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CROSS-REFERENCE TO RELATED APPLICATION
Reference is made to the related application Ser. No. 555,897, filed
concurrently herewith in the names of J. D. Menken, B. A. Brastand and T.
C. Barthel, entitled "Apparatus for Sensing and Transmitting a Pacemaker's
Stimulating Pulse" and assigned to the assignee of this invention.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to apparatus for modulating a carrier with
information to be transmitted over a relatively low-bandwidth transmission
medium in a manner to permit its detection at a receiver and in particular
to such apparatus adapted to transmit data indicative of the width of a
pacemaker stimulating pulse to a remote location, whereby the pulse width
may be accurately determined to provide an indication of the pacemaker's
energy source, e.g. a battery.
2. Description of the Prior Art
Heart pacemakers such as that described in U.S. Pat. No. 3,057,356, issued
in the name of Wilson Greatbatch and assigned to the assignee of this
invention, are known for providing electrical stimulus to the heart
whereby it is contracted at a desired rate in the order of 72 beats per
minute. A heart pacemaker is capable of being implanted in the human body
and operative in such an environment for relatively long periods of time,
to provide cardiac stimulation at relatively low power levels by utilizing
a small, completely implanted, transistorized, battery-operated pacemaker
connected via flexible electrode wires directly to the myocardium or heart
muscle. The electrical stimulation by this pacemaker is provided at a
relatively fixed rate.
Such cardiac pacers of the implantable variety have found wide acceptance
for patients suffering from complete heart block. As a result, the use of
these pacers has increased the life expectancy of those patients with
implants, from a 50% probability of one (1) year to nearly the life
expectancy of physically-comparable humans not suffering from the same
heart disorder.
Typically, such cardiac pacers are encapsulated in a substance
substantially inert to body fluids, and are implanted within the patient's
body by a surgical procedure wherein an incision is made in the chest
beneath the patient's skin and above the pectoral muscles or in the
abdominal region, and a pacemaker is implanted therein. Due to the
inconvenience, expense and relative risk to the patient's health, it is
highly desired to extend the life of the power source or battery, whereby
the number of such surgical procedures is limited. The resultant problem
for the attendant doctor is to determine when the batteries should be
replaced, keeping in mind the relative risk or probability of premature
pacer failure due to battery depletion.
A number of solutions to this problem have been proposed, one being
replacement at predetermined intervals, thus accepting an empirically
determined risk or failure of the pacemaker batteries. Another proposed
solution is to establish pacer "clinics" where photographic analysis
techniques are used to detect imminent failure. These solutions are not
entirely satisfactory for detecting, simply and positively, a degradation
of pacer system performance. The risk of undetected premature failure
associated with periodic replacement intervals at predetermined intervals
is obviously undesirable. Photoanalysis techniques are complicated, not
positive in detection and require that the patient be physically present
in the physician's office. Further, such techniques are not readily
available to physician and patient on short notice, but rather, as
mentioned previously, would be available only at special clinics.
In U.S. Pat. No. 3,618,615, assigned to the assignee of this invention,
there is disclosed an artificial cardiac pacemaker for generating at
regular intervals a train of stimulating pulses, one of which is of
significantly lower energy than the other pulses. If the heart responds to
the reduced energy stimulating pulse, an adequate safety factor remains,
but if the heart does not respond, e.g. no beat is detected in response to
the lower energy or test pulse, marginal operation and possible imminent
failure is ascertained.
Another method of ascertaining the pending failure of a pacemaker energy
source is described in U.S. Pat. No. 3,713,449, assigned to the assignee
of this invention, describing an artificial pacemaker including means for
varying selectively the pulse width of its stimulating pulse. Control of
the pulse width is made preferably by a mechanism external of the body by
an attending physician. By such mechanism, the physician varies the pulse
width of the implanted pacemaker until capture is lost. As the physician
has previously measured the pulse width at the time of pacemaker implant,
the pulse width at a subsequent time may be varied until capture is lost,
whereby the state of the battery can be determined with respect to its
replacement.
In an alternative approach to the problem of accurately determining battery
depletion, there are artificial pacemakers such as described in U.S. Pat.
No. 3,842,844 having a battery or cell depletion indicator that increases
the pulse width of the output signal as their batteries deplete, i.e.
their voltage amplitude decreases. Further, as the power source or battery
depletes, the pulse repetition rate of such artificial cardiac pacemakers
also decreases. For example, at the time of implantation, an artificial
cardiac pacemaker may produce stimulating pulses at 70 beats per minute
(BPM), plus or minus two beats, with a pulse width in the order of 0.5
msec. After a period of service illustratively in the order of 2-4 years,
the BPM changes in the order of 5-10%, i.e. a decrease of 5-7 beats from
the original BPM, and the pulse width may increase to a value in the order
of 1 msec. Dependent upon the known histories of such batteries, such a
change in the BPM as well as a change in pulse width indicates that one of
a plurality (e.g. 4 or 5) cells has failed, and that it is time to replace
the batteries within the implanted pacemaker to assure continued heart
stimulation of a sufficient level.
Pulse width increase is desired in order that as the amplitude of the
voltage provided from the pacemaker battery decreases, the total energy in
the stimulating pulse remains substantially constant. It is understood
that the voltage level of the pacemaker battery may decrease below a level
at which the heart may not respond regardless of the pulse width. Further,
as the pulse width increases to compensate for decreases in the voltage
level, the current drain upon the battery increases, thereby increasing
the rate of battery depletion.
As indicated above, one of the disadvantages of the prior art schemes with
regard to determining pacemaker energy source depletion, is that it
requires the patient to be physically present in the doctor's office,
hospital or clinic. At the present time, there are systems such as
described in U.S. Pat. No. 3,923,041, assigned to the assignee of this
invention, for sensing via electrodes attached to the patient's body the
electrical activity of the heart including the stimulating pulses applied
thereto from an artificial cardiac pacemaker and for transmitting such
signals over readily available transmission media such as the telephone
lines, to a remote station in the doctor's office, hospital or clinic, for
example, where a trained person, such as the physician or his aide, may
conveniently monitor the heart activity. In this manner, regular checkups
of the patient's heart may be made without requiring the patient to travel
to the hospital, clinic or doctor's office.
In FIG. 4 of the drawings, there is shown a typical graph of the electrical
activity of a patient's heart which is stimulated by pulses derived from
an artificial cardiac pacemaker. The heart activity or electrocardiogram
(EKG) of the patient is indicated in FIG. 4 by the letter "B," whereas the
stimulating pulse is identified by the letter "A," whose pulse width is
indicated with the letters "PW." Noting that the threshold for stimulation
of the heart varies from patient to patient, the minimum pulse width of
the stimulating pulse is in the order of 100.mu.sec and its minimum
voltage amplitude is in the order of 0.5V. Typically, the amplitude of the
stimulating pulse would be in the order of 6V.
In view of the narrow bandwidth of telephone transmission lines, low
carrier frequencies in the order of 1500Hz are used to transmit such
signals. As a result, it is not possible to transmit with accuracy, the
width of the pacemaker stimulating pulse. Typically, the pulse width of an
artificial cardiac pacemaker may vary from 0.15 msec to about 3 msec. A
significant portion of a pulse of such narrow width is lost when
superimposed upon a low carrier frequency. Thus, though it would be
possible to transmit directly such a pulse to a remote station, the
detected pulse width would not be reproduced with sufficient accuracy to
indicate with the desired precision the state or life of the pacemaker
energy source.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide an accurate
indication of the state of an artificial pacemaker power source, without
requiring a patient with an implanted pacemaker to be present in the
doctor's office, clinic or hospital.
It is a further object of this invention to provide apparatus for
transmitting with significant accuracy the artificial pacemaker
stimulating pulse as detected from the patient, whereby its pulse width
may be accurately determined and displayed at a remote station.
It is a still further object of this invention to provide a relatively
inexpensive circuit for accurately measuring the width of the stimulating
pulse and for effectively multiplying the detected pulse width before
transmission over a relatively narrow bandwidth medium to a receiver
station.
In accordance with these and other objects, there is disclosed apparatus
for detecting through electrodes attached to the patient's body, the
electrical activity of the patient's heart and the stimulating pulses
applied thereto by an implanted cardiac pacemaker, and for trannsmitting
these signals upon relatively narrow bandwidth transmission media such as
telephone lines, to be received and suitably displayed, whereby the width
of the artificial pacemaker stimulating pulse may be accurately
determined. In particular, the stimulating pulse is detected and amplified
and then subsequently multiplied by a factor selected in view of the
relatively low carrier frequency to ensure that upon subsequent detection
and display; other cardiac electrical activity is not obscured. The
multiplied pulse is imposed upon the carrier frequency without loss of
sufficient accuracy as to its width. In particular, the multiplication is
achieved in an illustrative analog embodiment, wherein the leading and
trailing edges of the stimulating pulse are detected to initiate and
terminate a first integration period in which a first reference voltage is
applied to an integrator to be integrated at a first, relatively high
rate. The first integration period is terminated with the detection of the
trailing edge of the stimulating pulse, and a second integrating period is
commenced thereby. Multiplication is achieved during a second, subsequent
period in which the integrator integrates a second reference voltage at a
second, significantly slower rate. A control signal is developed for a
multiplied period corresponding to the sum of the first and second
integration periods and is applied to actuate a voltage-controlled
oscillator which generates a signal for the multiplied period to be
transmitted upon the low bandwidth transmission lines.
BRIEF DESCRIPTION OF THE DRAWINGS
These and other objects and advantages of the present invention will become
more apparent by referring to the following detailed description and
accompanying drawings, in which:
FIG. 1A is a simplified, block diagram of the transmitter in accordance
with the teachings of this invention;
FIG. 1B is a simplified diagram showing the transmitter as shown in FIG. 1A
for transmitting the electrical activity of the patient's heart along a
transmission medium to a receiver;
FIG. 2 is a more detailed, block diagram of the transmitter as generally
shown in FIG. 1A;
FIGS. 3A, 3B and 3C are detailed, schematic drawings of the transmitter
more generally shown in FIGS. 1A and 2, and are interrelated as shown in
FIG. 3;
FIG. 4 is a graphical representation of the electrical activity of the
patient and the heart pacemaker stimulating pulses inserted thereon, as
transmitted and detected by the receiver in accordance with the teachings
of this invention;
FIGS. 5A to 5L show the various signals as provided in the block diagram of
the transmitter of FIG. 2; and
FIGS. 6A and 6B show a simplified block diagram of the receiver as
generally shown in FIG. 1B.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings and in particular to FIG. 1B, there is shown
a simplified diagram illustrating the relationship of a transmitter 10 in
accordance with the teachings of this invention for detecting and
transmitting the electrical activity of the patient's heart, e.g. the
patient's electrocardiogram (EKG) upon which has been superimposed the
detected stimulating pulse multiplied by a selected factor, across a
relatively low bandwidth transmission medium 50 to be received by a
receiver 60, which operates to separate the multiplied stimulating pulse
and to display it with significant accuracy, whereby the attending
physician can determine accurately the width of the stimulating pulse and
the state of the pacemaker's energy source. It is evident that the
transmitter 10 may be a relatively portable unit that is adapted to be
coupled to a telephone set, whereby the desired signals are transmitted
over the telephone lines or transmission medium 50. At the end of the
transmission medium 50, the receiver 60 is adapted to be coupled to
another telephone set, whereby the transmitted signal may be converted to
electrical signals within the receiver 60 to be processed and displayed as
to be described in more detail.
In FIG. 1A, there is shown a simplified schematic of the transmitter 10,
shown in block form in FIG. 1B. In particular, a pair of electrodes 11 is
attached to the patient, as upon the inside of his forearms, whereby the
electrical activity of the patient's heart and the stimulating pulse as
generated and applied to the patient by the implanted artificial
pacemaker, are detected and applied to an amplifier 12. The detected
signal is amplified by the amplifier 12 before being applied to the EKG
channel comprised of a filter 13 and a voltage-controlled oscillator 18.
The filter 13 serves to block the pacemaker stimulating pulse from the EKG
channel. Further, the amplified signal is also applied to a pulse
stretcher 20 whereby the stimulating pulse is detected and amplified to be
subsequently stretched by a fixed factor, dependent upon the carrier
signal to be transmitted along the transmission medium 50 to a display,
e.g. a strip recorder, within the receiver 60, whereby the pulse width as
displayed thereon does not mask other information of interest to the
patient's doctor. Illustratively, the factor is selected to be within the
range of 30-50 with a preferred value being in the order of 40. By
selecting a multiplication factor in the noted range, the superimposed,
multiplied pulse is of sufficient duration with respect to the frequency
of the carrier signal, that no significant amount of the width of the
stimulating pulse is lost. At the same time, even with a factor of 50, a
pulse of normal width, e.g. 1.0 msec, as received and displayed by the
receiver 60 does not unduly mask the other information as displayed at the
remote station. When an abnormal pulse of extended width, e.g. 3 msec, is
detected and displayed by the receiver 60, the stretched pulse may obscure
other data of interest. However, the display of such a wide pulse is
desired in that it provides a warning that the pacemaker battery may be
nearly depleted.
In FIG. 2, there is shown a more detailed block diagram of the transmitter
10 generally shown in FIGS. 1A and 1B. In particular, the electrical
activity of the heart with the stimulating pulse imposed thereon is sensed
by the electrodes 11 and applied thereby to an EKG channel comprised of
the amplifier 12, a slew rate limiter 14 and an amplifier 16. Likewise,
the detected activity is applied by conductor 15 to a pulse stretcher or
multiplier 20, comprised of a detector-amplifier 23 which serves to
differentiate and amplify the pacemaker stimulating pulse, shown in FIG.
5A, to provide first and second spikes for each stimulating pulse width
received. As illustrated in FIG. 5B, the leading edge of the detected
stimulating pulse causes detector-amplifier 23 to generate a
positive-going pulse, whereas the trailing edge of the stimulating pulse
causes detector-amplifier 23 to generate a negative-going pulse. As shown
in FIG. 2, the series of positive- and negative-going pulses are applied
by a conductor 25 to first and second one-shot multipliers 22 and 24. In
FIG. 2, letters "A," "B," "C," etc. are used to denote the signals
appearing at various points in the circuit and correspond to these
signals' waveforms as shown in FIGS. 5A to 5L. For example, the series of
positive- and negative-going pulses as shown in FIG. 5B are applied to
both of the one-shot multipliers 22 and 24. The positive-going pulse
corresponding to the leading edge of the stimulating pulse triggers the
one-shot multiplier 22 to generate a pulse-like signal of fixed duration,
e.g. 8 msec, as shown in FIG. 5C, whereas the negative-going pulse
corresponding to the trailing edge of the stimulating pulse causes
one-shot multiplier 24 to generate a pulse of fixed duration, e.g. 8 msec.
As shown by a comparison of FIGS. 5A, 5C and 5D, the leading edge of the
fixed duration pulses as derived from the one-shot multipliers 22 and 24
are spaced from each other by a period corresponding to the width of the
stimulating pulse.
As shown in FIG. 2, the fixed pulses derived from one-shot multipliers 22
and 24 are both applied to a differentiating circuit 26, whose output is
coupled through an OR circuit 30 to apply an output as shown in FIG. 5E
comprised of a series of positive-going pulses to the set terminal of a
first flip-flop 32. The differentiating circuit 26 differentiates the
pulse-like signals, providing spikes corresponding to the leading edge of
the fixed pulse width signals, thereby rendering the OR circuit 30
relatively insensitive to changing DC levels but responsive to the rising,
spiked signals. The output of the OR circuit 30, as shown in FIG. 5E,
comprises a series of positive-going pulses spaced apart from each other a
distance corresponding to the pulse width of the stimulating pulse and
serve to set the first flip-flop 32, whose output goes high, as shown in
FIG. 5F, upon being set.
Further, the fixed pulse width signals derived from the one-shot
multipliers 22 and 24 are both applied to an AND circuit 28, which upon
coincidence of its input signals, applies an output signal as shown in
FIG. 5G, to reset the first flip-flop 32 and to set a second flip-flop 34.
As shown in FIG. 2, the output of the AND circuit 28 is applied to the
reset terminal of the first flip-flop 32 along the conductor 39, and to
the second flip-flop 34 along conductor 41. As can be seen from a
comparison of FIGS. 5A, 5D and 5G, an output from the one-shot multiplier
24 corresponds to the trailing edge of the stimulating pulse and serves to
set the AND circuit 28, whereby the first flip-flop 32 is reset. Thus, the
output of the first flip-flop 32 lasts for a period corresponding to the
width of the detected stimulator pulse.
Further, the AND circuit 28 serves to distinguish stimulator pulses from
other, extraneous signals that may be imposed upon the electrodes 11,
requiring coincidence of the fixed pulse width signals before resetting
the first flip-flop 32. In particular, the fixed pulse width of the
signals derived from the one-shot multipliers 22 and 24 is set
illustratively at 8 msec, and the pulse width of a real stimulating pulse
is typically less than 8 msec. Thus, if an extraneous signal having a
pulse width greater than 8 msec is detected, a coincidence condition will
not be sensed by AND circuit 28 to reset flip-flop 32 and a stretched
pulse will not be transmitted by the transmitter 10.
Further, the output of the one-shot multiplier 24 is applied by way of the
differentiating circuit 26 and the OR circuit 30 to tend to set the first
flip-flop 32. However, the differentiator circuit 26 responds to the
leading edge of the output of the one-shot multiplier 24 to provide a
sharp pulse-like signal. By comparison, the reset pulse derived from AND
circuit 28 is of relatively long duration, i.e. the total overlap between
the outputs of the one-shot multipliers 22 and 24, whereby the tendency to
set flip-flop 32 is overriden and in fact, the first flip-flop 32 is
reset.
As shown in FIG. 2, the output derived from the first flip-flop 32 is
applied by a conductor 47 to control a first switch 36, whereby when the
output of the flip-flop 32 is high, the first switch 36 applies a negative
reference signal -Vref to an integrator 40, tending to integrate the
reference signal in a positive direction, as shown in FIG. 5J. As
explained above, the first flip-flop 32 is set and therefore the first
switch 36 is turned on for a first period of time corresponding to the
width of the stimulating pulse to be stretched or multiplied. Thus, in the
first period of time, the integrator 40 integrates the negative reference
voltage -Vref at a first, relatively high rate whereby the amplitude of
the integrator 40 at the termination of the first period is proportional
to the width of the stimulating pulse. The first period is terminated upon
the first flip-flop 32 being reset to turn off the first switch 36, at
which time the second flip-flop 34 is set to provide a high output as
shown in FIG. 5H to a second switch 38, which is turned on to apply a
positive reference potential +Vref to the integrator 40. In response to
the positive ref potential, the integrator 40 integrates in a
negative-going direction at a second, significantly slower rate than the
first rate for a second period of time.
As shown in FIG. 2, the output of the integrator 40 is applied to a
comparator 42, which compares the output of integrator 40 with a reference
level, illustratively set at ground potential. Thus, when the output of
the integrator 40 is in excess of the reference or ground potential, the
output from the comparator 42 goes high, as seen in FIG. 5K. The output of
the comparator 42 is applied by a conductor 43 to reset the second
flip-flop 34. In particular, as shown in FIGS. 5K and 5H, the trailing
edge of the output of the comparator 42 serves to reset the second
flip-flop 34, whereby the second switch 38 is rendered non-conductive and
the integrator 40 ceases to integrate the +Vref signal, as shown in FIG.
5J. As indicated in FIG. 5J, the integrator 40 integrates during the first
period the negative-going reference potential -Vref at a first, relatively
high rate, e.g. +2v/ms and integrates during the second period the
positive reference potential +Vref at a second, relatively slow rate, e.g.
-2v/39ms. Thus, if the width of the stimulating pulse is "ta", as shown in
FIG. 5A, the sum of the first and second periods corresponds to 40 ta,
thus providing a means for multiplying the stimulating pulse for a
preselected factor, e.g. 40. It is understood that the integrating rates
for the first and second periods may be set at other values, whereby the
multiplication factor may be set in the range of 30-50. Thus, the output
of the comparator 42 representing the multiplied stimulating pulse, as
shown in FIG. 5K, goes high for a period corresponding to 40ta, and is
further applied to a summing point 17.
Turning our attention back to the EKG channel as shown in FIG. 2, the
detected electrical activity of the patient's heart along with the
pacemaker, stimulating pulse superimposed thereon, is applied to the
amplifier 12 which amplifies the input signal by a fixed gain, e.g. 11, to
provide the amplified output to a slew rate limiter 14, which operates to
limit the amplitude of signals passing therethrough, whereby the
stimulating pulses are substantially eliminated from the EKG channel. In
particular, the slew rate limiter 14 limits the rate of increase of the
input signals to a value, e.g. -100v/sec, whereby the EKG signals varying
in amplitude in the range of 1 to 5mV and of a frequency in the range of 0
to 50 or 70 Hz pass thereby and the stimulating pulses are effectively
blocked. The limited signals are further amplified by an amplifier 16 to
be applied with the multiplied stimulating pulse inserted at summing point
17 to a voltage-controlled oscillator (VCO) 18. The VCO 18 is responsive
to the amplitude of the input signal derived from the summing point 17 to
provide an output of a frequency proportional to its voltage level. As a
result, the amplified signal derived from the amplifier 16 indicative of
the EKG activity controls the voltage-controlled oscillator to provide an
output whose frequency varies about a nominal or center frequency in the
order of 1500 Hz. As shown in FIGS. 5K and 5L, the high output of the
comparator circuit 42 increases the center frequency of the VCO 18 to
about 2100 Hz for a period corresponding to that of the stretched pulse.
The VCO output is applied to be amplified by an audio amplifier 19,
whereby a transducer such as an audio speaker 21 produces a carrier signal
in the audio range, capable of being transmitted via a relatively low
bandwidth transmission line such as a telephone line 50. Though not shown,
it is understood that the speaker 21 would be disposed adjacent or coupled
to a telephone mouthpiece, whereby the tone is converted to electrical
signals to be transmitted via the telephone lines.
As explained above, the output of the comparator 42 is applied by a
conductor 45 to the summing point 17. When the output of the comparator 42
is high as shown in FIG. 5J, corresponding to a period equal to the width
of the stimulating pulse multiplied by a fixed factor, e.g. 40, the input
of the VCO 18 is clamped by the comparator output, substantially ignoring
the output of the amplifier 16, whereby the output of the VCO 18 is driven
to a frequency dependent primarily upon the comparator output, e.g. a
frequency of 2100 Hz, as illustrated in FIG. 5L. As a result, the VCO 18
applies a signal of a frequency of 2100 Hz to the speaker 21, whereby a
corresponding electrical signal is transmitted via the telephone lines 50
to be detected and processed by the receiver 60 to provide an indication
as upon a digital, LED display of the period of the 2100 Hz tone burst,
corresponding to the pulse width of the stimulating pulse.
In FIGS. 3A, 3B and 3C, there is shown a detailed, schematic diagram of the
circuit elements making up the transmitter as shown in block form in FIG.
2, wherein the elements composing each of the blocks shown in FIG. 2 are
outlined in dotted line and are identified with corresponding numbers. In
particular, the electrical activity of the patient's heart including the
superimposed stimulating signals are applied by electrodes 11 to the
amplifier 12, whose output is coupled to the EKG channel comprised of the
slew rate limiter 14, the amplifier 16, the summing point 17 and the VCO
18. As indicated in FIG. 3A, the summing point or circuit 17 includes a
variable resistor 150, whereby the output of the comparator 42 shown in
FIG. 3B and applied by a conductor 45 may be adjusted to derive an output
from the VCO 18 of selected frequency. In turn, the output of the VCO 18
is passed through a low-pass filter 25 (not shown in FIG. 2) before being
amplified by the audio amplifier 19 to energize the speaker 21. The
low-pass filter 25 removes extraneous signals to render the energizing
signal applied to the speaker 21 more sinusoidal, thus making the signal
detected at the receiver 60 more readable, as upon an oscilloscope.
As shown in detail in FIG. 3B, the sensed electrical activity of the
patient's heart along with the detected stimulating pulse applied thereto
is also applied by the conductor 15 to the detector-amplifier 23. In
particular, the signal is derived from a sensitivity adjustment
potentiometer 152 (see FIG. 3A) and applied by the conductor 15 to be
differentiated by a first differentiating circuit comprised of capacitor
156 and resistor 157, before being amplified by amplifier 153.
Subsequently, the signal is differentiated by a differentiating circuit
comprised of capacitor 158 and resistor 159 before being amplified by
amplifier 154. In turn, the output of amplifier 154 is differentiated by a
differentiating circuit comprised of capacitor 160 and resistor 161,
before being amplified by amplifier 155. The output of the
amplifier-detector 23 as derived from amplifier 155, is sufficiently
amplified and shaped to precisely trigger the control circuitry of the
pulse stretcher 20, now to be described in detail. In particular, the
amplifier output is applied by the conductor 25 to each of the one-shot
multipliers 22 and 24. The output of the one-shot multiplier 22 is applied
through the RC differentiating circuit 26 to the OR circuit 30 whose
output is applied by conductor 37 to set the first flip-flop 32. The
outputs of the one-shot multipliers 22 and 24 both are applied to the AND
circuit 28 comprised of three diodes and a transistor, whose output is
applied by conductor 39 to an OR circuit 31, which resets the first
flip-flop 32. Further, the output of the OR circuit 30 is also applied
along conductor 51 to a trailing edge monitor circuit 35, to initiate the
timing of the predetermined period, e.g. 5 msec. The output of the AND
circuit 28 indicative of the presence of the trailing edge is applied
along conductor 55 to inhibit the trailing edge monitor circuit 35. In the
absence of a signal from the AND circuit 28, the trailing edge monitor
circuit 35 generates a reset signal to be applied along conductor 53 to
the OR circuit 31, whereby flip-flop 32 is reset. Thus, in the presence of
extraneous noise where no defined trailing edge is detected, the trailing
edge monitor 35 serves through its associated OR circuit 31 to reset the
flip-flop 32 in preparation for receiving and processing the next detected
stimulating pulse.
In FIG. 3C, there is shown the detailed circuitry of the switches 36, 38,
the integrator 40 and the comparator 42. In particular, the positive
reference voltage is developed across a zener diode 158 to which is
applied a voltage, e.g. 9V, developed by an unregulated power source. In a
similar manner, the negative reference voltage -Vref is developed across a
light-emitting diode 160. Dependent upon which of the flip-flops 32 or 34
is energized, one of the switches 36 or 38 is actuated to apply its
reference voltage to the integrator 40, whose output in turn is applied to
the comparator 42. As explained above, the output of the comparator as
derived from the collector of transistor 162 is applied by conductor 45 to
the summing point 17 as shown in FIG. 3A.
In FIGS. 6A and 6B, there is shown a block diagram of the receiver 60,
generally shown in FIG. 1A. The receiver 60 is designed to decode and
display a frequency-modulated carrier signal as transmitted over the
transmission line 50 and converted to an audio sound by an ordinary
telephone mouthpiece. The receiver 60 displays the transmitted EKG
information on a strip chart recorder (not shown) and also provides a
display of the pacemaker rate and pulse width information on a separate,
typically digital display (not shown). The receiver 60 is intended for use
in the physician's office, clinic or hospital, for the reception and
analysis of EKG and stimulating pulse parameters of an implanted,
artificial pacemaker.
With regard to FIG. 6A, the earpiece of the receiver telephone (not shown)
is disposed adjacent a pick-up transducer 62, whereby the audio sound as
produced thereby of a frequency corresponding to the amplitude of the
patient's heart activity and the stimulating pulse, is converted to an
electrical signal. The electrical signal output of the pick-up 62 is
applied to a band-pass amplifier 64, which passes and amplifies only those
audio frequencies of interest, typically in the range of 1000-2500 Hz. The
band-pass amplifier output is applied to a high-pass amplifier 66, which
further reduces unwanted, high-frequency signals. In turn, the high-pass
amplifier output is applied to a high-gain amplifier 68 which further
amplifies the signal to a usable level. The filtered and amplified signal
output of the high-gain amplifier 68 is applied to a one-shot multiplier
70 which generates in response to the positive-going edge of the carrier
signal a pulse of a predetermined, relatively short duration in the order
of one-half the period of the carrier signal. The one-shot output is in
turn applied to a driver 72 which increases the amplitude to a desired
signal level in the order of .+-.8.0V.
The output of the driver 72 is applied to an EKG channel 80 and to an
artifact channel 100. The EKG channel comprises a no-signal clamp 82 which
"locks off" the EKG channel in the absence of an input signal so that the
output as derived therefrom and applied to its strip chart recorder is not
noisy. The no-signal clamp output is applied in turn to a demodulator 84,
which integrates the series of pulses applied to the EKG channel from the
driver 72 to recover the original signal indicative of the patient's heart
activity. The demodulated data is in turn applied to an EKG gate circuit
86, which blocks and prevents the stimulating pulses or pacemaker
artifacts from distorting the EKG data to be displayed upon the strip
chart recorder. During the time that a stimulating pulse is being
received, the EKG gate circuit 86 is closed and an EKG level memory 88
stores the last EKG data signal received, to provide continuity before and
after receipt of the stimulating pulse. Further, the EKG signal can be
applied successively to a 60 Hz notch and a 2 Hz roll-off filter 90 to
modify the frequency response of the receiver 60 at the option of the
user. A switch (not shown) is provided on the front panel of the receiver
60 to permit the notch and roll-off filters 90 to be incorporated into or
removed from the circuit of FIG. 6A.
The pulsed outputs of the driver 72 are also applied to an artifact channel
100 comprising a filter 102, which removes unwanted high frequencies and
produces a DC level proportional to the rate at which the train of input
pulses are received. In turn, the DC level output of the filter 102 is
applied to a high-pass filter 104 which produces a spike, when the DC
level at its input changes abruptly. In turn, the spike outputs of
high-pass filter 104, which correspond to the beginning and end of the
multiplied stimulating pulse, are applied to an artifact detector circuit
106 taking the form of a flip-flop which sets at the leading edge of and
resets at the trailing edge of the transmitted stimulating pulse, to
generate a corresponding pulse. Such a flip-flop includes a threshold
circuit rendering the circuit more sensitive to being reset than set. In
this manner, in a marginal detection situation, the artifact detector 106
would thereby tend to reset, avoiding a "hand-up" condition. The output of
the artifact detector 106 is applied to an artifact driver circuit 108,
which serves to amplify these signals and to apply its amplified output to
the artifact reinsertion amplifier 92, which recombines the amplified
stimulating pulse signal and the EKG signal and applies the combined
signals to a low-pass filter and driver circuit 96, which serves to remove
the remnants of the carrier signal and to am | | |
