Abstract:
An electronic system is disclosed which examines the first derivative of an applied electrocardiographic (ECG) signal, recognizes the R-wave event therefrom, and discriminates against signal components which lie outside predetermined signal parameters in producing a rectangular pulse output. In addition, muscle artifact and pacers are discriminated against. The system includes an automatic gain control to eliminate the need for operator controls.

Description:
BACKGROUND OF THE INVENTION 
     In electrocardiographic measuring and monitoring systems, it is desirable to provide a two-dimensional display of time-varying ECG signals on the screen of a cathode ray tube. Typically, the horizontal (X) axis of the display represents time while the vertical (Y) axis represents signal amplitude. It is essential in producing a meaningful display that the horizontal sweeping of the beam across the cathode ray tube screen be synchronized with the ECG signal in a manner similar to that of a triggered oscilloscope. 
     It is also desirable in such systems to provide an alarm in the event of heart malfunction. Reliable detection of the electrical signal related to heart pumping action is essential both to the implementation of the synchronized display and to the measurement of heart beat rate to monitor the condition of the heart. However, it is extremely difficult to obtain reliable triggering and heartbeat information in the presence of motion artifacts, muscle noise, fluctuations in R-wave amplitude, and electronic heart pacer devices. 
     In previous systems, it has been a common practice to employ a level comparator to provide a trigger for the sweep and for the rate detector whenever the ECG signal amplitude surpasses a predetermined amplitude level. The R wave is normally of greater amplitude than any other component of the ECG cycle, and therefore is generally used for such triggering and rate detection purposes. However, a major drawback to this scheme is imposed in the presence of electrical and muscle noise on the ECG signal, which may have sufficient amplitude to inadvertently effect triggering of the sweep. Furthermore, devices such as pacers continue to operate and produce a triggered sweep long after a patient has expired. Therefore, it is imperative in monitoring situations such as in intensive care units that only the physiologically-produced signals be recognized. 
     SUMMARY OF THE INVENTION 
     According to the present invention, electrocardiographic signals are applied to a system of discriminators for detection of the R-wave event. In most ECG signals, the QRS complex thereof has the characteristic of exhibiting the greatest time rate of change of signal voltage, hereinafter referred to as  dv  /dt, of any portion of the signal which has a unidirectional slope lasting 15 milliseconds or more. Muscle artifacts and pacers may have greater  dv  /dt, but the time duration is shorter in any one slope. P and T waves may have a longer time duration in one slope, but have much lower  dv  /dt. 
     The ECG signals are first amplified to a suitable level and then may be passed through a low-pass filter to eliminate much of the highfrequency muscle artifact components. A differentiator then accentuates slopes having a  dv  /dt greater than a predetermined value while attenuating components having a lower  dv  /dt. A full-wave rectifier may be employed to eliminate polarity sensitivity. The rectified signal is then applied to an amplitude discriminator, which rejects the lower amplitude components and produces pulses corresponding in time duration to the accepted signal slopes, which have the greatest  dv  /dt and greatest time duration. The pulses are finally applied to a time discriminator which rejects pulses having a duration shorter than a predetermined time, and producing a rectangular pulse output when a qualifying pulse is recognized. Because of ECG signal characteristics, this rectangular pulse is coincident with one of the slopes in the R-wave component. The time discriminator, once accepting a qualifying slope, locks out further inputs for a predetermined time interval to prevent multiple pulse output in the event that two or more slopes within the Q to T interval may qualify. 
     The system may include an automatic gain control for the amplifier to which the ECG signals are applied in order to maintain a substantially consistent signal proportion for R-wave detection when the ECG signal is fluctuating or differs in amplitudes from patient to patient. A novel approach in this system is to provide a selective automatic gain control by applying the first derivative of the ECG signal to a second amplitude discriminator, which produces a control voltage in response to the time duration of a portion of the differentiated R wave. The control voltage is applied to a control amplifier to appropriately vary the gain of the ECG amplifier. In this manner, only the R-wave component of the ECG signal is affected by the automatic gain control. In addition, the automatic gain control may include a provision to cause the gain of the ECG amplifier to rapidly increase to its maximum value in the event of loss of an output signal from the system, so that the system can quickly recover to detect a low-amplitude ECG signal. 
     A pacer detector may be employed to detect signals having a  dv  /dt greater than that which could be physiologically produced. A pulse coincident with the detected pacer pulse is applied to the amplitude discriminators to block entry of the pacer signals into the discriminators. 
     It is therefore one object of the present invention to detect the R-wave event in an applied electrocardiographic signal. 
     It is another object of the present invention to provide a rectangular pulse output coincident with the R-wave event in an electrocardiographic signal. 
     It is a further object of the present invention to provide reliable triggering for a physiological monitor on a beat-to-beat basis. 
     It is yet another object of the present invention to provide reliable heart beat rate information for purposes of heart malfunction detection. 
     It is yet a further object of the present invention to detect the R-wave event in an electrocardiographic signal in the presence of a pacer signal. 
     It is still another object of the present invention to detect the R-wave event in an electrocardiographic signal in the presence of motion artifact and muscle noise. 
     It is still a further object of the present invention to provide an R-wave detector which is insensitive to signal polarity. 
     It is another object of the present invention to provide an R-wave detector which will automatically respond to ECG signals of varying amplitudes. 
     It is a further object of the present invention to provide a novel automatic gain control to eliminate the necessity of operator controls. 
     Further objects, features, and advantages will be apparent from consideration of the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of the R-wave detector system according to the present invention; 
     FIG. 2 illustrates a typical electrocardiographic waveform, identifying the P, Q, R, S, and T waves; and 
     FIG. 3 is a waveform ladder diagram showing the time relationship of various waveforms throughout the system of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Turning now to the drawings, FIG. 1 shows a block diagram of the system according to the present invention. Each block of the block diagram comprises circuitry well known in the art, obviating an exhaustive dissertation of circuit operation. Instead, each block will be discussed in accordance with its contribution to the overall system. 
     An electrocardiographic waveform similar to that shown in FIG. 2 is applied to an input terminal 1. The waveform shown in FIG. 2 is a typical ECG waveform whose amplitude is plotted against time. The P, Q, R, S, and T components are identified, and it can be seen that the R wave has an amplitude and time rate of change greater than any other portion of the waveform. For this description, however, the ladder diagram of FIG. 3 will be used to show the waveform relationships throughout the system, and therefore the ECG waveform is shown in expanded time scale in FIG. 3A. 
     This ECG waveform may be amplified by a suitable variable gain amplifier 3 and then may be passed through a low pass filter 5 to remove extraneous signals having frequency components higher than those which would be considered part of the ECG waveform. In this manner, ECG waveforms of varying amplitudes can be amplified to a substantially consistent level, and much of the motion artifact and muscle noise which may be riding on the ECG waveform may be removed. 
     The ECG waveform is then applied to a differentiator 7, which may be a conventional resistive-capacitive differentiating network, to produce the first derivative of the ECG signal. The differentiated ECG waveform, shown in FIG. 3B, is then applied to a full-wave rectifier 9 to produce a waveform having components all of one polarity, for example, the positive components shown in FIG. 3C. 
     The rectified first deriviative of the ECG signal is then applied to amplitude discriminator 11, which may for example be an amplifier which is normally cut off and then biased into conduction when the amplitude of the rectified and differentiated signal exceeds a predetermined level. This predetermined level, hereinafter referred to as the recognition level, is shown as the first recognition level in FIG. 3C. The first recognition level is chosen to reject lower-amplitude components of the rectifier output. When the rectifier output exceeds the recognition level, the amplitude discriminator 11 produces an output voltage in the form of a pulse or a series of pulses as shown in FIG. 3D. 
     The output pulses from amplitude discriminator 11 are applied to a short time discriminator 13. The time discriminator 13 will act only on pulses having a time interval which exceeds a predetermined recognition interval, rejecting any pulse which has a duration shorter than the duration of the slope of a typical R wave. The recognition interval, then, can be in the range of from about 5 to 25 milliseconds, and any pulse having a time duration which exceeds the recognition interval will produce an output from discriminator 13. As can be seen in FIG. 3E, the rising portion of the time discriminator output is nearly coincident with the peak of the R wave of the ECG waveform shown in FIG. 3A. 
     The output of short time discriminator 13 may be applied to long time discriminator 15, which produces a voltage in the form of a rectangular pulse in response to the output from discriminator 13. Long time discriminator 15 operates for a time interval which is approximately the length of time required for one complete Q-through T-wave cycle which may vary from 100 to 250 milliseconds. The rectangular pulse output is available at output terminal 16, and may also be applied to short time discriminator 13 to inhibit further pulses from amplitude discriminator 11 once the time discriminator 13 has recognized a qualifying pulse during a single ECG waveform cycle. The time interval of long time discriminator 15 may be adjusted to accomodate the maximum expected heart beat rates. 
     The R-wave detection system according to the present invention may include an automatic gain control to electronically vary the gain of variable gain amplifier 3. The automatic gain control includes a second amplitude discriminator 17 connected to receive the rectified first derivative of the ECG waveform from rectifier 9, a low pass filter 19 through which the output of the amplitude discriminator 17 is passed, and a control amplifier 21 which produces a control voltage in response to the output of the low pass filter 19 to electronically control the gain of amplifier 3. 
     Since a consistent signal amplitude from one heart beat to another is desired for the first amplitude discriminator 11 to act upon, the recognition level of amplitude discriminator 17 for the automatic gain control is set to approximately the desired amplitude, or slightly less. FIG. 3C shows the second recognition level superimposed on the rectifier output waveform. The amplitude discriminator 17 will produce an output when the amplitude of the signal from rectifier 9 exceeds the second recognition level. If the second recognition level is not reached, there is no output from amplitude discriminator 17. Under these conditions, the control amplifier 21 increases the gain of variable gain amplifier 3. Eventually, the amplitude of the rectifier 9 output will surpass the recognition level of amplitude discriminator 17, and an output will be produced and passed through the low pass filter 19 to hold the input of control amplifier 21 at the level required to maintain the gain of variable gain amplifier 3 at a level which will produce a consistent signal amplitude from beat to beat. Low pass filter 19 averages the output of amplitude discriminator 17 between beats of ECG signals so as to provide the proper level restoration at the input of control amplifier 21 to maintain the proper amount of gain of variable gain amplifier 3. 
     In addition to the foregoing, the automatic gain control may include a provision to cause the variable gain amplifier 3 to rapidly increase to its maximum value if the rectangular pulse output from the system falls below a predetermined rate. Loss of an output is indicative of a loss of the ECG signal. This provision may include an integrator 25 and a comparator 27 connected between the system output terminal 16 and the low pass filter 19. Integrator 25 receives the rectangular pulses produced by time discriminator 15 and produces a sawtooth voltage which is reset to zero with the occurrence of each pulse and rises slowly between pulses. Comparator 25 which may be a conventional comparator having a predetermined reference level, receives the sawtooth voltage from integrator 25. If the output pulses from discriminator 15 cease or fall below a predetermined rate, which may be, for example, 10 to 20  output pulses per minute, the sawtooth voltage from integrator 25 will rise to the reference level of comparator 27, causing comparator 27 to produce an output which is applied through a portion of the low pass filter 19 to control amplifier 21. Under this condition, the input to control amplifier 21 is of a level which will cause the gain of variable gain amplifier 3 to immediately increase to its maximum value, facilitating a high-sensitivity system to detect a low-amplitude ECG signal if such is available. 
     The R wave detection system according to the present invention may also include a circuit for detecting the presence of a pacer or similar device and prevent this device from having any affect on the system. The ECG waveform at terminal 1 may also be applied to pacer detector 23. Pacer detector 23 may include, for example, a frequency selective amplifier which responds only to signals having a time rate of change greater than that which could be physiologically produced, and a switch which produces a short-duration output pulse coincident with such a detected signal. The output from detector 23 is applied simultaneously to amplitude discriminator 11 and amplitude discriminator 21 to block entry of the rectifier 9 output into the discriminators. Thus, the pacer has no effect on the operation of the system to detect R waves. 
     It will be obvious to those skilled in the art that many changes may be made in the details of the above-described preferred embodiment of the present invention without departing from the spirit of the invention. For example, there are many circuit configurations that can be arranged to perform the amplitude and time discriminator functions.