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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to electronic devices implantable within the human
body and in particular to cardiac monitoring and stimulating apparatus.
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. Such a heart pacemaker is capable of being implanted in the human
body and operative in such an environment for relatively long periods of
time. Typically, such pacemakers are implanted within the chest beneath
the patient's skin and above the pectoral muscles or in the abdominal
region by a surgical procedure wherein an incision is made in the selected
region and the pacemaker is implanted within the patient's body. Such a
pacemaker provides cardiac stimulation at 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 provided by this pacemaker is provided
at a fixed rate.
In an article by D. A. Nathan, S. Center, C. Y. Wu and W. Keller, "An
Implantable Synchronous Pacemaker for the Long Term Correction of Complete
Heart Block", American Journal of Cardiology, 11:362, there is described
an implantable cardiac pacemaker whose rate is dependent upon the rate of
the heart's natural pacemaker and which operates to detect the heart beat
signal as derived from the auricular sensor electrode and, after a
suitable delay and amplification, delivers a corresponding stimulus to the
myocardium and in particular, the ventricle to initiate each heart
contraction.
Such cardiac pacemakers, separately or in combination, tend to alleviate
some examples of complete heart block. In a heart block, the normal
electrical interconnection in the heart between its atrium and its
ventricle is interrupted whereby the normal command signals directed by
the atrium to the ventricle are interrupted with the ventricle contracting
and expanding at its own intrinsic rate in the order of 30-40 beats per
minute. Since the ventricle serves to pump the greater portion of blood
through the arterial system, such a low rate does not provide sufficient
blood supply. In normal heart operation, there is a natural sequence
between the atrial contraction and the ventricular contraction, one
following the other. In heart block, there is an obstruction to the
electrical signal due, perhaps, to a deterioration of the heart muscle or
to scar tissue as a result of surgery, whereby a block in the nature of a
high electrical impedance is imposed in the electrical flow from the
atrium to the ventrical.
Where the heart block is not complete, the heart may periodically operate
for a period of time thus competing for control with the stimulation
provided by the artificial cardiac pacemaker. Potentially dangerous
situations may arise when an electronic pacemaker stimulation falls into
the "T" wave portion of each natural complete beat. As shown in FIG. 1,
the "T" wave follows by about 0.2 seconds each major beat pulse (or "R"
wave causing contraction of the ventricles of the heart). Within the "T"
wave is a critical interval known as the "vulnerable period" and, in the
case of a highly abnormal heart, a pacemaker impulse falling into this
period can conceivably elicit bursts of tachylcardia or fibrillation,
which are undesirable and may even lead to a fatal sequence of
arrhythmias.
Cardiac pacemakers of the demand type are known in the prior art such as
that disclosed by United Kingdom Pat. No. 826,766 which provides
electrical pulses to stimulate the heart only in the absence of normal
heartbeat. As disclosed, the heartbeat is sensed by an acoustical device
disposed external of the patient's body, responding to the presence of a
heartbeat to provide an inhibit signal defeating the generation of heart
stimulating pulses by the pacemaker. In the absence of the patient's
natural heartbeat, there is disclosed that the pacemaker generates pulses
at a fixed frequency.
In U.S. Pat. No. Re. 28,003, of David H. Gobel, assigned to the assignee of
this invention, there is disclosed an implantable demand cardiac pacemaker
comprising an oscillator circuit for generating a series of periodic
pulses to be applied via a stimulator electrode to the ventricle of the
heart. The stimulator electrode is also used to sense the "R" wave of the
heart, as derived from its ventricle to be applied to a sensing portion of
the cardiac pacemaker wherein, if the sensed signal is above a
predetermined threshold level, a corresponding output is applied to an
oscillator circuit to inhibit the generation of the stimulator pulse and
to reset the oscillator to initate timing a new period. The following
patents, each assigned to the assignee of this invention, provide further
examples of demand type heart pacemakers: U.S. Pat. No. 3,648,707 of
Wilson Greatbatch; U.S. Pat. No. 3,911,929 of David H. Gobeli; U.S. Pat.
No. 3,927,677 of David H. Gobeli et al; U.S. Pat. No. 3,999,556 of Clifton
Alferness; and U.S. Pat. No. 3,999,557 of Paul Citron et al.
Demand type pacemakers are particularly adapted to be used in patients
having known heart problems such as arrhythmias. For example, if such a
patient's heart develops an arrhythmia, failing to beat or to beat at a
rate lower than a desired minimum, the demand type pacemaker is activated
to pace the patient's heart at the desired rate. Of particular interest to
the subject invention, are those patients that have recently undergone
heart surgery; typically, these patients are apt to develop any and all
known arrhythmias in the immediate post-operative period. Current therapy
for such patients involves the implanting at the time of surgery of
cardiac leads with their electrodes connected to the patient's heart and
the other ends of the leads being connected to an external pacemaker to
provide pacing for arrhythmia management.
In addition, the same pacemaker leads that interconnect the internally
planted electrodes and its external pacemaker, are also connected to an
external monitoring unit for providing signals indicative of the patient's
heart activity to the external monitoring unit. A significant advantage of
such pacemaker leads is that they may be used for recording of direct
epicardial electrograms, which provide high quality precision data as to
the patient's heart activity. The study of such wave shapes, i.e.,
morphology, is an invaluable aid in a diagnosis of arrhythmias. In this
regard, it is understood that a normal EKG having its electrodes attached
to various portions of the patient's skin does not provide the high
quality output signal for diagnosis of arrhythmias as is obtained by
cardiac electrodes attached directly to a patient's heart. For example,
the output signal as obtained from such directly attached electrodes has a
bandwidth in the order of 500 Hz and a signal to noise ratio in the order
of 40 to 1, with no more than 30 db frequency loss. Such a high quality
EKG signal cannot be obtained from a standard EKG monitor as is attached
only to the outer skin of the patient.
However, the use of pacemaker leads directed through the patient's skin
presents certain problems. Typically, if the external leads are left in
the patient for any length of time, e.g., 5 to 7 days, an infection may
develop at the exit side of the leads, and the leads may be accidentally
pulled with subsequent damage to the patient's heart. Further, such leads
present micro and macro shock hazards to the patient. For example, there
are small residual charges on many objects within a surgical environment
and if the leads are accidentally exposed to such a charge, it will be
applied via the leads to the patient's heart possibly inducing an
arrhythmia therein. Further, relatively high voltage such as carried by an
AC powerline are typically found in the operating room; the electrogram
recording apparatus is so powered and the contemplated accidental contact
of the external leads with such an AC powered line would have serious
consequences for the patient. In addition, it is necessary to remove the
cardiac leads approximately 5 to 7 days after their surgical implantation.
Further, there is considerable electrical environmental noise within an
intensive care unit where a post-operative cardiac patient would be
placed. Illustratively, such noise results from fluorescent lights or
other electrical equipment typically found in an intensive care unit and
is capable of inducing millivolt signals into such cardiac leads of
similar amplitude to those signals derived from the patient's heart. Thus,
such environmental noise-induced signals may serve to inhibit the external
pacemaker from pacing, even though the patient's heart may not be beating.
Further, it is contemplated that after the surgical implantation of such
demand pacemakers, that the connections of the atrial and ventrical leads
to the external pacemaker may be reversed, with resulting pacer-induced
arrhythmias.
the prior art has suggested artificial pacemakers having a transmitter or
unit disposed externally of the patient's body and a receiver surgically
implanted within the patient, having leads directly connected to the
patient's heart. For example, in the West German Auslegeschrift No. 25 20
387, entitled Testing Arrangement for Artificial Pacemakers, there is
described a pacemaker having an external transmitter for transmitting
external energy by radio frequency (RF) waves to an internally planted
unit for supplying electrical stimulation to the heart. Further, it is
disclosed that the internally planted unit is capable of transmitting
information to a monitoring device disposed externally of the patient's
body, for indicating various characteristics of the pacemaker.
Further, in a pair of articles entitled "A Demand Radio Frequency Cardiac
Pacemaker", by W. G. Holcomb et al, appearing in Med. & Biol. Eng., Vol.
VII, pp. 493-499, Pergamon Press, 1969, and "An Endocardio Demand (P&R)
Radio Frequency Pacemaker", by W. G. Holcomb et al, appearing in the 21st
ACEMB, page 22A1, November 18-21, 1968, there is described a
demandpacemaker including an external transmitter 10' as shown in FIG. 2,
labeled PRIOR ART, for generating an RF signal from its primary coil or
antenna 16' to be received by a receiver 12' internally implanted within
the patient's skin 14'. In addition, the receiver 12' in turn transmits
heart activity in terms of the currents of the heart's "R" wave to
synchronize the activity of a pulse generator 26 within the external
transmitter 10'. As shown in FIG. 2, the receiver 12' includes two
separate electronic circuits each sharing common leads connected to the
pacemaker electrodes, which are surgically connected to the patient's
heart. The first circuit, i.e., the EKG transmitter section, consists of a
rectifying circuit of diodes D20-D23 for providing power to a transistor
amplifier Q10, to which is applied the EKG signal; the amplified EKG
signal is applied in turn to a coil 34'a for transmission to the
transmitter 10'. The primary coil or antenna 16' receives and applies the
EKG signal via a detector 30, to be amplified by an amplifier 30, which
provides the indicated EKG signal to be analyzed upon a display not shown.
The second electronic circuit of the receiver 12' is the stimulus
receiver, which furnishes the stimulating pulse to the cardiac electrodes.
In particular, the output of the pulse generator 26 of the transmitter 10'
is applied via closed switch 24 to superimpose a high voltage pulse upon
the output of the 2 MHz oscillator, which is subsequently amplified by
amplifier 20 and applied via detector 18 to the antenna 16'. The high
voltage pacemaker pulse as superimposed upon the RF carrier, is received
by the coil 34'b and rectified by the diode D25 and the capacitor C25 to
actuate an electronic switch primarily comprised of transistors Q12 and
Q13, which are closed thereby to apply the high voltage pulse via FET Q11
to the pacemaker electrodes, the FET Q11 serving to regulate the current
passing to the pacemaker electrodes. The transistors Q12 and Q13 are
voltage-responsive and disconnect the coil 34'b from the pacemaker
electrodes in the absence of the high voltage pacemaker pulse.
It is understood that the RF carrier as derived from the oscillator 22 is
continuously applied to the coil 16'. The secondary coil 34'a receives a
continuous RF wave from the primary coil 16'. An EKG signal is derived
from the pacemaker electrodes and is applied to the base of the amplifier
transistor Q10, which in turn provides a correspondingly varying load to
the coil 34'a, whereby a corresponding voltage fluctuation is induced
across the coil 16'. In other words, the voltage appearing across the coil
16' is amplitude modulated in accordance with the patient's heart activity
or EKG signal. Though the circuitry shown in FIG. 2 provides a relatively
simple circuit of energizing the receiver 12' implanted within the
patient, the EKG signal as derived from the patient does not contain
sufficient precision to provide a diagnostic quality display of the
patient's EKG. Typically, to provide a diagnostic quality display of the
patient's EKG it is necessary to transmit the EKG signal with a bandwidth
of 100 Hz with a signal to noise ratio in the order of 40 to 1 and with no
more than a 3 db frequency loss; the circuitry shown in FIG. 2 does not
provide such quality primarily due to the amplitude modulation type of
signal transmission, which is sensitive to the relative positions in terms
of distance and angle of orientation between the coils 16' and 34'a. In
this regard, if the distance or the angle between the axes of the coils
16' and 34'a vary due to the patient's movement, the amplitude of the
signal as seen by the detector 18 also will vary. Thus, in an amplitude
modulation system, this body movement will provide a distortion in the EKG
signal detected. In addition, the extraneous noise to which such a
pacemaker would be exposed such as radiation from fluorescent lights or AC
power lines, as well as other extraneous artifacts, may appear as
amplitude modulation to introduce further errors in the signal received
from the transmitter 10'.
SUMMARY OF THE INVENTION
It is therefore an object of this invention to provide a cardiac
stimulating device implantable within a patient capable of being energized
from a power source external of the patient's body, and for applying
stimulating signals to the atrial and ventrical of the patient's heart.
It is a further object of this invention to provide an artificial heart
pacemaker comprised of an external transmitter or power source for
transmitting by RF coupling to a receiver implanted within the patient and
operative independent of a local battery by the RF signals transmitted
from the transmitter to pace the patient's heart in a variety of modes.
In accordance with these and other objects of the invention, there is
provided cardiac stimulating apparatus comprising an external device or
transmitter for transmitting an electromagnetic signal to an internal unit
or receiver disposed within the patient's body, whereby power is supplied
to the internal unit. The internal unit is coupled by electrodes to the
atrial and ventrical portions of the patient's heart for applying
stimulating signals to first and second sites (e.g., the atria and
ventricles) of the patient's heart. In particular, the internal unit is
operative to stimulate the heart in varying modes of operation and
includes decoder means responsive to encoded signals transmitted from the
external unit for controlling the manner of heart stimulation. In one
illustrative embodiment of this invention, the internal unit is operative
in a first mode to provide stimulating pulses to the atrium of the
patient's heart and in a second mode for providing stimulating pulses to
the ventrical of the patient's heart. Further, the decoder means may be
actuated to pace synchronously or demand pace the atrium or to
asynchronously or demand pace the ventrical of the patient's heart, and
further to sequentially pace the atrial and the ventrical with an
adjustable delay between the application of pulses thereto.
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. 1 illustrates the voltage wave produced by a human heart during one
complete heartbeat;
FIG. 2 is a schematic drawing above described of a demand heart pacemaker
circuit of the prior art;
FIG. 3 is a pictorial showing of the manner in which an artificial heart
pacemaker in accordance to the teachings of this invention, is implanted
within the patient's body;
FIG. 4 is a block diagram of a transmitter or external unit and a receiver
or internal unit in accordance with the teachings of this invention;
FIG. 5 is a detailed circuit diagram of the receiver as shown in FIGS. 3
and 4; and
FIGS. 6 and 7 comprise a detailed circuit diagram of the transmitter as
shown in FIGS. 3 and 4.
DESCRIPTION OF THE PREFERRED EMBODIMENT
With regard to the drawings and in particular to FIG. 3, there is shown an
artificial pacemaker and monitoring system in accordance with the
teachings of this invention, including a transmitter or external unit 10
that generates stimulating pulses to be applied via a pair 15 of
conductors to an incapsulated coil 16 whereby electromagnetic energy in
the form of RF radiation is tramsmitted through the skin 14 of the patient
to be sensed by an internal unit 12 and in particular, as shown in FIG. 4,
a coil 34a. The internal unit or receiver 12 is solely powered by the RF
radiation transmitted to it for stimulating in various modes of operation
the atrium 40 and ventrical 42 of the patient's heart, as by the leads 17
and 19, respectively.
Further, the external unit 10 is connected via conductor 59 to a monitoring
unit 63, illustratively taking the form of a 78000 series unit of
Hewlett-Packard for providing a display of the atrial and ventrical
activity of the patient's heart.
With reference now to FIG. 4, there is shown the transmitter 10 including a
timing and mode control 50 for controlling a variety of selected pacing
functions and capable of operating in a demand or asynchronous mode for
atrial, ventrical, or atrial-ventrical sequential pacing. In an
illustrative embodiment of this invention, the control 50 may be
appropriately adjusted to effect asynchronous atrial pacing from 50 to 800
BPM and demand atrial pacing from 50 to 180 BPM. It is contemplated that
for use with postsurgical patients, arrhythmias may readily develop and by
the application of heart stimulating pulses in the range of 180 to 800 BPM
that the patient's heart may be forced out of its arrhythmic beating
pattern. Further, the control 50 is adapted for asynchronous or demand
ventrical pacing in the range of 50 to 180 BPM. In an atrial-ventrical
sequential pacing mode, the control 50 may be adjusted to provide
stimulation from 50 to 180 BPM with a delay between the atrial and
ventrical pulses adjustable from 0 to 300 ms. The control 50 selectively
applies the output of an R-F transmitter 52 by an antenna switch 56 via
the set 15 of leads to the primary coil or antenna 16 for transmission of
a corresponding electromagnetic wave to be received and detected by the
receiving coil or antenna 34a.
Significantly, the transmitter or external unit 10 operates in first mode
for effecting pacing and in a second mode for processing of EKG or
electrograph information derived from the implanted receiver 12. In the
second mode of operation, pulse width modulated data indicative of the
amplitude of the heart's activity is transmitted from the coil or antenna
34b of the internal unit 12 to the coil 16 of the external unit 10 to be
applied via the antenna switch 56 to a pulse-width demodulator 54 and a
demultiplexer 55. As will be explained in detail later, the internal unit
or receiver 12 is capable of sensing and monitoring the atrial and
ventrical activity of the patient's heart and for transmitting pulse width
modulated signals indicative thereof in timed sequence with a timing
signal, to the transmitter or external unit 10. In this regard, the
demodulator 54 and the demultiplexer 55 separates the atrial and ventrical
signals transmitted from the internal unit 12, as well as demodulates the
pulse width modulated signals to provide corresponding output signals
indicative of the amplitude of the atrial and ventrical signals, via
conductor 59 to the external monitoring device 63, as shown in FIG. 3,
whereby a graphical display thereof may be provided with a diagnostic
quality, so that the attending physician may accurately analyze the
patient's heart activity. With such information, the physician is able to
predict pending heart failure or arrhythmias. In this regard, the subject
invention is capable of achieving diagnostic quality displays, i.e. is
able to transmit to the receiver unit 10, heart signals with a bandwidth
frequency of 100 Hz, a signal noise ratio in the order of 40 to 1 with no
more than 3 db frequency loss. Further, a sensing amplifier 58 provides an
inhibit signal to the control 50 whereby the control 50 is inhibited from
operation during spontaneous cardiac activity.
As shown in FIG. 4, the receiver or internal unit 12 comprises an RF
detector 60 for receiving the RF signal as derived from the input coil or
antenna 34a, which separates the detected RF signal into power and control
components, the power component energizing the power storage circuit 66.
As will be explained in detail later with respect to FIG. 5, the power
storage circuit 66 provides power for energizing the elements of the
receiver 12. The RF detector 60 also provides a data signal to a pacing
pulse and command decoder 62, which decodes the control signal transmitted
from the transmitter 10 to detect the mode of pacing in which the receiver
12 is to be operated in and for applying energizing pulses to a pacing
output circuit 64. As shown in FIGS. 4 and 3, the output of the pacing
output circuit 64 is connected via leads 17 and 19 to the atrial and
ventrical portions 40 and 42 of the patient's heart. In addition, the
atrial and ventrical leads 17 and 19 are also connected to a monitoring or
second portion of the internal unit 12, which comprises two operational
amplifiers 68 and 70 for amplifying and applying respectively atrial and
ventrical signals to a multiplexer-modulator circuit 74. As will be
explained in detail with respect to FIG. 5, the circuit 74 operates to
energize sequentially the coil or antenna 34b and thereby transmit via the
coil 16 to the receiver 10 signals indicative of the atrial and ventrical
activity of the patient's heart, accompanied by at least one timing
signal. In addition, the circuit 74 modulates the ventrical and atrial
signals in a manner that is not adversely effected by environmental noise,
as occurs to amplitude modulated signals. In an illustrative embodiment of
this invention, circuit 74 pulse width modulates each of the signals
before transmitting same to the receiver 10. It is contemplated that the
circuit 74 may also frequency modulate the atrial and ventrical signals to
transmit them accurately to the external unit 10.
In FIG. 5, there is shown a detailed schematic diagram of the receiver 12
wherein the functional blocks as shown in FIG. 4, are shown and identified
with similar numbers. The transmitter 10 transmits from its primary coil
16 to the secondary coil 34a an RF signal comprised of a train of
amplitude modulated pulses. Each signal of such train comprises a first or
power pulse that is stored in the power storage circuit 66 to provide
energization for the elements of the receiver 12. The initial power pulse
has a width in the order of at least 20 ms that is detected by the
detector 60 in the form of a tuning capacitor C1 connected in parallel
with the antenna or secondary coil 34a. A positive voltage derived from
capacitor C1 is applied through a diode D1 to charge a capacitor C3 to a
value determined by a zener diode D3a, illustratively a plus 10 volts.
Further, a negative voltage is established through diode D3 to charge a
capacitor C4 to a value limited by the zener diode D4a, illustratively a
negative 10 volts. As a review of the schematic of FIG. 5 indicates, these
negative and positive voltages energize the elements of the receiver 12
and are applied to various points throughout the receiver 12.
Illustratively, the first power pulse has a pulse width in excess of 20 ms
and an amplitude in excess of 30 volts (peak to peak), whereby the
capacitors C3 and C4 are charged with a voltage that will not be
discharged for a period in the order of 800 ms, which is long enough to
permit the monitoring portion of the receiver 12 to pick up a P wave and
an R wave, as shown in FIG. 1, of approximately 75 BPM. Further, the first
power pulse is respectively derived from the control 50 of the external
unit 10 once each 800 ms to continue the energization of the internal unit
12 to monitor the patient's heart. When the apparatus is operating in the
telemetry mode only, the pulse width of the initial power pulse is
increased to about 100 ms in a manner to be described. Similarly, during
the transition period between the pacing and telemetry mode and during the
refractory period of the patient's heart, a second power pulse, again of
100 ms length, is transmitted to power the receiver 12.
Further, as indicated in FIG. 5, the train of pulses also include a series
of between 0 and 3 command pulses. If no command pulses are transmitted,
the receiver 12 is commanded to operate in its monitoring mode and no
pacing will be provided. If a single command pulse occurs within 2 ms
after the initial power pulse, the pacing pulse and command decoder 62
decodes such instruction to cause the receiver 12 to pace the atrium at a
pulse width equal to the width of the command pulse. If two pulses follow
within 4 ms of the initial power pulse, the decoder 62 causes the receiver
12 to pace the atrium on the occurrence of the first command pulse, and to
arm the ventricular output circuit 64b on the occurrence of the second
command pulse. If there is a third command pulse occurring within a period
of up to 300 ms of the trailing edge of the initial power pulse, the
decoder 62 will effect a corresponding delayed actuation of the
ventricular output circuit 64b to apply a pacing pulse to the patient's
ventricle. By providing a variable delay before the occurrence of the
ventricular pacing pulse, a sequential atrial-ventricular pacing mode can
be effected.
The command pulses are derived from the capacitor C1 and applied to a
detector circuit comprised of resistor R1 and capacitor C2 which responds
only to the envelope of the power and command pulses, typically having a
width in the order of 20 ms and 0.5 ms, respectively, to turn on
transistor Q1, when a power or command pulse has been so detected. The
voltage established upon capacitor C4 is coupled across resistor R3 and
transistor Q1, whereby its output as derived from its collector is limited
to a value less than that to which capacitor C4 is charged. As explained,
the collector of transistor Q1 is turned on in response to power or
command pulses, whereby positive going pulses of a duration correspondent
to the power or command pulses are applied via a data line 81 to actuate
the atrial and ventrical output circuits 64a and 64b in a manner to be
explained.
Upon the occurrence of the initial power pulse, a corresponding negative
going signal is developed at the collector of the nonconductive transistor
Q1 and is applied via a diode D5 and resistor R4 connected in parallel and
NAND gate 80 to set a one shot 82 upon the trailing edge of the initial
power pulse. The resistor R4, a capacitor C5 and NAND gate 80 act as a
discriminator to prevent the passage of any pulses shorter than
approximately 20 ms, i.e., any command pulses. As a result, the one shot
82 only responds to the power pulse and derives at its output terminal 13
a timing pulse of approximately 2 ms commencing at the trailing edge of
the initial power pulse and being applied to input terminal 2 of the NAND
gate 102, thereby enabling the NAND gate 102 for a period of 2 ms. Thus,
if a command pulse appears upon the data line 81 within this timing window
of 2 ms after the trailing edge of the initial power pulse, an output is
derived from the NAND gate 102 and inverted by digital inverter 104 to
actuate the atrial output circuit 64a and in particular to render
conductive switch 91, whereby a pacing pulse is applied via the lead 17 to
stimulate the atrium 40 of the patient.
In addition, the 2 ms pulse derived from pin 13 of the one shot 82 is
inverted by inverter 100 and is applied to input terminal 3 of flipflop
84, which responds to its trailing edge, that is, at the end of the window
in which the atrial trailing pulse can be activated. The one shot 82 also
generates at its output terminal a 4 ms negative pulse that is applied to
the reset input terminal R of the flipflop 84, which in turn provides a
positive output signal at its output Q following the termination of the
output pulse derived from terminal 13 of one shot 82. This output pulse
derived from flipflop 84 provides an enabling signal to the NAND gate 86,
whereby the second, ventricular arming command pulse may be applied via
the enabled NAND gate 86 to the input terminal 11 of a flipflop 88, which
in turn applies a positive enabling signal from its output terminal 13 to
a NAND gate 90. In this manner, a second window is defined illustratively
in a period between 2 ms and 4 ms following the trailing edge of the first
power pulse, in which window the second ventricular arming command pulse
may appear to arm the ventricular output circuit 64b in preparation to be
actuated by the third command pulse.
At this time, the NAND gate 90 is enabled or armed for a period
illustratively up to 333 ms to wait the third, ventricular pacing command
pulse on the data line 81 and applied to pin 8 of the NAND gate 90. If the
third, ventricular pace command pulse occurs during the 333 ms window, it
is passed via the enabled NAND gate 90, inverted by an inverter 92 to
render conductive a switch 93 of the ventrical output circuit 64b, whereby
a negative ventricular stimulating pulse is applied via the conductive
switches 93 and 95, a capacitor C9 and lead 19 to energize the ventrical
42 of the patient's heart. The switches 91, 93, and 95 are well known in
the art and may illustratively take the form of commercially available
IC's.
As a further feature of this invention, the rate at which the patient's
ventrical can be paced is limited to 180 ventricular BPM by the provision
of a one shot 106. As seen in FIG. 5, the ventricular stimulating pulse as
derived from the output of the inverter 92 is also applied to reset the
one shot 106, which responds thereto by providing a one shot output pulse
of a period of 333 ms from its output terminal 10 to be applied to the
reset terminal R of the flipflop 88 for a corresponding period, whereby
flipflop 88 may not be set to enable the above described ventrical pulse
path (including NAND gate 90 and inverter 92) for a like period. In this
manner, it is assured that the patient's ventricle 42 may not be paced at
a rate above 180 BPM or more often than once each 333 ms, as may occur in
the event of a failure of a circuit component.
Thus, depending on the coded signal transmitted to the internal unit 12,
the artificial pacemaker is capable of operating to apply pacing pulses to
either of the patient's atrium 40 or ventrical 42 or to operate in an
atrial-ventrical sequential pacing mode, wherein the atrium 40 is first
paced and after a selected time delay, the ventrical 42 is paced. In
addition, if no pacing is desired, only the initial power pulse is
applied, which provides a power energization for a subsequent period in
which the remaining elements of the internal unit 12 remain energized, and
a second monitoring portion of this circuit, as will now be described, is
energized for transmitting to the external unit 10, atrial P-type and
ventricular R-type waves as sensed by electrodes applied directly to the
patient's heart. In the following, the monitoring portion of the receiver
12 is explained whereby the atrial and ventricular signals are time
multiplexed and pulse width modulated, to provide a train of pulses
energizing the coil 34b, to induce similar signals into the primary coil
16 of the external unit 10, whereby corresponding atrial and ventricular
signals may be separated and applied to the external monitor 63, as shown
in FIG. 3. During periods of RF transmission when the antenna switch 56 as
shown in FIG. 4 is in a position to transmit only the RF transmission, the
monitoring portion of the receiver 12 is not operative to transmit the
atrial and ventricular signals, because the magnetic field created by the
RF transmission from the coil 16 is of significantly greater magnitude
than that of the atrial and ventricular signals emanating from the coil
34b.
As shown generally in FIG. 4 and in detail in FIG. 5, the electrodes
connected to the atrium 40 and ventrical 42 of the patient's heart are
connected by the leads 17 and 19 to operational amplifiers 68 and 70,
whereby the atrial and ventricular signals are amplified and applied to
the multiplexer/modulator circuit 74. In particular, the atrial and
ventricular signals are applied to a time multiplexing portion of the
circuit 74 for placing these signals in a desired time sequence, along
with a timing signal. In particular, the circuit 74 comprises an
operational amplifier 94 having internal feedback and operated as a
free-running oscillator to provide a square wave output that is applied to
a counter 110, from whose three output terminals are derived in sequence
three timing signals, which are applied in turn to three corresponding
switches 111, 113 and 115 sequentially enabling the three switches in a
timed sequence. The switches 111, 113 and 115 may take the form of
commercially available IC's. In particular, the output of the atrial
operational amplifier 68 is applied to the switch 111, while the output of
the ventricular operational amplifier 70 is applied to the switch 111. A
negative voltage, as derived from the capacitor C3 is applied to the
switch 115 to provide the desired timing signal. Thus, the multiplexer
modifies the square wave output of the oscillator 94 to provide therefrom
in sequence a first signal indicative of the atrial activity of the
patient's heart, a second signal indicative of the ventricular activity of
the patient's heart and a third positive going timing pulse to be applied
to a pulse width modulating circuit essentially comprised of the
operational amplifier 98.
In an illustrative embodiment of this invention, the three timing outputs
of the counter 110 are 1.0 ms in width, occurring every 3 ms to thereby
sample the P-wave signal appearing at the patient's atrium 40 or the
R-wave signal appearing at the patient's ventrical 42. In the illustrative
embodiment, the sampling frequency is 333 Hz. Thus, with the average
P-wave signal lasting longer than 20 ms, the P-wave is sampled about six
times as it rises and falls to provide about six sample amplitudes of
signals to be applied to the amplifier 98. In similar fashion, a number of
sampled amplitudes of the R-wave are derived and applied in sequence with
the pulses indicative of instantaneous P-wave amplitudes to the amplifier
98. The instantaneous sampled amplitudes are transformed into pulse width
modulated signals by amplifier 98.
In particular, the output of the square wave generator formed by the
oscillator 94 is applied to a second free running oscillator 96, which in
turn generates a triangle wave output to be applied via resistor R16 to a
first input of the operational amplifier 98 acting as a comparator and
pulse width modulator. A second input to be summed with the first, is
derived via a resistor R15 from the commonly connected output electrodes
of the switches 111, 113 and 11 | | |
