Electrocardiographic means

Nov. 2, 1965

3,215,136

N. J. HOLTER ETAL

ELECTBOCARDIOGRAPHIG MEANS

Filed July 6, 1962

Sheets-Sheet 1

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Nov. 2, 1965

3,215,136

N. J. HOLTER ETAL

ELECTROCARDIOGRAPHIC MEANS

Filed July 6, 1962

Sheets-Sheet 2

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United States Patent Office

3,215,136

Patented Nov. 2, 1965

3,215,136 ELECTROCARDIOGRAPHIC MEANS

Norman J. Holter and Wilford R. Glasscock, Helena, Mont, assignors to The Holter Research Foundation, Inc., Helena, Mont, a corporation of Montana ⁵

Filed July 6, 1962, Ser. No. 207,963

Claims. (Cl. 128—2.06)

The present invention relates to means for processing electrocardiac signals and more particularly to means for ]θ obtaining large quantities of electrocardiac signals and to means for facilitating the processing and observing of large volumes of such signals in a short interval of time.

As is well known, there are electrical signals that circulate upon the surface of a person’s skin as a result of 15 the expansions and contractions of the cardiac muscle. These electrical signals are the so-called electrocardiac or EKG signals and have waveshapes that have many known relationships to the action of the cardiac muscle and to the condition thereof. By placing electrodes on the pa- 20 tient’s skin, these electrocardiac or EKG signals may be sensed and by employing a suitable cathode ray oscilloscope, electrocardiograph or similar device, the waveshapes of the EKG signals may be visually presented for viewing by a highly trained person such as an electro- 25 cardiologist. The electrocardiologist may then visually observe the electrocardiogram and endeavor to make a determination of the characteristics of the heart.

Although many heart disorders have been detected in this manner, it has been found that a failure to detect an ³⁰abnormal EKG signal has not been an altogether reliable indication of a normal and healthy heart. During their early stages, some forms of disorders produce abnormal EKG signals at only random or infrequent intervals and/ or only when the patient is engaged in certain forms of ³³activity. As a result, to obtain a record of such abnormal EKG signals, it is necessary to make EKG recordings that extend over protracted periods of time while the patient is engaged in a wide variety of activities. Since heretofore it has been necessary for the patient to ⁴⁰be directly connected to the electrocardiograph as a practical matter, it has been extremely difficult if not impossible to accumulate EKG recordings over an extended period of time and especially while the patient is active during the period. Moreover, if such large volumes of ⁴⁵recordings could be made, it would be impractical for a highly trained person, such as an electrocardiologist, to expend the excessive amounts of time required for carefully reviewing the recordings. It is also extremely difficult, if not impossible, for an operator to manually ex- ⁵⁰amine the large volume of data and accurately identify all abnormalities that are of a very random and infrequent nature. It may thus be seen that although electrocardiographs have been a very valuable research and clinical tool, they have not been entirely satisfactory as they are ⁵⁵subject to numerous operating difficulties that have limited their usefulness, particularly in routine check-ups for detecting aliments during their early stages.

It is now proposed to provide electrocardiographic means that will overcome the foregoing difficulties. More ^bUparticularly, it is proposed to provide electrocardiographic means that will permit the accumulation of large volumes of EKG signals from a patient even though he is engaged in virtually unlimited types of activities and that will permit the observing and analyzing of large volumes of ⁶⁵EKG signals in short intervals of time, and will also insure a more accurate detecting and identifying of abnormal beats which may occur only infrequently and/or randomly during an extended recording period. After detecting and locating abnormal beats, the cardiologist ⁷may concentrate his entire attention on such individual beats, thereby making a more efficient use of his skills. This is to be accomplished by providing electrocardiographic means wherein the EKG signals may be recorded by means of a small, compact recorder worn by the patient while he engages in any desired activity. The EKG signals may then be visually presented on a device such as a cathode ray oscilloscope for subsequent study. Although the recordings may be recorded in real time, over an extended time interval, they may be reproduced at a greatly accelerated rate. This will not only permit reproducing the EKG signals in a small fraction of the time required to accumulate them, but will also permit the waveforms to be superimposed upon one or more of the preceding waveforms.

Since EKG signals are of a generally aperiodic nature, i.e., occur at random time intervals, heretofore it has been impossible to cause the signals to provide a visual display wherein the waveform will remain stationary. It is, therefore, also proposed to provide means for triggering the display means from a predetermined portion of the signal so that the display of the waveform may be made to remain stationary. In one form of the present electrocardiac means, the reproduction portion includes means that are effective to produce a pair of EKG signals wherein one of the signals is delayed from the other. This will permit triggering the display means from the earlier of the signals whereby the display means will include any desired amount of the waveform of the EKG waveform and at least the important portons of the waveform will remain stationary.

It may thus be seen that the entire waveforms of succeeding EKG may be superimposed on each other. As long as the EKG signals are all substantially identical, the display pattern will be substantially steady. However, in the event of the presence of one or more abnormal EKG signals, the display pattern will include an erratic portion that differs from the steady waveform portion and will appear as an abrupt and perceptible change in the pattern. As a result, an observer may review large volumes of EKG signals in a much shorter time than has heretofore been possible and may readily observe the presence of even random and erratic beats. In addition, means may also be provided which will simultaneously, audibly reproduce a sound which corresponds to the EKG signal. In the event of an abnormal EKG signal, the sound will have a corresponding variation. Thus, the operator may simutaneously audibly and visually perceive any abnormalities.

Although it has been possible to determine the average pulse rate over an extended period of time due to the aperiodic or random nature of the pulse rate, it has been extremely difficult if not impossible to determine the instantaneous pulse rate or fluctuations in the rhythm thereof. It is, therefore, also proposed to provide means that will sense the time interval between successive heart beats and will be effective to produce a visual display of the time intervals. This display will thus provide a continuous indication of the instantaneous pulse rate and fluctuations in the rhythm thereof.

These and other features and advantages and objects of the present invention will become readily apparent from the following detail description of one form of the present invention particularly when taken in connection with the accompanying drawings wherein like reference numerals refer to like parts, and wherein:

FIGURE 1 is a block diagram of a recording portion of electrocardiographic means embodying one form of the present invention;

FIGURE 2 is a block diagram of a reproducing portion of electrocardiographic means embodying another form of the present invention;

FIGURE 3 is a view of one of the visual display pat-

3,315,138 terns produced by one section of the reproducing portion of FIGURE 2; and

FIGURE 4 is a view of another of the visual display patterns produced by another section of the reproducing portion of FIGURE 2.

Referring to the drawings in more detail, the present invention is embodied in electrocardiographic means 10 for obtaining electrocardiac or EKG signals and displaying them for observation. In the present instance,_ this means 10 includes a recording portion 12 for receiving and recording EKG signals and a reproduction 14 for reproducing the recorded EKG signals and producing a visual and audio reproduction of the signals.

The recording portion 12 of the electrocardiographic means which is shown in FIGURE 1 is preferably a substantially conventional tape recorder for recording signals on a magnetic tape 16. However, it is preferably a selfcontained unit having its own power supply so as to be capable of operating for extended periods of time. In addition, the tape recorder is also preferably miniaturized to be sufficiently small to be worn by a patient without materially interfering with his actions. As a result, it will be possible for the patient to indulge in a wide variety of activities while his EKG is being recorded on the tape 16. For example, the EKG signals recorded may be those produced while the patient engages in his normal day-to-day living or, if desired, they may be those produced as a result of the patient engaging in certain prescribed exercises designed to reveal predetermined characteristics of the heart.

The input to the recorder 12 includes an amplifier 18 which may be of conventional design for receiving EKG signals and amplifying them to a more useful level. The input 20 of the amplifier 18 may include one or more electrical conductors for being electrically connected to one or more electrodes for being secured to the patient. The electrodes may be disposed in any suitable location on the patient’s skin such, for example, as the so-called unipolar position.

Although the various characteristics of EKG signals obtained in the foregoing manner may vary throughout a wide range, as a general rule in a normal or healthy person the EKG signal will have a waveform that includes, in the following sequence, a P wave, a QRS complex, a T wave that is separated from the QRS complex by an ST segment. Although there may be several additional waves present in a normal EKG signal, since they will have little or no affect on the operation of the present invention, for purposes of simplicity, the present description will be confined to a waveform of this nature.

The P wave is normally a small, positive pulse that corresponds to the initial impulse that triggers, the commencement of the heartbeat. Shortly after the P wave, there is a quiescent or isoelectric portion of substantially uniform amplitude which separates the P wave from the QRS complex. The QRS complex is substantially coincident with the actual expansion and contraction of the cardiac muscle producing the actual pumping action. This complex commences and terminates with the socalled Q and S waves, respectively. These waves are generally relatively small negative pulses, and are separated from each other by the intervening R wave. The R wave, which is the most conspicuous portion of the EKG signal has the appearance of a positive “spike” with a sharp rise and fall and a relatively short duration, normally on the order of up to 0.03 to 0.04 seconds.

Following the QRS complex, there will usually be a T wave which is separated from the S wave by the socalled ST segment. The T wave represents the end of the beat and is usually followed by a substantially quiescent signal. This quiescent condition will continue to exist until the following EKG signals occurs as indicated by the commencement of the succeeding P wave.

As previously stated, the input 20 of the amplifier 18 may be connected directly with the pick-up electrodes by one or more conductors. However, if desired, it may be indirectly connected with the electrodes by means of a telemetry system wherein a radio transmitter worn by the patient is connected with the electrodes so as to radiate a signal containing the EKG signal and a radio receiver receives the radiated signals and feeds the EKG signal to the input 20 of the amplifier 18.

The amplifier 18 may be of any conventional design provided it has a substantially uniform amount of gain over an adequate bandwidth to effectively amplify all of the components in the EKG signal without any distortions thereof. The signal at the output 22 from the amplifier 18 will thus be a faithful reproduction of the EKG signal but of increased amplitude.

The output 22 from the amplifier 18 may be connected with one of the inputs 24 to a mixer 26. The mixer 26 be of a conventional variety such as is employed in tape recording devices. The mixer 26 is effective to mix the EKG signal on the input 24 with the signal on a second input 28. The second input 28 may be connected to a bias oscillator 30 which may be of a variety commonly employed in tape recorders. It may thus be seen that the signal on the output 32 of the mixer 26 will be an EKG signal suitable for recording directly onto the magnetic tape 16.

The output 32 is electrically connected to a pair of recording heads 34 and 36. These heads 34 and 36 may be substantially identical to each other and connected in series so that identical signals will flow through each. The heads 34 and 36 are particularly adapted to have the magnetic tape pass therethrough so that each head may separately record signals on separate laterally displaced tracks extending axially along the tape 16.

The magnet tape 16 may have a portion wound upon a supply reel 38 and a portion wound on a take-up reel 40 so that the portion therebetween may be disposed in the heads 34 and 36. Driving means may be operatively connected to one or both of the reels 38 and 40 so as to drive the tape through the heads 34 and 36. In order to obtain a substantially constant tape velocity through the heads, it has been found desirable to employ a synchronous motor 42 that is controlled by a substantially constant frequency oscillator 44.

In order to permit recording of large volumes of EKG signals over extended periods of time and to facilitate the subsequent reproducing the EKG signals at an accelerated rate, it is desirable for the tape speed during recording to be slow. Although the tape speed may be any desired amount, by way of example, it has been found that a speed on the order of 7½ inches per minute will permit a faithful recording of the EKG and will permit recording over extended time intervals on a short length of tape. However, the tape speed may be faster or slower if so desired.

As previously stated, the two recording heads 34 and 36 are physically laterally displaced so that they will record on separate tracks and they are electrically interconnected so that the recordings on the two tracks will be substantially identical. As will subsequently become apparent, it is also desirable for the two heads 34 and 36 to be “staggered” or displaced longitudinally of the tape 16. The recording in the two tracks will thus be displaced from each other by an amount corresponding to the spacing between the heads. By way of example, in one operative embodiment, the two tracks were displaced from each other by a distance of approximately 1½ inches.

The reproduction portion 14 of the electrocardiac means 10 is preferably a separate unit from the recording portion 72. It may be larger and heavier so as to be suitable for use in a doctor’s office or in a laboratory. The reproduction portion 14 includes a substantially conventional tape deck 40 having a supply reel 42 and a take-up reel 44 with the tape 16 extending therebetween. A. motor 46 is interconnected with one or both, of the reels,

3,215,138 so as to drive the tape 16 therebetween. Although the speed of the tape 16 may be any desired amount, it has been desirable to employ a drive mechanism and/or motor that will cause the speed of the tape 16 to be equal to the recording speed for example 7½ inches per minute and a higher speed. The high speed is preferably a standard tape speed such as 7½ inches per second as this will permit the tape deck 40 and transport mechanism to be of substantially conventional design. If the recording speed is on the order of 7½ inches per minute and the playback speed is on the order of 7½ inches per second, this will produce a change in speed of approximately 60 to 1. As a result, one minute of playback time will represent sixty minutes or one hour of recording time and twenty-four hours of real time recordings may be reproduced in twenty-four minutes. As a result, this will make it practical to review large volumes of recordings in a relatively short time interval. It should be noted that if it is so desired, the speed ratios may be larger or small so as to reduce or increase the reproducing time. A pair of playback heads 48 and 50 may be provided for scanning the tape 16 as it passes therethrough.

The first head 48 may act as a trigger head and is operatively interconnected with a horizontal trigger branch 52 for actuating the horizontal sweep circuits in a cathode ray oscilloscope 54. The second head 50 may act as a signal head and is operatively interconnected with a vertical branch 56 for actuating the vertical sweep circuits of the cathode ray oscilloscope 54.

These heads 48 and 50 are displaced laterally to scan only one of the two recorded tracks. In addition, the two heads are “staggered” axially of the tape 16 so that the recorded EKG signal on one track will pass through the first head 48 a predetermined time interval before the other head 50. As a result, the signal in the head 50 will be delayed from the signal in the head 48 by a predetermined time interval. One or both of the heads may be adjustable longitudinally of the tape 16 so as to vary the amount of this time delay.

The signal head 50 is interconnected with the vertical deflection branch 56 by means of a reversing switch 58. This reversing switch 58 may be movable between a first set of fixed contacts and a second set of fixed contacts so as to effectively reverse the polarity of the signal produced by the head 50. The movable contact in the switch 58 is operatively interconnected with the input to a signal preamplifier 60.

The preamplifier 60 may be of conventional design for increasing the amplitude of the signal to a more useful level. It should be noted that since the signal reproduced by the head 50 is a function of the magnetic recordings travelling past the head, the signal fed into and through the preamplifier 60 will be a derivative of the original recorded EKG signal. Accordingly, the output from the preamplifier 60 may be interconnected with a compensating amplifier 62. This amplifier 62 in addition to further amplifying the signal may include compensating or integrating means for effectively restoring the signal to its original EKG waveshape. The output 64 from the compensating amplifier 62 may be connected to one of the fixed contacts in a selector switch 66.

If so desired, a non-compensating amplifier 68 may also be provided. This amplifier 68 is interconnected with the output of the signal preamplifier 60 so as to receive the derivative signal. The non-compensating amplifier 68 may be free of any integrating characteristics. As a result, the signals from the amplifiers 62 and 68 will be of equivalent amplitude, but the signal from the amplifier 68 will still be the differentiated signal. The output 7® of the compensating amplifier 68 may be interconnected with a second fixed contact in the selector switch 66.

The movable contacts in the switch 66 may be interconnected with the input to an audio amplifier 71 which will be effective to amplify the signal supplied thereto and drive a loudspeaker 72. The loudspeaker 72 will thus be effective to produce an audible signal which will correspond to the EKG signal or the first derivative thereof depending upon the setting of the selector switch 66. In the event that the playback speed is on the order of 60 times the recording speed, and the original pulse rate was on the order of 60-150 beats per minute, the fundamental frequency of the audio signal will be on the order of 60-150 cycles per second. Thus, the audible signal will have the characteristics of a low frequency growl. If the successive EKG signals are substantially identical and occur at a substantially constant rate, this growl will have a correspondingly uniform characteristic. However, in the event there are any irregularities in the EKG signals, there will be a corresponding variation in the audible signal. As a result, the operator will very quickly perceive that a change has occurred.

In addition, the movable contact in the switch 66 is operatively interconnected with one side of a differential amplifier 74. This amplifier 74 may be of a conventional design and is effective to amplify only the difference between the signals on the opposite sides of the amplifier. The opposite side of the amplifier 74 may be interconnected with a variable reference bias source 76. This source 76 will be effective to supply a D.C. bias signal for maintaining one side of the amplifier 74 at a desired level. Since the differential amplifier 74 will be effective to amplify only the difference between the EKG signal on the one side and the reference bias on the opposite side, varying the reference bias will be effective to vary the D.C. level of the output signal without in any way varying the configuration or waveform of the signal

The output of the amplifier 74 may be operatively interconnected with the input 78 to the vertical deflection circuitry of a suitable display device such as the cathode ray oscilloscope 54. The electron beam in the oscilloscope 54 will thus be deflected vertically in response to the signal from the amplifier 74. In addition, the vertical position or D.C. level of the signal may be varied by adjustment of the reference bias source 76.

The second or trigger head is operatively interconnected with the horizontal branch 52 by means of a reversing switch 80. This switch 80 may be substantially identical to the first reversing switch 58. The switch includes movable contacts that may be positioned to engage a first set of fixed contacts or a second set of fixed contacts so that the polarity of the signal on the movable contacts may be reversed.

The movable contact in the switch 80 may be interconnected with the input to a trigger preamplifier 82. This amplifier 82 may be substantially identical to the amplifier 60 and is effective to amplify the signal from the playback head 48 to a more useful level. The amplifier 82 is preferably free of any form of compensating or integrating circuitry so that the signal in the output 84 of the amplifier 82 will be a derivative of the EKG signal as originally recorded on the tape 16.

More particularly, the signal in the output 84 is shown at A. If the switch 80 is set to feed a positive signal to the amplifier 82, normally, the derivative signal will include a large positive pulse corresponding to the beginning of the R wave. This results from the rapidly rising leading edge of the R wave. This will be followed immediately by a large negative pulse. This pulse results from the rapidly falling trailing edge of the R wave. It may thus be seen that the negative pulse is coincident with the termination of the R wave and beginning of the S wave.

If desired, a calibrating switch may be connected to the input of the preamplifier 82 for operatively interconnecting the amplifier with a source of 60 cycles. In the event the reproduction portion 14 operates 60 times the speed of the original recording, a 60 cycle calibrating signal will correspond to a pulse rate of 60 beats per minute and will thus provide a known reference for checking the electrocardiac means 10.

The output 84 from the trigger preamplifier 82 is inter3,215,136 connected with the input to a clipper circuit 86 so as to feed the derivative signal A thereto. This clipper circuit 86 may be of a substantially standard variety for suppressing or clipping all negative portions of a signal. As a result, the signal on the output 88 will be only a positive pulse such as shown at B. It may thus be seen that with the switch 80 in the position shown, the signal B in the output 88 from the clipper circuit 86 will be a positive pulse corresponding to the commencement of the R wave. However, if desired, the switch 80 may be reversed to invert the signal A on the output 84 from the trigger preamplifier 82. The positive portion of the derivative signal A will then correspond to the trailing edge of the R wave and the signal B from the clipper circuit 86 will be a positive pulse substantially coincident with the termination of the R wave and/or the beginning of the S wave.

The output 88 of the clipper circuit 86 may be connected to the input of a multivibrator 90. This multivibrator 90 may be of the one-shot variety. That is, each time a triggering signal such as a positive pulse is applied to the input thereof, the multivibrator 90 will change its state for a predetermined time interval. The duration of the time interval will, of course, be determined by the various characteristics such as the time constants of the circuit. In the present instance, a variable capacitance 92 may be included in the multivibrator 90 so that the time constant may be varied throughout a predetermined range. It will thus be seen that the output signal C from the multivibrator 90 will be a positive squarewave having a time duration determined by the setting of the capacitance 92.

The output from the multivibrator 90 may be operatively interconnected with the input to a differentiator 94. The differentiator 94 is effective to differentiate the squarewave pulse C from the multivibrator 90 and produce a positive pulse and a negative pulse corresponding to the commencement and termination of the squarewave. However, the positive pulse is clipped or otherwise suppressed so that only the negative pulse D will remain. The timing of this pulse will correspond to the termination of the squarewave C. Accordingly, the pulse D will be delayed behind the positive portion of the derivative signal A by the duration of squarewave C.

This negative pulse D from the differentiator 94 may be fed into a saw-tooth generator 96. The potential of the output 98 of the saw-tooth generator will gradually increase at a substantially uniform rate once the generator is triggered by the pulse D. This pulse will reset the generator 96 to zero and cause a new saw-tooth waveform to build up at a substantially uniform rate. This build-up will be free to continue until a succeeding pulse is applied to the input thereof at which time the potential will return to zero and start to repeat the build-up. It may be seen that the maximum amplitude of the saw-tooth will correspond to the length of time that the waveform is permitted to build up. Thus, the amplitude of each saw-tooth will be proportional to the time between the successive pulses D applied thereto. The output 98 of the generator 96 may be interconnected with a fixed contact in a selector switch 100.

In addition, a second saw-tooth generator 102 may be operatively interconnected with a second fixed contact in the selector switch 100 for supplying a series of saw-tooth waveforms thereto. This generator 102 is preferably of the free-running variety. As a result, the period of the saw-tooth waves will be substantially constant. Although the period may be any desired amount, it is preferably several times as long as the real time interval between the EKG signals as recorded from the patient. By way of example, the period may be on the order of ah interval of approximately 2½ seconds.

The movable contact in the selector switch 10® is operatively interconnected with one side of a differential amplifier 104. Although this amplifier may be of any suitable design, in the present instance it is substantially iden8 tical to the first differential amplifier 74. That is, it is effective to amplify only the difference between the two sides. The second side of the amplifier 104 may be operatively interconnected with an adjustable reference bias 5 source 106. The source 106 will permit manual adjustment of the D.C. level in the output of the second side. Since the output signal will represent the difference between the saw-tooth and the reference bias 106, the output signal will include a D.C. component that may be varied without affecting the shape of the saw-tooth wave.

The output 108 of the differential amplifier 104 may be interconnected to the horizontal deflection system of the cathode ray oscilloscope 54. The saw-tooth potential on the output 108 will thus cause the electron beam to be 15 swept across the face of the oscilloscope 54 at a substantially uniform rate. It should be noted that since the generator 96 will be triggered from the trailing edge of the squarewave pulse, the horizontal sweep will be synchronized with either the commencement of the R wave 20 or the S wave, depending upon the setting of switch 80, but will be delayed therefrom by the duration of the pulse C from the multivibrator 90. Accordingly, the commencement of the horizontal sweep in the oscilloscope 54 will be coordinated with a particular portion of the signal 25 from the signal reproduction head 50 even though the signal is of an aperiodic or random nature.

In addition, if it is so desired, a second oscilloscope 108 may be provided. The horizontal sweep circuitry of this oscilloscope 108 preferably is effective to require an 30 extended period of time. For example, the horizontal circuitry may be connected to the output of the free-running saw-tooth generator 102. This will require 2 or 3 seconds to complete a sweep and, during this time, one hundred or more EKG signals will be reproduced.

The vertical deflection circuitry may be interconnected to the output side of the differential amplifier 104. As will be remembered, the output signal from amplifier 104 will be a saw-tooth waveform that has a peak amplitude which is proportional to the time between each EKG 40 signal.

As a result, it may be seen that the display pattern on the face of the oscilloscope 108 will be similar to FIGURE 4 and will consist of a plurality of substantially vertical lines. The oscilloscope preferably includes a memory ₄₅ such as a long persistence screen whereby a large number of lines will always remain on the screen. The height of each line will correspond to the instant avenues pulse rate and uniformity of the upper edge of the pattern will indicate the rhythm.

₅₀ In order to employ the present electrocardiac means 10, the recording unit 12 may be interconnected with electrodes attached to the patient and placed on the patient in an operative condition. If the recording unit 12 is sufficiently “miniaturized” it will not materially inter55 fere with the normal activities of a patient. As a.result, the patient may engage in certain preselected activities or exercises that are specifically designed to reveal certain characteristics of the EKG signal. Or, if desired, the patient may carry' on his normal daily routine so as to 0Q determine the ability of the heart to withstand such a routine. Because of the large capacity of the recorder and the rapid processing of the EKG signals, it will now be practical to accumulate recordings of the patient’s EKG over extended periods of time. Thus, even if the patient 05 has only random and infrequent abnormal beats or has abnormal beats that occur only during particular types of activities, a sufficient volume of recordings may be obtained to permit a reasonably accurate determination of the patient’s cardiac condition.

Once an adequate volume of recordings has been made under the desired circumstances, the recording unit 12 may be removed from the patient and the reel of recorded tape 16 may be placed in the reproduction portion 14. The tape 16 will then be stripped off of the supply reel 75 42, drawn through the playback heads 48 and 50 and

3,215,136 wound onto the take-up reel 44. Although the speed at which the tape 16 travels through the heads 48 and 50 may be any desired amount, it has been found desirable to employ a relative standard speed such as 7½ inches per second. This speed will permit the use of a substan- _gtially standard tape deck. In addition, it will permit the use of a tape speed of 7½. inches per minute to be used in the recording unit 12 whereby the playback time will be sixty times shorter than the recording time. Thus, one second of playback time will represent one minute of real time recording and one minute of playback will represent ¹⁰one hour of recording. As a result, the EKG recordings for an entire twenty-four hour day may be reproduced in a period of only twenty-four minutes. This will now make the reviewing of an entire day’s EKG economically fea- ,sible.

As the tape 16 travels through the two heads 48 and 50, the EKG signal on the leading track will pass through the trigger head 48 first. The signal from the trigger head 48 will be amplified in and pass through the trigger pre- ₂₀amplifier 82. Since this amplifier 82 does not include any form of compensating or integrating circuitry, the signal A at the output 84 will be a signal that corresponds to the derivative of the original EKG. Its primary characteristic of this signal will be first a sharp positive pulse, if ₂₅switch 80 is in the position shown. This pulse corresponds to the beginning of the R wave. Secondly, there will be a sharp negative pulse corresponding to the ending of the R wave. This derivative signal will then pass through the clipper 86 and the negative pulse will be elim- ₃₀mated. The output 88 will thus include only a positive pulse such as signal B. The time of this pulse will correspond to the beginning of the R wave unless the switch 80 is reversed in which case it will correspond timewise to the S wave. This pulse B will then trigger the multivibra- 35 tor 90 and produce the squarewave pulse C. This pulse C will have a predetermined time duration regulated by the setting of the time control 92.

The pulse C from the multivibrator 90 will be fed into the differentiator 94. The differentiator 94 will be effec- 40 tive to produce a negative pulse D corresponding to the ending of the pulse C. This negative pulse will then be fed into the input of the saw-tooth generator 96 so as to cause it to gradually increase the saw-tooth wave. This saw-tooth wave will then be fed into the differential am- _4gplifier 104. As the potential of the saw-tooth wave changes about the reference level set by the bias source 106, the output potential will produce a corresponding change. This saw-tooth wave will then pass into the horizontal deflection circuitry of the oscilloscope 54 and cause the elec- _ggtron beam to be horizontally swept across the tube at a substantially uniform velocity.

At the same time that the beam is being swept horizontally across the face of the oscilloscope tube, a signal from the signal head 50 will feed a signal into the vertical _ggbranch 56. This signal will be amplified in the amplifiers 60 and 62 and converted into a form substantially identical to the original EKG. Assuming the selector switch 66 is positioned as shown, the differential amplifier 74 will then feed the EKG signal to the oscilloscope 54 and θθ cause the electron beam therein to be deflected vertically. The inter-action of the vertical and horizontal deflections of the beam will be effective to cause a trace or oscillogram to be visually displayed on the face of the oscilloscope that corresponds to the waveform of the EKG. _6gIt should be noted that the horizontal sweep will be triggered as a result of the R wave portion of the EKG signal on the trigger track passing through the trigger head 48. However, the commencement of the sweep will be delayed from the instant the R wave passes through the head by the time duration of the squarewave pulse C from the multivibrator 90. Since the recordings in the trigger track lead those in the signal track by a distance corresponding to the spacing between the recording heads 34 and 36 by properly positioning the playback heads 48 75 and 50, the R wave portion of the signal may pass through the trigger head 48 prior to the time the corresponding signal begins to pass through the signal head 50. It may thus be seen that prior to the beginning of an EKG signal in the vertical branch 56, the horizontal branch 52 will be triggered. The length of this time will be determined by the displacement between the two recording tracks and the relative displacement of the two- heads 48 and 50 and the duration of the pulse C. Thus, the horizontal sweep can be made to commence just prior to the beginning of each EKG signal.

Normally, the control 92 will be set such that the horizontal sweep will commence just prior to the P wave. As a result, each of the EKG waveforms displayed on the oscilloscope will commence at substantially the same point in the waveform. Thus, all of the waveforms will be synchronized with each other even though the signals are aperiodic or of a random nature. As may be seen from the display pattern shown in FIGURE 3, the waveform of each EKG signal will be superimposed over the preceding waveform. Since the EKG signals will be created at a rate on the order of at least 60 per second, the persistence of the oscilloscope tube and the operator’s vision will cause the waveforms to blend into a somewhat broad but well-defined pattern. In the event all of the EKG signals have waveforms that are substantially identical, the display pattern will show substantially the same waveform. It may then be determined by observing the pattern and general characteristics of the EKG waveform. However, in the event there are a few random or erratic EKG signals having waveforms differing from the remaining signals, they will produce a corresponding variation in the display pattern and simultaneously a change in the audible signal from the loudspeaker 72.

More particularly, assuming that practically all of the EKG signals are normal, they will produce a broad pattern such as the pattern 110 of FIGURE 3. Upon the occurrence of an ectopic beat and/or ST segment depression, the electron beam will be deflected outside of the normal pattern and produce an individual trace 112. Althrough the persistence of this individual trace 112 may be too short for the operator to study the details thereof, he will be immediately aware of its existence and the approximate time thereof. If so desired, the appropriate portion of the tape 16 may be re-played at its original recording speed, 7½ inches per minute in the present example, and the switch -100 may be set to cause the freerunning saw-tooth generator 102 to supply the horizontal sweep signals. Since the EKG signals will now be reproduced at their original rate and it will take several seconds, for example 2½ seconds, for a horizontal sweep to be made, several EKG waveforms will be displayed on the face of the oscilloscope. In addition, they will be created at -a sufficiently slow -rate to permit studying each individual waveform. Thus, each -abnormal EKG waveform may be located and identified and, if desired, may be permanently recorded, for example, by photographing the display pattern while the waveform or waveforms are present thereon.

While the reproducing portion 14 is running at 7½ inches per second, the oscilloscope 108 will be producing a display pattern such as shown in FIGURE 4. This display pattern will include a -large number of substantially vertical lines with the length of each line corresponding to the peak amplitude of the saw-tooth wave from the generator 96. Since this is dependent upon the time interval between each beat, the lines will be a function of the instantaneous -pulse rate. Thus, if the pulse rate is uniform and regular and fast, the -pattern will have -an appearance of the section 114. As the pulse rate slows down, the lines will lengthen such as in section 116. In the event the heart skips a beat, there will be a conspicuously long line such as 118, or if it provides a double beat, it will produce an equally consp-icous short line 120.

It may thus be seen that electrocardiac means have

3,215,136 been provided that now permit the accumulation of large volumes of EKG signals under virtually any desired type of operating condition and that also permit observing and accurately analyzing all of the accumulated EKG signals. Although only a single embodiment of the present invention has been disclosed and described herein, it will be readily apparent to persons skilled in the art that numerous changes and modifications may 'be made thereto without departing from the spirit thereof. Accordingly, the foregoing disclosure and description are for illustrative purposes only and do not limit the invention which is defined only by the claims that follow.