GB2034046A

GB2034046A - Cardiac Arrhythmia Detector and Recorder

Info

Publication number: GB2034046A
Application number: GB7932004A
Authority: GB
Original assignee: BENNISH A
Current assignee: BENNISH A
Priority date: 1978-09-18
Filing date: 1979-09-14
Publication date: 1980-05-29
Also published as: GB2034046B; CA1138531A

Abstract

Cardiac arrhythmias are detected by measuring the duration of the first derivative of the QRS component signal of a cardiac rhythm by using a counter 32 and comparing such duration to a running average duration, and/or by measuring the duration of the R-R interval by using a counter 34 and comparing such interval to a running average interval by using comparators 40, 46. A cassette recorder 48, 56, 58 is activated via a gate 42 in response to an arrhythmia detected by either one of the above techniques. The cardiac rhythm signal is fed to the recorder 48, 56, 58 through a delay network such that the recorded signal brackets the detected arrhythmia. The time and date of the arrhythmia may also be recorded on another channel of the cassette. A manual record switch 60 is also provided for separate wearer actuation to record what he considers to be unusual heart activity. <IMAGE>

Description

SPECIFICATION Cardiac Arrhythmia Detector and Recorder The present invention is directed to monitoring and recording apparatus and methods and, more particularly, to a method and apparatus for monitoring the heart activity of a cardiac patient, detecting cardiac arrhythmias and recording such arrhythmias in real time for later analysis.

Prior art apparatus of the subject type are illustrated in the following United States Patents: 3,759,248; 3,824,990; 3,832,994; 3,858,034; 3,861,387 and 4,023,564. Reference may also be had to Kosowsky, "Holter Monitoring", Journal of Continuing Education in Cardiology, Viol. 14, No. 2, February 1978, pages 13~21; and Dreifus et al, "Newer Techniques in Cardiac Monitoring", Heart and Lung, July-August 1975, pages 568-572.

Desirability of providing electronic apparatus for automatically detecting cardiac arrhythmias has heretofore been recognized, as exemplified in the above-noted prior art. Similarly, it has been recognized as desirable to provide combined detection and recording apparatus as a single unit which may be worn by a cardiac outpatient to monitor heart activity while the patient follows his daily routine. Such portable apparatus should be compact, rugged and lightweight, and yet responsive to a wide variety of cardiac arrhythmia types. Similarly, the apparatus should be responsive to critical arrhythmias indicative of cardiac pathology without being sensitive to extraneous false positives caused by noise or noncardiac muscular activity.The techniques and apparatus proposed in the prior art are not considered to satisfy the above-noted and other desirable, and in some cases critical, features in apparatus of the subject type.

Moreover, some portable apparatus which have achieved some commercial acceptance contemplate continuous recording of heart activity for a specific time duration, such as twenty-four hours. Where the activity is recorded on a magnetic tape cassette, for example, the cassette must be played back and monitored on a CRT by a skilled technician for pathological events. Such a technique is very expensive and does not achieve optimum reliability. Other prior art apparatus contemplate only manuallyactivated recording when a wearer thinks that he is experiencing unusual heart activity.

Accordingly, it is a general object of the present invention to provide improved cardiac monitoring apparatus which overcomes and satisfies the foregoing problems and difficulties.

More specific objects of the invention are to provide cardiac monitoring apparatus which may be worn by a cardiac outpatient without substantially interfering with his daily routine, which is automatically responsive to a wide variety of cardiac pathological events, which records such events in real time together with preceding and/or subsequent "normal" heart activity for later analysis, which simplifies arrhythmia detection techniques by recording only clinically significant events while minimizing false spurious recording, which significantly improves the effectiveness and efficiency of a battery-operated portable cardiac recorder, which has an enhanced signal-to-noise ratio for reduced sensitivity to extraneous or false positives, and/or which includes provision for manual activation of the recorder by the wearer both for recording what he considers to be unusual heart activity and for recording a "normal" cardiac signal at desired intervals for purposes of comparison.

A further object of the invention is to provide cardiac arrhythmia detection circuitry which is particularly well adapted for use in a portable cardiac monitoring apparatus and yet may be used to advantage in a full-scale hospital cardiac care unit.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which.~ Fig. 1 is a functional block diagram of a presently preferred embodiment of the cardiac arrhythmia detection and recording apparatus provided by the invention; and Figs. 2-9 are graphical waveforms useful in understanding operation of the invention, Figs.

3-9 being drawn to scale.

Fig. 1 illustrates a presently preferred embodiment of the detection and recording apparatus 20 provided by the invention as comprising a differential input amplifier 22 connected to conventional cardiac electrodes 24 suitably positioned on a patient 26. The output of amplifier 22 is connected through a high pass filter 28 to the input of a differentiating amplifier 30. The output of differentiating amplifier 30 is connected through respective zero-crossing detection circuits 31, 33 to the inputs of a QRS duration counter 32 and an R-R interval counter 34. Counters 32, 34 also have a counting input connected to a one-kilohertz clock oscillator 36.

One output of counter 32 is connected to a QRS duration average computation circuit 38 which preferably receives and stores a selected number of successive QRS duration signals from counter 32 and computes therefrom a running average QRS duration. In a preferred embodiment of the invention, averager 38 is responsive to the QRS components of the three cardiac rhythm signals immediately preceding the duration signal stored in counter 32. The outputs of averager 38 and counter 32 are connected to a comparator 40 which provides a first signal to an OR gate 42 when the duration indicated in counter 32 is greater or less than the running average duration indicated by averager 38 by an amount equal to or greater than twenty percent of the running average.

Similarly, R-R interval counter 34 is connected to an R-R interval averager 44 for computing a running average R-R interval over a plurality of, preferably three, immediately preceding QRS cardiac rhythmic signals. Counter 34 and averager 44 are connected to a comparator 46 which provides a second signal to OR gate 42 when the R-R interval indicated by counter 34 is greater or less than the running average R-R interval indicated by averager 44 by an amount equal to or greater than thirty percent of the average interval. OR gate 42 provides control signal to a tape drive circuit 48 in response to a signal from either comparator 40 or comparator 46, or both.

The output of input amplifier 22 is also connected through an analog-to-digital convertor 50 which provides digital signals in real time to a shift register memory 52 indicative of the amplitude of the cardiac rhythm signal. The gate input of memory 52 is connected to clock oscillator 36. Preferably, memory 52 comprises a 3000-word serial shift register memory which, when combined with a one-kilohertz clock oscillator frequency, provides digital signals at the memory output as a sampled replica of the cardiac rhythm input signals effectively delayed by a period of three seconds. Clock oscillator 36 is also connected to the sampling or gating input of a time and date memory circuit 54.The outputs of memories 52, 54 are connected through correspondingly respective digital-to-analog convertors 53, 55 to two input channels of a three-channel tape recorder circuit 56 which is responsive to a control signal from tape drive 48 to record the analog signals from convertors 53, 55 on two channels of a cassette tape illustrated schematically at 58. A manual record switch 60 which may be activated by the patient 26 has one output connected to a tape drive 48 and a second output connected to a third channel of tape recorder 56.

The output of tape drive 48 is a retriggerable pulsed signal 62 of predetermined duration, preferably on the order of six seconds. Thus, when the tape drive is activated by an control signal from OR gate 42, cardiac rhythm signals are recorded on cassette 58 which effectively bracket the detected pathological event. Stated differently, the delayed output from memory 52 precedes the detected cardiac arrhythmia by approximately three secpnds, while the tape drive signal 62 has a duration of about six seconds, so that the recorded rhythm signals includes approximately three seconds preceding and three seconds following the pathological event. Time and date of the detected event indicated by memory 54 are recorded on a second tape channel.When tape drive 48 is activated by manual record switch 60, a six-second cardiac rhythm signal is recorded, together with time and date on the second tape channel and an indication on a third tape channel that the recording was initiated by the manual record switch. If a second arrhythmia is detected during the recording interval, pulsed signal 62 is retriggered and the recording will continue for an additional six seconds. The entire circuit, including the tape drive, is battery operated.

Overall operation of the preferred embodiment of the invention illustrated in Fig. 1 will be evident from the foregoing discussion and need only be outlined briefly in connection with a "typical" cardiac rhythm signal of the type illustrated in Fig.

2. The rhythm signal of Fig. 2 includes an atrial P component which has a small positive amplitude, as on the order of 50 to 100 microvolts, and a relatively short duration, as on the order of forty to eighty milliseconds. Thereafter, following a brief interval of quiescence on the order of 1 50 milliseconds, the signal cycles through a QRS complex corresponding to depolarization of the cardiac muscle in which the signal swings briefly negative in the Q component, then a relatively sharp positive spike of about one millivolt in the R component, and thereafter through a brief negative swing in the S component. A nominal formal QRS duration of 100 milliseconds is typical. After another brief quiescent interval on the order of 200 milliseconds, a slight positive swing corresponding to the T component indicates repolarization of the cardiac muscle.The interval between cardiac rhythm signals is the inverse of the pulse rate and would be one second, for example, for a typical cardiac rhythm at sixty beats per minute.

The frequency cutoff of high pass filter 28 is preferably selected so as to block the low frequency P and T signal components from amplifier 30, the arrhythmia detection circuitry thereby being responsive solely to the QRS rhythm signal components. The input to differential amplifier 30 is set to be responsive to signals above a minimum threshold, and thereby co-operates with filter 28 to block low frequency and/or low voltage high frequency noise signals generated by patient muscular activity. It will be appreciated that the arrhythmia detection circuit is responsive to the derivative of the input signal via amplifier 30, and is essentially independent of signal amplitude.Thus, filter 28 and amplifier 30 co-operates to overcome problems inherent in prior art techniques which are responsive in whole or in part to the low frequency and low voltage P and T signal components and/or to QRS signal amplitude.

Zero crossing detection circuit 31 is preferably responsive to the "peak" of the first pulse component, i.e. the Q component in Fig. 2 with the P component having been blocked by filter 28 and amplifier 30. QRS duration counter begins counting at this first "peak" and continues counting until the differentiated signal stabilizes at zero, i.e. at the end of the S signal component.

Circuit 33 is responsive to the second zero crossing of successive differentiated rhythm signals, i.e. to the peak of the R signal component illustrated in Fig. 2. The QRS duration in counter 32 is compared with the average duration in averager 38 and is thereafter loaded into the averager between QRS rhythm signals for maintaining the running average. The R-R interval counter 34 is responsive to detection circuit 33 sequentially to stop the preceding R-R interval count, compare such count with the running average interval in averager 44, load the preceding count into averager 44 for maintaining a running interval average and then beginning the succeeding R-R interval count.All of such operations are, of course performed in microseconds and have no practical effect upon the successive R-R interval count.

While arrhythmia monitoring is taking place as described, all of the P, Q, R, S and T components are being continuously sampled and loaded into shift register memory 52 through convertor 50 for later recording if required. When analysis of recorded information is desired, cassette 58 may be taken to a clinic and played back on a tape reader connected to a conventional EKG strip chart recorder. The result will be a series of permanently recorded cardiac rhythm signals exhibiting potential pathological events, each preceded by a corresponding time signal, which may be analyzed by a medical clinician. Thus, the invention both provides for automatic portable recording and eliminates any requirement for scanning of several hours' recording searching for potentially pathological events.

It will be appreciated by persons skilled in the analysis and treatment of cardiovascular disorders that Fig. 2 illustrates a somewhat idealized rhythm signal for a healthy individual. For patients exhibiting some form of cardiac disorder the rhythm signal may vary substantially from that shown in Fig. 2. Indeed, for patients who have suffered permanent heart damage, one or more of the Q, R and S components may be exaggerated or obliterated. Thus, for purposes of the present description and the appended claims, the term "QRS" with reference to rhythm signal components must be read in its broadest aspects as representing the signal which results from the patient's electrical ventricular depolarization.

Similarly, the term "R-R interval" signifies the interval between successive rhythm signals, and need not necessarily be measured between identifiable "R" signal components.

In accordance with an important feature of the invention, the arrhythmia detection circuitry, as distinguished from the recording circuitry, is responsive only to the ventricular depolarization or QRS signal and blocks or ignores the atrial depolarization or P component and the ventricular repolarization or T component. (The atrial repolarization signal is masked by the QRS signal and, in any event, is of sufficiently low frequency as to be blocked by filter 28). An important feature of the present invention lies in recognition of the fact that all clinically significant arrhythmias may be detected using only the ventricular depolarization or QRS signal components, namely QRS duration and R-R interval.In this connection, arrhythmia detection for initiating recording must be distinguished from arrhythmia diagnosis, the latter requiring analysis of all signal components including P and T. All of the P, Q, R, S and T components are recorded for diagnosis on cassette 58 via convertors 50, 53 and memory 52 whenever an arrhythmia is detected. According to one important aspect of the present invention, it has been recognized that all clinically significant cardiac events which must be diagnosed by reference to the P and/or T signal components vary sufficiently in QRS duration and/or R-R interval to be detected by the present invention without monitoring the P and T components per se, and thereby eliminating the source of problems inherent in prior art systems that attempt to detect P and T components directly.Thus, the present invention not only simplifies detection techniques but also improves reliability problems inherent in prior art techniques which attempt to detect arrhythmias by, in effect, diagnosing various arrhythmia types.

In accordance with another important feature of the invention, it has been recognized that it is not so much the absolute value of the duration of or interval between QRS signals for a particular patient that is important as measured against fixed standards, as is the relative duration or interval for the particular patient as compared to what is "normal" for him. Indeed, a patient who has suffered heart damage may have a "normal" QRS duration and R-R interval which departs significantly from the above-noted times for a nominally healthy person. Comparison of a patient's QRS duration and R-R interval in the present invention to corresponding running average measurements effectively provides arrhythmia detection for each individual patient by comparing each cardiac event to what is "normal" for him.

Although it may be desirable to measure the actual duration of the QRS signal from the beginning of the 0 component, it is convenient and presently preferred to begin measurement at the "peak" of the Q signal utilizing a differentiated signal and conventional zero crossing detection circuitry. As noted above, it is not the absolute duration of the QRS signal that is important but comparison of each duration to past durations.

Thus, the first half of the Q component may be ignored so long as QRS duration begins and ends in the same manner for each successive rhythm signal. Similarly, R-R interval may be measured even when there is no discernable R signal component so long as measurement takes place in a similar manner for each successive rhythm signal. In some cases, "R--R interval" may in fact be an S-S interval, for example.

The use of a running average for comparison of QRS duration and R-R interval has the advantage of tracking relatively slow changes in cardiac rhythm associated with changes in patient activities without giving false alarm indications.

Average computation using three duration counts and three interval counts has been empirically selected in the present invention as providing reliable results of desired sensitivity at low cost.

However, greater or possibly even fewer counts could be utilized in computing average QRS duration and/or R-R interval without departing from the invention, although it is presently believed that at least three counts are required.

The allowable range of plus or minus twenty percent in QRS duration comparator 40 is recognized in the art as a convenient cutoff point for detection of pathological events. The plus or minus thirty percent range in R-R interval comparator 46 has been empirically selected as providing reliable indications of actual pathological events without being overly sensitive to normal variations in cardiac rhythm. Other comparator ranges for QRS duration and R-R interval may be utilized where desired.

The foregoing and other functional features of the invention may be further appreciated with reference to Figs.3~9 which illustrates various "typical" pathological events, all of which may be detected and recorded by the preferred embodiment of the invention illustrated in Fig. 1.

Each set of cardiac rhythm signals in Figs. 3-9 is six seconds in duration and is drawn to scale. Fig.

3 illustrates a "normal sinus rhythm" at a relatively slow rate of fifty-four beats per minute interrupted by a single premature ventricular contraction (PVC) at 62. The normal R-R interval in Fig. 3 is 1.36 seconds. The premature ventricular contraction at 62 occurs 560 milliseconds following the preceding R signal component, which is well below seventy percent of the average R-R interval (950 milliseconds) to which comparator 46 is responsive. Additionally, the QRS duration of the premature ventricular contraction is 170 milliseconds, which is substantially greater than one hundred twenty percent of the average QRS duration of eight milliseconds. Thus, in this example, OR gate 42 is energized by both comparators 40 and 46.

This combination of a thirty percent R-R interval range with use of a running average interval for comparison purposes has a particularly synergistic effect in detection of PVC's. More specifically, a PVC may be just within the thirty percent range and therefore not be detected upon occurrence. However, the same PVC will so effect the running average of three preceding R-R intervals that gate 42 will be activated by the next QRS complex. It is felt that the R-R interval range of thirty percent will alone detec#t ninety percent of all clinically significant PVC's, and that the use of the running average for comparison purposes will pick up the other ten percent.

Fig. 4 illustrates an electrocardiographic abnormality comprising frequent unifocal premature ventricular contractions with a configuration of so-called "ventricular trigeminy".

The normal R-R interval in this cardiac rhythm is 600 milliseconds, while the premature R-R interval is 500 milliseconds. Thus comparator 46 is not activated. However, the QRS duration of the premature ventricular contractions at 64 is 120 milliseconds, which is substantially greater than 120 percent of the normal QRS duration fifty milliseconds. Thus OR gate 42 is activated by comparator 40.

Fig. 5 illustrates a cardiac rhythm abnormality called "atrial fibrillation". In this relatively common abnormality, the patient's atrial activity indicated by the P signal components are chaotic and difficult to discern. The R-R intervals are constantly changing in the illustrated ventricular rhythm from 155 to 170 beats per minute, with an average of about 160 beats per minute.

Although the R-R intervals are irregular, they do not depart from the average by plus or minus thirty percent and therefore do not indicate an alarm condition. Thus, the present invention may be used in a patient with atrial fibrillation but reasonably consistent ventricular response without constantly indicating an alarm condition.

A premature ventricular contraction as illustrated at 66 in Fig. 5 having a duration of 120 milliseconds, as compared with a seventy millisecond average QRS duration, provides an alarm signal through comparator 40 and gate 42.

Fig. 6 illustrates a pattern of normal sinus rhythm interrupted by frequent premature atrial contractions 68. The QRS duration of the premature atrial contractions are usually well within normal and relatively constant. However, each premature atrial contraction occurs in the range of forty to two hundred forty milliseconds after the preceding signal complex, which is substantially less than the running average 760 milliseconds R-R interval. Each premature atrial contraction, therefore, activates comparator 46.

Figs. 7 and 8 illustrate examples of second degree heart blockage in which the ventricular cardiac muscle responds inconsistently to electrical atrial activity, which is to say that each P wave is not followed by a QRS complex. In second degree heart blockage of the Wenckebach, variety illustrated in Fig. 7, the P R intervals progressively increase until a QRS complex is dropped or "blocked", whereupon the P-R interval again shortens. Fig. 7 illustrates a four-to-three block, i.e., one wherein four P wave components result in only three QRS complexes.

The QRS complexes are not distinguished by abnormally prolonged duration, but the range change is signifcant in that the R-R interval containing the missed QRS complex is almost twice as long as the average preceding intervals.

Thus as illustrated in Figs. 6 and 7 combined, the present invention is adapted to be responsive both to short and long R-R intervals indicative, respectively, of premature and blocked heart rhythms. Fig. 7 also illustrates the aforementioned feature of the invention whereby a clinically significant arrhythmia diagnostically related to the P signal component is detected, and recorded for later diagnosis, without attempting to monitor the low voltage and frequency P component per se. It will also be recognized that the present invention is adapted to respond to Q, R and S signals of "normal" polarity, as well as complexes of opposite polarity of the type illustrated in Fig. 7.

Fig. 8 illustrates a two-to-one second degree heart block with a period in mid-strip which displays a one-to-one response. Since the patient's normal rhythm displays a two-to-one heart block, the brief one-to-one response is, in fact, double the patient's average rate and an alarm signal is generated.

Fig. 9 displays a commonly encountered problem with a patient wearing an intermittently functioning or demand-type electronic pacemaker. The QRS duration following a pacemaker "blip" is normally prolonged because of the position of the pacemaker at the tip of the right ventricle. The pacemaker spike, when first activated, provokes a prolonged QRS duration at the onset of pacing, therefore generating an alarm condition and recording the three-second interval preceding pacing and the first three seconds after the onset of pacing. Similarly, absence of the pacemaker spike following turnoff or failure of the pacemaker, or failure of a pacemaker spike to provoke a subsequent QRS results in a sensed short QRS duration which would result in an alarm signal and recording event.Filter 28 (Fig.1) passes the high frequency pacemaker spike to the arrhythmia detector circuitry. Where use with a pacemaker is not contemplated, high pass filter 28 may conveniently be replaced by a band pass filter to block both the low frequency P and T signal components and high frequency noise.

Counters 32, 34, averagers 38, 44, comparators 40, 46, gate 42, tape drive 48 and memories 52, 54 may be provided in the form of conventional low-cost digital integrated circuitry, while the remainder, including amplifiers 22, 30, filter 28 and convertors 50, 53, 55 may be provided in whole or in part by readily available integrated linear circuit components. Indeed, miniature cassette recorders which include both tape drive circuitry 48 and recording circuitry 56 are commercially available. The integrated circuit components and the tape drive and recording head may be readily provided in a compact and rugged portableto be worn by a cardiac outpatient as in the shirt pocket, for example.It will also be appreciated that the recording fidelity and speed, typically on the order of one-eighth inch per second, of a conventional cassette recorder inherently performs a smoothing function between the channel input and the corresponding recording head, thereby eliminating any requirement for demodulation circuitry between the sampled output of memory 52 and the channel input.

It will also be appreciated that the arrhythmia detection circuitry provided by the invention and previously described in responsive to the derivative of the cardiac rhythm signal and is therefore substantially independent of the amplitudes of the signal components. Thus the signal-to-noise ratio is substantially enhanced as compared with prior art monitoring techniques which are responsive in whole or in part to signal amplitude.

It will be appreciated that the present invention offers particular advantages when utilized in a portable battery operated unit since it is maintained in a non-recording state until an arrhythmia is detected, at which time recording begins but continues only for a short duration bracketing the detected arrhythmia. The recorder then returns to non-recording conditions until the next detected arrhythmia. This minimizes battering drain and, very importantly, allows monitoring over long intervals, greater than 24 hours, since the amount of tape used depends only upon the number and frequency of detected arrhythmias.These highly desirable attributes are optimized by the present invention because, as previously described, the present invention is based in large part on the recognition that clinically significant arrhythmias can be detected using QRS components (only), namely QRS duration and R-R interval, based on a differentiated waveform, with preselected ranges of permissable deviations from a running average standard, in order to separate important abnormal events. However according to the present invention, QRS duration and R-to-R interval are preferably, if not necessarily, detected from this differentiated waveform for example, in the case of QRS interval detection using techniques of the type disclosed in U.S. Patents 3,552,386, 3,598,110,3,616,791 and 3,903,873.

Although the present invention is particularly well adapted for use in a compact and portable cardiac monitoring units, the principles thereof, particularly the arrhythmia detection circuitry embodied in filter 28 to OR gate 42, may readily be incorporated into full scale hospital coronary care units. The invention is intended to embrace the above-noted and all other alternatives, modifications and variations as fall within the spirit and broad scope of the appended claims.

Claims

1. A cardiac arrhythmia detector comprising means for receiving a series of analog cardiac rhythm signals, first means responsive to duration of each said rhythm signal substantially independently of signal amplitude for providing a first signal when said duration is either greater or less than a first standard by more than a first preselected amount, second means responsive to intervals between successive rhythm signals for providing a second signal when a said interval is greater or less than a second standard by more than a second preselected amount, and means for providing third signal in response to either said first or said second signal.

2. The detector set forth in claim 1, wherein said receiving means includes means for differentiating said analog cardiac rhythm signals, said first and second means being responsive to the derivative of said analog cardiac rhythm signals.

3. The detector set forth in claim 2, wherein said first means includes first averaging means for determining an average rhythm duration over a first preselected number of rhythm signals and first comparison means for comparing each successive rhythm signal duration to said average duration to provide said first signal when said successive duration is greater or less than said average duration by more than said first preselected amount.

4. The detector set forth in claim 3, wherein said first averaging means determines a running average rhythm duration over said first preselected number of cardiac rhythm signals.

5. The detector set forth in claim 4, wherein said preselected number is three.

6. The detector set forth in claim 4, wherein said first preselected amount is plus or minus 20 percent of said running average duration.

7. The detector set forth in claim 2 or 3, wherein said second means includes second averaging means for determining an average interval over a second preselected number of intervals between said cardiac rhythm signals and second comparison means for comparing each successive rhythm interval with said average interval for providing said second signal when said successive rhythm interval is greater or less than said average interval by more than said second preselected amount.

8. The detector set forth in claim 7, wherein said second average means computes a running average interval over said second preselected number of intervals.

9. The detector set forth in claim 8, wherein said second preselected number is three.

10. The detector set forth in claim 8, wherein said second preselected amount is plus or minus 30 percent of said running average interval.

11. The detector set forth in claim 1, wherein each said cardiac rhythm signal normally includes at least Q, R and S signal components, wherein said first means is responsive to the duration of said Q, R and S components collectively, and wherein said second means is responsive to the interval between R components of successive rhythm signals.

12. The detector set forth in claim 7 further comprising recorder means and means responsive to a said third signal for activating said recorder means to record a series of said cardiac rhythm signals which includes the particular rhythm signal which caused the said third signal.

13. The detector set forth in claim 12 further comprising delay means connected to said receiving means for providing to said recorder means a replica of said analog cardiac signals delayed by a predetermined time, and wherein said means responsive to said third signal activates said recorder means for at least said predetermined time.

14. The detector set forth in claim 13, wherein said delay means includes analog-to-digital convertor means for providing digital signals as a function of amplitude of said cardiac rhythm signals and digital shift register means for continuously sampling and storing said digital signals.

1 5. The detector set forth in claim 13, wherein said predetermined delay time is substantially three seconds, and wherein said means responsive to said third signal activates said recorder means for substantially six seconds.

16. The detector set forth in claim 13 adapted to be worn as a portable unit by a cardiac out patient or the like wherein said recorder means comprises a magnetic tape cassette recorder.

17. The detector set forth in claim 1 6 further comprising means adapted to be manually operable by a wearer of said detector for activating said recorder independently of said third signal.

1 8. The detector set forth in claim 1 7, wherein said recorder comprises a multiple-channel cassette recorder, said detector further comprising means for recording data indicative of time of day when said recorder is activated and whether said recorder has been activated by said third signal or said manually operable means.

19. The detector set forth in claim 2 further comprising recorder means and means responsive to a said third signal for activating said recorder means to record a series of said cardiac rhythm signals which includes the particular signal rhythm which caused the said alarm signal.

20. The detector set forth in claim 1 9 adapted to be worn as a portable unit by a cardiac outpatient or the like wherein said recorder means comprises a magnetic tape cassette recorder.

21. The detector set forth in claim 20 further comprising means adapted to be manually operable by a wearer of said detector for activating said recorder independently of said alarm signal.

22. A method of detecting cardiac arrhythmias comprising the steps of (a) developing a series of cardiac rhythm signals, (b) monitoring only that portion of said series of cardiac rhythm signals corresponding to cardiac ventricular depolarizations, (c) providing a control signal when a selected characteristic of one or more of said ventricular depolarization signals departs from a predetermined standard, and (d) responsive to said control signal, recording a series of cardiac rhythm signals which includes the particular rhythm signal which resulted in said control signal and, which further includes both said ventricular depolarization portions and other portions of said rhythm signals.

23. A portable cardiac monitor and recorder comprising electrode means for developing cardiac rhythm signals, means for monitoring only that portion of said cardiac rhythm signals corresponding to ventricular depolarization, said monitoring means including first means for providing a first control signal when a duration of said portion departs from an average duration and second means to provide a second control signal when an interval between successive ones of said portions departs from an average interval, and a magnetic cassette recorder responsive to said first and second signals for recording a series of cardiac rhythm signals which includes the particular rhythm which resulted in one of said first and second control signals and which also includes the entire rhythm signal developed by said electrode means including said portion, said recorder being normally inoperative in the absence of one of said control signals.

24. The device set forth in claim 23, wherein said monitoring means is arranged and constructed such that control signals generated thereby cause actuation of said recorder only in response to either said first control signal or said second control signal.

25. A cardiac arrhythmia detector constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

26. A method of detecting cardiac arrhythmias substantially as hereinbefore described with reference to the accompanying drawings.

27. A portable cardiac monitor and recorder constructed substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.