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CROSS REFERENCE TO COPENDING APPLICATIONS
Attention is drawn to the following copending, commonly assigned
applications, all filed on the same date and further describing the
overall system in which the subject invention is incorporated:
"CARDIAC PACEMAKER HAVING A RATE LIMIT", by David L. Thompson, Ray S.
McDonald, and Yan Sang Lee, Ser. No. 957,828, filed Nov. 6, 1978;
"DEMAND CARDIAC PACEMAKER HAVING REDUCED POLARITY DISPARITY" by Gerome P.
Hartlaub and Ray S. McDonald, Ser. No. 957,812; filed Nov. 6, 1978;
"DIGITAL CARDIAC PACEMAKER" by David L. Thompson, Gerome P. Hartlaub, Ray
S. McDonald, and Martin A. Rossing, Ser. No. 957,960, filed Nov. 6, 1978;
"FREQUENCY TO VOLTAGE CONVERTER FOR CARDIAC TELEMETRY SYSTEM" by Stanley L.
Gruenenwald, Ser. No. 958,202, filed Nov. 6, 1978;
"PROGRAMMING AND MONITORING APPARATUS FOR A PROGRAMMABLE PACING GENERATOR"
by Robert Smith, Ser. No. 958,063 (now indicated to be allowed), filed
Nov. 6, 1978.
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to systems for detecting the occurrence of the
leading and trailing edges of pulses and in particular of those pulse-like
signals applied by a pacemaker to a patient's heart.
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 stimulating pulses to a
patient's 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 stimulating pulses by
this pacemaker are provided at a relatively fixed rate.
Typically, such heart pacemakers 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.
After surgical implantation of an artificial heart pacemaker by known
surgical techniques, the patient is required to have periodic checkups so
that the heart pacemaker function may be monitored for possible battery or
other failure.
A major problem with these devices is that battery failure is not precisely
predictable statistically and while statistics do exist they are
unfortunately gathered after pacemaker failure has occurred. Further,
present heart pacemakers available have a functional life expectancy of
about two to five years, but individual ones may not exceed this, and in
fact may rather unpredictably fail before this statistical determined
period. Usually approximately 90 percent of heart pacemaker failures are
battery failures, and the remaining 10 percent are a result of other types
of failures, the next most common failure being in the leads themselves.
Electronic component failure in artificial cardiac pacers is generally a
very small factor. However, all of these factors must be considered when
diagnosing a possible malfunction.
The number and kinds of variables that exist make an accurate a priori
prediction of the lifetime of a given heart pacemaker difficult. The
problem therefore is that of measuring the heart pacemaker pulse, the
interval between impulses and some characteristics or set of
characteristics which will allow determination in advance of a critical
situation, i.e., when the heart pacemaker is about to fail.
One such characteristic is that as the battery starts to fail, the voltage
output of the pulses starts to drop and generally as the voltage drops the
width of the pacemaker pulse changes. Further, in most heart pacemakers
the rate of firing changes.
Most pacemakers generally are powered by 4 or 5 miniature batteries.
Present monitoring techniques are geared to detect when the first of those
5 batteries has failed, which means that the safety factor is decreased.
In any event, a failure of not only one cell but generally two can be
tolerated before the patient is in any danger. It should be cautioned
however that when a battery does fail, it fails very rapidly. The battery
voltage remains almost constant throughout the lifetime of the battery.
Therefore, changes may be detected in the pacemaker output pulses by
comparing measurements from one checkup to another.
In one approach to the problem of accurately determining battery depletion,
pacemakers such as described in U.S Pat. No. 3,842,844 are provided with 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, a heart 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 to 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.
In order to detect the patient's electrical heart activity, electrodes are
attached for example to the patient's body including his right arm, left
arm, left leg, chest and right leg. The electrical activity, as shown in
FIG. 2, includes the patient's ECG signal upon which is impressed the
pacer pulse appearing before the QRS wave form, which is generated
naturally by the heart's activity. The pacer pulse is usually of large
amplitude and very small width. Though noting that it is desired to
measure the width of the pacer pulse, it is very difficult to determine
the width with accuracy. Conventional monitors, for example, do not have a
sufficient band width to pass the pacer pulse with any reasonable degree
of fidelity. Further, it is necessary to distinguish the pacer pulse from
the patient's QRS wave, as well as 60 cycle noise and muscle artifacts.
One of the distinguishing characteristics of the pacer pulse is that it
has a very fast rise time, being typically in the microsecond range. By
contrast, the electrical impulses normally originated in the heart or
other noise sources such as 60 Hz line noise, have rise times in the order
of 10-20 milliseconds. Other common noise sources may be generated by
electrical appliances being operated from the same power line. These
generally have fast rise times but very short duration. Since these
individual pulses generally are only of fractions of microseconds long,
they are distinguished from heart pacemaker pulses principally by the
pulse widths since heart pacemaker pulses are commonly in the 1
millisecond range. Thus, the heart pacemaker pulses may be identified by
their relatively fast time and their relatively long pulse width in the
order of 0.5-4 milliseconds.
In U.S. Pat. No. 3,885,552 of Kennedy, there is disclosed a cardiac
monitoring system for monitoring the heart activity and in particular for
measuring among other parameters the width of the heart pacemaker pulses.
In particular, there is disclosed a pacer pulse selection logic circuit
including a differentiator having a time constant of approximately 100
microseconds for providing an output if the applied input has a rise time
shorter than 100 microseconds. Further, the noted logic circuit also
includes an integrator circuit providing a signal going low indicating
that the pulse width of the applied signal is greater than a selected
minimum pulse width of 250 microseconds. The Kennedy circuit functions to
provide an output identifying the presence of a pacer pulse upon the
occurrence of both a signal from the aforementioned differentiator and
integrating circuits. For a similar disclosure of a system for detecting
the presence of a pacer pulse, attention is also drawn to U.S. Pat. No.
3,871,363 of Day which similarly discloses the use of a differentiator
circuit and an integrator circuit for respectively measuring rise times
smaller than a selected minimum and pulse widths in excess of a
predetermined width to identify thereby heart pacemaker pulses.
Reference is made to FIG. 5, which generally shows an implanted pacemaker
of the type generally described above having an output capacitor C3 that
is coupled by suitable electrodes to the patient's heart, which for the
sake of simplicity may be considered as presenting an essentially
resistive impedance to the output of the heart pacemaker. In typical
operation, the pacemaker generates a series of timing pulses applied to
the base of transistor Q thereby rendering this transistor Q conductive
and "dumping" the charge established upon capacitor C3 across the heart's
resistance R2. It is recognized, that such a circuit is essentially a
differentiation circuit, whereby the electrical signal discharged across
the resistor R2 has an essentially sloping or curved wave-form as
indicated in FIG. 4A. As shown in FIG. 4A, the pulse appearing between
times t1 and t2 has a very fast rise time beginning at time t1, generally
sloping down to time t2 with the fast fall time at that instant; this
pulse has relatively sharp, well defined leading and trailing edges making
its detection relatively easy. However, in practice, the detected wave
shape of a pacemaker pulse is more as shown in the time interval between
t3 and t5, of FIG. 4A. Generally, the differentiation process effected by
the output capacitor C3 and the heart's resistance R3 accounts for the
relatively poor wave shape quality, i.e., attenuated quality of this
pulse. Thus, it may be observed that the leading edge is relatively well
defined and thus may be accurately detected. On the other hand, the
trailing edge is of a degraded wave form making its detection more
difficult. Thus, it is difficult to accurately detect and measure the
pulse width of such a heart pacemaker pulse.
In the above-noted prior circuits for detecting pulse widths, a
differentiator circuit is used to detect the leading and trailing edges.
Typically, there is no problem in detecting the occurrence of the leading
edge of a pacer pulse, which under normal circumstances has a sufficiently
fast rise time to actuate normal differentiator circuits to provide a
defined output pulse. Similarly, prior art differentiator circuits are
capable of providing an output corresponding to the trailing edge, even of
a degraded pulse as shown in FIG. 4A. The problem then arises when the
attenuated or degraded slope of the pulse occurring between the leading
and trailing edges may have a sufficiently fast fall time such that the
output of the differentiator circuit may have a sufficient amplitude so as
to cause the associated threshold circuit to provide a premature output
indicative of the trailing edge. As shown in FIG. 4A, the drooping
waveform portion of the pacemaker pulse is essentially capacitive in
nature being attenuated by the indicated expression. Therefore, the droop
or attenuation associated with the detected artifact pulse is predictable,
assuming that a reasonable range of capacitive coupling is made to the
patient's heart. The worst contemplated case of attenuation is considered
to be a decrease in impulse amplitude of 50% within a time period of 200
microseconds or 80% in 1 millisecond. In other words, the leading edge of
a degraded pulse may have a time constant in the order of 3 or 4
microseconds, the trailing edge a time constant in the order of 70
microseconds, and the attenuated or drooping intermediate waveform portion
may have a time constant in the order 620 microseconds. Though many
multiples of either the leading or trailing edge, the time constant of the
drooping portion may be sufficient to provide a premature indication of
the trailing edge and therefore an incorrect indication of the pulse
width.
SUMMARY OF THE INVENTION
The system of this invention functions to detect the leading and trailing
edges of an electrical pulse and in particular of a pulse applied by a
heart pacemaker to a patient's heart. The system of this invention
functions to distinctly detect each of the relatively fast rising leading
edge and the relatively slow rising trailing edge, while preventing a
premature detection of the trailing upon the attenuated or drooping
portion of the pacing pulse therebetween. The patient's heart activity
signals comprising the pacing pulse is applied along with extraneous noise
including the patient's QRS wave, 60 Hz noise, muscle artifacts, etc., are
applied to a differentiator or wave shaping amplifier whose gain or
transfer function is configured to reject signals having low frequency
components, i.e., the noise signals mentioned above. The output of the
differentiator is applied to a first input of a differential amplifier,
while the aforementioned heart activity signals are applied to a second
input; the differential amplifier circuit essentially subtracts the
amplifier output from the heart activity signals to remove the DC
component from the output of the amplifier circuit. If the amplifier
output is above a predetermined amplitude, the differential amplifier
output is applied to a first threshold circuit or leading edge detector to
provide an output signal indicative of the occurrence of the leading edge,
and to set a one shot circuit, which in turn provides a signal initiating
the detection of the trailing edge. The output of the wave shaping
amplifier is also applied via a limiter or amplifier to a second threshold
circuit, which responds to signals above a predetermined threshold to
provide a signal indicative of the occurrence of the trailing edge. Noting
that the output of the waveshaping amplifier is less to the trailing edge
than the leading edge, the gain of the amplifier, as well as the time
constant of the waveshaping amplifier and the threshold level of the
second threshold detector is set to detect the trailing edge of the
pacemaker pulse. The output of the first threshold circuit corresponding
to the leading edge and the output of the second threshold circuit
corresponding to the trailing edge are applied to a logical circuit to
provide a defined output pulse whose leading and trailing edges sharply
define the leading and trailing edges of the pacer pulse and therefore the
pulse width therebetween.
In an illustrative embodiment of this invention, the pulse width detection
circuit is incorporated into a low bandwidth transmission system, wherein
the heart activity of the patient is transmitted over a low band width
transmission medium, e.g., a telephone line, to a remote station as in a
physician's office whereat the physician may observe not only the pulse
width of the cardiac pulse but also the heart's ECG signal. At the
transmitter of such a system, the ECG signals are amplified and applied to
a voltage controlled oscillator (VCO) to provide an audio output signal of
a frequency dependent upon the amplitude of the input voltage. The heart
activity signal is also applied to the aforementioned pulse width
detection circuit whereby the pulse width of the pacemaker pulse is
accurately detected and subsequently stretched by a given factor to cause
the aforementioned VCO to oscillate at a predetermined frequency. Thus, a
signal corresponding to the ECG of the patient's heart and a signal
indicative of the pulse width of the pacer pulse, i.e., the signal of
predetermined frequency, are transmitted via a transducer and the low
bandwidth transmission medium to the remote station, where they are
detected and appropriately displayed.
BRIEF DESCRIPTION OF THE DRAWINGS
The 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 functional block diagram showing a transmitter for detecting
and transmitting heart activity signals via a low bandwidth transmission
medium to a receiver, as shown in FIG. 1B;
FIG. 2 is a graph showing a patient's ECG signal upon which there has been
imposed pacer pulse signals;
FIG. 3 is a functional block diagram of the transmitter particularly
illustrating the pulse width detector in accordance with the teachings of
this invention;
FIGS. 4A-4H show the wave forms of various signals as imposed upon the
pulse width detector of FIG. 3;
FIG. 5 is a functional block diagram of the circuit formed by the output of
a heart pacemaker and the patient's heart;
FIGS. 6A, B and C are detailed circuit diagrams of the transmitter and
pulse width detector as more generally shown in FIG. 3;
FIG. 7 is a graph showing the transfer function or graph of the wave
shaping amplifier or differentiator of FIGS. 3 and 6C; and
FIGS. 8A and 8B show respectively a theoretical model of electrical
operation of the wave shaping amplifier, and the desired and undesired
responses of such an amplifier.
DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to the drawings and in particular to FIG. 1B, there is shown
a simplified diagram illustrating the relationship of a transmitter 10 for
detecting and transmitting the electrical activity of the patient's heart,
e.g., the patient's electrocardiogram (ECG) upon which has been
superimposed the detected stimulating pulse in accordance with the
teachings of this invention, across a relatively low bandwidth
transmission medium 50 to be received by a receiver 60, which operates to
separate the multipled or stretched 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 or legs,
whereby the electrical activity of the patient's heart and the stimulating
pulse as generated and applied to the patient by the implanted heart
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 14 and a voltage controlled oscillator 18.
The filter 14 serves to block the pacemaker stimulating pulses from the
ECG channel. Further, the amplified signal is also applied to a pulse
width detector 18 in accordance with the teachings of this invention, for
providing an output sharply defining the pacer pulse width, and a pulse
stretcher 20 whereby the detected pulse is 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. The transmitter and
receiver as generally shown in FIGS. 1A and 1B are more specifically
described in U.S. Pat. No. 3,986,496 of Brastad, which is assigned to the
assignee of this invention.
It is contemplated that in accordance with the teachings of this invention,
that the pulse width of the pacer pulse may be accurately detected and
then transmitted to the remote station at the physician's office where the
physician or a trained technician may accurately observe the patient's
heart activity as shown in FIG. 2. In FIG. 2 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 heart pacemaker.
The heart activity or electrocardiogram (ECG) of the patient is indicated
in FIG. 2 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
0.5 V. Typically, the amplitude of the stimulating pulse would be in the
order of 6 V.
Referring now to FIG. 3, there is shown an illustrative embodiment of the
transmitter 10 generally shown in FIG. 1A and in particular an
illustrative embodiment of the pulse width detector 18 in accordance with
the teachings of this invention. The patient's ECG signals and the
pacemaker pulse or artifact signals are sensed by a pair of electrodes 11
worn on the patient's arms and if utilized, an electrode 9 worn on the
patient's right leg (or other location of the body) and applied to an
input amplifier 12. The input amplifier 12 amplifies the input signals
rejecting errors caused by common mode signals of large amplitudes that
may be present. In a normal amplifier such common mode signals would tend
to mask out the desired low level ECG and pacemaker pulse signals and even
cause amplifier malfunctions such as saturation. The input amplifier 12
has a high input impedance, a low output impedance and typically greater
than unity gain, for example, a gain of 2. The high input impedance is
required because of the high source impedance which can occur between the
electrodes 11 and the patient's skin. The signals via the electrode 9 are
driven by an amplifier (not shown) that provides a sense common mode
voltage to the input amplifier 12. Such a system is well known in the art
and may take the form of a Hewlett-Packard driver lead EGC system. The
input amplifier 12 provides a buffered, single ended output to the
following signal processing circuit now to be described.
As apparent from FIG. 3, the buffered outputs are applied to the pulse
width detector 18 wherein the pacemaker pulse or artifact signals are
detected and whose pulse width is accurately determined, as well as to
circuitry for amplifying and processing the ECG signals, now to be
described. In particular the output of the input amplifier 12 is applied
to an ECG amplifier 22 that converts the ECG wave form to current and
provides a sufficient gain to realize the desired sensitivity of the
current control oscillator 16. The gain of the ECG amplifier 22 is
controlled by the setting of the potentiometer R2. The amplified ECG
signal is applied to a CCO control circuit 24 that controls the mode of
operation of the CCO. As seen in FIG. 3, an input is received from the
pulse stretcher 20, which upon being rendered high, causes the CCO 16 to
provide an output of selected frequency, e.g., 2250 Hz, for a period of
time corresponding to the multiplied or stretched pulse width of the
pacemaker pulse. The output of the CCO control circuit 24 is applied to
the current control oscillator 16 which modulates the frequency of its
output in accordance with the amplitude of the input current signal to
provide an output that is amplified by an audio amplifier 19 to drive a
transducer 21 in the form of a speaker. It is contemplated as shown in
FIG. 1B, that the transducer 21 would be coupled to a telephone receiver,
whereby the audio signals derived from the transducer 21 are reconverted
to electrical signals to be transmitted over the medium 50 to the receiver
60. Further, there is included a timer 30 which automatically disconnects
the power voltage as derived from a voltage regulator 28 from the
remaining portions of the system to thereby terminate transmission of the
patient's heart signals after a time sufficient to provide the physician
an adequate sample thereof. Typically, this may be in the order of about
30 seconds. Further, there is included a low voltage inhibit circuit 26,
which senses the power source potential to thereby disable the current
control oscillator 16 when the voltage level of the power source falls
below a desired level. In this regard, it is contemplated that the
transmitter 10 may be a portable unit powered by a power source such as a
battery. A capacitor bias amplifier 25 is coupled to the VCC regulator 28
to keep the voltage disposed across the capacitor C1 of the filter 14,
which capacitor C1 is the low frequency cut off the ECG amplifier 22, as
low as possible to avoid leakage effects from causing errors in the
frequency of the output of the CCO 16. Further, the capacitor bias
amplifier 25 also supplies a node that follows the voltage V.sub.CC as
derived from the regulator 28 so that capacitors may be tied to it instead
of ground, thereby to avoid inducing supply related transients across
these capacitors.
As shown in FIGS. 1A and 3, the heart signals are applied from the input
amplifier 12 also to the pulse width detector 18 and in particular to a
differential amplifier circuit 34 to provide an output as seen in FIG. 4C.
In this regard it is understood that the pacemaker pulse or artifact
signal as derived from the input amplifier 12 generally resembles those as
shown in FIG. 4A. It would be desired to have a well defined pacer pulse
as shown in the time period from t1 to t2, but as explained above its wave
shape is often degraded to appear as shown by that pulse appearing in FIG.
4A between times t3 and t5. The artifact signals are also applied to a
wave shaping amplifier or differentiator circuit 32, whose output is shown
in FIG. 4B. More specifically, the differentiator 32 responds to the
leading edge appearing at either times t1 or t3 to provide a sharply
rising spike on the positive going edge of the pacer pulse. The positive
going spike is followed by a shelf which corresponds to the DC amplitude
of the input pacer pulse as shown in FIG. 4A. It is noted that the
amplitude X will vary from patient to patient, dependent upon the level of
stimulation by the implanted pacemaker, as well as the efficiency of the
electrical connection between the patient's body and input amplifier 12.
At time t2 the differentiator circuit 32 provides a negative going spike
corresponding to the trailing edge of the input pulse appearing at times
t2 and t5. In the time period t3 to t5, a poor quality pacemaker pulse is
sensed which may have a drop or fall of 80% within one millisecond. The
differential amplifier 32 processes such an input pulse and provides an
output as shown in FIG. 4B between the time period t3 and t4. It is noted
that at t4, the wave form of FIG. 4B falls to a point slightly below the
reference level and then resumes a shelf to time t5 at which time there is
provided a negative going spike.
The heart activity signals including the pacing pulses are also applied to
a first input of a differential amplifier circuit 34, while the output of
the differentiator 32 is applied to a second input of the differential
amplifier 34. Essentially, the differential amplifier 34 amplifies the
difference of these two signals, thus removing the DC level component as
shown in FIG. 4B of the differentiator output, to provide an output as
shown in FIG. 4C. The amplifier output including a first spiked signal
indicative of the leading edge of the pacing pulse and a second spiked
signal indicative of the trailing edge of the pacer pulse, is applied to a
leading edge detector or first threshold level circuit 36, which provides
an output upon its line D when the first spiked signal appearing at t1 is
positive going and exceeds a predetermined positive level, and an output
upon its E line when the first spiked signal is negative going and exceeds
a negative, similar level.
It is noted that the input pacemaker pulses may be either negative or
positive going; therefore the leading edge detector 36 is capable of
detecting negative and positive going spikes of a similar predetermined
level. The outputs on the D and E lines are applied to a gate 38 that is
normally enabled to pass these signals to an OR gate 40 to set a one shot
multiplier 42, and to a leading edge flip-flop 44 to set the flip-flop 44
if a positive going leading edge is detected and to reset the flip-flop 44
if a negative leading edge is detected. The outputs of the leading edge
detector 36 are respectively shown in FIGS. 4D and 4E.
As shown in FIG. 3, the output of the one shot multiplier 42 is also
applied to enable a second, normally disabled gate 52 to initiate the
detection and processing of the second spiked signal corresponding to the
trailing edge of the pacemaker pulse. The output of the differentiator
circuit 32, as shown in FIG. 4B, is applied to a limiter circuit or
amplifier 48, which functions to amplify this input with a selective gain
and to apply the amplified wave forms similar to that shown in FIG. 4B to
a pulse width detector or second threshold circuit 51. The pulse width
detector 51 detects positive and negative going trailing edges to provide
upon its output lines F and G, respectively, signals if the amplitude of
the input spikes exceed a predetermined level.
In operation, a pacemaker pulse as shown in FIG. 4A is applied to each of
the first and second differential circuits 34 and 32 respectively.
Considering first the detection of the leading edge as occurs at time t3,
the differentiator circuit 32 provides an output as shown in FIG. 4B that
is applied along with the pacemaker pulse to the inputs of the
differential amplifier circuit 34 to provide an output as shown in FIG.
4C. The differential circuit 34 in a sense removes the DC component
notably the shelf between the positive and negative going spikes to
provide an output as shown in FIG. 4C. If as shown in FIG. 4C, the initial
spiked signal is positive going, the leading edge detector 36 provides an
output signal upon its D line if the amplitude of the positive going spike
is above its threshold level. Assuming so, the signal on the D line is
passed by the first gate 38 and applied via the OR gate 40 to set the one
shot multiplier 42, thereby inhibiting the first gate 38 and enabling the
second gate 52. At the same time, the output of the first gate 38 is
applied to set the leading edge flip-flop 44 whereby its Q output goes
high. Conversely, it may be understood that if the leading edge of the
pacemaker pulse and therefore the initial spiked signal as derived from
the differential amplifier 34 is negative going, an output is provided on
the E line and applied to reset the leading edge flip-flop 44, driving its
Q output negative.
At a point in time between t3 and t5, the second gate is been enabled,
permitting the passage of signals to a pulse width flip-flop 54. Assuming
that the output of the second differentiator circuit 32 is as shown in
FIG. 4B, i.e., the trailing edge is a negative going signal, that negative
signal is amplified by the limiter 48 and applied to the pulse width
detector 51, whereby if the amplitude of the negative going spike is above
the threshold level of the pulse width detector 51, an output is developed
upon its G line and applied via the enabled second gate 52 to reset the
pulse width flip-flop 54, thereby driving its Q output negative. Thus, it
is seen that at the time t3 when a positive going spike appears, a high
signal is applied from the flip-flops 44 and 54 to the inputs of the
exclusive NOR gate 46, which then provides a high output. When as in the
example discussed above, the trailing edge is evidenced by the negative
going spike of FIG. 4B at time t5, the flip-flop 54 is reset driving the
exclusive NOR gate 46 low. Thus, the NOR gate 46 provides a sharply
defined output as shown in FIG. 4H going high at time t3 and going low at
time t5, thereby defining the leading and trailing edges of the pacemaker
pulse.
As shown in FIGS. 1A and 3, the output pulses of FIG. 4H are applied from
the exclusive NOR gate 46 to the pulse stretcher 20 to be multiplied or
stretched by a given factor before being applied to the CCO controller 24,
whereby the current control oscillator 16 is disposed into its mode of
operation where it generates a signal of fixed frequency for a period
corresponding to the pulse width of the pacemaker pulse. As indicated
above, with respect to FIG. 1B, this signal may be transmitted via the
transmission medium 50 to a receiver 60, whereat it is detected and
suitably displayed for the benefit of the physician or his technician.
Referring now to FIGS. 6A, B and C, there is shown detailed drawings of the
circuit elements making up the block diagrams more generally shown in FIG.
3. Each of the block diagrams shown in FIG. 3 will now be discussed below
with regard to FIGS. 6A, 6B and 6C.
A significant aspect of this invention resides in accurately detecting an
attenuated pacer pulse as seen in FIG. 4A by separately sensing and
processing the leading and trailing edges of the pulse, while taking
particular precautions not to sense the droop or attenuated waveform
portion of the pulse between the leading and trailing edges, which
otherwise might provide a false, premature indication of the trailing
edge. To this end, the characteristics and parameters of the elements of
the pulse width detector 18 and in particular the differential amplifier
34, the differentiator or wave shaping amplifier 32, the limiter or
amplifier 48, and the threshold detectors 36 and 51 are critically
selected. As shown i | | |
