Compressed storage of data in cardiac pacemakers

An implantable medical device for human implant. The device includes a telemetry transmitter and receiver for communicating information from the implanted device to an external programmer or monitor and for receiving commands or other information from an external programmer. The device is provided with one or more sensors and means for monitoring, recording and storing the recordings of physiologic signals after implant. The device is provided with a waveform compression and storage system which stores monitored signals in the form of defined analog voltages maintained within the device, rather than employing more traditional digital storage techniques.

FIELD OF THE INVENTION 
This invention relates generally to the field of cardiac pacemakers, and 
more particularly relates to the compression and storage of analog signals 
therein. 
BACKGROUND OF THE INVENTION 
A wide variety of cardiac pacemakers are known and commercially available. 
Pacemakers are generally characterized by which chambers of the heart they 
are capable of sensing, the chambers to which they deliver pacing stimuli, 
and their responses, if any, to sensed intrinsic electrical cardiac 
activity. Some pacemakers deliver pacing stimuli at fixed, regular 
intervals without regard to naturally occurring cardiac activity. More 
commonly, however, pacemakers sense electrical cardiac activity in one or 
both of the chambers of the heart, and inhibit or trigger delivery of 
pacing stimuli to the heart based on the occurrence and recognition of 
sensed intrinsic electrical events. A so-called "VVI" pacemaker, for 
example, senses electrical cardiac activity in the ventricle of the 
patient's heart, and delivers pacing stimuli to the ventricle only in the 
absence of electrical signals indicative of natural ventricular 
contractions. A "DDD" pacemaker, on the other hand, senses electrical 
signals in both the atrium and ventricle of the patient's heart, and 
delivers atrial pacing stimuli in the absence of signals indicative of 
natural atrial contractions, and ventricular pacing stimuli in the absence 
of signals indicative of natural ventricular contractions. The delivery of 
each pacing stimulus by a DDD pacemaker is synchronized with prior sensed 
or paced events. 
Pacemakers are also known which respond to other types of physiological 
based signals, such as signals from sensors for measuring the pressure 
inside the patient's ventricle or measuring the level of the patient's 
physical activity. These are labeled "VVIR" for a single chamber version 
or "DDDR" for a dual chamber version. 
The complexity of modern pacemakers, the occurrence of rare device 
failures, or, more commonly, physiologic changes, and device variables or 
drift dictate the need for numerous programmable parameters accessible 
noninvasively via an externally operated programmer. The need to assess 
system performance or troubleshoot the patient, device and/or lead system 
in an acute, clinical setting or long-term, while the patient is 
ambulatory, is increasing. 
Ambulatory EKG monitoring is the most effective way of determining 
satisfactory pacemaker or cardioverter/defibrillator function. In the 
presence of a malfunction that has occurred or has been provoked by daily 
activity, there is no technique that provides better accuracy for 
determination of the state of the function of the implanted device and its 
interaction with the patient. Passive EKG monitoring with provocative 
testing, can frequently detect a malfunction of the implanted device and 
is the basic technique utilized today. Additionally, the storage of data 
in counters or, more recently, rate or trend histograms, and the use of 
telemetry markers to indicate device function may aid in the diagnosis of 
malfunction or may allow optimization of device performance. However, it 
is often impossible in an acute, clinical setting to duplicate specific 
daily events and activities, thus many problems are unresolved. 
Additionally, it is impossible to record for prolonged periods of time and 
even for short periods of time with adequate resolution. Lastly, event 
counters and histograms do not provide the temporal relationship between 
events to enable diagnosis of transiently occurring problems. 
Episodic events such as transiently brief runs of pacemaker mediated 
tachycardia (PMT), supraventricular tachycardia (SVT) or syncope can be 
detected as a source of clinically significant symptoms by ambulatory 
monitoring and rarely occur during clinical passive EKG evaluation. Events 
related to daily activity, electromagnetic interference (EMI), loss of 
capture via specific body position or activity, and the effect of sleep or 
activity on patient/device interaction may be readily demonstrated by 
ambulatory monitoring. 
However, ambulatory "holter" monitoring typically entails attaching tape-on 
electrodes to a patient and monitoring the surface EKG via a tape recorder 
or integrated circuit (IC) memory recorder worn on the patient's belt for 
a 24-hour period. This "strapped on" device causes patient discomfort and 
limits activity. If the transient event is not captured, the trial must be 
repeated or the troubleshooting process must be curtailed or changed to a 
trial and error method for problem resolution. Additionally, the 
evaluation of the 24 hours of stored data is a time intensive and 
expensive process. Lastly, standard holter monitoring has no capability 
for determining device function simultaneous with the stored EKG 
artifacts. 
Devices have been proposed for the electronic storage and transmission of 
the analog information in an implantable medical device to solve the above 
listed problems and shortcomings. The most common method is to digitize 
(i.e. change to digital format) an analog signal for storage or 
transmission. For example, U.S. Pat. No. 4,223,678 by Langer, et al., 
discloses the recording of EGM data prior to and following the detection 
of an arrhythmic event and subsequentially delivered shock. Pre-event 
analog data is converted by an analog to digital converter (ADC) and 
stored in auxiliary memory while post-event data is stored in main memory. 
Both memories are frozen upon storing of the single episodic event. 
U.S. Pat. No. 4,407,288 by Langer, et al., discloses a two micro-processor 
based implantable defibrillator with ECG recording capability. Pre-event 
detection data is stored via a low speed processor into memory via direct 
memory access (DMA). Upon event detection, the data for a single event is 
frozen. 
U.S. Pat. No. 4,625,730 by Fountain, et al., discloses the storage of ECG 
data via DMA to memory. One event may be stored for a total time duration 
of 10.24 seconds. Also, the '730 patent discloses a hand-held patient 
programmer used as an actuator to trigger the storage of an event. 
U.S. Pat. No. 4,295,474 by Fishell discloses the recording of ECGs as in 
the '678 patent along with the recording of time and number of arrhythmic 
episodes since the previous office visit. The ECG is converted by a six 
bit ADC at 50 Hz for the storage and memory of one event for a total 
duration of 80 seconds. Ten seconds of continuously stored data is 
contained in a section of memory to enable the freezing and storage of 10 
seconds of pre-event data and 70 seconds of post-event data. 
The prior art listed above may be typically characterized by digitizing 
data with an ADC, the storage of a single event with limited duration 
(10-80 seconds), no provision for extended storage times or storage of 
multiple events/episodes, generally low fidelity signals, and extended 
telemetry transmission times to external peripherals. 
To obtain extended storage capability (long storage time or multiple 
storage of events), digitized analog signals, such as an ECG, would 
require very large memory storage capability--140 million bits of memory 
per day--(see Chapter 1, Part 4 "The Third Decade of Cardiac Pacing"). 
Memory of that size would require a large number of integrated circuits 
and would not fit in a typically sized implantable pulse generator, would 
cause excessive current drain from the battery during operation and 
require nearly five hours to uplink to a peripheral utilizing a 
"state-of-the-art" telemetry system. Proposed solutions to the large 
amount of memory required include data reduction techniques such as 
blanking out all signals except for the signals indicative of the atrial 
and ventricular depolarization and repolarization (PQRST interval) which 
is then converted via standard techniques. This results in a reduction of 
the storage requirements of only approximately 50 percent. Additionally, 
compression techniques such as Coordinate-Reduction-Time-Encoding-System 
(CORTES) or Amplitude-Zone-Epoch-Time-Coding (AZTEC) may be used to 
achieve up to 10:1 data compression (see Data Compression--Techniques and 
Applications, Pg. 256-259, T. J. Lynch). However these techniques are not 
suitable for use in implantable medical devices because of the extensive 
processing power required to compress and store data real time. 
Additionally, data stored in an implantable medical device's volatile 
memory can be erased or contaminated (flawed) by device failure, EMI, 
cautery or defibrillation procedures. Therefore, most implantable pulse 
generators have a power-on reset (POR) circuit that resets memory when 
power supply glitches occur. Any stored data is then erased and lost to 
the follow-up clinician. 
Alternative methods proposed for the storage of analog signals in 
implantable pulse generators include magnetic bubble memories (see Chapter 
3, Part 4 "The Third Decade of Cardiac Pacing") and Charge Coupled Devices 
(CCD). Present problems with bubble memories include the limited size of 
the memory (or alternatively, the large size of the integrated circuit), 
the difficulty in the read/write mechanism required and the complexity of 
the interface circuitry to the rest of the implantable pulse generator 
circuit (typically constructed of CMOS circuitry). Alternatively, in a 
CCD, the signal to be recorded is stored as a charge on an integrated 
capacitor. However analog information cannot be stored on a CCD for very 
long because the capacitor leakage rate on the CCD is too high to maintain 
accuracy for any significant length of time. 
What is needed is a device that can electrically store analog information 
in an implantable medical device with reasonable precision, for long 
terms, with substantially reduced complexity, minimized current drain from 
the battery, greatly reduced memory requirements, increased telemetry 
transmission rates and the integration of signals marking specific 
function of the implanted device simultaneous with the recording of the 
analog signal. The system of the present invention provides for an 
efficient electronic recording and playback system for an implantable 
medical device which stores signal information in considerably less 
memory, with reduced complexity, and reduced current drain than that 
required for digital storage, and includes markers indicative of device 
function. 
SUMMARY OF THE INVENTION 
It is, therefore, an objective of the present invention to provide an 
apparatus for placing and storing analog data into nonvolatile memory of 
an implantable medical device. 
It is furthermore an objective of the present invention to compress the 
analog data prior to storage, whereby less memory is required. Adequate 
fidelity for clinical use is maintained during the compression operation. 
It is furthermore an objective of the present invention to provide for the 
transmission of the analog data to an external peripheral for subsequent 
clinical use. 
It is furthermore an objective of the present invention to incorporate 
signals indicative of implanted device function interleaved with the 
stored analog data. 
It is furthermore an objective of the present invention to utilize the 
stored analog data to enable arrhythmia detection and treatment. 
It is furthermore an objective of the present invention to utilize the 
stored analog data to enable the detection of an evoked response in an 
auto capture mode of operation. 
The above and other objectives that will herein after appear, and the 
nature of the invention, will more clearly be understood by reference to 
the following description, the amended claims and the accompanying 
drawings.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS OF THE INVENTION 
The present invention will now be more fully described with reference to 
the various Figures of the drawings, FIG. 1 showing generally how a 
pacemaker 10 in accordance with the present invention may be implanted in 
a patient 12. A pacemaker lead 14 is electrically coupled to pacemaker 10 
and extends into the patient's heart 16 via a vein 18. The distal end of 
lead 14 includes one or more exposed conductive electrodes for receiving 
electrical cardiac signals and for delivering electrical pacing stimuli to 
the patient's heart 16. In accordance with the invention to be hereinafter 
described, the distal end of pacemaker lead 14 may also incorporate a 
pressure transducer (not shown in FIG. 1 due to the small scale of that 
Figure) for producing electrical signals representative of the pressure 
inside heart 16. 
Turning to FIG. 2, a block diagram of pacemaker 10 from FIG. 1 is shown. 
Although the present invention is described in conjunction with a 
pacemaker 10 having a microprocessor-based architecture, it will be 
understood that it could be implemented in any logic based, custom 
integrated circuit architecture, if desired. It will also be understood 
that the present invention may be utilized in conjunction with other 
implantable medical devices, such as cardioverters, defibrillators, neural 
stimulators, cardiac assist systems, and the like. 
In the embodiment shown in FIG. 1, pacemaker 10 includes an activity sensor 
20, which may be, for example, a piezoelectric element bonded to the 
inside of the pacemaker's housing. Sensor 20 provides a sensor output 
which varies as a function of a measured parameter that relates to the 
metabolic requirements of patient 12. In addition, pacemaker 10 includes a 
pressure sensor 22 disposed at the distal end of lead 14, as previously 
noted, which may be similarly used to ascertain the metabolic requirements 
and/or cardiac output of patient 12. Pressure sensor 22 may be a 
piezoelectric element such as is disclosed in U.S. Pat. No. 4,407,296 to 
Anderson, entitled "Integral Hermetic Implantable Pressure Transducer," or 
U.S. Pat. No. 4,485,813 to Anderson et al., entitled "Implantable Dynamic 
Pressure Transducer System," each assigned to the assignee of the present 
invention and incorporated herein by reference. 
Pacemaker 10 is schematically shown in FIG. 2 to be electrically coupled 
via a pacing lead 14 to a patient's heart 16. Lead 14 includes an 
intracardiac electrode 24 and pressure sensor 22 located near its distal 
end and positioned within the right ventricular (RV) chamber of heart 16. 
Lead 14 can carry either unipolar or bipolar electrodes as is well known 
in the art. In the presently disclosed embodiment, lead 14 which couples 
pacemaker 10 to the ventricula endocardium can comprise a steroid-tipped, 
unipolar lead with an integral pressure transducer of the type described 
in the aforementioned references. Electrode 24 is coupled via suitable 
lead conductor 14a through input capacitor 26 to node 28 and to 
input/output terminals of an input/output circuit 30. Output from first 
sensor 20 is coupled to input/output circuit 30. Output from pressure 
sensor 22 is also coupled to input/output circuit 30 via suitable lead 
conductor 14b. 
Input/output circuit 30 contains the analog circuits for interface to the 
heart 16, activity sensor 20, pressure sensor 22, and antenna 52, as well 
as for the application of stimulating pulses to heart 16 to control its 
rate as a function thereof under control of the software-implemented 
algorithms in a microcomputer circuit 32. 
Microcomputer circuit 32 comprises an on-board circuit 34 and an off-board 
circuit 36. On-board circuit 34 includes a microprocessor 38, a system 
clock circuit 40, and on-board RAM 42 and ROM 44. Off-board circuit 36 
includes an off-board RAM/ROM unit 46. Microcomputer circuit 32 is coupled 
by data communication bus 48 to a digital controller/timer circuit 50. 
Microcomputer circuit 32 may be fabricated of custom integrated circuit 
devices augmented by standard RAM/ROM components. Data communication bus 
48 is also coupled to an analog memory integrated circuit 80 which 
includes a DAC 82, an address decode circuit 84, a sample and hold circuit 
86, a high voltage supply and associated switches 88 and EEPROM memory 
cells 90. 
It will be understood that the electrical components represented in FIG. 2 
are powered by an appropriate implantable battery power source, not shown, 
in accordance with common practice in the art. 
An antenna 52 is connected to input/output circuit 30 for purposes of 
uplink/downlink telemetry through RF transmitter/receiver (RF TX/RX) unit 
54. Telemetering both analog and digital data between antenna 52 and an 
external device, such as an external programmer (not shown), is 
accomplished in the presently disclosed embodiment by means of all data 
first being digitally encoded and then pulse-position modulated on a 
damped RF carrier, as substantially described in co-pending U.S. patent 
application Ser. No. 07/468,407, filed on Jan. 22, 1990, entitled 
"Improved Telemetry Format," which is assigned to the assignee of the 
present invention and which is incorporated herein by reference. 
A crystal oscillator circuit 56, typically a 32,768-Hz crystal-controlled 
oscillator, provides main timing clock signals to digital controller/timer 
circuit 50. A Vref/Bias circuit 58 generates a stable voltage reference 
and bias currents for the analog circuits of input/output circuit 30. An 
analog-to-digital converter/multiplexor (ADC/MUX) unit 60 digitizes analog 
signals and voltages to provide "real-time" telemetry of pressure and 
intracardiac signals and battery end-of-life (EOL) replacement function. A 
power-on-reset (POR) circuit 62 functions as a means to reset circuitry 
and related functions to a default condition upon detection of a low 
battery condition, which will occur upon initial device power-up or will 
transiently occur in the presence of electromagnetic interference, for 
example. 
The operating commands for controlling the timing of pacemaker 10 are 
coupled by bus 48 to digital controller/timer circuit 50 wherein digital 
timers and counters are employed to establish the overall escape interval 
of the pacemaker, as well as various refractory, blanking, and other 
timing windows for controlling the operation of the peripheral components 
within input/output circuit 30. 
Digital controller/timer circuit 50 is coupled to a sense amplifier 64 and 
an electrogram amplifier 66 for receiving amplified and processed signals 
picked up from electrode 24 through lead conductor 14a and capacitor 26 
representative of the electrical activity of the patient's heart 16. Sense 
amplifier 64 amplifies sensed electrical cardiac signals and provides this 
amplified signal to peak sense and threshold measurement circuitry 65, 
which provides an indication of peak sensed voltages and the measured 
sense amplifier threshold voltage on multiple conductor signal path 67 to 
digital controller/timer circuit 50. The amplified sense amplifier signal 
is also provided to a comparator 69. The electrogram signal developed by 
EGM amplifier 66 is used in those occasions when the implanted device is 
being interrogated by an external programmer, not shown, in order to 
transmit by uplink telemetry a representation of the analog electrogram of 
the patient's electrical heart activity as described in U.S. Pat. No. 
4,556,063, issued to Thompson et al., assigned to the assignee of the 
present invention and incorporated herein by reference. An output pulse 
generator 68 provides the pacing stimulus to the patient's heart 16 
through coupling capacitor 74 in response to a pacing trigger signal 
developed by digital controller/timer circuit 50 each time the escape 
interval times out, or an externally transmitted pacing command has been 
received, or in response to other stored commands as is well known in the 
pacing art. 
Digital controller/timer circuit 50 is coupled to an activity circuit 70 
for receiving, processing, and amplifying signals received from activity 
sensor 20. Activity circuit 70 produces an activity signal which is 
representative of the patient's metabolic requirements. Similarly, digital 
controller/timer circuit 50 is coupled to a pressure circuit 72 for 
receiving, amplifying and processing sensor output from pressure sensor 
22. In the presently disclosed embodiment of the invention, pressure 
circuit 72 produces an amplified, filtered analog pressure signal which is 
received by digital controller/timer circuit 50. In conjunction with 
ADC/MUX 60, digital controller/timer circuit samples and digitizes the 
pressure signal from pressure circuit 72 to obtain a digital 
representation of the peak value of intracardiac pressure during each 
cardiac cycle. This value is provided to microprocessor 34, which 
maintains a running average over a previous number of cardiac cycles (e.g. 
sixteen) of the intracardiac pulse pressure. 
With continued reference to FIG. 2, input/output circuit 30 further 
includes sensitivity control circuitry 75 coupled between digital 
controller/timer circuit 50 and sense amplifier circuit 64. Sensitivity 
control circuit 75 controls the sense amplifier gain and thus the sensing 
threshold of sense amplifier 64 as instructed by digital controller/timer 
circuit 50. 
With reference to memory 80 of FIG. 2, digital to analog converter (DAC) 82 
converts a digital representation of a signal into an analog signal. 
Address decode 84 controls the addressing of the row and column of analog 
memory 90 for both writing and reading data. Sample and hold circuit 86 
samples an analog signal at a periodic rate to enable the storage of an 
analog signal in analog memory 90. High voltage (HV) and switch circuit 88 
generates an approximate 20 volt DC voltage and, via the switches, stores 
a representation of an analog signal in analog memory 90. Analog memory 90 
is a standard EEPROM memory utilized in this application to store analog 
signals. 
Turning now to FIG. 3, the preferred embodiment for the compression and 
storage of an analog signal in the pacemaker of FIG. 2 is shown. The 
signal to be stored may be one or both of the intracardiac signal from the 
EGM amplifier 66 of FIG. 2 and/or a signal from a sensor such as the 
pressure sensor 22 shown in FIG. 2. The EGM amplifier 66 and pressure 
circuit 72 amplify and filter the appropriate physiologic signal from the 
patient and provide a voltage level suitable for storing in memory 80. The 
flow diagram of FIG. 3 is initiated at block 102. The data to be stored is 
sampled and held at block 104 by sample and hold circuit 86 of FIG. 2. At 
block 106, the data is stored in a section of analog memory 90 via address 
decode 84 and high voltage supply (HV) and switch circuit 88, all of FIG. 
2. Digital data indicative of device function may also be stored 
interdispersed among the analog signal as herein described later in 
association with FIGS. 9 and 10. Analog memory 90 may be of the type 
incorporated in ISD 1016, a telecommunication monolithic integrated 
circuit from Information Storage Devices, Inc. A preliminary specification 
dated March, 1991 is incorporated herein by reference in its entirety. 
Analog data is continuously stored in a circular buffer (a section of 
analog memory 90), enabling 30 seconds of pre-event data to be frozen upon 
an event detection period. 
Automatic event detection is tested for in block 108 by microcomputer 
circuit 32 of FIG. 2 which monitors the intracardiac electrogram for 
arrhythmias, PVCs or runs of PVCs, as shown in co-pending U.S. patent 
application Ser. No. 07/881,996, filed on May 1, 1992 entitled "Diagnostic 
Function Data Storage and Telemetry Out for Rate Responsive Cardiac 
Pacemaker," and assigned to the assignee of the present invention and 
incorporated herein by reference in its entirety. Alternatively, patient 
activation by reed switch closure with a hand-held magnet placed over the 
implant site may also trigger storage of signals. Upon automatic detection 
of the event, or patient activation, the analog data is converted to 
digital data at block 110 by 8 bit ADC 60 of FIG. 2. During the data 
compression process, the sample and hold 104 and storage in buffer 106 are 
bypassed and the analog data is compressed in real-time. At block 112, the 
digital data is compressed as will herein below be described. The 
compressed data is reconverted to analog voltage levels at block 114 via 
DAC 82 of FIG. 2. At block 116 the marker data from the microcomputer 32 
and the reconverted data from block 114 are stored in analog memory 90 via 
address decode 84 and high voltage supply (HV) and switch circuit 88, all 
of FIG. 2. A test for 2 more data values is evaluated at block 118. If 
`YES`, the flow diagram returns to block 110. If `NO`, the flow diagram 
moves to block 120 where data temporarily stored in a section of analog 
memory 90 used as a circular buffer is converted, compressed, reconverted 
and stored as herein described above. Upon completion of storage of the 
data temporarily stored in the circular buffer, the flow diagram exits at 
block 122. 
In the present embodiment, five minutes of data may be stored along with 
the 30 seconds of pre-event data from the circular buffer and the time of 
occurrence from a system clock from microcomputer circuit 32 of FIG. 2. 
Nine separate events including pre-event, post-event and time of 
occurrence of each may be stored in an ISD 1016 (a 128K byte) analog 
memory 80. More or longer episodes may be stored if larger memory capacity 
or multiple integrated circuits are available. Memory 80 may operate in a 
freeze mode whereupon being filled with nine separate events, the data is 
frozen under command of microcomputer 32 until interrogation and reset by 
an external programmer, not shown. Alternatively, memory 80 may operate in 
a circular buffer or over-write mode whereby the most recent nine events 
will be stored in memory and may be read and reviewed by a clinician upon 
interrogation by the programmer. In either storage mode, the external 
programmer may reset and reinitiate the storage function under program 
commands as is well known in the art. The reading of the stored data from 
analog memory 90 is under control of microcomputer 32, address decode 84 
and high voltage and switch circuit 88 whereby a specific memory bit is 
addressed and the analog signal is reconstructed sequentially, one bit at 
a time. This signal is routed through bus 48 to input/output circuit 30 
for transmission to an external programmer (not shown) via RF 
transmit/receive circuit 54 and antenna 52. 
It should be apparent that other methods of patient activation may be used 
such as tapping on the pacemaker can with a signal generated by the 
activity sensor 20 of FIG. 2 being detected to trigger the recording 
episode. Additionally, a simple patient programmer may be utilized to 
trigger the storage event. 
Turning now to FIG. 4, a flow diagram is shown which illustrates the 
compression process 110 of FIG. 3 in accordance with the presently 
disclosed embodiment of the invention. The flow diagram of FIG. 4 begins 
with the starting of the compression process at block 200 by event 
detection 104 of FIG. 3. At block 202 a counter, COMPRESS.sub.-- CYCLE, is 
set to 1. The first voltage value data point (n.sub.1) is saved at block 
204. The following two data points, n.sub.2 and n.sub.3, are obtained in 
block 206 from block 110 of FIG. 3. Data point n.sub.2 is compared to the 
saved data point n.sub.1 at block 208. If n.sub.2 is equal to, or greater 
than, n.sub.1, block 208 returns a `YES` and the larger of n.sub.2 or 
n.sub.3 is saved at block 210. If n.sub.2 is smaller than n.sub.1, then 
the smaller of n.sub.2 or n.sub.3 is saved at block 220. More data is 
tested for at block 212. If at least 2 more data values are available, the 
flow diagram returns to block 206. If block 212 returns a `NO`, the 
COMPRESS.sub.-- CYCLE counter is incremented by 1. At block 216, if the 
COMPRESS.sub.-- CYCLE counter is greater than 2 the flow diagram is exited 
at block 218. If the COMPRESS.sub.-- CYCLE counter is less than, or equal 
to 2 the flow diagram returns to block 204. As described in this preferred 
embodiment, the flow diagram is serially repeated a second time on the 
data compressed and stored by the algorithm of FIG. 4 to enable the 
compression of data by a factor of 4 to 1. The flow diagram of FIG. 4 
retains good fidelity by storing the maximum or minimum transition points 
in a waveform while reducing the stored data by 75%. 
Turning now to FIG. 5, a flow diagram is shown which illustrates an 
alternative embodiment whereby the implantable device 10 of FIG. 1 
incorporates an implantable 24 hour holter monitor. The signal to be 
stored may be one or both, the intracardiac signal from EGM amplifier 66 
of FIG. 2 and/or the signal from a sensor such as the pressure sensor 22 
shown in FIG. 2. The flow diagram is initiated at block 302 by the methods 
as herein described above (e.g. event detection, programming or patient 
activation). The analog signal is converted to a digital word in block 304 
via the 8-bit ADC 60 of FIG. 2. At block 306, the data is compressed via 
the technique as described herein above and shown in FIG. 4. At block 308, 
the compressed digital data is reconverted to voltage levels via the DAC 
82 shown in FIG. 2. At block 310, the marker data from the microcomputer 
circuit 32 of FIG. 2 and the reconverted data from block 308 are stored in 
analog memory 90 of FIG. 2. A test for more data is evaluated at block 
312. If `YES`, the flow diagram returns to block 304. If `NO`, the flow 
diagram exits at block 314. Block 310 may store data in a freeze mode or 
an over-write mode under control of microcomputer 32, FIG. 2. 
Referring now to FIG. 6, a flow diagram is shown which illustrates an 
alternative embodiment whereby the implantable device 10, FIG. 1, 
incorporates storage of analog EGM data directly without an intervening 
conversion to digital data and subsequent reconversion to analog data. The 
signal to be stored may be one or both, the intracardiac signal from EGM 
amplifier 66 of FIG. 2 and/or the signal from a sensor such as the 
pressure sensor 22 shown in FIG. 2. The flow diagram is initiated at block 
402 by the methods as described herein above (e.g. event detection, 
programming or patient activation). The analog data is sampled and held at 
block 404 by a sample and hold circuit 86 of FIG. 2. The sampling may 
occur at a 128 Hz rate for adequate clinically useful data. At block 406, 
the data is stored in analog memory 90 via address control 84 and high 
voltage supply (HV) and switches 88 of FIG. 2. At block 408, a test for 
more data is entered. If `YES`, the flow diagram returns to sample and 
hold block 404. If `NO`, the flow diagram is exited at block 410. This 
embodiment is simpler, requires less circuitry and less current drain from 
the battery because of the elimination of the analog to digital conversion 
304, the compression of data 306 and the digital to analog conversion 308 
from FIG. 5. The embodiment shown in FIG. 6 will have improved fidelity 
and will compress data to 8:1 versus a digital equivalent. 
Finally turning to FIG. 7, a block diagram is shown which illustrates an 
alternative embodiment whereby the implantable device 10, FIG. 1, 
incorporates the storage and compression of analog data without the 
conversion of the analog data to digital data and the subsequent 
reconversion to analog data as described in the embodiment of FIG. 5. The 
signal to be stored may be one or both, the intracardiac signal from EGM 
amplifier 66 of FIG. 2 and/or the signal from a sensor such as the 
pressure sensor 22 shown in FIG. 2. A sample and hold circuit 502, sampled 
at a 128 Hz rate, generates analog voltage data that is stored temporarily 
in analog memory cells 506, 508, and 510 under control of MUX control 
circuit 504. 
The first analog voltage value/data is stored in cell 510 and in the first 
memory location. The next two data samples are stored in cells 506 and 
508, respectively. Comparator 512 compares the voltage level of analog 
cells 506 and 508 indicating, by a true logic level, if the voltage in 
cell 506 is greater than 508. Alternatively, a false logic level indicates 
that the voltage in cell 508 is greater than the voltage in cell 506. 
Similarly, comparator 514 compares the voltage level of analog cells 508 
and 510 indicating by a true logic level if the voltage in cell 508 is 
greater than 510. Alternatively, a false logic level indicates that the 
voltage in cell 510 is greater than the voltage in cell 508. Microcomputer 
32, FIG. 2, determines by a simple process flow, shown in FIG. 8, which 
value is stored in analog memory 516 by control/MUX circuit 504. 
Referring to FIG. 8, a block diagram is shown which illustrates a flow 
diagram associated with the compression of data from FIG. 7. The flow 
diagram of FIG. 8 begins by initiating the compression process at block 
600 by event detection 104 of FIG. 3. The first data value (C) is stored 
in temporary cell 510 and the first address in memory in block 602. The 
next two values of data are stored in cells 508 (B) and 506 (A) 
respectively by block 624. If comparator 514 is `TRUE` at block 604, the 
flow diagram tests if comparator 512 (D) is TRUE in block 606. If `TRUE`, 
the analog value stored in 506 is stored in memory and in cell 510 by 
block 618. If at block 606, comparator 514 (E) is `FALSE`, the analog 
value stored in cell 508 is stored in memory and cell 510 by block 618. If 
at block 604, comparator 514 is `FALSE` and comparator 512 is `TRUE`, the 
analog value in cell 506 is stored in memory and cell 510 at block 618. If 
at block 612, comparator 512 is `FALSE`, the analog value stored in cell 
508 is stored in memory and cell 510 by block 618. Block 620 tests for 2 
more data values; if `YES`, the flow diagram returns to block 624. If 
`NO`, the compression process exits and stops at block 622. 
After the analog signals are stored as herein above described, the analog 
data is converted to a signal for transmission to an external peripheral, 
such as a programmer, at a subsequent follow-up either via a clinic or 
hospital office visit or transtelephonically. Telemetry techniques are 
well known to those skilled in the art. Specific reference is made to 
converting the stored analog voltage to a pulse position modulated format 
as taught in Medtronic U.S. Pat. No. 4,556,063 to Thompson et. al, and 
incorporated herein by reference. Alternatively, the stored analog voltage 
may be digitized by an ADC and transmitted via the method taught in 
copending U.S. patent application Ser. No. 07/468,407 filed on Jan. 22, 
1990, entitled "Improved Telemetry Format," which is assigned to the 
assignee of the present invention and which is incorporated herein by 
reference. The high data rate telemetry system of the '407 application 
would be a preferred method for transmitting the stored data to a 
peripheral allowing data to be stored at a low rate (e.g., 32 samples per 
second to even one sample per hour or per day) and transmitted to a 
peripheral at a very high rate. 
FIGS. 9 and 10 demonstrate the incorporation of marker or event indication 
information into the stored data. A marker indication of device function 
is entered into the stored analog data stream by utilizing a single bit 
indicator of -V.sub.MAX followed by a selectable voltage value indicating 
a marker of specific device function. In present day embodiments, 
intracardiac signal data is lost when digital marker information is 
simultaneously transmitted with the intracardiac signal because signal 
data bits are removed and replaced with digital data indicative of device 
function. FIG. 9 shows an example of the stored voltage 700 in the 
preferred embodiment of FIG. 3. At 702, the marker indicator of -30 mVolts 
(-V.sub.MAX) is indicated. At 704, a value of -10 mVolts indicates a 
ventricular event. In a preferred embodiment for a dual chamber (DDD) 
pacemaker, the following are selectable voltage levels for data bit 704: 
______________________________________ 
Event Voltage 
______________________________________ 
.sup.V Sense -10 mV 
.sup.V Refractory Sense 
-20 mV 
.sup.V Pace -30 mV 
.sup.A Sense +10 mV 
.sup.A Refractory Sense 
+20 mV 
.sup.A Pace +30 mV 
______________________________________ 
FIG. 10 shows the reconstructed and displayed EGM signal 802 with a 
ventricular sense (V.sub.Sense) marker indicator 800 displayed on an 
external peripheral/programmer graphics screen or printed or plotted to a 
paper hardcopy. Note that the marker information has been removed from the 
analog data signal and the adjacent analog data points reconnected. 
Similarly, the time of occurrence of each stored event may be encoded and 
stored as herein described above. 
In addition to the storage of analog data and subsequent telemetry to a 
programmer as taught herein above, a stored arrhythmia episode may be 
processed by microcomputer 32 to initiate a pacing, cardioversion or 
defibrillation therapy as is known in the art. Additionally, a stored 
signal indicative of capture, non-capture or a fusion beat may be compared 
on a periodic, or a beat-by-beat basis to the signal seen by the sense 
amplifier 64 after a pacing stimulus from output circuit 68. If an evoked 
response is not indicated (non-capture), the output stimulus pulse width 
and/or amplitude may be increased to regain capture as disclosed in U.S. 
Pat. No. 4,858,610 issued to Callaghan, et al., U.S. Pat. No. 4,878,497 
issued to Callaghan, et al., and U.S. Pat. No. 4,729,376 issued to Decote, 
all of which are incorporated herein by reference in their entireties. The 
compression function may be performed by microcomputer 32 by pattern 
matching the periodic signal within a window from 5 to 80 mSec after 
output stimulus to the stored representation of the signal indicative of 
capture. Alternatively, the negative magnitude of the stored non-capture 
signal may be summed with the periodic signal with the resultant signal 
being indicative of an evoked response, if present. 
From the foregoing detailed descriptions of particular embodiments of the 
invention, it should be apparent that a pacemaker has been disclosed which 
is provided with the capability of the compression and storing of a 
significant amount of analog data such as an intracardiac electrogram. 
While the particular embodiments of the present invention have been 
described herein in detail, it is to be understood that various 
alterations, modifications, and substitutions can be made therein without 
departing from the spirit and scope of the present invention, as defined 
in the claims, which follow.