Arrhythmia recorder for use with an implantable defibrillator

What is claimed is:

1. Apparatus dedicated to a single patient for monitoring the operation of an automatic defibrillator implanted in the patient, the apparatus comprising:

first detector means responsive to deliverance of defibrillating energy by said implanted defibrillator for triggering the storage of data related to said deliverance of the defibrillating energy;

storage means for durably storing said data;

external receiving means for receiving said data; and

telemetry means for transmitting said data from the patient to said external receiving means.

2. The device recited in claim 1 wherein said storage means is implanted with said defibrillator.

3. The device recited in claim 1 wherein said storage means is external to the recipient.

4. The device recited in claim 1 wherein said are disturbances in cardiac electrical activity.

5. The device recited in claim 1 wherein said data are defibrillation pulses.

6. The device recited in claim 1 wherein said data is a representation of an ECG signal.

7. The device recited in claim 1 wherein said data is a time tag.

8. The device recited in claim 1 wherein said telemetry means is external to said recipient, and includes external electrodes on said recipient for sensing the electrical activity of the heart.

9. The device recited in claim 1 wherein said telemetry means is implanted with said defibrillator.

10. The device recited in claim 1 and further comprising means for permanently recording said data transmitted to said external receiving.

11. The device according to claim 1, wherein said storage means comprises:

first storage means having a predetermined small capacity for temporarily storing and updating a small portion of said data; and

second storage means for durably storing the portion of the data occurring after the deliverance of defibrillating energy.

12. The device according to claim 11 wherein said first storage means and said second storage means comprise the same storage device.

13. The device according to claim 11 wherein said receiving device includes a tape recorder having a tape for durably storing the data stored in said first storage means and said data stored in said second storage means.

14. The device according to claim 11 wherein said first storage means includes a random access memory.

15. The device according to claim 11 wherein said first storage means includes a shift register.

16. The device according to claim 11 wherein said first storage means and said second storage means each include a random access memory.

17. The device according to claim 1 further comprising means for converting said data into a digital signal, and means for delivering said digital signal to said receiving device.

18. The device of claim 1, further comprising second detector means responsive to events characteristic of a possible fibrillation episode for triggering the storage of data related to said possible fibrillation episode, and wherein said storage means durably stores said data related to said possible fibrillation episode, said external receiving means receives said data related to said possible fibrillation episode, and said telemetry means transmits said data related to said fibrillation episode from said patient to said external receiving means.

19. The device of claim 18, further comprising means for formatting said received data related to said possible fibrillation episode and said deliverance of defibrillating energy to provide information indicative of whether or not said deliverance of defibrillating energy was made in response to the detection of fibrillation by said implanted defibrillator.

20. The device of claim 1, further comprising means for monitoring data relating to cardiac electrical activity, and wherein said first detector means, in response to said deliverance of defibrillating energy, triggers the storage of data related to said cardiac electrical activity, said external receiving means receives said data related to said cardiac electrical activity, and said telemetry means transmits said data related to said cardiac electrical activity from said patient to said external receiving means.

21. The device of claim 20, further comprising means for formatting said received data related to said cardiac electrical activity and said deliverance of defibrillating energy to provide information indicative of whether or not said deliverance of defibrillating energy was made in response to the detection of fibrillation by said implanted defibrillator.

22. The device of claim 21, further comprising second detector means responsive to events characteristic of a possible fibrillation episode for triggering the storage of said data related to said cardiac electrical activity.

23. Apparatus dedicated to a single patient for recording data pertinent to the operation of an automatic defibrillator implanted in the patient, said apparatus comprising:

means for sensing electrical activity associated with the patient's heart as a first signal;

means for converting said first signal to a digital signal;

first memory means having a predetermined number of memory locations for storing said digital signal;

means responsive to said first signal for providing a fibrillation signal at the detection of fibrillation;

detector means responsive to the deliverance of defibrillating energy into the patient's heart for producing a defibrillation signal;

means responsive to said fibrillation signal for disabling said first memory means, and for causing said first memory means to hold the digital signal already stored therein;

second memory means having a predetermined number of memory locations for storing said digital signal;

means responsive to said fibrillation signal for enabling said second memory means; and

means responsive to said defibrillation signal for enabling said second memory means.

24. The device according to claim 23 further comprising:

first memory address means for addressing said memory locations in said first memory means;

means responsive to said fibrillation signal for storing the address of said first memory means; and

second memory address means for addressing said memory locations in said second memory means.

25. The device according to claim 23 further comprising means for reproducing in eye-readable format the portion of the first signal stored in said first and second memory means.

26. The device of claim 23 wherein said second storage means includes a tape recorder for durably storing the output of said first storage means.

27. Apparatus dedicated to a single patient for monitoring the operation of an automatic defibrillator implanted in the patient, the apparatus comprising:

a first unit adapted to be worn by the patient, said first unit comprising,

sensing means for sensing electrical activity associated with the patient's heart as a first signal, and

transmitting means for transmitting said sensed first signal; and

a second unit capable of easily being carried and moved, said second unit comprising,

receiving means for receiving said transmitted first signal,

detector means in circuit with said receiving means and responsive to a defibrillation attempt for triggering the storage of data related to said attempt, and

storage means for durably storing said data related to said attempt.

28. The device of claim 27 wherein said detector means is an arrhythmia detector.

29. The device according to claim 27 wherein said second unit further comprises means for retrieving from said storage means said durably stored data related to said attempt, and

means for displaying said retrieved output in eye-readable format.

30. The device according to claim 27 wherein said first unit further comprises internal timing means for providing a time tag, and means responsive to said detector means for causing said time tag to be durably stored in said storage means.

31. The device according to claim 30 wherein said internal timing means is a digital watch chip.

32. The device according to claim 27 wherein said sensing means is a set of chest electrodes.

33. The device according to claim 27 wherein said second unit further comprises:

means responsive to said received first signal for producing a detection signal when said received first signal falls below a predetermined magnitude,

audio means for producing an audible tone in response to said detection signal, and

means operatively associated with said storage means for disabling said storage in response to said detection signal.

34. The device according to claim 27 further comprising means in circuit between said receiving means and said ECG storage means for converting said received ECG signal into a digital signal.

35. The device of claim 34 wherein said storage means includes a random access memory.

36. The device of claim 34 wherein said storage means includes a shift register.

37. The device of claim 27 wherein said first unit further comprises encoding means in circuit between said sensing means and said transmitting means for encoding said sensed first signal, and said second unit further comprises decoding means in circuit with said receiver means and said storage means for decoding said received first signal.

38. The device of claim 37 wherein said encoding means is a pulse frequency modulation encoder and said decoding means is a pulse frequency modulation decoder.

39. The device of claim 37 further comprising a pulse sensor means responsive to said sensed ECG signal for providing to said encoding means a defibrillation signal produced when said pulse sensor means senses a defibrillation pulse.

40. The device of claim 37 wherein said decoding means is also in circuit between said receiver means and said detecting means.

41. The device according to claim 27 wherein said storage means includes a cassette tape recorder having a magnetic tape.

42. The device according to claim 41 wherein said second unit further comprises means for retrieving the information stored on said magnetic tape, and means for displaying said retrieved information in eye-readable format.

43. The apparatus of claim 39, wherein said second unit further comprises,

means responsive to said received first signal for producing a second signal when said received first signal falls below a predetermined magnitude, and

means for producing an audible tone in response to said second signal to alert the patient that said first signal is not being received by said second unit.

44. Apparatus dedicated to a single patient for monitoring the operation of an automatic defibrillator implanted in the patient, the apparatus comprising:

first detector means responsive to events characteristic of a possible fibrillation episode for triggering the storage of data related to said possible fibrillation episode;

second detector means responsive to deliverance of defibrillating energy by said implanted defibrillator for triggering the storage of data related to said deliverance of defibrillating energy;

storage means for durably storing the data related to said possible fibrillation episode and said deliverance of defibrillating energy;

external receiving means for receiving the data related to said possible fibrillation episode and said deliverance of defibrillating energy; and

telemetry means for transmitting the data related to said possible fibrillation episode and said deliverance of defibrillating energy from the patient to said external receiving means.

45. An implantable automatic defibrillator including treatment verification capabilities, said defibrillator comprising:

sensing means for sensing electrical activity associated with the heart of a wearer;

detector means receiving said electrical activity from said sensing means for detecting cardiac arrhythmias including fibrillation;

signal-generating means for issuing signals in response to the detection of arrhythmias by said detector means;

storage and discharge means for first storing and then automatically discharging defibrillating energy through the heart of the wearer;

charging means for charging said storage and discharge means with said defibrillating energy;

means creating an electrical link between said signal-generating means and said charging means for activating said charging means upon detection of fibrillation by said detector means;

storage means for durably storing the electrical activity sensed by said sensing means; and

means creating an electrical link between said signal-generating means and said storage means for activating said storage means upon detection of an arrhythmia by said detector means.

46. The defibrillator of claim 45, wherein said storage and discharge means and said charging means are activated simultaneously in response to a single signal produced by said signal-generating means.

Description Submit all comments and votes

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a device for recording and subsequently reproducing desired portions of an ECG signal produced by a heart prior to and during the occurrance of various disturbances in cardiac electrical activity.

2. Description of the Prior Art

During the past several decades, coronary heart disease has come to occupy the first position among the causes of death in the developed areas of the world. Although the precise cause of sudden death in coronary heart disease has not yet been entirely clarified, the available evidence permits the medical field to ascribe death in the majority of these cases to a grave disturbance in cardiac electrical activity resulting in ventricular fibrillation.

While it is not possible to predict with unerring exactness which patient suffering from coronary heart disease will be the victim of sudden death, several high risk groups of patients can be recognized. For example, patients who have experienced myocardial infarction, even though they may be surviving in good health, run a substantial risk of dying suddently, a risk several times greater than that associated with the general population. Further, if patients with myocardial infarction have a history of serious ventricular arrhythmias and/or of cardiac arrest, or if evidence of persistent myocardial irritability is present, it may be logically assumed that the risk of sudden death is increased substantially. Patients like those described above would greatly benefit from an automatic, standby or demand defibrillator.

Another recognizable class of patients particularly in need of an automatic defibrillator is the class composed of those who have not shown prior histories of myocardial infarction but who show severe symptoms of coronary heart disease, such as ventricular arrhythmias resistant to medical treatment or angina pectoris.

Finally, there are scores of individuals walking the streets today who experience recurring episodes of atrial fibrillation, atrial flutter, or tachycardia. While not life-threatening, these supra-ventricular arrhythmias can become debiliting and lead to complications, and hence require treatment when present. Such individuals require frequent electrical or pharmacological conversion under the care of their physicians to return their hearts to normal sinus rhythm.

Great strides are presently being made to develop an automatic, fully implantable ventricular defibrillator. See, for example, U.S. Pat. Nos. Re. 27,652 and 27,757, where the first concept of the automatic implantable ventricular defibrillator is described. Recent advances have also been made in enhancing the reliability of fibrillation detectors. In this latter regard, see copending U.S. Pat. Applications Ser. Nos. 878,005 and 878,006, each filed on Feb. 15, 1978. Furthermore, as outlined in copending U.S. Pat. Application Ser. No. 801,300, filed on May 27, 1977, steps have been taken to improve the reliability of the implanted defibrillator by the provision of circuitry which interrogates the implanted electronics to verify proper operation before a defibrillating shock is delivered.

Notwithstanding the substantial steps which have been taken to develop the automatic, fully implantable defibrillator and to ensure the operation of the sensing and defibrillating circuitry, it must not be forgotten that the implantable defibrillator is in its infancy. Accordingly, there is a current need for data which either verifies the accuracy or which uncovers the failings of the sensing and defibrillating circuits. Specifically, there is a need for a practical device capable of providing data by recording and subsequently reproducing desired portions of an electrocardiogram (ECG) signal produced by a heart prior to and during the occurrence of various disturbances in cardiac electrical activity. With such a device, not only could the operation of the implanted defibrillator be verified, but valuable information about the patient's heart activity prior to and during cardiac arrhythmias could be obtained. Furthermore, there is a need for a practical device which could be worn by a patient to monitor heart activity even in the absence of an implanted defibrillator.

It is toward the object of meeting the foregoing needs that the present invention is directed.

SUMMARY OF THE INVENTION

The subject invention relates in general to a device for recording and subsequently reproducing desired portions of an ECG signal produced by a heart prior to and during the occurrence of various disturbances in cardiac electrical activity. Through the use of an appropriate transducer, the electrical activity of a patient's heart is detected and converted into a typical ECG signal.

More specifically, in the first (implanted) embodiment of the subject invention, the ECG signal is converted to a convenient digital form. The digital signal representing the ECG signal is stored on a FIFO (first in, first out) basis in a first memory having a predetermined small capacity. A fibrillation detector which continuously monitors the ECG signal produces a fibrillation detected logic signal at some time after the onset of a fibrillation episode. Circuitry is provided to cause the first memory to stop storig the digital representation of the ECG signal and to hold the previously stored ECG signal when this logic signal occurs. A second, large capacity memory is made operative in response to the fibrillation detected logic signal, and is provided for storing subsequent ECG signals for an extended period of time.

In addition, circuitry is provided for reading out the data stored in the first memory, the data representing heart action taking place prior to the fibrillation episode, and for reading out data from the second memory, the data representing heart action taking place during and subsequent to the fibrillation episode. Usually the readout operation is performed by a physician or assistant at the physician's office or at a hospital in conjunction with a display for subsequent interpretation.

In the second (external) embodiment of the present invention, the means used to sense the heart's cardiac electrical activity as an ECG signal takes the form of chest electrodes placed on the anterior chest wall of the patient. Conventional telemetry techniques are used to broadcast the ECG signal from the patient. This signal is received for recording and subsequent playback by the circuitry of the subject invention, which is housed in a convenient container such as a briefcase. In this embodiment, the received ECG signal is converted to a convenient digital form. The digital signal representing the ECG signal is stored on a FIFO basis in a storage device having a predetermined capacity. An arrhythmia detector which continuously monitors the received ECG signals produces an arrhythmia detected logic signal at the occurrence of a disturbance in cardiac electrical activity. By way of example, such a disturbance may be produced by ventricular tachycardia, bradycardia, asystole, ventricular flutter, ventricular fibrillation, and ectopic beats. The arrhythmia detected logic signal turns on a tape recorder which records the output of the storage device. After the disturbance has ceased and the heart has returned to normal cardiac electrical activity the arrhythmia detected logic signal ceases. The recorder continues to record the output of the storage device for a predetermined time period equal, for example, to the time interval necessary for the storage device to once read out its entire contents. After this has taken place the recorder shuts down. Thus the recorder now possesses on a magnetic tape in digital form, the desired portion of the received ECG signal produced by the heart prior to and during the occurrence of the disturbance in cardiac electrical activity.

The tape is played back by a physician or trained assistant at the physician's office or at a hospital in conjunction with a display device for subsequent interpretation.

Customarily, the term electrocardiogram (ECG) implies the use of electrodes on the body surface to obtain electrical signals indicative of heart activity. The term electrogram, on the other hand, generally refers to measurements made at the surface of the heart. As used herein, "ECG" is defined broadly, and refers to any measurement of the electrical activity of the heart, notwithstanding the source or technique of the measurement.

It is accordingly an object of the present invention to provide a device for recording and subsequently reproducing desired portions of an ECG produced by a heart prior to and during the occurrence of a disturbance in cardiac electrical activity.

It is another object of the present invention to provide a low power implantable device for preserving valuable information about the patient's heart activity as represented by an ECG signal occuring prior to and during a fibrillation episode.

It is still another object of the present invention to provide a lightweight, external device employing radio telemetry techniques for recording and subsequently reproducing a desired portion of an ECG signal produced by a heart prior to and during the occurrence of a disturbance in cardiac electrical activity.

It is yet another object of the present invention to provide a low power implantable device for recording and subsequently reproducing portions of ECG signals relating to multiple fibrillation episodes as experienced by a patient.

It is a further object of the present invention to provide a lightweight external device for recording and subsequently reproducing portions of ECG signals relating to multiple disturbances in cardiac electrical activity as experience by a patient.

It is yet a further object of the present invention to provide a device for recording and subsequently reproducing information pertinent to a defibrillation attempt by an implantable defibrillator.

It is yet another object of the present invention to provide treatment verification capabilities as part of an automatic implantable defibrillator, which has a device for storing and discharging defibrillating energy through the heart of a wearer and a device for charging the storage and discharge device upon detection of fibrillation.

It is still another object of the present invention to provide a device which will aid in recognizing the need for a fully implantable ventricular defibrillator in a patient suffering from coronary heart disease.

It is yet another object of the present invention to provide a device which will aid in treatment of patients suffering from cardiac arrhythmias.

It is still a further object of the present invention to provide a device for verifying the operation of an implanted defibrillator.

Other objects and advantages of this invention will further become apparent hereinafter and in the drawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a general block diagram of a preferred embodiment of the subject invention.

FIG. 2 is a detailed block diagram of another preferred embodiment of the subject invention.

FIG. 3 is a detailed block diagram of the recording or storing portion associated with the FIG. 1 embodiment.

FIG. 4 is a detailed block diagram of the reproducing or retrieving portion association with the FIG. 1 embodiment.

FIG. 5 is a schematic diagram of an embodiment of the arrhythmia detector associated with the FIG. 2 embodiment.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In describing the preferred embodiments of the invention illustrated in the drawings, specific terminology will be resorted to for the sake of clarity. However, it is not intended to be limited to the specific terms so selected, and it is to be understood that each specific term includes all technical equivalents which operate in a similar manner to accomplish a similar purpose.

The first embodiment of the subject invention will be described in general with reference to FIG. 1. A portion of the device implanted within a patient is indicated generally at 10 and is adapted to associate with an external portion of the device generally designated at 12. The numeral 14 schematically represents the skin of the patient and hence shows the separation between the implanted device 10 and the external device 12.

A sensing probe 16 senses the heart's cardiac electrical activity as an electrocardiogram (ECG) signal. This signal is received by a fibrillation detector 18, which includes a signal generator 21 that issues a fibrillation detected (FD) signal. A record/playback device 22 also receives the ECG signal from the probe 16 on line 27 along with a fibrillation detected (FD) signal from the fibrillation detector 18 on line 25. A defibrillator 15 also receives the FD signal from the fibrillation detector 18, on line 23. The defibrillator 15 is of conventional design and includes a storage and discharge device 13 for storing and discharging defibrillating energy through the heart of a wearer, and a charging device 11 for charging the storage and discharge device upon receipt of the FD signal on line 23. When a fibrillation is detected the defibrillator issues a defibrillating shock to the heart via a shock delivering probe 17. In this embodiment the defibrillator 15, the fibrillation detector 18, and the record/playback device are encased in the same implantable housing 19. The record/playback device 22 records a desired portion of the ECG signal produced by the heart prior to and during a fibrillation episode.

The information stored in the record/playback device 22 may be retrieved and converted to an eye-readable form for subsequent examination through the use of the external device 12. A code word command is transmitted to the record/playback device 22 through a transmitter 24 via a modulator 26. The information stored in the record/playback device is received by receiver 28. The output of receiver 28 passes through a demodulator 30. The output of the demodulator is fed to a display device 34 via a readout decoder 32 to display the information stored in the record/playback device in an eye-readable format.

How the desired portion of the ECG signal produced by a heart prior to and during a fibrillation episode is recorded or stored may be explained with reference to FIG. 3.

The sensing probe 16 provides an ECG signal. This signal is amplified through an amplifier 60 and fed into a fibrillation detector 18. The fibrillation detector is preferrably of the type disclosed in copending U.S. Application Ser. No. 878,006. The output of the amplifier 60 is also fed to an analog-to-digital (A/D) converter 64, which converts the ECG signal into a digital representation. The digital representation is made up of a series of words wherein each word contains four to six bits (hereinafter four for convenience). A clock 66 provides an INITIATE CONVERSION signal as well as gating pulses to the A/D converter 64 via lines 68. The digital output of the A/D converter is written into an auxiliary delay memory, or storage device 70 by a WRITE strobe produced by the A/D converter on lines 63 when conversion has been completed. In the preferred embodiment the auxiliary memory 70 contains a 4K (assuming a four bit word) random access memory (RAM). In order to operate with the lowest possible power dissipation the RAM is of the complementary metal oxide semi-conductor (CMOS) type. As stated before each word of digital data contains 4 bits. Thus the auxiliary memory has the capacity to store 1,024 words or 4,096 bits of digital information. The auxiliary memory also contains the usual read/write control circuitry for writing information into the RAM and for reading information out of the RAM. The auxiliary memory also contains the usual row and column decode circuitry for addressing a currently selected memory location for reading or writing. Stored data is read out of the auxiliary memory onto a data bus 74 in response to a READ signal received on lines 75. How the READ signal is provided will be explained in detail hereinafter. An auxilliary memory address (AMA) counter 76 provides a 10-bit address word to the auxiliary memory on address bus 78. The clock 66 applies clock pulses on lines 80 to increment the AMA counter 76. The AMA counter 76 provides address data to sequentially address all of the RAM locations in the auxiliary memory. As long as clock pulses are received on lines 80, the AMA counter will continously repeat the address sequence.

The address data is also received by an auxiliary memory address (AMA) store 82 via the address bus 78. In this way the address of the currently selected word in the auxiliary memory is presented to the AMA store. The AMA store 76 may be any conventional storage device which is capable of storing an address from the address bus 78 in response to a WRITE signal and which is capable of reading out the stored address in response to a READ signal. In addition the AMA store could be a dedicated portion of the auxiliary memory 70, or the main memory 94. The address stored is read from the AMA store on store bus 91. The AMA store receives such a WRITE signal on lines 81 and such a READ signal on lines 83. How these signals are produced will be explained in detail hereinafter.

The output of the fibrillation detector 18 is fed to the set input of a bistable flip-flop 88 and to an episode counter 90. The Q output of flip-flop 88 is fed to the start input of a 48-hour timer 92. The Q output of flip-flop 88 is fed to a reset input of the 48-hour timer. The 48-hour timer may be either a count-up or count-down counter. When 48 hours has passed the timer produces an output signal which is fed to the reset input of flip-flop 88.

The Q output of flip-flop 88 is also fed to the auxiliary memory 70, to AMA store 82, and to a main memory chip selector/formatter 92. How the Q output affects these three elements will be explained hereinafter with reference to the operation of the subject invention.

A main memory or storage device 94, which contains a plurality of RAM chips, receives data from the A/D converter 64 via the data bus 96. The number and capacity of the RAM chips is determined by the amount of information desired to be stored. In a preferred embodiment there are three 4K RAM chips. The main memory 94 also receives data from an internal timer 98 on time bus 100. The internal timer, which as an example may be a digital watch chip, provides a continuous read out of desired information such as time and date in the form of a digital signal referred to as a time tag. Data is read from the main memory onto data bus 102 in response to a READ signal received on lines 101. How the READ signal is provided will be explained in detail hereinafter. A main memory address (MMA) counter 104 provides address data on address bus 106 to sequentially address the word locations in the main memory in the same way as the AMA counter 76. Alternatively, the AMA counter can be used to address the main memory if the counter is cleared just after the pointer to the last word in the auxiliary memory is strobed into the AMA store, all as discussed in detail in the description of FIGS. 3 and 4.

The selector/formatter 92 is responsive to the Q output of flip-flop 88 and the output of episode counter 90. The selector/formatter is used to select and enable a particular RAM chip in the main memory. The selector/formatter counts the number of words in the chip selected. When the selector/formatter determines that the chip is full it disables that chip and selects and enables another chip.

In order to keep power dissipation to a minimum, only the auxiliary memory 70, the A/D converter 64, the clock 66, the AMA counter 76, and the time clock 98 are continuously powered. These elements are enclosed within the dotted line 41. The remaining elements enclosed within the dotted line 43 are powered, and thereby made operative, only after the issuance of a fibrillation detected signal from the fibrillation detector 18.

The elements used to configure the reproduction or data retrieval circuitry of the embodiment shown in FIG. 1 will be explained with reference to FIG. 4. As previously mentioned, the portion of the device generally designated as 10 is implanted within the patient and the device generally designated at 12 is external to the patient. A transmitter 120 and a receiver 122 are located near to but below the skin 14 of the patient. A receiver 28 and a transmitter 24 are located external to the patient. Power and information signals are transmitted by telemetry through the skin of the patient by way of transmitter 24 and receiver 122 as well as receiver 28 and transmitter 120. This type of information transfer is well known in the art and operates under the principle of transformer coupling between primary and secondary windings. When any one of the transmitters is activated a signal is transformer coupled across the skin of the patient. This signal is received through the secondary winding of the coupling transformer at the associated receiver.

A code word command to initiate retrieval is fed into a modulator 26. The output of the modulator 26 is fed into the transmitter 24. The output of the transmitter 24 is received by the receiver 122 which produces two output signals, one of which is used to activate a power regulator 124 and the other of which is fed to a demodulator 126. The output of the demodulator is fed to a code word comparator 128. The code word comparator produces two output signals. The first signal is fed to take-over control 130. The second signal is fed to the reset input of flip-flop 88. A read-out clock 132 provides timing signals to the take-over control 130. The t