Abstract:
Disclosed are systems and methods for detecting an onset of true labor in a pregnant mammal. In accordance with a method of this invention the following steps are executed: (a) detected electrical signals from a succession of muscular contractions associated with labor; (b) determining an average frequency of the electrical signals for each contraction; (c) determining a discriminant (δ) from the determined average frequency for each contraction so as to provide a set of discriminants, individual ones of the set of discriminants being associated with an individual one of the contractions; and (d) indicating a beginning of true labor when a slope of at least two discriminants of the set of discriminants exceeds a first threshold value and when a magnitude of a last determined discriminant exceeds a second threshold value. The onset of true labor can also be indicated when a value of a critical determinant exceeds a threshold value, the value of the critical determinant being obtained from a combination of the slope of at least two discriminants of the set of discriminants and a magnitude of a last determined discriminant.

Description:
CROSS-REFERENCE TO A RELATED PATENT APPLICATION 
     This patent application is a continuation-in-part of U.S. patent application Ser. No. 08/585,790, filed Jan. 16, 1996, which will issue on Jul. 28, 1998 as U.S. Letters Pat. No. 5,785,664, the disclosure of which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF THE INVENTION 
     This invention relates generally to medical and veterinary instrumentation and, in particular, to medical and veterinary instrumentation for use in diagnosing premature labor. 
     BACKGROUND OF THE INVENTION 
     The risks to babies born prematurely are great and the economic costs to society continue to increase. At present, infants less than 22 weeks gestation are not predicted to survive, and at 28 weeks, only 77% survive. The total yearly cost for preterm babies in the U.S. is in excess of five billion dollars. In order for preterm babies to have a better chance of survival, premature labor must be detected prior to any dilation of the cervix, so that effective drug therapy can be instituted. 
     In more than 50% of cases of threatened premature labor, the patient&#39;s contractions are those of false labor and will stop spontaneously. However, when the patient calls her physician, she will be advised to go to the hospital. If, when the patient arrives at the hospital, her cervix is not significantly dilated, she will be observed for a period of time. If the contractions stop, the patient will be sent home. If, however, they continue, the cervix can dilate further and either make it more difficult to stop the labor, or make the birth of the baby inevitable. The drugs used to stop contractions of premature labor pose a significant risk, both to the mother and baby. Before a physician will start administering these drugs, he must be absolutely sure that the patient is in true labor. 
     Every muscle produces an electric signal when it contracts. The electric signal from the heart can be measured as an electrocardiogram (EKG), and from the uterus as an electrohysterogram (EHG). This EHG will vary during pregnancy and labor. 
     In my U.S. Pat. No. 5,301,680, issued Apr. 12, 1994, entitled &#34;Apparatus and Method for the Diagnosis of Labor&#34;, there are described systems and methods that determine a periodicity of an electrical signal in an abdominal region. A uterine contraction vector having a magnitude and propagation direction is then determined and is employed to distinguish true labor from false labor, and to detect the onset of true labor. Methods and apparatus are also disclosed for determining the extent of cervical dilation. 
     It is known to those skilled in the art that, as labor progresses, there is a shift toward higher frequencies in the frequency spectrum of the electrical activity of the uterus. 
     By example, Marque and Duchene (&#34;Human Abdominal EHG Processing for Uterine Contraction Monitoring&#34;, in Applied Biosensors, D. L. Wise Ed., Butterworth Publisher, Boston, 187-226 (1989)), have disclosed a technique for the detection of premature labor by calculating a criterion. The criterion is based on the energy in a total band (0.2-3.0 Hz), a low frequency band (0.2-0.45 Hz), a high frequency band (0.8-3.0 Hz), and the length of a recorded EHG burst. These authors calculate and graph this criterion during contractions, and show that if it remains negative it is an &#34;effective&#34; contraction, and if it becomes positive it is an &#34;ineffective&#34; contraction. 
     Reference can also be had to U.S. Pat. Nos. 5,446,953, 5,623,939 and 5,776,073 wherein Garfield describes the use of action potential measuring electrodes to acquire electromyographic signals. Frequency components of the electromyographic signals are analyzed to characterize uterine activity. 
     SUMMARY AND OBJECTS OF THE INVENTION 
     The inventor has discovered that more meaningful diagnostic information can be obtained from the electrical activity of the uterus, by calculating a parameter representative of each entire uterine contraction, and by measuring a change in this parameter with respect to parameters calculated from subsequent contractions. 
     It is thus one object of this invention to provide a method, and an apparatus for accomplishing this method, whereby uterine activity is characterized by obtaining an average frequency of a burst of electrical activity measured from a surface of the abdomen, and by determining a parameter characteristic of the measurement. 
     It is a further object of this invention to predict the onset of true labor by measuring a temporal change in the determined parameter. 
     It is another object of this invention to provide an apparatus which may be worn by a pregnant mammal which will signal the onset of true labor. 
     A still further object of the invention provides instrumentation for remotely monitoring a pregnant mammal to aid in diagnosing true labor from false labor. 
     The foregoing and other problems are overcome, and the objects of the invention are realized, by methods and apparatus utilizing an electromyographic detection of electrical fields inherent in muscular activity for the diagnosis of premature labor. 
     In accordance with this invention the apparatus includes a system for measuring the average frequency of electrical activity present in a burst of electrical activity from the abdominal region, and for processing the measurements to indicate and predict the onset of true labor. The system senses electrical activity indicating a smooth muscle contraction, such as a uterine contraction, and calculates the duration of the electrical activity and the average frequency of the electrical activity. Additionally, the electrical activity may be displayed to a practitioner, and/or recorded and stored for subsequent analysis, and/or transmitted to another location for analysis and/or storage. 
     In one aspect the invention teaches a method having steps of: (a) detecting electrical signals from a succession of muscular contractions associated with labor; (b) determining an average frequency of the electrical signals for each contraction; (c) determining a discriminant (δ) from the determined average frequency for each contraction so as to provide a set of discriminants, individual ones of the set of discriminants being associated with an individual one of the contractions; and (d) indicating a beginning of true labor when a slope of at least two discriminants of the set of discriminants exceeds a first threshold value and when a magnitude of a last determined discriminant exceeds a second threshold value. Alternatively, the onset of true labor can also be indicated when a value of a critical determinant exceeds a threshold value, the value of the critical determinant being obtained from a combination of the slope of at least two discriminants of the set of discriminants and a magnitude of a last determined discriminant. 
     This invention furthermore teaches the remote monitoring of a pregnant mammal, in combination with the methods and apparatus for enabling a practitioner to remotely make a diagnosis so as to distinguish true labor from false labor. 
     By automatically analyzing the electromyographic signals obtained from a pregnant mammal, this invention detects differences in the signals and enables a discrimination to be made between false labor contractions (Braxton-Hicks) and true labor contractions. 
     In accordance with the teachings of this invention the discriminant is obtained by first determining the average frequency (f avg ) of the electrical activity within a burst, and then employing constants as follows: 
     
         δ=κf.sub.avg -Λ 
    
     where the constants κ and Λ are selected so that δ has a first value, e.g. a negative value, for false labor contractions, and a second value, e.g. a positive value, for true labor contractions. This can be accomplished by taking measurements from a representative sample of pregnant mammals, and thus establishing the values of the constants accordingly. 
     The average frequency can be determined by, for example, performing a Discrete Fourier Transform (DFT) of the measured signal to obtain the frequency in accordance with: 
     
         f.sub.avg =Σn.sub.i f.sub.i /n.sub.T, 
    
     where n i  is the value of the Fourier Transform at f i , and n T  is the total number of points. 
     In a presently preferred embodiment of this invention the frequency range of interest is from about 0.1 Hz to about 5.0 Hz. Braxton-Hicks activity is generally characterized by a frequency in the range of about 0.2 Hz to about 0.3 Hz, while the onset of true labor may be indicated by an increase in the measured frequency above about 0.4 Hz to a maximum frequency in the range of about 1.2 Hz to about 1.3 Hz, although other frequencies and ranges of frequencies may be encountered. 
     The method further includes the steps of digitizing and storing the detected electrical signals; and the step of indicating includes a further step of transmitting at least the stored, digitized electrical signals to a remote location. 
     The step of indicating a beginning of true labor may be accomplished when a value of a critical determinant exceeds a threshold value, with the value of the critical determinant being obtained from a combination of the slope of at least two discriminants of the set of discriminants and a magnitude of a last determined discriminant. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: 
     FIG. 1A is a pictorial representation of a Labor Onset Monitor (LOM) that is constructed and operated in accordance with the invention; 
     FIG. 1B is a block circuit diagram of a Labor Onset Monitor that is constructed and operated in accordance with the invention; 
     FIG. 2 illustrates a first display (Display1) of typical waveforms of the uterine electromyogram as detected by the electrodes that are coupled to the system of FIG. 1B; 
     FIG. 3A illustrates a second display (Display2) showing a change in a discriminant (δ) parameter as a function of time; 
     FIG. 3B is an example of a display (Display3) that is responsive to the change in the δ parameter as a function of time, as detected by the embodiments of FIGS. 1B, 4B, 5 and 6, for indicating a false or a true labor condition; 
     FIG. 4A is a pictorial representation of a second embodiment of the invention which is designed to be worn by a pregnant mammal; 
     FIG. 4B is a block circuit diagram of a second embodiment of the invention which is designed to be worn by a pregnant mammal; 
     FIG. 5 is a block circuit diagram of a third embodiment of the invention which is designed to be worn by a pregnant mammal, that provides for remotely monitoring the pregnant mammal over a communications link; 
     FIG. 6 is a block circuit diagram of a embodiment of a practitioner&#39;s console that forms a portion of the labor onset monitor system shown in FIG. 5; and 
     FIG. 7 is a diagram useful in explaining a technique to obtain an average frequency of a burst in accordance with this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The disclosure of my above-mentioned U.S. Pat. No. 5,301,680, issued Apr. 12, 1994, entitled &#34;Apparatus and Method for the Diagnosis of Labor&#34;, is incorporated by reference herein in its entirety. 
     As employed herein, the term &#34;pregnant mammal&#34; is not intended to be limited to only a pregnant human female, in that the methods and apparatus of this invention may be utilized by both medical and veterinary practitioners. 
     Also, as employed herein, the term &#34;diagnostic information&#34; is intended to refer to data and/or signals (audible and/or visible) emanating from the apparatus of the invention, the data and/or signals being the result of the processing by the apparatus, in accordance with the methods of the invention, of the voltages detected by suitable transducers, such as but not limited to electrodes, that are attached to a pregnant mammal. These voltages may be detected and then processed in close proximity to the pregnant mammal, or may be transmitted over a suitable telemetry or communications link and processed remotely from the pregnant mammal. Furthermore, the data and/or signals are presented to a practitioner in such a manner so as to aid the practitioner in at least distinguishing true from false labor. The data and/or signals are additionally presented to a practitioner in such a manner so as to aid the practitioner in determining and predicting the onset of true labor. 
     FIGS. 1A and 1B illustrate the Labor Onset Monitor (LOM) 33 that is constructed and operated in accordance with the invention. The LOM 33 includes a set of electrodes 1. The electrodes 1 may be EKG or equivalent electrodes, or may be capacitively coupled electrodes of type described by W. H. Ko and J. Hynecek in &#34;Dry Electrodes and Electrode Amplifiers&#34;, Biomedical Electrode Technology-Theory and Practice, pgs. 169-181, ed. Miller, H. A. and Harrison, D. C., Academic Press, New York (1974). In general, any suitable transducer for detecting electrical energy can be employed. 
     The set of electrodes 1 are positioned on the abdomen of the pregnant mammal, midway between the fundus and the symphysis pubis, at an inter-electrode spacing of from approximately 1 to 7 cm. A ground electrode is positioned on a part of the body remote from the abdomen; e.g., the thigh. The electrodes 1 are connected to a differential amplifier 2, with the following typical characteristics: 
     CMRR&gt;100 dB 
     Gain 50,000 
     Input Impedance&gt;1,000 MegOhms 
     The amplified electrode signal is converted to digital form with an analog to digital (A/D) converter 3, at an A/D sampling rate in a range of 8 to 256 conversions per second. The digitized signal is digitally filtered by a filter 5 in a selected frequency range; e.g. 0.1 to 5.0 Hz, and may be displayed on an appropriate display device 6, (Display1, FIG. 2), such as a conventional CRT, and also stored in a storage device 7, such as semiconductor memory and/or a conventional fixed or removable disk. 
     The presence of electrical activity from the uterus is detected by an envelope detector 4, which determines and stores the time (t1) of the start of the electrical activity, and which also determines and stores the end of the electrical activity (t2). The envelop detector 4 activates a control loop 27, which causes the storage device 7 to initiate an incremental transfer of the currently acquired data to a Fourier transform (FT) or Power Spectral Density (PSD) module 8. This incremental transfer continues until the envelop detector 4 senses the end of the contraction at t2, at which time the envelop detector 4 deactivates the control loop 27. 
     A Fourier transform or a Power Spectral Density 8 is then incrementally calculated for the filtered signal, and the incremental calculations are summed in a summation device 9. A suitable embodiment for the FT/PSD block 8 is discussed below. The FT or PSD is integrated, by an integration module 25, in a plurality of selected frequency ranges; e.g., α, β, γ and ε, selected from the range of filtering, to give the energy in the ranges H.sub.α, H.sub.β, H.sub.γ, and H.sub.ε, respectively. Suitable, but not limiting, frequency ranges are as follows: α=0.1-1.25 Hz; β=1.25-2.5 Hz; γ=2.5-3.75 Hz; and ε=3.75-5 Hz. The duration, λ=t2-t1, of the electrical activity is obtained from the envelope detector 4. 
     Next, a discriminant δ for the electrical activity is determined, in a calculation module 26, from a combination of the energies, as for example: 
     
         δ=ρH.sub.α +σH.sub.β +τH.sub.γ +φH.sub.ε, 
    
     where constants ρ, σ, τ and φ are selected so that δ has a negative value for false labor contractions, and a positive value for true labor contractions. This is preferably accomplished by taking measurements from a representative sample of pregnant mammals, and thus establishing the values of the constants accordingly. 
     In a further technique in accordance with this invention the discriminant is obtained by first determining the average frequency (f avg ) of the electrical activity within a burst, and then employing constants as follows: 
     
         δ=κf.sub.avg -Λ 
    
     where the constants κ and Λ are selected, as above, so that δ has a negative value for false labor contractions, and a positive value for true labor contractions. As before, this can be accomplished by taking measurements from a representative sample of pregnant mammals, and thus establishing the values of the constants accordingly. 
     The average frequency can be determined by, for example, performing a Discrete Fourier Transform (DFT) of the measured signal to obtain the frequency in accordance with: 
     
         f.sub.avg =Σn.sub.i f.sub.i /n.sub.T, 
    
     where n i  is the value of the Fourier Transform at f i , and n T  is the total number of points (see FIG. 7). The average frequency may also be obtained by measuring a number of crossings of the signal through the x-axis per unit time, or by any suitable technique. 
     However determined, the discriminant δ may, for example, range in value from -10 to +10. 
     The value of δ is stored, along with the time T1 of the termination of the electrical activity, as an ordered pair (T1,δ 1 ) in a storage device 11, and may also be displayed on a suitable display device 12 (Display2, FIG. 3A). 
     At the end of the next contraction, the slope Σ of the two discriminants δ (i.e., the discriminant obtained for the contraction just ended (T2, δ 2 ) and the discriminant from the preceding contraction (T1, δ 1 ) is calculated as: 
     
         Σ=(δ.sub.2 -δ.sub.1)/(T.sub.2 -T.sub.1). 
    
     That is, the change in the discriminant δ is determined between two successive contractions. Alternatively, a plurality of the ordered pairs (Tn,δ n ) can be accumulated for a plurality of successive contractions, and the slope determined over the plurality of ordered pairs. 
     If the value of the slope Σ exceeds a critical (c) value Σ c , and if δ exceeds a critical (c) value δ c , as determined by a comparator 13, an audible and/or visual warning signal is given using the display device 14 (Display3, FIG. 3B) to indicate the onset of true labor. Otherwise, the output of the comparator 13 is employed to display a &#34;normal&#34; signal on the display device 14, indicating that true labor has not begun. 
     By example, and referring to FIG. 3A, the critical values may be established such that the onset of true labor is indicated when the discriminant line passes, with a predetermined positive slope, through a range of discriminant values located on or near the x-axis. 
     It is also within the scope of the teaching of this invention to employ a combination (e.g., a linear combination) of Σ and δ to obtain a value of a critical determinant. When the value of the critical determinant is determined to exceed a threshold value, the onset of true labor is indicated. 
     A further embodiment of the LOM (designated 34) is shown in FIGS. 4A and 4B. The operation of this embodiment is similar to that as described for the embodiment as shown in FIG. 1, except for the absence of the display devices 6 and 12. This embodiment is intended to be carried on, by, or in close proximity to the pregnant mammal within a suitable battery-powered electronics enclosure 15. In this embodiment of the invention, when the comparator 13 senses the onset of true labor an audible and/or visual signal is given. 
     A pictorial representation of the embodiment described in FIG. 4B is shown in FIG. 4A. This representation depicts the electronics enclosure 15, electrodes 1 and associated wiring and connectors, an ON/OFF indicator 16, a low battery indicator 17, and the labor warning signal 18. That is, in this embodiment the Display3 is embodied in the labor indicator 18. 
     A further embodiment of the LOM, designated as 34&#39;, is shown in FIG. 5. This embodiment is intended to enable the remote monitoring of a pregnant mammal via a communications link. In this embodiment of the invention, the output of the A/D converter 3 is stored in a storage device 20. When the comparator 13 senses the start of true labor, a signal is sent via a control link 13a to a transmitter 21. The transmitter 21 accesses the storage device 20 and transmits the data contained therein via a telemetry link 22 to a practitioner&#39;s console 30. The telemetry link 22 can be any suitable data transmission means, such as an RF link, a telecommunications link, and a data link. 
     FIG. 6 is a block circuit diagram of the practitioner&#39;s console 30. A receiver 23 receives the data from the telemetry link 22 and stores the received data in a storage device 24. The data is then replayed, at high speed, into the envelope detector 4, and is subsequently processed as described with regard to FIG. 1B. 
     Alternatively, and referring again to FIG. 5 in the LOM 34&#39;, the input to the transmitter is coupled instead to the storage device 7, or to the storage device 11, thereby simplifying the construction of the practitioners console 30. That is, in these other embodiments the practitioner&#39;s console 30 relies on the processing of the contraction-related signals that is performed by the LOM 34&#39;. In these other embodiments it may still be desirable to provide the storage device 20, and to also transmit the &#34;raw&#34; A/D converter data from the storage device 20 to the practitioner&#39;s console 30. In this manner the practitioner&#39;s console 30, is enabled to check the accuracy of the calculations performed by the LOM 34&#39;, and/or to employ other and/or more sophisticated calculations on the raw A/D converter data. 
     The practitioners console 30 is modified, in accordance with the instant invention, so as to determine the discriminant values by using the average frequency of the electrical activity, as described above. 
     As was also described above, it is preferred to calculate some form of the Fourier transform (e.g., a fast Fourier Transform, a discrete Fourier Transform, a Power Spectral Density (PSD)) in the block 8 of FIGS. 1B, 4B, and 5. In a presently preferred, but not limiting, embodiment of this invention the LOM employs a discrete Fourier transform (DFT) when processing the filtered spectral data obtained from the electrodes 1. 
     By example, one suitable software package for performing a discrete Fourier transform is available from National Instruments, and is described in a publication entitled &#34;Fast Fourier Transforms and Power Spectra in Lab Windows®&#34;, Application Note 020, J. Pasquarette, National Instruments Corporation (8/91). Furthermore, and as is described at page 10 of this publication, the software package includes a function named Spectrum that enables the power spectrum of a signal to be determined. Thus, the PSD blocks 8 of FIGS. 1B, 4B, 5 and 6 can also be implemented with this particular software package. 
     In more detail, the power spectrum, S xx , of a time domain signal is defined by: S xx  (k)=|X(k)| 2  /n 2 , where S xx  (k) is the discrete power spectrum of a discrete time domain signal x(i), X(k) is the discrete Fourier transform of x(i), and n is the number of data points. 
     Although described above in the context of discrete circuit elements such as envelope detectors, digital filters, etc., it should be appreciated that all or most of the illustrated blocks could be implemented with a suitably programmed data processor, such as a digital signal processor (DSP). In like manner the various separate memories could all be contained within a single memory device or medium. Thus, while the invention has been particularly shown and described with respect to presently preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.