Cardiac signal augmentation apparatus

Abstract

Apparatus for receiving pacemaker artifact signals and EKG signals having means for separating the artifact from the EKG, standardizing the artifact, and recombining the signals for use, for example, in telephone transmission. The apparatus also includes means for monitoring the pacemaker rate.

Description

BACKGROUND OF THE INVENTION

The use of cardiac pacers to sustain life in heart disease cases is well-known. It is also well-known that cardiac pacers provide an electrical stimulation pulse to the heart, hereinafter referred to as pacer artifact pulses or pacer pulses, and that when the heart beats normally or in response to an electrical stimulation pulse it provides an electrical wave form called an electrocardiogram pulse, hereinafter referred to as EKG signals or pulses.

In the prior art, various means of sensing and recording the pacer artifact and EKG have been available. It has also been found advantageous to provide equipment which makes the information concerning the sensed artifact and EKG pulses available to the patient, and to transmit such information to a remote station such as a doctor's office, for example by a telephone system. One example of a telephone transmission system is U.S. patent application Ser. No. 235,252, filed: Mar. 16, 1972, entitled EVALUATION SYSTEM FOR CARDIAC STIMULATORS and assigned to the Assignee of this invention.

It has been discovered that a major problem exists in the recording and transmission of pacemaker artifacts and EKG signals; the problem is the result of the large voltage ratio between the pacer artifact signal and the EKG signal. When this ratio is large, the EKG signal baseline will be shifted to the discharge baseline of the pacer artifact signal, thus distorting the EKG pulses. A large artifact signal can also produce offscale deflection of a recorder with possible damage to the recording mechanism and dynamic aberrations due to such problems as amplifier overloads.

To overcome this problem, the apparatus of this invention separates and separately processes the EKG and pacer artifact signals, and augments the pacer artifact by providing a uniform or standardized pacer artifact pulse.

BRIEF DESCRIPTION OF THE INVENTION

Briefly described, the apparatus of this invention provides means for receiving the cardiac pacer artifact pulses and EKG pulses. First and second electrical channels are provided: the first channel having apparatus for substantially blocking pacer artifacts while passing the EKG pulses; the second channel having means for blocking the EKG pulses while passing the pacemaker artifact pulses, and means responsive to the pacemaker artifact for providing a uniform or standardized pacemaker artifact pulse. In the preferred embodiment the amplified EKG signal from the first channel and the standardized pacemaker artifact pulse from the second channel are recombined or summed prior to output. The output is then available to be attached to recorder units, or provided to transmission apparatus such as a telephone system. Also provided in the preferred embodiment is a rate monitor system for detecting the rate of operation of the cardiac pacer and providing a signal when the pacer rate is within predetermined boundaries.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of the apparatus of this invention;

FIG. 2 is a schematic drawing of a portion of the apparatus of this invention referred to as a slew-rate limiter;

FIG. 3 is a combined schematic and block diagram of a portion of the apparatus of this invention referred to as the rate monitor.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In FIG. 1 there is shown an input 10 which is adapted to be connected to means such as electrodes which are in turn adapted to be connected to or placed in receiving position relative to a body for the purpose of receiving pacemaker artifact pulses and EKG pulses. Input 10 is connected to the input of a DC amplifier 11. The output of amplifier 11 is connected to the input of a slew-rate limiter 12 and to the input of a high pass filter 18.

Limiter 12 is connected through a capacitor 13 to the input of an AC amplifier 15. The input of amplifier 15 is connected through a resistor 14 to ground. The output of amplifier 15 is connected to a first input on a summing circuit 16. The output of summing circuit 16 is connected to an output 26.

The output of filter 18 is connected through a capacitor 19 to the input of another AC amplifier 22. The input of AC amplifier 22 is connected through a resistor 21 to ground. The output of amplifier 22 is connected to the input of an absolute value amplifier 23. The output of amplifier 23 is connected to the input of a monostable multivibrator 24. The output of multivibrator 24 is connected to a junction 25. Junction 25 is connected to another input of summing circuit 16, and is connected to the input of a rate monitor 28.

Output 26 is adapted to be connected to recording or transmission apparatus, such as an input 50 connected to the input of a voltage controlled oscillator 51 which has an audio output device 52 for use with a telephone transmission system.

In operation of the apparatus of FIG. 1, the cardiac pacer artifacts and EKG pulses appearing at input 10 will be amplified by DC amplifier 11 and presented to limiter 12 and filter 18. At this point the pacer pulses and EKG pulses are separated for individual processing and augmentation. The EKG pulse channel includes slew-rate limiter 12 which limits the pacer pulse to a maximum rate of rise of a predetermined value, selected in the preferred embodiment to be approximately 10 volts per second. The operation of slew-rate limiter 12 insures that the large amplitude of the short, sharp pacer artifact is limited and therefore will not introduce drastic EKG pulse baseline shifts. Signals will pass undistorted through slew-rate limiter 12 as long as the signals do not exceed the amplitude-frequency limits of limiter 12.

The EKG signal which appears at the output of limiter 12 is AC coupled through capacitor 13 and resistor 14 into AC amplifier 15 for the purpose of bringing the signal up to a level where it can be used to modulate devices such as voltage controlled oscillator 51. The AC coupling of capacitor 13 and resistor 14 is found to be highly desirable because if a DC component were present it could saturate amplifier 15 due to the high gain in the total system.

The pacer pulse and EKG signal also appear at the input of high pass filter 18. This filter will substantially attenuate the EKG pulses to substantially block them from this channel. The output of filter 18 is AC coupled, through capacitor 19 and resistor 21, to the input of AC amplifier 22. The AC coupling also serves to differentiate the artifact to give pulse spikes at the output of amplifier 22, which spikes correspond to the leading and trailing edges of the cardiac pacer artifact pulse. The output of amplifier 22 is then sent through an absolute value amplifier 23, of a design well-known to those skilled in the art. Amplifier 23 will provide an output of positive going pulses regardless of the actual polarity of the pacemaker artifact. The function of absolute value amplifier 23 is necessary because it is well-known that the pacer artifact pulse can be either negative or positive going as referenced at input 10. Further, the artifact pulse may have only one sharp leading edge.

The output of absolute value amplifier 23 triggers monostable multivibrator 24 which provides at its output a uniform or standardized pacer pulse with predetermined pulse width and pulse amplitude.

The output of multivibrator 24 and the output of amplifier 15 are summed or combined in summing circuit 16, in the preferred embodiment, to provide an output signal at output 26 that is representative of the input signal at input 10. The output signal replaces the actual pacer artifact pulse with a standardized pacer artifact pulse thus providing clear signals capable of transmission and recording.

As shown in FIG. 1, in the preferred embodiment the combined signals at output 26 are adapted to be connected to input 50 of voltage controlled oscillator 51. The combined signals will frequency modulate oscillator 51, the output of which is acoustically coupled into the telephone system through apparatus 52. The signals may then be received at a remote station, such as a physician's office, and be deciphered by any one of many systems well-known to those skilled in the art. Additional features, such as a call-back feature may be readily provided.

Again referring to FIG. 1, the output of multivibrator 24, which constitutes the standardized pacer pulse, is also connected to the input of rate monitor 28. Monitor 28 will detect a change in pacer pulse rate and provide a visual indication to the patient if the rate has changed by a predetermined amount. The rate detection is done in a manner to be more fully described below in the discussion of FIG. 3.

In the above description of the apparatus of this invention as depicted in FIG. 1 it becomes apparent that there are two channels in the circuitry, one channel for accepting each of the cardiac pacer artifact pulses or the EKG pulses. In the EKG channel, a device called a slew-rate limiter is described which substantially removes the pacer artifact to avoid a baseline shift of the EKG signal. It is apparent that though a slew-rate limiter is used in the preferred embodiment, the advantages of applicant's unique structure can be achieved by using any device which will substantially block the pacer artifact while passing the EKG pulses substantially undistorted.

In the cardiac pacer artifact pulse channel a high pass filter is used to substantially block the EKG pulses while passing the artifact pulse. Thus each artifact will pass through the channel to cause a response in the form of a standardized or uniform pulse representative of the artifact. This uniform pulse is selected to be of a voltage and pulse width especially adaptable for transmission and recording. Such a uniform pulse is acceptable for the purposes of determining operation and rate of the cardiac pacer. The EKG signal, on the other hand, is preferably passed undistorted to appear in the truest possible form after transmission and recording so that a physician may examine the pulses and have available all of the information normally given in an electrocardiogram.

Referring now to FIG. 2 there is shown a schematic diagram of the slew-rate limiter used in the apparatus of this invention. The slew-rate limiter indicated generally at 12 comprises an input terminal 30 adapted to receive an input voltage designated V.sub.in. A resistor 31 is connected between terminal 30 and a first negative input of an OP-AMP 32. Another positive input on OP-AMP 32 is connected through a resistor 33 to ground. The output of OP-AMP 32 is connected to a junction 35. A diode 36 and a diode 37 have their cathodes connected, respectively, to junction 35 and another junction 40. Another set of diodes 41 and 42 have their anodes connected, respectively to, junctions 35 and 40. A first current source I.sub.1 is indicated as having a flow into the anodes of diodes 36 and 37, and another current source designated I.sub.2 is indicated as having a current flow out of the cathodes of diodes 41 and 42. Junction 40 is connected to an output terminal 45 designated as having a voltage output V.sub.out. A capacitor 46 is connected between terminal 45 and ground. A resistor 33 is connected between the output of OP-AMP 32 and the first mentioned input of OP-AMP 32. A capacitor 34 is connected across resistor 33, and a resistor 47 is connected between terminal 45 and the junction between resistor 33 and the first mentioned input of OP-AMP 32.

The purpose of the slew-rate limiter 12 is to limit the excursion of input electrical amplitudes to a specified rate of change, once a predetermined limit has been exceeded. When V.sub.in =O and I.sub.1 =I.sub.2 the system is in equilibrium and V.sub.out =O (neglecting the offset of the OP-AMP 32). When a positive going signal is applied at terminal 30, the output of OP-AMP 32 will go negative and the current I.sub.2 will be sunk by OP-AMP 32. The current I.sub.2 will sink current from capacitor 46. As long as predetermined slewing conditions are not reached, OP-AMP 32 will force V.sub.out to follow V.sub.in. However, if a positive pulse or a signal exceeding the predetermined slew-rate limits is applied to terminal 30, the output of OP-AMP 32 will go negative, its rate limited only by the slew-rate characteristics of OP-AMP 32. The voltage across capacitor 46 will now be discharged at a rate determined by I.sub.2. During this slewing operation, the output of OP-AMP 32 will go into saturation. The values of the feedback network comprising resistor 33 and capacitor 34 are selected to avoid high frequency oscillation. The magnitude of resistor 47 is chosen such that its current is less than the magnitude of current sources I.sub.1 and I.sub.2, to avoid loading capacitor 46. The slewing-rate can be adjusted by proper selection of component values.

Referring now to FIG. 3 there is shown a schematic and block diagram of the circuitry of rate monitor 28. There is shown a power input terminal 60 adapted to be connected to a power source indicated as V. A Zener diode 61 is connected between terminal 60 and ground to provide stabilization of the power source. Serially connected across diode 61 are a resistor 62, a potentiometer 63 and a resistor 64. An amplifier 65 is connected between the wiper arm of potentiometer 63 and a junction 66. Junction 66 is connected to ground through serially connected resistors 71, 72 and 73. A switch 75 has its wiper arm connected to move between a calibrate point connected to the junction between resistor 71 and 72, and a run point connected between resistor 72 and 73. The arm of switch 75 is connected to one input of an amplifier 76.

Terminal 60 is connected through a resistor 81 to a first input on an amplifier 82, which has a second input connected to ground. A capacitor 83 is connected between the output of amplifier 82 and the first input of amplifier 82. The output of amplifier 82 is also connected to an input of an inverting amplifier 87. The output of amplifier 87 is connected to an input of a comparator 88 and to an input of a comparator 76. Another input of comparator 88 is connected to junction 66. The outputs of comparators 76 and 88 are connected to a logic network 89 which is in turn connected to an indicator lamp 90.

A normally opened contact 84 of a switch 85 is connected across capacitor 83. The input switch 85 is connected to junction 25 as shown in FIG. 1.

Still referring to FIG. 3, the rate indicator is designed in such a manner that as long as the rate of the cardiac pacer falls within a predetermined region, a visual indication of this fact is given. Resistors 71, 72 and 73 set the percentage of excursion for the rate. On the occurrence of a standardized pacer pulse at junction 25, switch 85 will operate to close contacts 84 and thus discharge capacitor 83. As soon as contacts 84 open at the end of the occurrence of the standardized artifact pulse, capacitor 83 will begin to charge and the output of amplifier 82 will operate as an integrator providing an increasing voltage ramp. During this operation, switch 75 will be connected to the run position, and thus different voltages will appear at one of the terminals of comparators 76 and 88. The output of the integrator formed by capacitor 83 and amplifier 82 will be inverted by amplifier 87 and applied to the other input of each of comparators 76 and 88. As the output of the integrator rises, amplifier 76 will be the first to provide a positive output. At a later time, amplifier 88 will provide a positive output.

As can be seen, each standardized pacer pulse acts as a reset to the integrator. If this reset occurs while the output of comparator 76 is high and the output of comparator 88 is low, then the pacer artifact pulse is within preset limits and this will be recognized by logic circuitry 89 to turn on indicator light 90. If the reset pulse occurs at a later time, when the outputs of both amplifiers 76 and 88 are high, the logic circuitry will recognize this and not cause indicator light 90 to turn on. The limits are preset to allow a desired percentage of change of rate before indicating the problem.

To set the repetition rate desired, switch 75 is moved to the calibrate position and potentiometer 63 is rotated until indicator light 90 starts blinking. Switch 75 is then switched back to the run position and the system will now be ready to monitor the patient who is wearing it at the time the calibration occurred.

From the above description of the operation of the apparatus of this invention and of various circuitry it involves, it becomes apparent that other circuit designs could be utilized without departing from the spirit of the invention which includes the separation of cardiac pacer artifact pulses from EKG pulses, the provision of a standardized artifact pulse, and the adaptability of the output for transmission and recordation.

Claims

What is claimed is:

1. In cardiac instrumentation apparatus including electrode means adapted to be connected to a body to receive cardiac pacer pulses and EKG pulses, the improvement comprising: input means for connection to the electrode means to receive the pulses; output means; EKG channel circuit means; cardiac pacer pulse channel circuit means; the EKG channel means including first means for substantially blocking pacer pulses; the pacer pulse channel means including second means for substantially blocking EKG pulses; means connecting the first and second means to the input means; third means for providing uniform pacer pulses in response to pacer pulses; means connecting the third means to the second means; summing means connecting the first and third means to the output means for summing the EKG pulses with the uniform pacer pulses prior to output; and the output means connected to means for receiving and displaying pulse information representative of the input signal.

2. The apparatus of claim 1 in which: the first means comprises limiter means for preventing undistorted passage of pulses having a rate of rise over a predetermined rate.

3. The apparatus of claim 2 in which: the limiter means comprises a slew-rate limiter.

4. The apparatus of claim 1 in which: the second means comprises filter means for passing cardiac pacer pulses and substantially blocking EKG pulses.

5. The apparatus of claim 1 in which the third means includes: absolute value amplifier means; monostable multivibrator means; the amplifier means connected between the second means and the multivibrator means; and the multivibrator means connected between the amplifier means and the summing means.

6. The apparatus of claim 1 including: rate circuit means for sensing the rate of cardiac pacer pulses; the rate circuit means including rate display means; and means connecting the rate circuit means to the third means.

7. The apparatus of claim 6 in which: the rate display means comprises visual indicator means for providing a signal dependent on the pacer pulse rate.

8. In cardiac instrumentation apparatus including electrode means adapted to be connected to a body to receive cardiac pacer pulses and EKG pulses, the improvement comprising: input means for connection to the electrode means to receive the pulses; DC amplifier means, slew-rate limiter means; AC amplifier means; summing means; means connecting the DC amplifier means between the input means and the limiter means; means connecting the AC amplifier means between the limiter means and the summing means; means connecting the summing means to the output means; high pass filter means; second AC amplifier means; absolute value amplifier means; monostable multivibrator means; means connecting the filter means between the DC amplifier means and the second AC amplifier means; means connecting the absolute value amplifier means betweeen the AC amplifier means and the multivibrator means; means connecting the multivibrator means to the summing means; and output means connected to the summing means for receiving and displaying information therefrom.

9. The apparatus of claim 8 including: means connected to the multivibrator means for sensing cardiac pacer pulse rate; and means for visually indicating when the rate has changed by a predetermined percentage.