g., every 30 minutes), but may alternatively be printed only once. At step  82  of the heart beat coincidence algorithm (FIG.  2 B), the algorithm generates a display signal which is one of a coincidence signal and a divergence signal. Strip chart device  34  indicates that a coincidence signal is received by printing a coincidence indicia  114  (e.g., two overlapping heart icons). Other indicia may be used, such as, the text “COINCIDENCE DETECTED”, an audible tone, or some combination thereof. Divergence may be indicated by a different indicia, such as indicia  116  (e.g., two non-overlapping heart icons) or by no indicia. Indicia  116  indicates that the coincidence was resolved. Coincidence indicia  114  and divergence indicia  116  may be printed periodically to approve the current status, or may be printed only when the status changes. 
     According to a further advantageous feature, the coincidence or divergence status may also be output via communications link  36 . 
     While the embodiments and application of the invention illustrated in the figures and described above are presently preferred, it should be understood that these embodiments are offered by way of example only. For example, alternative algorithms may be employed to compare the heart beat occurrences. Further, the method steps presented may be employed in a different order. Accordingly, the present invention is not limited to a particular embodiment, but extends to various modifications that nevertheless fall within the scope of the appended claims.